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resourcerer

resourcerer is a library for declaratively fetching and caching your application's data. Its powerful useResources React hook or withResources higher-order React component (HOC) allows you to easily construct a component's data flow, including:

Additional features include:

Getting started is easy:

  1. Define a model in your application (these are classes descending from Model or Collection):
// js/models/todos-collection.js
import {Collection} from 'resourcerer';

export default class TodosCollection extends Collection {
  url() {
    return '/todos';
  }
}
  1. Create a config file in your application and add your constructor to the ModelMap with a key:
// js/core/resourcerer-config.js
import {register} from 'resourcerer';
import TodosCollection from 'js/models/todos-collection';

// choose any string as its key, which becomes its ResourceKey
register({todos: TodosCollection});

// in your top level js file
import 'js/core/resourcerer-config';

  1. Use your preferred abstraction (useResources hook or withResources HOC) to request your models in any component:

    1. useResources

      import {useResources} from 'resourcerer';

// tell resourcerer which resource you want to fetch in your component
const getResources = (props) => ({todos: {}});

function MyComponent(props) {
  const {
    isLoading,
    hasErrored,
    hasLoaded,
    todosCollection
  } = useResources(getResources, props);
  
  // when MyComponent is mounted, the todosCollection is fetched and available
  // as `todosCollection`!
  return (
    <div className='MyComponent'>
      {isLoading ? <Loader /> : null}

      {hasErrored ? <ErrorMessage /> : null}

      {hasLoaded ? (
        <ul>
          {todosCollection.toJSON().map(({id, name}) => (
            <li key={id}>{name}</li>
          ))}
        </ul>
      ) : null}
    </div>
  );
}

    2. withResources

      import React from 'react';
import {withResources} from 'resourcerer';

// tell resourcerer which resource you want to fetch in your component
@withResources((props) => ({todos: {}}))
class MyComponent extends React.Component {
  render() {
    // when MyComponent is mounted, the todosCollection is fetched and available
    // as `this.props.todosCollection`!
    return (
      <div className='MyComponent'>
        {this.props.isLoading ? <Loader /> : null}

        {this.props.hasErrored ? <ErrorMessage /> : null}

        {this.props.hasLoaded ? (
          <ul>
            {this.props.todosCollection.map((todoModel) => (
              <li key={todoModel.id}>{todoModel.get('name')}</li>
            ))}
          </ul>
        ) : null}
      </div>
    );
  }
}

There's a lot there, so let's unpack that a bit. There's also a lot more that we can do there, so let's also get into that. But first, some logistics:

Contents

  1. Installation
  2. Nomenclature
  3. Tutorial
    1. Intro
    2. Other Props Returned from the Hook/Passed from the HOC (Loading States)
    3. Requesting Prop-driven Data
    4. Changing Props
    5. Common Resource Config Options
      1. params
      2. options
      3. noncritical
      4. force
      5. Custom Resource Names
      6. prefetches
      7. data
      8. lazy
      9. minDuration
      10. dependsOn
      11. provides
    6. Data mutations
    7. Serial Requests
    8. Differences between useResources and withResources
    9. Using resourcerer with TypeScript
    10. Caching Resources with ModelCache
    11. Declarative Cache Keys
    12. Prefetch on Hover
    13. Refetching
    14. Cache Invalidation
    15. Tracking Request Times
  4. Configuring resourcerer
  5. FAQs
  6. Migrating to v2.0

Installation

$ npm i resourcerer or yarn add resourcerer

resourcerer requires on React >= 16.8 but has no external dependencies.

Note: Resourcerer is written in TypeScript and is compiled to ESNext. It does no further transpiling—including import/export. If you are using TypeScript yourself, this won't be a problem. If you're not, and you're not babelifying (or similar) your node_modules folder, you'll need to make an exception for this package, ie:

// webpack.config.js or similar
module: {
  rules: [{
    test: /\.jsx?$/,
    exclude: /node_modules\/(?!(resourcerer))/,
    use: {loader: 'babel-loader?cacheDirectory=true'}
  }]
}

Nomenclature

  1. Props. Going forward in this tutorial, we'll try to describe behavior of both the useResources hook and the withResources HOC at once; we'll also rotate between the two in examples. Note that if we talking about a passed prop of, for example isLoading, that that corresponds to an isLoading property returned from the hook and a this.props.isLoading prop passed down from the HOC.

