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Inside Macintosh: More Macintosh Toolbox /

Chapter 1 - Resource Manager / Using the Resource Manager

Releasing and Detaching Resources

When you've finished using a resource, you can call ReleaseResource to release the memory associated with that resource. For a given resource, the ReleaseResource procedure releases the memory associated with the resource, setting the handle's master pointer to NIL, thus making your application's handle to the resource invalid. (This is similar to the situation shown in Figure 1-8.) After releasing a resource, use another Resource Manager routine if you need to use the resource again. For example, the code in Listing 1-3 first uses GetResource to get a handle to a resource, manipulates
the resource, then uses ReleaseResource when the application has finished
using the resource. If the application needs the resource later, it must get a valid handle to the resource by reading the resource into memory again (using GetResource, for example).

Listing 1-3 Releasing a resource

PROCEDURE MyGetAndPlaySoundResource(resourceID: Integer);
VAR
   myHndl: Handle;
BEGIN
   myHndl := GetResource('snd ', resourceID);
   {use the resource}
   {when done, release the resource}
   ReleaseResource(myHndl);
END; 

Your application can also use the DetachResource procedure to replace a resource's handle in the resource map with a handle whose value is NIL. However, the DetachResource procedure does not release the memory associated with the resource. You can use DetachResource when you want your application to access the resource's data directly, without the aid of the Resource Manager, or when you need to pass the handle to a routine that does not accept a resource handle. (For example, the AddResource routine used in Listing 1-4 on page 1-18 takes a handle to data, not a handle to a resource.) Once you detach a resource, the Resource Manager does not recognize the resource's handle in the resource map in memory as a valid handle to a resource, but your application can still manipulate the resource's data through its own handle to the data.

Figure 1-9 shows how both your application and the Resource Manager have a handle to a resource after your application calls GetResource. The figure also shows how the Resource Manager replaces the handle in the resource map in memory with a handle whose value is NIL when your application calls DetachResource.

Figure 1-9 Detaching a resource

You can also easily copy a resource by first reading in the resource using GetResource, detaching the resource using DetachResource, then copying the resource by using AddResource (and specifying a new resource ID). Listing 1-4 uses this technique to copy a resource within the current resource file.

Listing 1-4 Detaching a resource

PROCEDURE MyCopyAResource(resourceType: ResType; 
                           resourceID: Integer;
                           VAR myHndl: Handle);
VAR
   newResourceID: Integer;
BEGIN
   myHndl := GetResource(resourceType, resourceID);
   DetachResource(myHndl);                {detach the resource}
   newResourceID := UniqueID(resourceType);
   AddResource(myHndl, resourceType, newResourceID, '');
END;