线程死锁浅析

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情形1:

dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"不会执行");
    });

如果改成异步就可以了，因为当前主线程的一次runloop会马上返回，下一次runloop则会执行主队列里的block。

dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"会执行");
    });

还有一个用于在主线程更新ui的方法，如果在主线程执行它，即使是同步的，也并不会造成死锁。

[self performSelectorOnMainThread:@selector(logTest) withObject:nil waitUntilDone:YES];

关于operationqueue的。

NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"111");
    }];
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"222");
    }];
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"333");
    }];
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"444");
    }];
    NSArray *arr = [NSArray arrayWithObjects:
                    op1,
                    op2,
                    op3,
                    op4,
                    nil];
    [[NSOperationQueue mainQueue]addOperations:arr waitUntilFinished:NO];

    NSLog(@"2");

结果：

2015-07-01 14:07:34.064 TestProject[1388:318202] 1
2015-07-01 14:07:34.065 TestProject[1388:318202] 2
2015-07-01 14:07:34.070 TestProject[1388:318202] 111
2015-07-01 14:07:34.071 TestProject[1388:318202] 222
2015-07-01 14:07:34.073 TestProject[1388:318202] 333
2015-07-01 14:07:34.073 TestProject[1388:318202] 444

NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"111");
    }];
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"222");
    }];
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"333");
    }];
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"444");
    }];
    NSArray *arr = [NSArray arrayWithObjects:
                    op1,
                    op2,
                    op3,
                    op4,
                    nil];
    [[NSOperationQueue mainQueue]addOperations:arr waitUntilFinished:YES];
    NSLog(@"2");

2015-07-01 14:10:48.577 TestProject[1423:335286] 1

addOperations 的返回，所以当前运行循环永远不会结束，所以主队列中的block永远不会执行到，程序就这样卡住了。

如果另外创建一个队列（不会是主队列），用同步执行的话：

<pre name="code" class="objc">NSLog(@"1");
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"111");
    }];
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"222");
    }];
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"333");
    }];
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"444");
    }];
    NSArray *arr = [NSArray arrayWithObjects:
                    op1,
                    op2,
                    op3,
                    op4,
                    nil];
    NSOperationQueue *queue = [[NSOperationQueue alloc]init];//并发,顺序不定
    [queue addOperations:arr waitUntilFinished:YES];
    NSLog(@"2");

<pre name="code" class="objc">2015-07-01 14:14:44.417 TestProject[1480:353564] 1
2015-07-01 14:14:44.418 TestProject[1480:353689] 111
2015-07-01 14:14:44.418 TestProject[1480:353705] 444
2015-07-01 14:14:44.418 TestProject[1480:353690] 333
2015-07-01 14:14:44.419 TestProject[1480:353685] 222
2015-07-01 14:14:44.419 TestProject[1480:353564] 2

如果改为异步执行，同样正常执行，1，2，3，4block执行顺序不定，但函数会立即返回。

<pre name="code" class="objc">NSLog(@"1");
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"111");
    }];
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"222");
    }];
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"333");
    }];
    NSBlockOperation *op4 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"444");
    }];
    NSArray *arr = [NSArray arrayWithObjects:
                    op1,
                    op2,
                    op3,
                    op4,
                    nil];
    NSOperationQueue *queue = [[NSOperationQueue alloc]init];//并发,顺序不定
    [queue addOperations:arr waitUntilFinished:NO];
    NSLog(@"2");

<pre name="code" class="objc">2015-07-01 14:17:53.111 TestProject[1513:365712] 1
2015-07-01 14:17:53.112 TestProject[1513:365712] 2
2015-07-01 14:17:53.112 TestProject[1513:365832] 333
2015-07-01 14:17:53.112 TestProject[1513:365831] 111
2015-07-01 14:17:53.112 TestProject[1513:365837] 444
2015-07-01 14:17:53.113 TestProject[1513:365830] 222

情形3:

<pre name="code" class="objc">NSLog(@"1");
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"=================1");
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"=================2");
        });
        NSLog(@"=================3");
    });
    NSLog(@"2");
    while (1) {
        
    }

运行结果：

<pre name="code" class="objc">2015-07-01 14:21:25.499 TestProject[1651:389143] 1
2015-07-01 14:21:25.499 TestProject[1651:389143] 2
2015-07-01 14:21:25.499 TestProject[1651:389172] =================1

<pre name="code" class="objc">NSLog(@"1");
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"=================1");
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"=================2");
        });
        NSLog(@"=================3");
    });
    NSLog(@"2");
    while (1) {
        
    }

<pre name="code" class="objc">2015-07-01 14:24:54.855 TestProject[1687:402116] 1
2015-07-01 14:24:54.855 TestProject[1687:402116] 2
2015-07-01 14:24:54.855 TestProject[1687:402152] =================1
2015-07-01 14:24:54.855 TestProject[1687:402152] =================3

在主线程中不要同步执行在主队列中的block。

线程死锁浅析

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原文地址：http://blog.csdn.net/ioszhuang2015/article/details/46698503