A binding defines a connection between a property on one object – the source – and a property on another object: the destination.
This pattern can simplify view controllers by entirely removing the need for glue code - code which updates views from the model or vice versa. It can also be used at the model layer, or the view layer, to enforce relationships.
Cocoa informs a client of value changes using different methods: KVO, target-action, notifications, and delegation. For this reason, a binding is made up of multiple triggers, where a trigger responds to any one of these methods.
The code for a simple binding might look like this:
// Set up the environment
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Storyboard" bundle:nil];
RDPersonViewController *personVC = [storyboard instantiateInitialViewController];
personVC.person = [[Person alloc] initWithName:@"Jimmy"];
personVC.bindings = [[BNDBindings alloc] init];
// Define the source property
BNDProperty *sourceProperty = [[BNDProperty alloc] initWithObject:[personVC person] keyPath:@"name"];
// Define the destination property
BNDProperty *destinationProperty = [[BNDProperty alloc] initWithObject:[personVC textField] keyPath:@"text"];
// Create a bind from the source property to the destination property
BNDBinding *binding = [[BNDBinding alloc] initWithSourceProperty:sourceProperty destinationProperty:destinationProperty];
// Add a trigger to update the binding when KVO notifications are sent for person's name property
BNDTrigger *trigger = [[BNDKVOTrigger alloc] initWithKeyPath:@"name" object:[personVC person] delegate:binding];
[binding addTrigger:trigger];
// Add to the VC
[[personVC bindings] addBinding:binding];The above code exerpt is incomplete without a reverse binding to ensure person's name matches the text field's text, however UITextField does not send KVO notifications when [UITextField name] is modified – it instead sends out an UITextFieldTextDidChangeNotification.
A trigger can update the binding when a notification is received:
// Create a binding as normal
BNDBinding *binding = [[BNDBinding alloc] initWithSource:[personVC textField] sourceKeyPath:@"text" destination:[personVC person] destinationKeyPath:@"name"];
// Create a trigger and add it to the binding
BNDTrigger *trigger = [[BNDNotificationTrigger alloc] initWithNotificationCenter:[NSNotificationCenter defaultCenter] notificationName:UITextFieldTextDidChangeNotification sender:[personVC textField] delegate:binding];
[binding addTrigger:trigger];
// Add it to the VC's bindings
[[personVC bindings] addBinding:binding];When the trigger receives a UITextFieldTextDidChangeNotification, it will update person's name using the new text.
Values regularly need to be converted from one format to another, especially when being passed from the view layer to the model layer. Your user interface components may recognize data in a different format to your models, for example the user enters digits into a string, and your model accepts a number. A transformer stands in place of a property object, converting values as they pass through:
------------ --------------- ----------- --------------- ------------
| Property | --> | Transformer | --> | Binding | --> | Transformer | --> | Property |
------------ --------------- ----------- --------------- ------------Transformers can convert the value either when it is retrieved from the source or when it is assigned to the destination - it does not matter. In the following example, destinationProperty is predefined as a property on an object:
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
destinationProperty = [[BNDStringToNumberTransformer alloc] initWithProperty:destinationProperty numberFormatter:formatter];destinationProperty can continue to be treated as if it defines a property on an object.
Transformers may be chained by initializing a transformer with another transformer in place of the property, for example:
transformer = [[CustomTransformer alloc] initWithProperty:transformer];Currently the only built in transformer is BNLStringToNumberTransformer. Suggestions and contributions are welcome.
To install Bindings in your project, add it to your Podfile: pod 'Bindings', '~> 0.2.0'.
Copyright (c) 2013 Ryan Davies
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