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PMHTTP

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PMHTTP is an HTTP framework built around URLSession and designed for Swift while retaining Obj-C compatibility.

We think URLSession is great. But it was designed for Obj-C and it doesn't handle anything beyond the networking aspect of a request. This means no handling of JSON, and it doesn't even provide multipart/form-data uploads. PMHTTP leaves the networking to URLSession and provides everything else. Features include:

PMHTTP was designed specifically for the HTTP functionality that Postmates needs. This means first-class REST support with a focus on JSON. But there's some functionality it doesn't handle which we may get around to doing at some point (see issues). Pull requests are welcome.

Table of Contents

Usage

A typical GET request looks like:

// https://api.example.com/v1/search?query=%s
let task = HTTP.request(GET: "search", parameters: ["query": "cute cats"])
    .parseAsJSON(using: { (response, json) in
        return try JSON.map(json.getArray(), Cat.init(json:))
    })
    .performRequest(withCompletionQueue: .main) { task, result in
        switch result {
        case let .success(response, cats):
            // Do something with the Cats.
        case let .error(response, error):
            // Handle the error. This includes network errors, JSON parse errors,
            // and any error thrown by Cat.init(json:).
        case .canceled:
            // The task was canceled. Ignore or handle as appropriate.
        }
}
// task can be canceled and can be queried for its state
// and this can be done from any thread.

A POST request might look like:

// https://api.example.com/v1/submit_cat
let task = HTTP.request(POST: "submit_cat", parameters: ["name": "Fluffles", "color": "tabby"])
    .parseAsJSON(using: { result in
        // POST parse blocks take a single `result` argument because 204 No Content is a valid
        // response. The `result` enum vends an optional `value` property, and has a
        // `getValue()` method that throws an error if the response was 204 No Content.
        return try SubmitCatResponse(json: result.getValue())
    })
    .performRequest(withCompletionQueue: .main) { task, result in
        switch result {
        case let .success(response, value):
            // value is a SubmitCatResponse.
        case let .error(response, error):
            // Handle the error. This could be a network error, a JSON parse error, or
            // any error thrown by SubmitCatResponse.init(json:).
        case .canceled:
            // The task was canceled.
        }
}

A multipart/form-data upload might look like:

// https://api.example.com/v1/submit_cat with photo
let req = HTTP.request(POST: "submit_cat", parameters: ["name": "Fluffles", "color": "tabby"])!
// We could add the image synchronously, but it's better to be asynchronous.
// Note: There is a convenience function to do this already, this is just an example.
req.addMultipartBody { upload in
    // This block executes on a background queue.
    if let data = UIImageJPEGRepresentation(catPhoto, 0.9) {
        upload.addMultipart(data: data, withName: "photo", mimeType: "image/jpeg")
    }
}
let task = req.parseAsJSON(using: { try SubmitCatResponse(json: $0.getValue()) })
    .performRequest(withCompletionQueue: .main) { task, result in
        // ...
}

Setup

You can modify the properties of the global HTTPManager object at any time, but to make setup easier, if your UIApplicationDelegate or NSApplicationDelegate object conforms to the HTTPManagerConfigurable protocol it will be asked to configure the HTTPManager the first time the HTTP global variable is accessed. This might look like:

extension AppDelegate: HTTPManagerConfigurable {
    public func configure(httpManager: HTTPManager) {
        httpManager.environment = HTTPManager.Environment(string: /* ... */)
        let config = URLSessionConfiguration.default
        config.timeoutIntervalForRequest = 10
        // PMHTTP defines a default User-Agent but we can supply our own
        config.HTTPAdditionalHeaders = ["User-Agent": myUserAgent]
        httpManager.sessionConfiguration = config
        if let (username, apiKey) = getAPICredentials() {
            httpManager.defaultCredential = URLCredential(user: username, password: apiKey, persistence: .forSession)
        }
        httpManager.defaultRetryBehavior = HTTPManagerRetryBehavior.retryNetworkFailureOrServiceUnavailable(withStrategy: .retryTwiceWithDefaultDelay)
    }
}

Detailed Design

PMHTTP was designed with 6 goals in mind:

HTTPManager

The overall manager class for PMHTTP is HTTPManager. This is the class that allows you to configure various global properties and to create new requests. Multiple managers can be created if desired, but a single global instance is provided under the global property HTTP (for Obj-C this is [HTTPManager defaultManager]). All properties and methods on this class are completely thread-safe.

Configuration of the shared HTTP instance can be done by adopting the HTTPManagerConfigurable protocol on your app delegate. This protocol provides a method that can be used to configure the shared HTTPManager object the first time the HTTP property is accessed. This design allows you to ensure the shared instance is properly configured prior to first use even if it's used prior to the normal entry point for your application (e.g. inside some class's +load method). Do note, however, that this method will be executed on whatever thread is first accessing the HTTP property, and so it should be safe to run from any thread.

