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a simple error message is written out (see the upcoming Exercises section for an extension on this point). In the Component s Find method (lines 37 42), the class relies on the actual type parameter for T having an Equals method. If it does not, the generic instantiation will fail at compile time. The Display method (lines 33 35) also assumes that the field accessed by the Name property has a ToString method defined. In fact, only string-like types will work properly here, so it is likely that the Display method should be defined later (see the upcoming Exercises section.) The Composite class also implements the IComponent interface (line 46). The Find and Remove methods of the Composite are more elaborate than those in the Component so that they can handle arbitrary structures of composites and components. Let s look at some of the more interesting statements: The Composite keeps as a list a local structure that consists of Components and Composites (line 47). When the contents are Composites, a new object is created, as well as a new list. The list is declared as:
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List <IComponent <T>> list;
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This shows that an open generic type can be used as a parameter to another generic type. The logic of Remove (lines 65 74) is that we first find the item in the structure and then, if it is there, we remove it from the list structure held locally in the Composite (line 69):
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(holder as Composite<T>).list.Remove(p);
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The holder variable is of type IComponent and needs to be cast to a Composite before the list can be accessed. An open generic type can still be used in a foreach loop, as in the Find and Display methods (lines 82 and 94):
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foreach (IComponent <T> c in list) { found = c.Find(s);
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The call to Find will go to the appropriate method, depending on the actual type of c at runtime. This supports the Composite pattern s ideal of having Components and Composites be treated the same. That completes the theoretical Composite pattern implementation. Apart from the concern with the Display method, the three preceding types can be used together for any elements. Thus, we can put them in a namespace called CompositePattern for use in the next example.Example: Photo Library In this example, we are concerned with collecting the filenames of digital photos into named sets. We will not use actual images in this example, just filenames as strings. The client is given a domain-specific set of commands with which to create and manipulate the library. Central to the manipulation of the library, from the user s point of view, is where we are. We start out at an empty set called Album. Some
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3: Structural Patterns: Composite and Flyweight
of the commands leave the system at the component that has just been adjusted, whereas others move it back to the first component in the set. The commands are:
AddSet
Add a new empty set with a name and stay there.
AddPhoto
Add a new named photo after the pointer and stay there.
Find
Find the named component (set or photo), or return null if it is not found.
Remove
Remove the named component (set or photo) and stay at the set from which it was removed.
Display
Display the whole structure.
Quit
Exit the program. Thus, the two operations that work on either Components or Composites are Find and Remove. Consider some examples of the workings of this system, illustrated in Example 3-2. The input commands are shown in the middle, the result of Display is shown on the left, and some commentary appears on the right.
The input file contains all the commands in the middle. The program will expect this file to be called Composite.dat.
AddSet Home AddPhoto Dinner.jpg AddSet Pets Going down another level AddPhoto Dog.jpg AddPhoto Cat.jpg Find Album Ensures Garden is at same level as Home AddSet Garden AddPhoto Spring.jpg AddPhoto Summer.jpg AddPhoto Flowers.jpg AddPhoto Trees.jpg Display Returns to start of Album Set Album length :2 --Set Home length :2 ----Dinner.jpg ----Set Pets length :2 ------Dog.jpg ------Cat.jpg --Set Garden length :4 ----Spring.jpg ----Summer.jpg
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