NSOperation NSOperationQueue

The NSOperation and NSOperationQueue classes alleviate much of the pain of multi-threading, allowing you to simply define your tasks, set any dependencies that exist, and fire them off. Each task, or operation, is represented by an instance of an NSOperation class; the NSOperationQueue class takes care of starting the operations, ensuring that they are run in the appropriate order, and accounting for any priorities that have been set.

NSOperation 和 NSOperationQueue类简化了多线程操作。创建一个NSOperationQueue对象，然后创建NSOperation添加到NSOperationQueue中。NSOperationQueue会为每一个NSOperation创建一个线程，并按照它们被添加的顺序执行（或者指定的顺序）。NSOperationQueue会自己处理autorelease pools 和其他的garbage，而不需要像在自己手动创建多线程时去费心管理这些事情。

NSOperationQueue *queue = [NSOperationQueue new];

NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self
    selector:@selector(methodToCall)
    object:objectToPassToMethod];

[queue addOperation:operation];
[operation release];

methodToCall会在新线程中被执行。