  2. ResourceKeys. These are the keys of the object passed to the register function in your top-level resourcerer-config.js file (discussed above in the introduction). The object is of type Record<ResourceKeys, new () => Model | new () => Collection>. These keys are passed to the executor functions and are used to tell the hook or HOC which resources to request.

  3. Executor Function. The executor function is a function that both the hook and HOC accept that declaratively describes which resources to request and with what config options. In these docs you'll often see it assigned to a variable called getResources. It accepts props as arguments and may look like, as we'll explore in an example later:

    const getResources = (props) => ({user: {path: {userId: props.id}}});

    or

    const getResources = (props) => {
  const now = Date.now();
  
  return {
    userTodos: {
      params: {
        limit: 20,
        end_time: now,
        start_time: now - props.timeRange,
        sort_field: props.sortField
      }
    }
  };
};

    It returns an object whose keys represent the resources to fetch and whose values are Resource Configuration Objects that we'll discuss later (and is highlighted below).

  4. Resource Configuration Object. In the object returned by our executor function, each entry has a key equal to one of the ResourceKeys and whose value we will refer to in this document as a Resource Configuration Object, or Resource Config for short. It holds the declarative instructions that useResources and withResources will use to request the resource.

Tutorial

Okay, back to the initial example. Let's take a look at our useResources usage in the component:

// `@withResources((props) => ({todos: {}}))`
const getResources = (props) => ({todos: {}});

export default function MyComponent(props) {
  const resources = useResources(getResources, props);
  
  // ...
}

You see that useResources takes an executor function that returns an object. The executor function takes a single argument: the current props, which are component props when you use withResources, but can be anything when you use useResources. The executor function returns an object whose keys are ResourceKeys and whose values are Resource Config objects. Where do ResourceKeys come from? From the object passed to the register method in the config file we added earlier!

// js/core/resourcerer-config.js
import {register} from 'resourcerer';
import TodosCollection from 'js/models/todos-collection';

// after adding this key, `todos` can be used in our executor functions to reference the Todos resource.
// The 'todos' string value will also be the default prefix for all todos-related return values.
// That's why we have `props.todosCollection`!
register({todos: TodosCollection});

(We can also pass custom prefixes for our prop names in a component, but we'll get to that later.)

Back to the executor function. In the example above, you see it returns an object of {todos: {}}. In general, the object it should return is of type {[key: ResourceKeys]: ResourceConfigObject}, where ResourceConfigObject is a generic map of config options. It can contain as many keys as resources you would like the component to request. In our initial example, the Resource Config Object was empty. Further down, we'll go over the plethora of options and how to use them. For now, let's take a look at some of the resource-related props this simple configuration provides our component.

Other Props Returned from the Hook/Passed from the HOC (Loading States)

Of course, in our initial example, the todosCollection won’t be populated with data immediately since, after all, the resource has to be fetched from the API. Some of the most significant and most common React UI states we utilize are whether a component’s critical resources have loaded entirely, whether any are still loading, or whether any have errored out. This is how we can appropriately cover our bases—i.e., we can ensure the component shows a loader while the resource is still in route, or if something goes wrong, we can ensure the component will still fail gracefully and not break the layout. To address these concerns, the useResources hook/withResources HOC gives you several loading state helper props. From our last example:

isLoading , hasLoaded , and hasErrored are not based on individual loading states, but are rather a collective loading state for the aforementioned-critical component resources. In the previous example, the todos resource is the only critical resource, so isLoading / hasLoaded / hasErrored are solely based on todosLoadingState. But we can also add a non-critical users resource, responsible, say, for only display users' names alongside their TODOs—a small piece of the overall component and not worth delaying render over. Here’s how we do that:

const getResources = (props) => ({
  todos: {},
  users: {noncritical: true}
});

function MyClassWithTodosAndAUsers(props) {
  const resources = useResources(getResources, props);
}

MyClassWithTodosAndAUsers will now receive the following loading-related props, assuming we've registered a usersCollection in our config file:

In this case, isLoading , et al, are only representative of todosLoadingState and completely irrespective of usersLoadingState . This allow us an incredible amount of flexibility for rendering a component as quickly as possible.

Here’s how might use that to our advantage in MyClassWithTodosAndAUsers :

import {Utils} from 'resourcerer';

function MyClassWithTodosAndAUsers(props) {
  const {
    isLoading,
    hasErrored,
    hasLoaded,
    todosCollection,
    usersCollection,
    usersLoadingState
  } = useResources(getResources, props);
  
  var getUserName = (userId) => {
    // usersCollection guaranteed to have returned here
    var user = usersCollection.find(({id}) => id === userId);
            
    return (
      <span className='user-name'>
        {user && user.id || 'N/A'}
      </span>
    );
  };
          
  return (
    <div className='MyClassWithTodosAndUsers'>
      {isLoading ? <Loader /> : null}
          
      {hasLoaded ? (
        // at this point we are guaranteed all critical resources have returned.
        // before that, todosCollection is still a Collection instance that can be
        // mapped over--it's just empty
        <ul>
          {todosCollection.map((todoModel) => (
            <li key={todoModel.id}>
              // pure function that accepts loading states as arguments
              {Utils.hasLoaded(usersLoadingState) ?
                getUserName(todoModel.get('userId')) :
                // if you're anti-loader, you could opt to render nothing and have the
                // user name simply appear in place after loading
                <Loader type="inline" />}
              {todoModel.get('name')}
            </li>
          )}
        </ul>
      ) : null}
          
      {hasErrored ? <ErrorMessage /> : null}
    </div>
  );

Here's a real-life example from the Sift Console, where we load a customer's workflows without waiting for the workflow stats resource, which takes much longer. Instead, we gracefully show small loaders where the stats will eventually display, all-the-while keeping our console interactive:

Noncritical Resource Loading

And here's what it looks like when the stats endpoint returns:

Noncritical Resource Returned

There’s one other loading prop offered from the hook/HOC: hasInitiallyLoaded. This can be useful for showing a different UI for components that have already fetched the resource. An example might be a component with filters: when a filter is changed after the initial resource is loaded (thus re-fetching the resource), we may want to show a loader with an overlay over the previous version of the component. See the Advanced Topics docs for more.

Requesting Prop-driven Data

Let's say we wanted to request not the entire users collection, but just a specific user. Here's our config:

// js/core/resourcerer-config.js
import {register} from 'resourcerer';
import TodosCollection from 'js/models/todos-collection';
import UserModel from 'js/models/user-model';

register({
  todos: TodosCollection,
  user: UserModel
});

And here's what our model might look like (NOTE: use a Model when you've got one resource instance, and a Collection when you've got a list of that resource):

// js/models/user-model.js
import {Model} from 'resourcerer';

export default class UserModel extends Model {
  url({userId}) {
    // `userId` is passed in here because we have
    // `path: {userId: props.id}` in the resource config object
    return `/users/${userId}`;
  }
  
  static dependencies = ['userId'];
}

The dependencies static property is important here, as we'll see in a second; it is a list of properties that resourcerer will use to generate a cache key for the model. It will look for the userId property in the following places, in order:

  1. the path object
  2. the data object
  3. the params object

All three of these come from via the Resource Configuration Object that is returned from our executor function; it might look like this:

const getResources = (props) => ({user: {path: {userId: props.id}}}) 

// hook
function MyComponent(props) {
  const resources = useResources(getResources, props);
  
  // ...
}

// HOC
@withResources(getResources)
class MyComponentWithAUser extends React.Component {}

Assuming we have a props.id equal to 'noahgrant', this setup will put MyComponentWithAUser in a loading state until /users/noahgrant has returned.

...and here's the best part:

Let's say that props.id changes to a different user. MyComponentWithAUser will get put back into a loading state while the new endpoint is fetched, without us having to do anything! This works because our model has dictated that its models should be cached by a userId field, which is passed to it in the path property.

Changing Props

In general, there are two ways to change props.id as in the previous example:

  1. Change the url, which is the top-most state-carrying entity of any application. The url can be changed either by path parameter or query paramter, i.e. example.com/users/noahgrant -> example.com/users/bobdonut, or example.com/users?id=noahgrant -> example.com/users?id=bobdonut. In this case, each prop change is indexable, which is sometimes desirable, sometimes not.

  2. Change internal application state. For these cases, useResources/withResources make available another handy prop: setResourceState. setResourceState is a function that has the same method signature as the useState we all know and love. It sets internal hook/HOC state, which is then returned/passed down, respectively, overriding any initial prop, ie setResourceState((state) => ({...state, id: 'bobdonut'})). This is not indexable.

