Figure 312 Properties View in Java

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Figure 312 Properties View
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There are two parties involved in the properties display: the Properties view and the selected object TheProperties view needs an interface to fetch the properties so that it can display them and the selected object needs to support this interface In Eclipse, the interface required by the Properties view is IPropertySource and is defined as follows:
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orgeclipseuiviewsproperties/IPropertySource
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public interface IPropertySource { public Object getEditableValue(); public IPropertyDescriptor[] getPropertyDescriptors(); public Object getPropertyValue(Object id); public boolean isPropertySet(Object id); public void resetPropertyValue(Object id); public void setPropertyValue(Object id, Object value); } The selected object somehow has to surface this interface to the Properties view A straightforward solution is to implement it directly in the class of the selected object:
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public interface IFile extends IPropertySource This solution has several problems:
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Adding many such service interfaces for other components to a class will result in a bloated interface The fact that a class implements a service is an implementation detail that you don't want to expose if the type is API Adding
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such an interface once the class is published is a breaking API change An IFile should not have to know about theProperties view, which is a concept from the UI layer This is precisely the coupling of model and user interface we want to avoid
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311 Extension Object/Extension Interface
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We need a mechanism that allows us to Add a service interface to a type without exposing it in that type Add behavior to preexisting types such as IFile
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[1] The pattern is called Extension Object and is also known as Extension Interface To quote its intent: "Anticipate that an object's interface needs to be extended in the future Extension Object lets you add interfaces to a class and lets clients query whether an object has a particular extension"
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See "Extension Object" in R Martin, Pattern Languages of Program Design 3 Addison-Wesley, Reading, MA, 1998 For a comprehensive description with several other implementation variations, see "Extension Interface" in D Schmidt, M Stal, H Rohner, F Buschmann, Pattern-Oriented Software Architecture Volume 2: Patterns for Concurrent and Networked Objects John Wiley & Sons, 2000 When implementing Extension Object, you have to answer several questions:
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Do you want to extend an individual object or a class With a class-based mechanism you add behavior to an entire class You can only add behavior but not state, that is, no fields How is an extension described and identified
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The Eclipse extension support is class-based That is, you can add behavior to existing classes, but not add state to its existing instances The additional behavior is described by an interface The key interface of the Eclipse extension support is IAdaptable Classes that support adaptability implement this interface While browsing the Eclipse source, you will find that IAdaptable is a popular interface, as illustrated in the following snapshot of the Hierarchy view shown in Figure 313
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Figure 313 IAdaptable Type Hierarchy
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What is behind the IAdaptable name When doing some Eclipse code archeology we found the class was originally called IExtensible This captured the idea that the mechanism existed to support extending a type with additional behavior Over time the name got changed to IAdaptable to emphasize the fact that the mechanism enables adapting an existing class to another interface IAdaptable is an interface with a single method that allows clients to dynamically query whether an object supports a particular interface:
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orgeclipsecoreruntime/IAdaptable
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public interface IAdaptable { public Object getAdapter (Class adapter); } The getAdapter() method answers a particular interface Callers of getAdapter() pass in the class object corresponding to the interface (see Figure 314)
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Figure 314 Extension Object
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The class object is typically retrieved with a class literal (for example, IPropertySourceclass) GetAdapter() could have taken aString argument instead, but then the compiler couldn't check to see whether the interface exists at compile time GetAdapter() returns an object castable to the given class (or null if the interface isn't supported) Here is an example of how the Properties view queries the currently selected object for its IPropertySource interface: