Key Value Observing (KVO) CPRE 388

Transcription

1 Key Value Observing (KVO) CPRE 388

2 MyObservedClass MyObserving Class Observed Property P Observer Callback Method

3 Key Value Observing KVO is a mechanism that allows objects to be notified of changes to specified properties of other objects. The controller layer binding relies heavily on KVO to be notified of changes in the model and controller layers. For apps that don t rely on the controller layer classes, KVO provides simplified methods for implementing inspectors and updating your user interface values

4 KVO in Cocoa IOS SDK frameworks have simple interfaces change a property and wait for the effects to ripple! If you want to change a label s font, you just set a property. If you want to add a new set of tabs to a UITabBarController, you simply have to assign an array of view controllers to the viewcontrollers property

5 KVO behind the scenes KVO does the framework control layer behind the scenes. Also available to you for your app controller layer. You might say Really? That s all it does? I can get the same by creating a custom setter!

6 KVO vs Custom setters It s easy to create one or two custom setters, but quite tedious if you have lots of properties you want to observe. Custom setters only work if you are monitoring properties under your control, but impossible if you want to monitor changes made to other objects you encapsulate. You have to implement your own retain/release/copy cycles manually, instead of simply

7 You have to manually keep track of what the old and new values of a property are, if they are important to you. Your code looks like spaghetti.

8 Central vs Distributed Unlike NSNotification, there is no central object that provides change notification for all observers notifications are sent directly to the observing objects when changes are made. NSObject provides this base implementation of KVO

9 You can observe any object properties including simple attributes, to one relationships, and to many relationships. Observers of to many relationships are informed of the type of change made as well as which objects are involved in the change.

10 KVO Compliance In order to be considered KVO compliant for a specific property, a class must ensure the following; The class must be key value coding compliant for the property as specified in Ensuring KVC Compliance (next slide). The class must allow automatic observer notifications for the property, or emits KVO change notifications for the property.

11 Ensuring KVC/KVO Compliance it must implement the methods forvalueforkey: and setvalue:forkey: to work for that property. attribute or a to one relationship, this requires that your class: Implement a method named <key>, is<key>, or have an instance variable <key> or _<key>. If the property is mutable, then it should also implement set<key>:

12 Registering for Key Value Observing The observed class must be key value observing compliant for the property that you wish to observe You must register the observing object with the observed object, using the method addobserver:forkeypath:options:context:. The observing class must implement observevalueforkeypath:ofobject:change:co ntext:.

13 MyObservedClass MyObserving Class addobserver:forkeypath:options:context: Observed Property P (1) Observer Callback Method (2) observevalueforkeypath:ofobject:change:context:

14 Registering the inspector as an observer of the openingbalance property (void) registerasobserver {// register "inspector" to receive change notifications // for the "openingbalance" property of the "account" object // and that both the old and new values of "openingbalance" // should be provided to the observer [account addobserver:inspector options:(nskeyvalueobservingoptionnew NSKeyValueObservingOptionOld) context:null]; }

15 Context When you register an object as an observer, you can also provide a context pointer The context pointer can be a C pointer or an object reference. The context pointer can be used as a unique identifier to determine the change that is being observed, or to provide some other data to the observer.

16 The context pointer is not retained, and it is the responsibility of the application to ensure that it is not released before the observing object is removed as an observer.

17 Receiving Notification of a Change When the value of an observed property of an object changes, the observer receives an observevalueforkeypath:ofobject:change:co ntext: message. All observers must implement this method. The observer is provided the object and key path that triggered the observer notification, a dictionary containing details about the change, and the context pointer that was provided when the observer was registered.

18 Change Dictionary The change dictionary entry NSKeyValueChangeKindKey provides information about the type of change that occurred If the value of the observed object has changed, the NSKeyValueChangeKindKey entry returns NSKeyValueChangeSetting. Depending on the options specified when the observer was registered, the NSKeyValueChangeOldKey and NSKeyValueChangeNewKey entries in the change dictionary contain the values of the property before, and after, the change

19 If the observed property is a to many relationship, the NSKeyValueChangeKindKey entry also indicates whether objects in the relationship were inserted, removed, or replaced by returning NSKeyValueChangeInsertion, NSKeyValueChangeRemoval, or NSKeyValueChangeReplacement, respectively.

20 Implementation of observevalueforkeypath:ofobject:change:context: (void) observevalueforkeypath:(nsstring *) keypath ofobject:(id) object change:(nsdictionary *)change context:(void *)context { if ([keypath isequal:@"openingbalance"]) { [openingbalanceinspectorfield setobjectvalue: [change objectforkey:nskeyvaluechangenewkey]]; }

21 // be sure to call the super implementation // if the superclass implements it [super observevalueforkeypath:keypath ofobject:object change:change context:context]; }

22 Removing the inspector as an observer of openingbalance (void) unregisterforchangenotification { [observedobject removeobserver:inspector forkeypath:@"openingbalance"]; }

23 KVO Implementation Automatic key value observing is implemented using a technique called isa swizzling The isa pointer points to the object's class which maintains a dispatch table. When an observer is registered for an attribute of an object the isa pointer of the observed object is modified, pointing to an intermediate class rather than at the true class. Instead of relying on the isa pointer your application should use the class method to determine the class of an object instance.

24 Another KVO Example UIView subclass that has a UIColor property that controls the display of several subviews. UIView ColorControlClass UIColor mycolor UIView SubView UIView SubView UIView SubView

25 (void)setcolor:(uicolor *)color { if (color!= color) { [color release]; color = [color retain]; Custom Setter } } mtitlelabel.textcolor = color; mdescriptionlabel.textcolor = color; [mbutton settitlecolor:color forstate:uicontrolstatenormal];

26 (id) initwithframe:(cgrect)frame { self = [super initwithframe:frame]; if (self) { [self addobserver:self forkeypath:@"color" options:0 context:nil]; } } KVO

27 (void)observevalueforkeypath:(nsstring*)keypath ofobject:(id)object change:(nsdictionary*)change context:(void*)context { if ([keypath isequaltostring:@"color"]) { mtitlelabel.textcolor = self.color; mdescriptionlabel.textcolor = self.color; [mbutton settitlecolor:self.color forstate:uicontrolstatenormal]; }

28 else { [super observevalueforkeypath:keypath ofobject:object change:change context:context]; } }