Objective-C 内存管理技巧与经验

http://blog.csdn.net/dboylx/archive/2009/02/15/3893770.aspx

 

自动释放进阶，让我们更深一步了解它的工作原理

  虽然自动释放对象的概念是如此简单，但了解它更多的工作原理还是很有必要的。不然在我们的嵌入式设备的开发中，仍然会走入内存漏洞深渊。

  其实，在我们的一个应用中，是有很多的”NSAutoreleasePool”对象实例的，就像它的命名一样，它们用来收集所有自动释放的对象。只要在调用”autorelease”方法后，它就会被加入到这个池中。在未来的某个时刻，一般指在”Foundation”与”AppKit”应用一个事件循环结束时，或者在响应完”WebObjects”类应用请求时，或调用”NSAutoreleasePool”对象的”release”方法。这里需要注意，”NSAutoreleasePool”并不止一个，为什么需要多个”NSAutoreleasePool”来管理内存呢？因为，在一个代码段内就回收所有自动释放对象是很有用处的，多线程应用中，每个线程可以拥有一个自动释放池的栈，当你创建了一堆临时对象时，而仅仅是在一段很短的上下文中，比如一个简单循环，你并不希望在下面的代码中，他们仍然占用保宝贵的内存资源，你就可以为这段短小紧凑本地上下文创建一个”NSAutoreleasePool”对象来管理他们:

- (id) findSomething
{
        id theObject = nil;
        // Whatever we're looking for
        NSAutoreleasePool *localPool = [[NSAutoreleasePool alloc] init];
        // Autoreleased objects are now automatically placed in localPool.

        // Loop that creates many temporary objects
        while ( theObject == nil )
        {
            ...
            if ( [temporaryObject matchesSomeCondition] )
            {
                theObject = [temporaryObject retain];
                // We want this one
            }
        }

        // Get rid of all those temporary objects
        [localPool release];

        return [theObject autorelease];
}

上段代码我们做了什么：

A,我们创建了一个”NSAutoreleasePool”对象，把它压到当前上下文中的内存管理池顶，在它下面的所有自动内存管理对象都被放入到这个池中。

B,我们调用了自动释放对象”temporaryObject”的”retain”方法，使它的生命周期超过本地池的管理。

C,释放池操作，同时把它从栈中POP出去。

D,紧接着，我们又在返回它前调用了 ”autorelease”方法，把这个对象放入当前池栈的TOP池中。


这东西有点绕，但原理其实还算简单。如果上面的能明白了，恭喜你，Objective-C的水平又上了一个台阶。

同理可证，还有一段更精练的代码：

- (NSArray *) findAListOfThings
{
        NSMutableArray *thingArray =
            [[NSMutableArray alloc] initWithCapacity: 25];
        // The list of 25 things we're looking for
        NSAutoreleasePool *outerPool = [[NSAutoreleasePool alloc] init];
        NSAutoreleasePool *innerPool = nil;
        NSArray *largeObjectArray = nil;
        id temporaryObject = nil;
        NSEnumerator *arrayEnumerator = nil;

        // Loops that create many temporary objects
        while ( [thingArray count] != 25 )
        {
            largeObjectArray = [self fetchLotsOfObjects];
            // largeObjectArray is autoreleased and contained in the
            // outer autorelease pool
            arrayEnumerator = [largeObjectArray objectEnumerator];
            // Note that the enumerator itself is a temporary object!
            // It will be released by the outerPool

            // Create the inner pool on each iteration. When
            // a pool is created, it automatically becomes the
            // "top" pool on the current thread's stack of pools.
            innerPool = [[NSAutoreleasePool alloc] init];
            // autoreleased objects now go into innerPool

            while ( temporaryObject = [arrayEnumerator nextObject] )
            {
                ...
                if ( [temporaryObject matchesSomeCondition] )
                {
                    [thingArray addObject: temporaryObject];
                    // Collections retain their members
                }            
            }

            // Dispose temporary objects created on this iteration;
            // Note that the objects added to thingArray during this
            // iteration are also in innerPool and thus sent a release
            // message, but are not destroyed because they have been
            // retained by thingArray and so have an additional reference
            // (their retainCount > 1)
            [innerPool release];
        }

        [outerPool release];

        return [thingArray autorelease];
}