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Creating New Image Representation Classes

If you want to add support for new image formats or generate images from other types of source information, you may want to subclass NSImageRep. Although Cocoa supports many image formats directly, and many more indirectly through the Image IO framework, subclassing NSImageRep gives you control over the handling of image data while at the same time maintaining a tight integration with the NSImage class. If you decide to subclass, you should provide implementations for the following methods:

• imageUnfilteredTypes

• canInitWithData:

• initWithData:

• draw

These methods provide the basic interface that the parent NSImageRep class needs to interact with your subclass. The methods provide information about what image data formats your class supports along with entry points for initializing your object and drawing the image.

Before your subclass can be used, it must be registered with the Application Kit. You should do this early in your application’s execution by invoking the registerImageRepClass: class method of NSImageRep. Registering your class lets Cocoa know that your class exists and that it can handle a specific set of file types. Your implementation of the imageUnfilteredTypes method should return an array of UTI types corresponding to the image file types your class supports directly.

Another method you should always override is the canInitWithData: method. Once your image representation class has been identified as handling a particular type of data, Cocoa may notify it when data of the appropriate type is received. At that time, Cocoa passes a data object to your canInitWithData: method. Your implementation of this method should examine the data quickly and verify that it can really handle the format.

Once your class is chosen to handle the image data, Cocoa looks for an initWithData: method and uses it to initialize your object with the image data. Your implementation of this method should retain the data and use it to initialize the object. At some point later, your draw method may be called to render the data in the current context. Your draw method should render the data at the current origin point and with the current size and settings specified by the NSImageRep parent class.

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