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Praeclarum.Bind

Bind gives you easy two-way data binding between properties of objects. These objects can be UI elements, plain old data, or complex model objects, whatever.

Values are automatically updated if the object classes implement property changed events.

This is especially useful when creating UI code where you want to display and edit model values.

using Praeclarum.Bind;

class PersonViewController : UIViewController
{
    UITextField nameEdit;

    Person person;

    public override void ViewDidLoad ()
    {
        Binding.Create (() => nameEdit.Text == person.Name);
    }
}

Installation

Bind can be included in your project by simply including Bind.cs in your project. It will work in any .NET 4.5 project.

There is even a Portable Class Library version of the project that works on Profile 78 (which includes everything except Silverlight 5).

Usage

Equality Binding

Equality binding is the simplest use of the library. Equality bindings are specified using the == operator in a call to Binding.Create:

Binding.Create (() => left == right);

where left and right are two values.

This binding will attempt to keep the values of left and right in sync. That is, if right changes, so will left.

When initialized, the binding will attempt to assign right to left. If left is constant, then it will do the reverse, assign left to right.

Left and right can be any expression ranging from simple constants up to long object walks:

Binding.Create (() => stateEdit.Text == person.Address.State);

When this binding is created, the value of person.Address.State is assigned to the edit control's Text property. If the user changes that text, the values will be written back to person.Address.State.

Bindings are symmetric, so you could just as well have written:

Binding.Create (() => person.Address.State == stateEdit.Text);

Then only difference occurs at initialization: the stateEdit.Text value is assigned to the person.Address.State value instead of the other way around.

Unbinding

Binding.Create returns a Binding object with one member Unbind. Calling this method permanently removes the bindings. If you want them back, you will need to re-create them.

var binding = Binding.Create (() => stateEdit.Text == person.Address.State);

...

binding.Unbind ();

Multiple Bindings

You can create multiple bindings by chaining them together with the and operator &&:

Binding.Create (() => 
    nameEdit.Text == person.Name &&
    stateEdit.Text == person.Address.State);

This is useful if you want to unbind a lot of data bindings all at once:

var multipleBindings = Binding.Create (() => 
    nameEdit.Text == person.Name &&
    stateEdit.Text == person.Address.State);

...

multipleBindings.Unbind ();

Complex Equality Binding

Sometimes you will want to bind a transformation or composition of data.

Consider the case of displaying a person's full name and allowing them to enter that data using text boxes:

class PersonViewController : UIViewController
{
    UITextField firstNameEdit;
    UITextField lastNameEdit;
    UILabel fullNameLabel;

    Person person;

    public override void ViewDidLoad ()
    {
        Binding.Create (() => 
            firstNameEdit.Text == person.FirstName &&
            lastNameEdit.Text == person.LastName &&
            fullNameLabel.Text == person.LastName + ", " + person.FirstName &&
            Title == person.LastName + ", " + person.FirstName);
    }
}

Here we have bound fullNameLabel.Text and the view controller's Title to a complex expression involving two variables. When either of these values change, the text will be automatically updated.

Complex expression disrupt two-way databinding - updates will only flow from the complex side to the simple side.

Bindings with complex expressions on both sides are meaningless. (Technically, they define a algebraic loop that must be solved. I haven't implemented this and probably never will.)

Change Tracking

You must call Invalidate to update a binding if the value comes from an object that does not implement [Automatic Change Tracking][].

Invalidate takes a lambda returning the property that changed:

Binding.Invalidate (() => obj.Property);

Automatic Change Tracking

Objects that implement the INotifyPropertyChanged interface or that has any of these events:

• PropertyChanged (where Property is the name of the property that causes the event)
• EditingDidEnd
• ValueChanged
• Changed

will be automatically change tracked.

Error Handling

If Bind runs into problems, it will raise the static event Binding.Error. The default behavior is to write a message to the debug console - binding errors do not raise exceptions.

If you want to debug these errors, create a global event handler and set a debug point:

Bind.Error += message => {
    // Set a breakpoint here
};