    Note that setResourceState is very useful for the withResources HOC because it allows you to 'lift' state above the fetching component that otherwise would not be possible. For useResources, it is a nice-to-have in some cases, but because you can always define your own useState above the useResources invocation, you may find that you use it less often.

Common Resource Config Options

params

The params option is passed directly to the sync method and sent either as stringified query params (GET requests) or as a body (POST/PUT). Its properties are also referenced when generating a cache key if they are listed in a model's static dependencies property (See the cache key section for more). Let's imagine that we have a lot of users and a lot of todos per user. So many that we only want to fetch the todos over a time range selected from a dropdown, sorted by a field also selected by a dropdown. These are query parameters we'd want to pass in our params property:

  @withResources((props) => {
    const now = Date.now();
      
    return {
      userTodos: {
        params: {
          limit: 20,
          end_time: now,
          start_time: now - props.timeRange,
          sort_field: props.sortField
        }
      }
    };
  })
  class UserTodos extends React.Component {}

Now, as the prop fields change, the params sent with the request changes as well (provided we set our dependencies property accordingly):

https://example.com/users/noahgrant/todos?limit=20&end_time=1494611831024&start_time=1492019831024&sort_field=importance

path

As referenced previously, all properties on an path object will be passed into a model's/collection's url function. This makes it an ideal place to add path parameters (contrast this with the params object, which is the place to add query (GET) or body (POST/PUT/PATCH) parameters). It will also be used in cache key generation if it has any fields specified in the model's static dependencies property (See the cache key section for more). Continuing with our User Todos example, let's add an path property:

const getResources = (props) => {
  const now = Date.now();
      
  return {
    userTodos: {
      params: {
        limit: 20,
        end_time: now,
        start_time: now - props.timeRange,
        sort_field: props.sortField
      },
      path: {userId: props.userId}
    }
  };
};

Here, this UserTodosCollection instance will get a userId property passed to its url function. We'll also want to add the 'userId' string to the collection's static dependencies array, because each cached collection should be specific to the user:

// js/models/user_todos_collection.js
export class UserTodosCollection extends Collection {
  url({userId}) {
    // userId gets passed in for us to help construct our url
    return `/users/${userId}/todos`;
  }
  
  static dependencies = ['userId'];
};

noncritical

As alluded to in the Other Props section, not all resources used by the component are needed for rendering. By adding a noncritical: true option, we:

force

Sometimes you want the latest of a resource, bypassing whatever model has already been cached in your application. To accomplish this, simply pass a force: true in a resource's config. The force-fetched response will replace any prior model in the cache.

  const getResources = (props) => ({latestStats: {force: true}});

  function MyComponentWithLatestStats(props) {
    const {latestStatsModel} = useResources(getResources, props);
  }

The resource will only get force-requested when the component mounts; the force flag will get ignored on subsequent updates. If you need to refetch after mounting to get the latest resource, use refetch.

This behavior is similar to the behavior you get with cache invalidation.

Custom Resource Names

Passing a resourceKey: <ResourceKeys> option allows you to pass a custom name as the withResources key, which will become the base name for component-related props passed down to the component. For example, this configuration:

const getResources = (props) => ({myRadTodos: {resourceKey: todos});

export default function MyComponentWithTodos {
  const {
    myRadTodosCollection,
    myRadTodosLoadingState,
    myRadTodosStatus,
    ...rest
  } = useResources(getResources, props);
}

would still fetch the todos resource, but the properties returned/props passed to the MyComponentWithTodos instance will be myRadTodosCollection, myRadTodosLoadingState, and myRadTodosStatus, etc, as shown. This also allows us to fetch the same resource type multiple times for a single component.

NOTE: when using resourcerer with Typescript (recommended), it will complain about custom resource names, but it will work. For now, you'll need to // @ts-ignore, but please submit a PR :).

prefetches

This option is an array of props objects that represent what is different from the props in the original resource. For each array entry, a new resource configuration object will be calculated by merging the current props with the new props, and the resulting request is made. In contrast to the original resource, however, no props representing the prefetched requests are returned or passed down to any children (ie, there are no loading state props, no model props, etc). They are simply returned and kept in memory so that whenever they are requested, they are already available.

