使用AVQueuePlayer的静态初始化程序的奇怪内存行为

Question

I have declared property

@property(nonatomic, strong) AVQueuePlayer *player;

and when I initialize it with + queuePlayerWithItems: it is not deallocated if I assign to player new object or nil. Even if I do it right after one row below. When I initialize player with – initWithItems: everything works as expected. I use ARC. Is it bug or are not static initializers autoreleasing or what is the difference? I remember the times before ARC when it would be like

+ (AVQueuePlayer *)queuePlayerWithItems:(NSArray *)items
{
    return [[[AVQueuePlayer alloc] initWithItems:items] autorelease];
}

So what is the matter?

Answer 1

With ARC your code may like look as this:

+ (AVQueuePlayer *)queuePlayerWithItems:(NSArray *)items 
{
    return [[AVQueuePlayer alloc] initWithItems:items];
}

The compiler may still put the created object into an autorelease pool, though. Whether or not this happens also depends on the level of optimization set, and whether the method family is correctly recognized.

You can help the compiler declaring the method like this:

+ (AVQueuePlayer *)queuePlayerWithItems:(NSArray *)items NS_RETURNS_RETAINED;

This will cause the compiler to assume that the method returns an allocated object and needs to be sent a balanced release message when appropriate, instead to put the object into the autorelease pool.

NS_RETURNS_RETAINED will expand to __attribute__((ns_returns_retained))

The definition should omit the attribute NS_RETURNS_RETAINED .

Edit:

If this is not your method and if you cannot change the declaration, you may use an explicit autorelease pool to get rid of the autoreleased object:

{
    AVQueuePlayer* player;
    @autoreleasepool {
       // the next line retains the returned object:
       player = [AVQueuePlayer queuePlayerWithItems:items];
    }  // here, the autorelease pool releases the returned object (it's still alive)
    // do something with variable player:
    ...
    [player ...];  // last usage of variable player
    // now, the last reference to player ceases to exist, it gets released and deallocated

}