相对于GCD。NSOperation更加灵活，能够自如地操作生命周期。

NSOperation是一个抽象的类，在使用时，使用的是他的两个子类NSInvocationOperation和

基本实现

NSInvocationOperation:target形式

- (void)baseUseDemo {
    NSInvocationOperation *op = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(doSomething) object:nil];
    [NSOperationQueue.mainQueue addOperation:op];
}

- (void)doSomething {
    NSLog(@"hello Operation");
}

NSBlockOperation:block形式

- (void)baseUseDemo {
    NSBlockOperation *blockOp = [[NSBlockOperation alloc] init];
    //添加执行代码块
    [blockOp addExecutionBlock:^{
        NSLog(@"This is BlockOperation!");
    }];
    
    //设置监听
    blockOp.completionBlock = ^{
        NSLog(@"Completion");
    };
    //添加到队列
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperation:blockOp];
}

优先级

    blockOp.qualityOfService = NSQualityOfServiceUserInteractive;

typedef NS_ENUM(NSInteger, NSQualityOfService) {
    NSQualityOfServiceUserInteractive = 0x21,
    NSQualityOfServiceUserInitiated = 0x19,
    NSQualityOfServiceUtility = 0x11,
    NSQualityOfServiceBackground = 0x09,
    NSQualityOfServiceDefault = -1
} API_AVAILABLE(macos(10.10), ios(8.0), watchos(2.0), tvos(9.0));

UserInteractive优先级最高，NSQualityOfServiceBackground优先级最低。

设置并发数

queue.maxConcurrentOperationCount = 2;

设置依赖关系

- (void)dependencyDemo {
    
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        [NSThread sleepForTimeInterval:0.5];
        NSLog(@"任务1");
    }];
    
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        [NSThread sleepForTimeInterval:0.5];
        NSLog(@"任务2");
    }];
    
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        [NSThread sleepForTimeInterval:0.5];
        NSLog(@"任务3");
    }];
    
    [op2 addDependency:op1];
    [op3 addDependency:op2];
    
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperations:@[op1, op2, op3] waitUntilFinished:YES];
}

挂起

[queue setSuspended:YES];
注意：suspended不能挂起已经被执行的任务。

[queue cancelAllOperations];
干掉所有任务。下次得重新添加任务