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]);
});
在主线程中运行它时，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，但它偶尔会在主队列上调用？
此日志表明您必须dispatch_apply从主线程调用。
当您说“将块提交到全局队列”时，请确保也将调用分派dispatch_apply到全局队列，因为这是一个同步 API。
当您将 调度dispatch_apply到某个后台队列时，请确保使用dispatch_async. 如果使用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]);
});
});
请注意，我正在使用dispatch_async并且将其dispatch_apply放入已调度块内，而不是相反。
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我来说两句