Important: The shared HTTP instance is a convenience intended for use by the application. If you're writing a shared component (e.g. a framework) that uses PMHTTP, you need to carefully consider whether using HTTP is appropriate or whether you should be using a separate instance of HTTPManager. The use of HTTP is only appropriate if you want to automatically adopt any configuration the application provides (including environment and default credential).

Environments

HTTPManager has a property environment of type HTTPManager.Environment. An environment is a simple wrapper around a URL and represents the base URL that requests should use if the request is not made with an absolute URL. You may wish to create your own extension that looks something like:

extension HTTPManager.Environment {
    // @nonobjc works around "a declaration cannot be both 'final' and 'dynamic'" error.
    @nonobjc static let Production = HTTPManager.Environment(baseURL: productionURL)
    @nonobjc static let Staging = HTTPManager.Environment(baseURL: stagingURL)
}

The environment is also used to determine whether a given request should adopt the default credential configured on the HTTPManager. Only requests for URLs that are prefixed by the environment will use the default credential. Requests for any other URL will have no credential by default, though a credential can always be added to any request.

Requests

Requests in PMHTTP are objects. In a pure-Swift world they'd be structs/protocols, but they're objects in order to be compatible with Obj-C. Unlike URLRequest, PMHTTP requests are inherently mutable (so they're like NSMutableURLRequest). They're also the only public component of PMHTTP that is not thread-safe, though it is safe to access a request concurrently as long as no thread is mutating the request (which is to say, reading values from the request does not perform any internal mutation).

Requests are split into a hierarchy of classes:

This hierarchy means that every class can provide only the methods/properties that make sense for all requests of that class type. For example, only HTTPManagerUploadFormRequest requests allow for adding multipart bodies.

Requests include properties for configuring virtually every aspect of the network request. A few properties inherit default values from the HTTPManager object, though these default values can always be overridden. One property of note is userInitiated, which is a boolean property that should be set if the request represents some action the user is waiting on. Setting this property to true causes the underlying network task to be executed at a high priority and causes all background queue processing to occur using QOS_CLASS_USER_INITIATED.

HTTPManagerUploadFormRequest provides support for creating multipart/form-data requests, which can be used for uploading files/images. These requests are implemented in a streaming fashion, so e.g. memory-mapped NSData objects won't be copied into a contiguous buffer, thus allowing you to upload files without concerns about memory use.

HTTPManagerRequest conforms to NSCopying so copies can be made of any request if necessary. Furthermore, when attaching a parse handler to a request (and therefore converting it into an HTTPManagerParseRequest<T>) the original request data is copied so subsequent mutations to the original request do not affect the parse request, and when a request is executed the request data is copied so the request can be immediately mutated without affecting the executing network task.

Requests are also designed such that they can be easily created and executed using a functional-style chain, as demonstrated by the Usage section above.

Parse requests always execute their parse handler on a background queue, with no option to run on a given queue (or the main queue). This constraint exists both to encourage parsing in the background, and for simplicity, as parsing on the main queue can always be accomplished by skipping the parse handler and parsing in the completion block instead.

Request completion blocks are similarly executed on a background queue by default (for requests with a parse handler, this will be the same queue that the parse handler executed on), although here a specific queue can be provided where the completion block should run, such as the main queue.

Network Tasks

Executing a request returns a value of type HTTPManagerTask. This class is the PMHTTP equivalent of URLSessionTask and is completely thread-safe. It provides properties for inspecting the current state of the request, including for accessing the underlying URLSessionTask, and it provides a cancel() method for canceling the request. Unlike URLSessionTask.cancel(), HTTPManagerTask.cancel() can be used to cancel a request while the parse handler is executing, not just canceling the networking portion. PMHTTP guarantees that if you execute HTTPManagerTask.cancel() from the same queue that the completion block is targeting, prior to the completion block itself executing, the completion block will always be given a result of .canceled even if it had already finished parsing before cancel() was invoked. This means that if you target the main queue for your completion block, you can be confident that a canceled task will never behave as though it succeeded or failed.

Like URLSessionTask, HTTPManagerTask supports key-value observing (although, like URLSessionTask, the KVO messages will occur on some background queue).

In the absence of automatic retrying, the networkTask property value will never change during the lifetime of the task. If automatic retrying has been configured, networkTask will change if the request is retried, and will broadcast any relevant key-value observing messages.

Network Activity Indicator

PMHTTP provides a callback you can use to implement support for the global network activity indicator. Each request object has a property affectsNetworkActivityIndicator (which defaults to true) that controls whether any tasks created from the request affect the callback. The callback itself is configured by assigning a block to HTTPManager.networkActivityHandler. This block is run on the main thread whenever the number of active tasks has changed. In order to display the global network activity indicator you can configure this like so:

HTTPManager.networkActivityHandler = { active in
    UIApplication.sharedApplication().networkActivityIndicatorVisible = active > 0
}

Automatic Retrying of Failed Requests

PMHTTP includes support for automatically retrying failed requests according to a configurable policy. The default retry policy can be configured with HTTPManager.defaultRetryBehavior, which can be overridden on individual requests with HTTPManagerRequest.retryBehavior. A few common retry policies are provided as convenience methods on HTTPManagerRetryBehavior, but any custom policy is supported as well. The convenience policies implement intelligent handling of the various NSURLErrorDomain errors, such as not retrying when encountering a non-transient error (such as NSURLErrorAppTransportSecurityRequiresSecureConnection), or retrying non-idempotent requests if the error indicates the server never received the request (e.g. NSURLErrorCannotConnectToHost). By default, retrying is disabled.

Cache Handling

PMHTTP implements intelligent cache handling for JSON responses. The HTTP standard allows user agents to cache responses at their discretion when the response does not include caching headers. However, this behavior is inappropriate for most REST API requests, and URLSession does not document its caching strategy for such responses. To handle this case, PMHTTP inspects JSON responses for appropriate caching headers and explicitly prevents responses from being cached if they do not include the appropriate cache directives. By default this behavior is only applied to requests created with .parseAsJSON(), .parseAsJSON(using:), or .decodeAsJSON(_:), although it can be overridden on a per-request basis (see HTTPManagerRequest.defaultResponseCacheStoragePolicy). Notably, requests created with .parse(using:) do not use this cache strategy as it would interfere with caching image requests.

Mocking

PMHTTP has built-in support for mocking network requests. This is done without swizzling (so it's safe to mock requests even in App Store builds), and it's done in a fashion that still creates a valid URLSessionTask (so any code that inspects HTTPManagerTask.networkTask will function as expected). Mocks can be registered on the HTTPManager as a whole, and individual requests can be independently mocked (so you can control whether a request is mocked based on more than just the URL in question).

Testing

PMHTTP itself has a comprehensive test suite, covering just about everything in the Swift API (the Obj-C–specific API is not currently tested, see issue #7). The tests are run against a custom HTTP/1.1 server implemented in the test bundle that listens on the loopback interface. This allows for testing all the functionality without any dependencies on external services and ensures the tests are very fast. The HTTP/1.1 server currently relies on CocoaAsyncSocket, which can be installed with carthage bootstrap. This dependency is not exposed to clients of PMHTTP as it's only used by the test suite.

The HTTP/1.1 server implements just about everything that I thought was useful. It has a few minor dependencies on PMHTTP itself (most notably, it uses HTTPManagerRequest.HTTPHeaders instead of reimplementing the functionality), but beyond that, it could actually be pulled out and used anywhere else that an HTTP/1.1 server is required. However, as this server was written for the purposes of testing and not production use, it does not have any built-in mitigation of DOS attacks beyond rejecting uploads greater than 5MiB (for example, it does not impose any limit on headers, which are kept in memory, and it does not have any sort of timeout on connection duration). It also does not have any tests itself, beyond the fact that it behaves as expected when used in the PMHTTP test suite.

Requirements

Requires a minimum of iOS 8, macOS 10.10, watchOS 2.0, or tvOS 9.0.

Installation

After installation with any mechanism, you can use this by adding import PMHTTP to your code.

Carthage

To install using Carthage, add the following to your Cartfile:

github "postmates/PMHTTP" ~> 4.0

This release supports Swift 4.0. For Swift 3.x you can use

github "postmates/PMHTTP" ~> 3.0

CocoaPods

To install using CocoaPods, add the following to your Podfile:

pod "PMHTTP", "~> 4.0"

This release supports Swift 4.0. For Swift 3.x you can use:

pod "PMHTTP", "~> 3.0"

License

Licensed under either of

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.

Version History

v4.5.0 (2019-11-27)

v4.4.3 (2019-11-14)

v4.4.2 (2019-08-13)

v4.4.1 (2019-04-24)

v4.4.0 (2019-04-23)

v4.3.3 (2019-04-07)

v4.3.2 (2018-11-14)

v4.3.1 (2018-08-01)

v4.3.0 (2018-07-26)

v4.2.0 (2018-07-10)

v4.1.1 (2018-06-21)

v4.1.0 (2018-06-15)

v4.0.1 (2018-05-17)

v4.0.0 (2018-02-23)

v3.0.5 (2017-09-11)

v3.0.4 (2017-09-05)

v3.0.3 (2017-08-18)

v3.0.2 (2017-05-01)

v3.0.1 (2017-03-28)

v3.0.0 (2017-02-27)

v2.0.1 (2017-01-05)

v2.0.0 (2017-01-03)

v1.0.4 (2016-10-20)

v1.0.3 (2016-09-23)

v1.0.2 (2016-09-22)

v1.0.1 (2016-09-12)

v1.0.0 (2016-09-09)

v0.9.3 (2016-09-09)

v0.9.2 (2016-09-09)

v0.9.1 (2016-08-17)

v0.9 (2016-08-05)

Initial release.