Question

我有以下代码：

dispatch_apply(10, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(size_t index) {
    [NSThread sleepForTimeInterval:index];
    NSLog(@"sleep %lu, in thread %@",index,[NSThread currentThread]);
});

在主线程中运行时，它会在控制台中打印：

2017-08-14 15:24:53.812 TestGCD[21480:4127352] sleep 0, in thread <NSThread: 0x600000067d40>{number = 1, name = main}
2017-08-14 15:24:54.816 TestGCD[21480:4127420] sleep 1, in thread <NSThread: 0x600000072100>{number = 3, name = (null)}
2017-08-14 15:24:55.816 TestGCD[21480:4127435] sleep 2, in thread <NSThread: 0x600000072180>{number = 4, name = (null)}
2017-08-14 15:24:56.815 TestGCD[21480:4127422] sleep 3, in thread <NSThread: 0x6080000731c0>{number = 5, name = (null)}
2017-08-14 15:24:57.813 TestGCD[21480:4127352] sleep 4, in thread <NSThread: 0x600000067d40>{number = 1, name = main}
2017-08-14 15:24:59.818 TestGCD[21480:4127420] sleep 5, in thread <NSThread: 0x600000072100>{number = 3, name = (null)}
2017-08-14 15:25:01.822 TestGCD[21480:4127435] sleep 6, in thread <NSThread: 0x600000072180>{number = 4, name = (null)}
2017-08-14 15:25:03.821 TestGCD[21480:4127422] sleep 7, in thread <NSThread: 0x6080000731c0>{number = 5, name = (null)}
2017-08-14 15:25:05.815 TestGCD[21480:4127352] sleep 8, in thread <NSThread: 0x600000067d40>{number = 1, name = main}
2017-08-14 15:25:08.824 TestGCD[21480:4127420] sleep 9, in thread <NSThread: 0x600000072100>{number = 3, name = (null)}

我发现在“sleep 0,4,8”中，当前线程是主线程。

sleep 0, in thread <NSThread: 0x600000067d40>{number = 1, name = main}

为什么我将块提交到全局队列但偶尔在主队列上调用？

Answer 1

此日志表明您必须从主线程调用dispatch_apply。

当您说“将块提交到全局队列”时，请确保将调用dispatch_apply调度到全局队列，因为这是一个同步API。
将dispatch_apply发送到某个后台队列时，请确保使用dispatch_async。如果您使用dispatch_sync，它可以使用当前线程作为优化。

你想要的是：

NSLog(@"starting on thread %@",[NSThread currentThread]);

dispatch_async(dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
    dispatch_apply(10, dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^(size_t index) {
        [NSThread sleepForTimeInterval:index];
        NSLog(@"sleep %lu, in thread %@",index,[NSThread currentThread]);
    });
});

注意，我正在使用dispatch_async并且我将dispatch_apply放在已调度的块中，而不是相反。

dispatch_apply偶尔将块提交到主队列

1 个答案: