iOS多线程控制最大并发数

iOS多线程最常用是GCD和NSOperation。一般简单的异步任务直接使用GCD,需要控制最大并发数和控制状态时我选择封装NSOperation(SDWebImage和AFN也是封装的NSOperation),NSThread一般和常驻线程使用(如AFN框架)。

NSOperation直接使用系统提供的maxConcurrentOperationCount可设置最大并发数，GCD则需要结合信号量手动去控制。

dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);

    dispatch_async(workConcurrentQueue, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"1");

        sleep(5);

        dispatch_semaphore_signal(semaphore);

    });

    dispatch_async(workConcurrentQueue, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"2");

        sleep(3);

        dispatch_semaphore_signal(semaphore);

    });

      dispatch_async(workConcurrentQueue, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"3");

        dispatch_semaphore_signal(semaphore);

    });

    dispatch_async(workConcurrentQueue, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"4");

        dispatch_semaphore_signal(semaphore);

    });