NSOpseration是抽象基类，所以要用子类实现它的方法。（内部使用gcd实现）

1.一个NSOperation 就代表一个任务，任务的代码分装在block里或者函数里。    OC提供子类：        NSInvocztionOperation：任务是一个函数，比较长        NSBlockOperation：任务是一个block，比较短

常用方法demo：（有关线程执行方式，队列类型看）

1.1NSBlockOperation:

- (void) testBlockOperation2{    //情况1.    NSBlockOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"startA:%@", [NSThread currentThread]);    }];    [operationA start];    NSBlockOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"startB:%@", [NSThread currentThread]);    }];    [operationB start];    NSBlockOperation *operationC = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"startC:%@", [NSThread currentThread]);    }];    [operationC start];        //情况2.    NSBlockOperation *operation = [[ NSBlockOperation alloc] init];    [operation addExecutionBlock:^{        NSLog(@"start1:%@", [NSThread currentThread]);    }];        [operation addExecutionBlock:^{        NSLog(@"start2:%@", [NSThread currentThread]);    }];    [operation addExecutionBlock:^{        NSLog(@"start3:%@", [NSThread currentThread]);    }];    [operation start];}

可以看到任务配置好以后，直接执行时同步的。而把任务block放到一起的时候是异步的。

- (void) testBlockOperation{    //情况3.    NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"start1:%@", [NSThread currentThread]);    }];        NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"start2:%@", [NSThread currentThread]);    }];        NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"start3:%@", [NSThread currentThread]);    }];        NSOperationQueue *queue = [[NSOperationQueue alloc] init];    [queue addOperations:@[operation3, operation2, operation1] waitUntilFinished:NO];}

放到全局队列里也是异步的（并发），如果放到主队列（[NSOperationQueue mainQueue]）里，那么是同步的。

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1.2NSInvocztionOperation：使用它和NSBlockOperation是一样的。都是NSOperation的子类……

- (void) testOperationInvocation{    NSOperationQueue *queue = [[NSOperationQueue alloc] init];    NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(operationFirst:) object:@"start"];    [operation1 start];        NSInvocationOperation *operation2 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(operationFirst:) object:@"newQ"];    [queue addOperation:operation2];}- (void) operationFirst:(NSString *) who{    NSLog(@"\n%@, %s, %@", who, __func__, [NSThread currentThread]);}

2.任务间依赖关系：也是用来实现异步线程间通信的一种约定

- (void) operationDependency{    NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"thread 1");    }];    [operation1 setCompletionBlock:^{        NSLog(@"finish thread1");    }];    NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"thread 2");    }];    NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{        NSLog(@"thread 3");    }];        [operation1 addDependency:operation2];    [operation2 addDependency:operation3];        NSOperationQueue *queue = [[NSOperationQueue alloc] init];    [queue addOperation:operation1];    [queue addOperation:operation2];    [queue addOperation:operation3];}

依据依赖顺序执行