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| <h4 class="subsection">8.9.4 Fast enumeration protocol</h4> |
| |
| <p>If you want your own collection object to be usable with fast |
| enumeration, you need to have it implement the method |
| |
| <pre class="smallexample"> - (unsigned long) countByEnumeratingWithState: (NSFastEnumerationState *)state |
| objects: (id *)objects |
| count: (unsigned long)len; |
| </pre> |
| <p>where <code>NSFastEnumerationState</code> must be defined in your code as follows: |
| |
| <pre class="smallexample"> typedef struct |
| { |
| unsigned long state; |
| id *itemsPtr; |
| unsigned long *mutationsPtr; |
| unsigned long extra[5]; |
| } NSFastEnumerationState; |
| </pre> |
| <p>If no <code>NSFastEnumerationState</code> is defined in your code, the |
| compiler will automatically replace <code>NSFastEnumerationState *</code> |
| with <code>struct __objcFastEnumerationState *</code>, where that type is |
| silently defined by the compiler in an identical way. This can be |
| confusing and we recommend that you define |
| <code>NSFastEnumerationState</code> (as shown above) instead. |
| |
| <p>The method is called repeatedly during a fast enumeration to retrieve |
| batches of objects. Each invocation of the method should retrieve the |
| next batch of objects. |
| |
| <p>The return value of the method is the number of objects in the current |
| batch; this should not exceed <code>len</code>, which is the maximum size of |
| a batch as requested by the caller. The batch itself is returned in |
| the <code>itemsPtr</code> field of the <code>NSFastEnumerationState</code> struct. |
| |
| <p>To help with returning the objects, the <code>objects</code> array is a C |
| array preallocated by the caller (on the stack) of size <code>len</code>. |
| In many cases you can put the objects you want to return in that |
| <code>objects</code> array, then do <code>itemsPtr = objects</code>. But you |
| don't have to; if your collection already has the objects to return in |
| some form of C array, it could return them from there instead. |
| |
| <p>The <code>state</code> and <code>extra</code> fields of the |
| <code>NSFastEnumerationState</code> structure allows your collection object |
| to keep track of the state of the enumeration. In a simple array |
| implementation, <code>state</code> may keep track of the index of the last |
| object that was returned, and <code>extra</code> may be unused. |
| |
| <p>The <code>mutationsPtr</code> field of the <code>NSFastEnumerationState</code> is |
| used to keep track of mutations. It should point to a number; before |
| working on each object, the fast enumeration loop will check that this |
| number has not changed. If it has, a mutation has happened and the |
| fast enumeration will abort. So, <code>mutationsPtr</code> could be set to |
| point to some sort of version number of your collection, which is |
| increased by one every time there is a change (for example when an |
| object is added or removed). Or, if you are content with less strict |
| mutation checks, it could point to the number of objects in your |
| collection or some other value that can be checked to perform an |
| approximate check that the collection has not been mutated. |
| |
| <p>Finally, note how we declared the <code>len</code> argument and the return |
| value to be of type <code>unsigned long</code>. They could also be declared |
| to be of type <code>unsigned int</code> and everything would still work. |
| |
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