The Workspace Composite in Java

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322 The Workspace Composite
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The Eclipse workspace provides resources stored in the file system A workspace consists of projects containing folders containing files, as shown in Figure 327
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You access the Singleton workspace instance from the static accessor ResourcesPlugingetWorkspace() An IWorkspaceRoot represents the top of the resource hierarchy in a workspace The workspace is a hierarchical structure and it therefore matches the intent of the Composite pattern well: "Compose object into tree structures to represent part/whole hierarchies Composite lets clients treat individual objects and compositions of objects uniformly"Figure 328 shows how the implementation of a workspace maps to Composite:
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Figure 328 IWorkspace Is a Composite of IContainers and IFiles
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Some observations on the workspace implementation:
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IResource provides access to its parent The getParent() method is a handle-only operation that can derive the parent from the path stored in the handle IContainer is the common base interface for the different composite classes It provides a method members() that returns its children as a typed IResource array
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You can traverse a resource tree using themembers() method provided by IContainer, but there is a better way
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323 Traversing the Resource Tree Visitor
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Traversing a resource tree manually using the members() method results in a lot of control-flow code in clients The control flow to traverse a resource tree can be extracted with a visitor When we check the intent of Visitor we find, "Represent an operation to be performed on the elements of an object structure Visitor lets you define a new operation without changing the classes of the elements on which it operates" This is all correct However, the main purpose here is to extract the common control flow and make it generally reusable IResourceVisitor is the visitor interface, which is accepted byIResource (see Figure 329)
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Figure 329 IResourceVisitor Visits IResources
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The accept() method implements the resource traversal and calls back the visitor for each resource The IResourceVisitor interface isn't type-specific; there are no separate methods for visiting a file or a folder If you need to distinguish between the visited resource types, you can do this inside the visit() method using the getType() method The code snippet from the Eclipse resource implementation illustrates the use of visitor
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orgeclipsecoreinternalresources/ResourceTree
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private void addToLocalHistory(IResource root, int depth) { IResourceVisitor visitor = new IResourceVisitor() { public boolean visit(IResource resource) throws CoreException { if (resourcegetType() == IResourceFILE) addToLocalHistory((IFile) resource); return true; } }; try { rootaccept(visitor, depth, false); } catch (CoreException e) { } } Returning true from visit() indicates that the children of a resource should be visited Returning false stops the traversal at the current resource While performance tuning, it was discovered that for some common traversals only a subset of the resource information is actually needed by the visitor IResourceProxyVisitor was introduced to reduce the information fetched from the file system It doesn't pass an IResource to the visit() method but an IResourceProxy
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orgeclipsecoreresources/IResourceProxyVisitor
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public interface IResourceProxyVisitor { public boolean visit(IResourceProxy proxy) throws CoreException; } IResourceProxy is an example of a virtual proxy It creates an expensive object on demand The expensive object is in this case the full workspace path of a resource The proxy is only valid during the call of the visit() method The Eclipse workspace has another advantage over accessing the file system directly comprehensive support for observing changes
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324 Tracking Resource Changes Observer
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Resources in the workspace can change either as a result of manipulating them inside Eclipse or from resynchronizing them with the local file system In both cases, observing clients need precise change information so that they can update themselves efficiently To observe changes, the workspace provides a resource listener, which is an Observer variation (see Figure 3210) Observers register with the workspace, which acts as the subject to be notified about changes