iOS多线程梳理-NSOperation

上一篇 iOS多线程梳理-GCD(3)

NSOperation

NSOperation 是苹果公司对 GCD 的封装，面向对象。 NSOperation 和 NSOperationQueue 分别对应 GCD 的 任务 和 队列 。使用如下：
1.将要执行的任务封装到一个 NSOperation 对象中
2.将此任务添加到一个 NSOperationQueue 对象中

添加任务

NSOperation 是一个抽象类，所以不能封装任务。但它有 2 个子类用于封装任务。分别是：NSInvocationOperation 和 NSBlockOperation 。创建一个 Operation 后，需要调用 start 方法来启动任务，它会 默认在当前队列同步执行。当然也可以在中途取消一个任务，只需要调用其 cancel 方法即可。

  //1.创建NSBlockOperation对象
  NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
      NSLog(@"task %@", [NSThread currentThread]);
  }];
  //2.开始任务
  [operation start];

NSBlockOperation 还有一个方法：addExecutionBlock: ，通过这个方法可以给 Operation 添加多个执行 Block，并且addExecutionBlock添加的Block 会并发异步执行。如下：

    NSLog(@"beagin %@",[NSThread currentThread]);
    NSBlockOperation *blockOp = [NSBlockOperation blockOperationWithBlock:^{//当前线程
       sleep(2);
        NSLog(@"first %@",[NSThread currentThread]);
    }];
    for (NSInteger i=0; i<3; i++) {
        [blockOp addExecutionBlock:^{//其他线程
            sleep(2);
            NSLog(@"section %@",[NSThread currentThread]);
        }];
    }
    [blockOp start];
/*输出结果：
     2017-10-12 17:15:30.638732+0800 GCDLearnAdvanced[5362:776321] beagin <NSThread: 0x608000076000>{number = 1, name = main}
     2017-10-12 17:15:32.640445+0800 GCDLearnAdvanced[5362:776360] section <NSThread: 0x60c000466e40>{number = 5, name = (null)}
     2017-10-12 17:15:32.640444+0800 GCDLearnAdvanced[5362:776363] section <NSThread: 0x60c000467a00>{number = 3, name = (null)}
     2017-10-12 17:15:32.640445+0800 GCDLearnAdvanced[5362:776361] section <NSThread: 0x600000270a00>{number = 4, name = (null)}
     2017-10-12 17:15:32.640469+0800 GCDLearnAdvanced[5362:776321] first <NSThread: 0x608000076000>{number = 1, name = main}
*/

创建队列NSOperation

• 主队列

  NSOperationQueue *mQueue = [NSOperationQueue mainQueue];
  

• 其他队列
  因为主队列比较特殊，所以会单独有一个类方法来获得主队列。那么通过初始化产生的队列就是其他队列了

• 串行队列
  NSOperationQueue 有一个参数 maxConcurrentOperationCount 最大并发数，可以用来设置最多可以让多少个任务同时执行。当把它设置为 1 的时候，就是为串行队列

任务依赖案例：

    //有 3 个任务：A: 从服务器上下载一张图片，B：给这张图片加个水印，C：把图片返回给服务器
    //1.任务一：下载图片
    NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"下载图片 - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //2.任务二：打水印
    NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"打水印   - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //3.任务三：上传图片
    NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"上传图片 - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //4.设置依赖
    [operation2 addDependency:operation1];      //任务二依赖任务一
    [operation3 addDependency:operation2];      //任务三依赖任务二
    //5.创建队列并加入任务
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperations:@[operation3, operation2, operation1] waitUntilFinished:YES];
    NSLog(@"其他task");
/*输出结果：
2017-10-12 17:33:27.035147+0800 GCDLearnAdvanced[5494:803023] 下载图片 - <NSThread: 0x600000274dc0>{number = 3, name = (null)}
2017-10-12 17:33:28.039947+0800 GCDLearnAdvanced[5494:803025] 打水印   - <NSThread: 0x60c000266e80>{number = 4, name = (null)}
2017-10-12 17:33:29.043323+0800 GCDLearnAdvanced[5494:803025] 上传图片 - <NSThread: 0x60c000266e80>{number = 4, name = (null)}
2017-10-12 17:33:30.048417+0800 GCDLearnAdvanced[5494:802986] 其他task
*/

• 不能相互依赖，比如A依赖B，B依赖A，这样会死锁
• 可以使用 removeDependency 来解除依赖关系
• addOperations：waitUntilFinished:当传入YES时会阻塞，等待任务完毕后才会继续往下执行

指定并发数量的多任务执行案例：

    NSLog(@"beagin %@",[NSThread currentThread]);
    NSOperationQueue *operationQ = [[NSOperationQueue alloc]init];
    operationQ.maxConcurrentOperationCount = 2;
    for (NSInteger i=0; i<5; i++) {
        NSBlockOperation *blockOp = [[NSBlockOperation alloc]init];
        [blockOp addExecutionBlock:^{
            sleep(2);
            NSLog(@"first%@ %@",@(i),[NSThread currentThread]);
        }];
        [operationQ addOperation:blockOp];
    }
/*输出结果：
     2017-10-12 16:55:18.363146+0800 GCDLearnAdvanced[5194:730174] beagin <NSThread: 0x60c00006ed40>{number = 1, name = main}
     2017-10-12 16:55:20.364293+0800 GCDLearnAdvanced[5194:730224] first0 <NSThread: 0x60400007f700>{number = 3, name = (null)}
     2017-10-12 16:55:20.364293+0800 GCDLearnAdvanced[5194:730228] first1 <NSThread: 0x6080002722c0>{number = 4, name = (null)}
     2017-10-12 16:55:22.368552+0800 GCDLearnAdvanced[5194:730225] first2 <NSThread: 0x600000269a00>{number = 5, name = (null)}
     2017-10-12 16:55:22.368552+0800 GCDLearnAdvanced[5194:730221] first3 <NSThread: 0x60400007d880>{number = 6, name = (null)}
     2017-10-12 16:55:24.372538+0800 GCDLearnAdvanced[5194:730228] first4 <NSThread: 0x6080002722c0>{number = 4, name = (null)}
*/

通过打印的时间我们可以看到最多只有两条线程同时异步执行。