Adapter
Adapter
Known Uses
The Motivation example comes from ET++Draw, a drawing application based on ET++. ET++Draw reuses the ET++ classes for text editing by using a TextShape adapter class.
InterViews 2.6 defines an Interactor abstract class for user interface elements such as scroll bars, buttons, and menus. It also defines a Graphic abstract class for structured graphic objects such as lines, circles, polygons, and splines. Both Interactors and Graphics have graphical appearances, but they have different interfaces and implementations (they share no common parent class) and are therefore incompatible---you can't embed a structured graphic object in, say, a dialog box directly.
Instead, InterViews 2.6 defines an object adapter called GraphicBlock, a subclass of Interactor that contains a Graphic instance. The GraphicBlock adapts the interface of the Graphic class to that of Interactor. The GraphicBlock lets a Graphic instance be displayed, scrolled, and zoomed within an Interactor structure.
Pluggable adapters are common in ObjectWorks\Smalltalk. Standard Smalltalk defines a ValueModel class for views that display a single value. ValueModel defines a value, value: interface for accessing the value. These are abstract methods. Application writers access the value with more domain-specific names like width and width:, but they shouldn't have to subclass ValueModel to adapt such application-specific names to the ValueModel interface. Instead, ObjectWorks\Smalltalk includes a subclass of ValueModel called PluggableAdaptor. A PluggableAdaptor object adapts other objects to the ValueModel interface (value, value:). It can be parameterized with blocks for getting and setting the desired value. PluggableAdaptor uses these blocks internally to implement the value, value: interface. PluggableAdaptor also lets you pass in the selector names (e.g., width, width:) directly for syntactic convenience. It converts these selectors into the corresponding blocks automatically.
Another example from ObjectWorks\Smalltalk is the TableAdaptor class. A TableAdaptor can adapt a sequence of objects to a tabular presentation. The table displays one object per row. The client parameterizes TableAdaptor with the set of messages that a table can use to get the column values from an object. Some classes in NeXT's AppKit use delegate objects to perform interface adaptation. An example is the NXBrowser class that can display hierarchical lists of data. NXBrowser uses a delegate object for accessing and adapting the data. Meyer's ``Marriage of Convenience'' is a form of class adapter. Meyer describes how a FixedStack class adapts the implementation of an Array class to the interface of a Stack class. The result is a stack containing a fixed number of entries.