上一篇 iOS多線程梳理-GCD(3)

NSOperation

NSOperation 是蘋果公司對 GCD 的封裝，面向?qū)ο蟆?NSOperation 和 NSOperationQueue 分別對應(yīng) GCD 的 任務(wù) 和 隊列 。使用如下：
1.將要執(zhí)行的任務(wù)封裝到一個 NSOperation 對象中
2.將此任務(wù)添加到一個 NSOperationQueue 對象中

添加任務(wù)

NSOperation 是一個抽象類，所以不能封裝任務(wù)。但它有 2 個子類用于封裝任務(wù)。分別是：NSInvocationOperation 和 NSBlockOperation 。創(chuàng)建一個 Operation 后，需要調(diào)用 start 方法來啟動任務(wù)，它會 默認(rèn)在當(dāng)前隊列同步執(zhí)行。當(dāng)然也可以在中途取消一個任務(wù)，只需要調(diào)用其 cancel 方法即可。

  //1.創(chuàng)建NSBlockOperation對象
  NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
      NSLog(@"task %@", [NSThread currentThread]);
  }];
  //2.開始任務(wù)
  [operation start];

NSBlockOperation 還有一個方法：addExecutionBlock: ，通過這個方法可以給 Operation 添加多個執(zhí)行 Block，并且addExecutionBlock添加的Block 會并發(fā)異步執(zhí)行。如下：

    NSLog(@"beagin %@",[NSThread currentThread]);
    NSBlockOperation *blockOp = [NSBlockOperation blockOperationWithBlock:^{//當(dāng)前線程
       sleep(2);
        NSLog(@"first %@",[NSThread currentThread]);
    }];
    for (NSInteger i=0; i<3; i++) {
        [blockOp addExecutionBlock:^{//其他線程
            sleep(2);
            NSLog(@"section %@",[NSThread currentThread]);
        }];
    }
    [blockOp start];
/*輸出結(jié)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}
*/

創(chuàng)建隊列NSOperation

• 主隊列

  NSOperationQueue *mQueue = [NSOperationQueue mainQueue];
  

• 其他隊列
  因為主隊列比較特殊，所以會單獨有一個類方法來獲得主隊列。那么通過初始化產(chǎn)生的隊列就是其他隊列了

• 串行隊列
  NSOperationQueue 有一個參數(shù) maxConcurrentOperationCount 最大并發(fā)數(shù)，可以用來設(shè)置最多可以讓多少個任務(wù)同時執(zhí)行。當(dāng)把它設(shè)置為 1 的時候，就是為串行隊列

任務(wù)依賴案例：

    //有 3 個任務(wù)：A: 從服務(wù)器上下載一張圖片，B：給這張圖片加個水印，C：把圖片返回給服務(wù)器
    //1.任務(wù)一：下載圖片
    NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"下載圖片 - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //2.任務(wù)二：打水印
    NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"打水印   - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //3.任務(wù)三：上傳圖片
    NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"上傳圖片 - %@", [NSThread currentThread]);
        [NSThread sleepForTimeInterval:1.0];
    }];
    //4.設(shè)置依賴
    [operation2 addDependency:operation1];      //任務(wù)二依賴任務(wù)一
    [operation3 addDependency:operation2];      //任務(wù)三依賴任務(wù)二
    //5.創(chuàng)建隊列并加入任務(wù)
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperations:@[operation3, operation2, operation1] waitUntilFinished:YES];
    NSLog(@"其他task");
/*輸出結(jié)果：
2017-10-12 17:33:27.035147+0800 GCDLearnAdvanced[5494:803023] 下載圖片 - <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] 上傳圖片 - <NSThread: 0x60c000266e80>{number = 4, name = (null)}
2017-10-12 17:33:30.048417+0800 GCDLearnAdvanced[5494:802986] 其他task
*/

• 不能相互依賴，比如A依賴B，B依賴A，這樣會死鎖
• 可以使用 removeDependency 來解除依賴關(guān)系
• addOperations：waitUntilFinished:當(dāng)傳入YES時會阻塞，等待任務(wù)完畢后才會繼續(xù)往下執(zhí)行

指定并發(fā)數(shù)量的多任務(wù)執(zhí)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];
    }
/*輸出結(jié)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)}
*/

通過打印的時間我們可以看到最多只有兩條線程同時異步執(zhí)行。