A great example of this is for pagination. Let's take our previous example and add a from property to go with our limit that is based on the value of a page prop (tracked either by url parameter or by setResourceState). We want to request the first page but also prefetch the following page because we think the user is likely to click on it:

const getResources = (props) => {
  const now = Date.now();
  const REQUESTS_PER_PAGE = 20;
      
  return {
    userTodos: {
      params: {
        from: props.page * REQUESTS_PER_PAGE,
        limit: REQUESTS_PER_PAGE,
        end_time: now,
        start_time: now - props.timeRange,
        sort_field: props.sortField
      },
      path: {userId: props.userId},
      // this entry is how we expect the props to change. in this case, we want props.page to be
      // incremented. the resulting prefetched request will have a `from` value of 20, whereas the
      // original request will have a `from` value of 0. The `userTodosCollection` returned (hook) or
      // passed down as props (HOC) will be the latter.
      prefetches: [{page: props.page + 1}]
    }
  };
};

When the user clicks on a 'next' arrow that updates page state, the collection will already be in the cache, and it will get passed as the new userTodosCollection. Accordingly, the third page will then get prefetched (props.page equal to 2 and from equal to 40). Don't forget to add from to the dependencies list!

If you're looking to optimistically prefetch resources when a user hovers, say, over a link, see the Prefetch on Hover section.

data

Pass in a data hash to initialize a Model instance with data before initially fetching. This is passed directly to the model constructor method, and is typically much less useful than providing the properties directly to the params property. One place it might be useful is to seed a model with an id you already have:

getResources = () => ({customer: {data: {id: props.customerId}}});

function MyCustomerComponent(props) {
  const {customerModel} = useResources(getResources, props);

  // now you can reference the id directly on the model
  console.log(customerModel.get('id')); // or the id shorthand, customerModel.id
}

You can also use data to take advantage of re-caching.

Like params and path, the data object will also be used in cache key generation if it has any fields specified in the model's static dependencies property (See the cache key section for more).

lazy

Lazy fetching is one of resourcerer's most powerful features, allowing you to get a reference to a model without actually fetching it. If the model is ultimately fetched elsewhere on the page, the component that lazily fetched it will still listen for updates.

A great example of when this would be useful is for search results. Search results are read-only, but if you modify the entity of a result somewhere else in the page, you'd like to see it reflected in your search results. Yet you don't want to fetch the entity details for every search result and spam your API. Enter lazy loading:

// todo_search.jsx
getResources = () => ({todosSearch: {params: someSearchParams}});

function TodoSearch(props) {
  const {todoSearchModel} = useResources(getResources, props);

  return (
    <Table>
      <thead>
        <TableHeader>Name</TableHeader>
        <TableHeader>Last Updated</TableHeader>
      </thead>
      <tbody>
        {todoSearchModel.get('results').map((todo) => <TodoSearchItem {...todo} />)}
      </tbody>
    </Table>
  );
}

// todo_search_item.jsx
// the todoModel is never actually fetched here, it's only listened on, allowing any changes made elsewhere in the page to be reflected here.
getResources = () => ({todo: {id: props.id, lazy: true}});

function TodoSearchItem(props) {
  const {todoModel} = useResources(getResources, props);

  return (
    <tr>
      <td>{todoModel.get('name') || props.name}</td>
      <td>{todoModel.get('updated_at') || props.updated_at}</td> 
    </tr>
  );

If the todo model has been fetched already, we'll read straight from that. And if it gets updated, this component, via resourcerer, is listening for updates and will re-render to keep our entire UI in sync. Otherwise, we'll just fall back to our read-only search results. WIN!

minDuration

Sometimes requests can be too fast for certain UIs. In these cases, spinners and other loading states can appear more like a jarring flicker than a helpful status indicator. For these, you can pass a minDuration equal to the minimum number of milliseconds that a request should take. This is great for save and destroy requests. It will work for fetch requests via useResources, as well, but beware: if multiple components use the same resource and there are different (or missing) values for minDuration, this will cause a race condition.

dependsOn

See the section on serial requests.

provides

See the section on serial requests.

