iOS 关于多线程的一些知识点（不断更新）

1.NSOperation

对于NSOperation，In OS X v10.6 and later, operation queues ignore the value returned by this method and always start operations on a separate thread.就是一个operation就新建一个thread。但是，这个说法是针对 NSOperationQueue *testQueue = [[NSOperationQueue alloc] init]; 这种queue来说的，如果向mainQueue中加入operation，那么这些operation一定会执行在main thread中，而且他们会串行执行。

下面是关于mainQueue方法的一部分说明：

The returned queue executes one operation at a time on the app’s main thread. 

下面看一段例子

 NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
    [mainQueue addOperationWithBlock:^{

        for(int i=;i<;i++)
        {
            NSLog(@"i is %d",i);
        }

    }];

    [mainQueue addOperationWithBlock:^{

        for(int m=;m<;m++)
        {
            NSLog(@"m is %d",m);
        }

    }];

以下是它的部分输出

-- ::55.000 OperationQueueTest[:60b] i is
-- ::55.001 OperationQueueTest[:60b] i is
-- ::55.001 OperationQueueTest[:60b] i is
-- ::55.002 OperationQueueTest[:60b] i is
-- ::55.002 OperationQueueTest[:60b] i is
-- ::55.003 OperationQueueTest[:60b] i is
-- ::55.004 OperationQueueTest[:60b] m is
-- ::55.004 OperationQueueTest[:60b] m is
-- ::55.004 OperationQueueTest[:60b] m is
-- ::55.005 OperationQueueTest[:60b] m is
-- ::55.005 OperationQueueTest[:60b] m is
-- ::55.006 OperationQueueTest[:60b] m is
-- ::55.006 OperationQueueTest[:60b] m is
-- ::55.007 OperationQueueTest[:60b] m is
-- ::55.017 OperationQueueTest[:60b] m is
-- ::55.017 OperationQueueTest[:60b] m is
-- ::55.018 OperationQueueTest[:60b] m is
-- ::55.018 OperationQueueTest[:60b] m is
-- ::55.019 OperationQueueTest[:60b] m is 

可以看出，加入mainQueue后，两个operation是顺序执行的。

再看下面一段代码

  NSOperationQueue *testQueue = [[NSOperationQueue alloc] init];
    [testQueue addOperationWithBlock:^{

        NSLog(@"111111  thread is %@",[NSThread currentThread]);
        for(int i=;i<;i++)
        {
            NSLog(@"i is %d",i);
        }

    }];

    [testQueue addOperationWithBlock:^{
        NSLog(@"222222 thread is %@",[NSThread currentThread]);
        for(int m=;m<;m++)
        {
            NSLog(@"m is %d",m);
        }

    }];

下面是它的部分输出

-- ::59.566 OperationQueueTest[:] i is
-- ::59.566 OperationQueueTest[:] i is
-- ::59.564 OperationQueueTest[:] m is
-- ::59.566 OperationQueueTest[:] i is
-- ::59.566 OperationQueueTest[:] i is
-- ::59.567 OperationQueueTest[:] i is
-- ::59.567 OperationQueueTest[:] i is
-- ::59.568 OperationQueueTest[:] i is
-- ::59.568 OperationQueueTest[:] i is
-- ::59.568 OperationQueueTest[:] i is
-- ::59.569 OperationQueueTest[:] i is
-- ::59.569 OperationQueueTest[:] i is
-- ::59.567 OperationQueueTest[:] m is
-- ::59.570 OperationQueueTest[:] m is
-- ::59.569 OperationQueueTest[:] i is
-- ::59.571 OperationQueueTest[:] i is
-- ::59.572 OperationQueueTest[:] i is
-- ::59.571 OperationQueueTest[:] m is
-- ::59.573 OperationQueueTest[:] m is
-- ::59.573 OperationQueueTest[:] m is
-- ::59.574 OperationQueueTest[:] m is
-- ::59.575 OperationQueueTest[:] m is
-- ::59.575 OperationQueueTest[:] m is
-- ::59.576 OperationQueueTest[:] m is
-- ::59.577 OperationQueueTest[:] m is 

可以看出Operation是同时在运行的，它们运行在2个不同的thread中！