iOS GCD不同场景的使用比较

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/**

* async -- 并发队列

* 会创建线程，一般同时开多条

* 并发执行任务

<span style="font-size:14px;">- (void)asyncGlobalQueue{    // 获得全局的并发队列    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);        // 将 任务 添加 全局队列 中去 异步 执行    dispatch_async(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);    });}</span>

<span style="font-size:14px;">-----下载图片4---<NSThread: 0x7fbd52c9b390>{number = 5, name = (null)}-----下载图片3---<NSThread: 0x7fbd52f00230>{number = 4, name = (null)}-----下载图片2---<NSThread: 0x7fbd52e3eed0>{number = 3, name = (null)}-----下载图片5---<NSThread: 0x7fbd52e2eb40>{number = 6, name = (null)}-----下载图片1---<NSThread: 0x7fbd52c06010>{number = 2, name = (null)}</span>

/**

* async -- 串行队列

* 会创建线程, 一般只开1条线程

* 串行执行任务（一个任务执行完毕后再执行下一个任务）

- (void)asyncSerialQueue{    // 1.创建一个串行队列    dispatch_queue_t queue = dispatch_queue_create("cn.heima.queue", NULL);        // 2.将任务添加到串行队列中 异步 执行    dispatch_async(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);    });}

-----下载图片1---<NSThread: 0x7fd4c5b00b50>{number = 2, name = (null)}-----下载图片2---<NSThread: 0x7fd4c5b00b50>{number = 2, name = (null)}-----下载图片3---<NSThread: 0x7fd4c5b00b50>{number = 2, name = (null)}-----下载图片4---<NSThread: 0x7fd4c5b00b50>{number = 2, name = (null)}-----下载图片5---<NSThread: 0x7fd4c5b00b50>{number = 2, name = (null)}

/**

* async -- 主队列

- (void)asyncMainQueue{    // 1.主队列(添加到主队列中的任务，都会自动放到主线程中去执行)    dispatch_queue_t queue = dispatch_get_main_queue();        // 2.添加 任务 到主队列中 异步 执行    dispatch_async(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);    });    dispatch_async(queue, ^{        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);    });}

-----下载图片1---<NSThread: 0x7faaca905000>{number = 1, name = main}-----下载图片2---<NSThread: 0x7faaca905000>{number = 1, name = main}-----下载图片3---<NSThread: 0x7faaca905000>{number = 1, name = main}-----下载图片4---<NSThread: 0x7faaca905000>{number = 1, name = main}-----下载图片5---<NSThread: 0x7faaca905000>{number = 1, name = main}

/**

* sync -- 主队列(会卡死)

- (void)syncMainQueue{    NSLog(@"syncMainQueue----begin--");        // 1.主队列(添加到主队列中的任务，都会自动放到主线程中去执行)    dispatch_queue_t queue = dispatch_get_main_queue();        // 2.添加 任务 到主队列中 异步 执行    dispatch_sync(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });        NSLog(@"syncMainQueue----end--");}

syncMainQueue----begin--

/**

* sync -- 并发队列

* 不会创建线程

* 串行执行任务（一个任务执行完毕后再执行下一个任务）

* 并发队列失去了并发的功能

- (void)syncGlobalQueue{    // 获得全局的并发队列    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);        // 将 任务 添加到 全局并发队列 中 同步 执行    dispatch_sync(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);    });}

-----下载图片1---<NSThread: 0x7fe5b2904c30>{number = 1, name = main}-----下载图片2---<NSThread: 0x7fe5b2904c30>{number = 1, name = main}-----下载图片3---<NSThread: 0x7fe5b2904c30>{number = 1, name = main}-----下载图片4---<NSThread: 0x7fe5b2904c30>{number = 1, name = main}-----下载图片5---<NSThread: 0x7fe5b2904c30>{number = 1, name = main}

/**

* sync -- 串行队列

* 不会创建线程

* 串行执行任务（一个任务执行完毕后再执行下一个任务）

- (void)syncSerialQueue{    // 创建一个串行队列    dispatch_queue_t queue = dispatch_queue_create("cn.heima.queue", NULL);        // 将 任务 添加到 串行队列 中 同步 执行    dispatch_sync(queue, ^{        NSLog(@"-----下载图片1---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片2---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片3---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片4---%@", [NSThread currentThread]);    });    dispatch_sync(queue, ^{        NSLog(@"-----下载图片5---%@", [NSThread currentThread]);    });}

-----下载图片1---<NSThread: 0x7fdb40f05020>{number = 1, name = main}-----下载图片2---<NSThread: 0x7fdb40f05020>{number = 1, name = main}-----下载图片3---<NSThread: 0x7fdb40f05020>{number = 1, name = main}-----下载图片4---<NSThread: 0x7fdb40f05020>{number = 1, name = main}-----下载图片5---<NSThread: 0x7fdb40f05020>{number = 1, name = main}

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