Data Mutations

So far we've only discussed fetching data. But resourcerer also makes it very easy to make write requests via the Model and Collection instances that are returned. These classes are enriched data structures that hold our API server data as well as several utilities that help manage the server data in our application. There are three main write operations via these classes:

  1. Model#save

    Use this to create a new resource object (POST) or update an existing one (PUT). Uses the return value of the isNew() method to determine which method to use. If updating, pass a {patch: true} option to use PATCH instead of PUT, which will also send over only the changed attributes instead of the entire resource.

    function MyComponent(props) {
  const {myModel} = useResources(getResources, props),
        onSave = () => myModel.save({foo: 'bar'})
          .then([model]) => // ...)
          .catch(() => alert('request failed'));
    
  return <button onClick={onSave}>Persist model</button>;
}

  2. Model#destroy

    Use this to make a DELETE request at a url with this model's id. Will also remove the model from any collection it is a part of.

    myModel.destroy().catch(() => alert('Model could not be destroyed));

  3. Collection#create

    If working with a collection instead of a model, .create() adds a new model to the collection and then persists it to the server (via model.save()). This is pretty convenient:

    function TodoDetails(props) {
  const {hasLoaded, todosCollection} = useResources(getResources, props),
        todoModel = todosCollection.get(props.id),
  
        onSaveTodo = {
          // set some loading state...
    
          if (!props.id) {
            // create new todo!
            return todosCollection.create(attrs)
              .then(([model]) => // ...)
              .catch(() => alert('create failed'));
              .then(() => // remove loading state);
          }
    
          // update existing
          todoModel.save(attrs).then(([model]) => ...).catch(() => alert('update failed'));
        };
  
   if (hasLoaded && props.id && !todoModel) {
     return <p>Todo not found.</p>;
   }

   return (
     // ...
     <button onClick={onSaveTodo}>Save</button>
   );
}

Each one of these methods exhibit the following behaviors:

  1. They automatically fire off the appropriate request with the right data and at the right url
  2. They will cause every component registered to that resource to re-render with the updated data, keeping the application in sync
  3. On error, they undo the changes that were done (and their registered components render again).

Note:

  1. All calls resolve an array, which is a tuple of [model, response]. All reject with just the response.
  2. All write calls must have a .catch attached, even if the rejection is swallowed. Omitting one risks an uncaught Promise rejection exception if the request fails.

Serial Requests

In most situations, all resource requests should be parallelized; but that’s not always possible. Every so often, there may be a situation where one request depends on the result of another. For these cases, we have the dependsOn resource config option and the provides resource config option. These are probably best explained by example, so here is a simplified instance from the Sift Console, where we load a queue item that has info about a user, but we can't get further user information until we know what user id belongs to this queue item.

@withResources((props) => ({
  user: {
    path: {userId: props.userId},
    dependsOn: !!props.userId
  },
  queueItem: {
    data: {id: props.itemId},
    provides: (queueItemModel) => ({userId: queueItemModel.get('userId')})
  }
}))
export default class QueueItemPage extends React.Component {}


In this simplified example, only `props.itemId` is initially present at the url `items/<itemId>`, and since the UserModel depends on `props.userId` being present, that model won’t initially get fetched. Only the QueueItemModel gets fetched at first; it has the `provides` option, which is function that takes an instance of the returned model or collection and returns a map of `{[key: string]: any}`. Each key is a new prop name, and each value the new prop's value.

So, in this case, the returned `queueItemModel` instance is passed as an argument, and we return a string that will be assigned to `props.userId` (or, more accurately, will be set as state via `setResourceState` as described in the previous section). At tthis point, `props.userId` exists, and the UserModel will be fetched. And we have serially requested our resources!

One thing to note here is that while the `queueItem` resource is being fetched, the user resource is in a `PENDING` state, which is a special state that does not contribute to overall component `isLoading`/`hasErrored` states (though it will keep the component from being `hasLoaded`). At this point, the `QueueItemPage` in the example above is in a `LOADING` state (`isLoading === true`) because `QUEUE_ITEM` is loading. When it returns with the user id, the `user` resource is put into a `LOADING` state, and the component then remains `isLoading === true` until it returns, after which the component has successfully loaded. If the `queueItem` resource happened to error for some reason, the `user` resource would never get out of its `PENDING` state, and the component would then take on the `ERROR` state (`hasErrored === true`) of `queueItem`. For more on `PENDING`, see [Thoughts on the PENDING State](/docs/advanced_topics.md#thoughts-on-the-pending-resource) in the [Advanced Topics document](/docs/advanced_topics.md).  

Finally, note that the `provides` function can return any number of fields we want to set as new props for other resources:

```js
const getResources = (props) => ({
  user: {
    options: {state: props.activeState, userId: props.userId},
    // userModel depends on multiple props from queueItemModel
    dependsOn: !!props.userId && props.activeState === "active"
  },
  queueItem: {
    data: {id: props.itemId},
    provides: (queueItemModel) => ({userId: queueItemModel.get('userId'), activeState: queueItemModel.get('state')})
  }
});
  
export default function QueueItemPage(props) {
  // activeState and userId are internal state within `useResources` and returned
  const {
    activeState,
    userId,
    userModel,
    queueItemModel
  } = useResources(getResources, props);
}

Differences between useResources and withResources

The hook and HOC largely operate interchangeably, but do note a couple critical differences:

  1. The withResources HOC conveniently contains an ErrorBoundary with every instance, but such functionality does not yet exist in hooks. This is a definite advantage for the HOC right now, since, if we're already setting hasErrored clauses in our components to prepare for request errors, we can naturally gracefully degrade when an unexpected exception occurs. You'll need to manage this yourself with hooks until the equivalent functionality is released.

  2. The executor function for a hook can be inlined in your component, which puts component props in its closure scope. So be extra careful to avoid this anti-pattern:

    function MyComponent({start_time, ...props}) {
  const {todosCollection} = useResources((_props) => ({todos: {params: {start_time}}}), props);
  
  // ...

    The subtle problem with the above is that the start_time executor function parameter is relying on a value in the function component closure instead of the _props parameter object; props passed to the executor function can be current or previous but are not the same as what is in the closure, which will always be current. This will lead to confusing bugs, so instead either read directly from the props parameter passed to the executor function:

    function MyComponent(props) {
  const {todosCollection} = useResources(({start_time}) => ({todos: {params: {start_time}}}), props);
  
  // ...

    or, even clearer, define your executor function outside of the component scope, as we've done throughout this tutorial (now you know why!):

    const getResources = ({start_time}) => ({todos: {params: {start_time}}});

function MyComponent(props) {
  const {todosCollection} = useResources(getResources, props);
  
  // ...

Caching Resources with ModelCache

resourcerer handles resource storage and caching, so that when multiple components request the same resource with the same parameters or the same body, they receive the same model in response. If multiple components request a resource still in-flight, only a single request is made, and each component awaits the return of the same resource. Fetched resources are stored in the ModelCache. Under most circumstances, you won’t need to interact with directly; but it’s still worth knowing a little bit about what it does.

The ModelCache is a simple module that contains a couple of Maps—one that is the actual cache {[cacheKey: string]: Model | Collection}, and one that is a component manifest, keeping track of all component instances that are using a given resource (unique by cache key). When a component unmounts, resourcerer will unregister the component instance from the component manifest; if a resource no longer has any component instances attached, it gets scheduled for cache removal. The timeout period for cache removal is two minutes by default (but can be changed, see Configuring resourcerer, or overridden on a model-class basis), to allow navigating back and forth between pages without requiring a refetch of all resources. After the timeout, if no other new component instances have requested the resource, it’s removed from the ModelCache. Any further requests for that resource then go back through the network.

Again, it’s unlikely that you’ll use ModelCache directly while using resourcerer, but it’s helpful to know a bit about what’s going on behind-the-scenes.

Declarative Cache Keys

As alluded to previously, resourcerer relies on the model classes themselves to tell it how it should be cached. This is accomplished via a static dependencies array, where each entry can be either:

  1. A string, where each string is the name of a property that the model receives whose value should take part in the cache key. The model can receive this property either from the path hash, the data hash, or the params hash, in that order.

  2. A function, whose return value is an object of keys and values that should both contribute to the cache key.

Let's take a look at the userTodos resource from above, where we want to request some top number of todos for a user sorted by some value over some time range. The resource declaration might look like this:

const getResources = (props) => {
  const now = Date.now();
      
  return {
    userTodos: {
      params: {
        limit: props.limit,
        end_time: now,
        start_time: now - props.timeRange,
        sort_field: props.sortField
      },
      path: {userId: props.userId}
    }
  };
};

And our corresponding model definition might look like this:

export class UserTodosCollection extends Collection {
  url({userId}) {
    return `/users/${userId}/todos`;
  }
  // ...
  
  static dependencies = [
    'limit',
    'userId',
    'sort_field',
     ({end_millis, start_millis}) => ({range: end_millis - start_millis})
  ];
};

We can see that limit and sort_field as specified in dependencies are taken straight from the params object that resourcerer transforms into url query parameters. userId is part of the /users/{userId}/todos path, so it can't be part of the params object, which is why it gets passed in the path object instead.

The time range is a little tougher to cache, though. We're less interested the spcecific end_time/start_time values to the millisecond—it does us little good to cache an endpoint tied to Date.now() when it will never be the same for the next request. We're much more interested in the difference between end_time and start_time. This is a great use-case for a function entry in dependencies, which takes the params object passed an argument. In the case above, the returned object will contribute a key called range and a value equal to the time range to the cache key.

The generated cache key would be something like userTodos~limit=50_$range=86400000_sort_field=importance_userId=noah. Again, note that:

Prefetch on Hover

You can use resourcerer's executor function to optimistically prefetch resources when a user hovers over an element. For example, if a user hovers over a link to their TODOS page, you may want to get a head start on fetching their todos resource so that perceived loading time goes down or gets eliminated entirely. We can do this with the top-level prefetch function:

import {prefetch} from 'resourcerer';

// here's our executor function just as we pass to useResources or withResources
const getTodos = (props) => {
  const now = Date.now();
      
  return {
    userTodos: {
      params: {
        limit: props.limit,
        end_time: now,
        start_time: now - props.timeRange,
        sort_field: props.sortField
      },
      path: {userId: props.userId}
    }
  };
};

// in your component, call the prefetch method with the executor and an object that matches
// what you expect the props to look like when the resources are requested without prefetch.
// attach the result to an `onMouseEnter` prop
<a href='/todos' onMouseEnter={prefetch(getTodos, expectedProps)}>TODOS</a>

Note, as mentioned in the comment above, that expectedProps should take the form of props expected when the resource is actually needed. For example, maybe we're viewing a list of users, and so there is no props.userId in the component that uses prefetch. But for the user in the list with id 'noahgrant', we would pass it an expectedProps that includes {userId: 'noahgrant'} because we know that when we click on the link and navigate to that url, props.userId should be equal to 'noahgrant'.

Refetching

resourcerer also returns a refetch function that you can use to re-request a resource that has already been requested on-demand. A couple examples of where this could come in handy:

  1. A request timed out and you want to give the user the option of retrying.
  2. You have made a change to one resource that may render an auxiliary resource stale, and you want to bring the auxiliary resource up-to-date.

The function takes a single or a list of ResourceKeys. Each entry will get refetched.

function MyComponent(props) {
  const {todosCollection, refetch} = useResources(({start_time}) => ({todos: {params: {start_time}}}), props);
      
  // ...
  
  return <Button onClick={() => refetch("todos")}>Refetch me</Button>;

NOTE:

Cache Invalidation

In some cases you may want to imperatively remove a resource from the cache. For example, you may make a change to a related resource that renders a resource invalid. For those cases, useResources returns an invalidate function that takes a single or a list of ResourceKeys:

function MyComponent(props) {
  const {todosCollection, invalidate} = useResources(({start_time}) => ({todos: {params: {start_time}}}), props);
      
  // ...
  
  return <Button onClick={() => invalidate(["todos"])}>Invalidate me</Button>;

Tracking Request Times

If you have a metrics aggregator and want to track API request times, you can do this by setting a measure static property on your model or collection. measure can either be a boolean or a function that returns a boolean. The function takes the resource config object as a parameter:

import {Model} from 'resourcerer';

class MyMeasuredModel extends Model {
  // either a boolean, which will track every request of this model instance
  static measure = true;

  // or a function that returns a boolean, which will track instance requests based on a condition
  static measure = ({data={}}) => data.id === 'noahgrant';
}

When the static measure property is/returns true, resourcerer will record the time it takes for that resource to return and pass the data to the track configuration method that you can set up, sending it to your own app data aggregator. This allows you to see the effects of your endpoints from a user’s perspective.

Configuring resourcerer

The same config file used to register your models also allows you to set custom configuration properties for your own application:

import {ResourcesConfig} from 'resourcerer';

ResourcesConfig.set(configObj);

ResourcesConfig.set accepts an object with any of the following properties:

FAQs

Migrating to v2.0

See the Migrating to v2.0 doc.