NSOperation的作⽤：配合使用NSOperation和NSOperationQueue也能实现多线程编程
NSOperation和NSOperationQueue实现多线程的具体步骤：
1）先将需要执行的操作封装到一个NSOperation对象中
2）然后将NSOperation对象添加到NSOperationQueue中
3）系统会⾃动将NSOperationQueue中的NSOperation取出来
4）将取出的NSOperation封装的操作放到⼀条新线程中执⾏

NSOperation是个抽象类,并不具备封装操作的能力,必须使⽤它的子类
使用NSOperation⼦类的方式有3种：
1）NSInvocationOperation
2）NSBlockOperation
3）自定义子类继承NSOperation,实现内部相应的⽅法

NSInvocationOperation 开启任务方法

//用 NSInvocationOperation 创建线程NSInvocationOperation *invocation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download:) object:@"NSInvocationOperation"];
//在当前线程中同步执行
[invocation start];- (void)download:(NSString *)url{NSLog(@"%@ --- %@", [NSThread currentThread], url);
}

将创建的任务添加到队列中,异步执行

- (void)invocationOperation{//用 NSInvocationOperation 创建线程NSInvocationOperation *invocation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download:) object:@"NSInvocationOperation"];  //创建全局队列(实现异步并发执行)NSOperationQueue *queue = [[NSOperationQueue alloc] init];//从对列中取出任务执行(异步)[queue addOperation:invocation];}- (void)download:(NSString *)url{NSLog(@"%@ --- %@", [NSThread currentThread], url);
}

注意:操作对象默认在主线程中执行，只有添加到队列中才会开启新的线程。即默认情况下，如果操作没有放到队列中queue中，都是同步执行。只有将NSOperation放到一个NSOperationQueue中,才会异步执行操作

NSBlockOperation 开启任务的方法

NSBlockOperation 添加任务
- (void)blockOperation2{//创建任务NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片1 --- %@", [NSThread currentThread]);}];[operation1 start];
}

注意:只要NSBlockOperation封装的操作数 > 1,就会异步执行操作

- (void)blockOperation2{//注意:只要NSBlockOperation封装的操作数 > 1,就会异步执行操作//创建任务NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片1 --- %@", [NSThread currentThread]);}];[operation1 addExecutionBlock:^{NSLog(@"下载图片2 --- %@", [NSThread currentThread]);}];[operation1 addExecutionBlock:^{NSLog(@"下载图片3 --- %@", [NSThread currentThread]);}];[operation1 start];
}

NSOperationQueue

NSOperationQueue的作⽤：NSOperation可以调⽤start⽅法来执⾏任务,但默认是同步执行的
如果将NSOperation添加到NSOperationQueue(操作队列)中,系统会自动异步执行NSOperation中的操作
添加操作到NSOperationQueue中，自动执行操作，自动开启线程
获取 NSOperationQueue 队列:

    NSOperationQueue *queue = [[NSOperationQueue alloc] init];//并发队列NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];//主队列

添加任务有两个方法:

- (void)addOperation:(NSOperation *)op;
- (void)addOperationWithBlock:(void (^)(void))block; 

实例代码

- (void)blockOperation{//创建全局队列(实现异步并发执行)NSOperationQueue *queue = [[NSOperationQueue alloc] init];//并发队列//NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];//主队列//创建任务NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片1 --- %@", [NSThread currentThread]);}];NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片2 --- %@", [NSThread currentThread]);}];NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片3 --- %@", [NSThread currentThread]);}];NSBlockOperation *operation4 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片4 --- %@", [NSThread currentThread]);}];NSBlockOperation *operation5 = [NSBlockOperation blockOperationWithBlock:^{NSLog(@"下载图片5 --- %@", [NSThread currentThread]);}];//将任务添加到队列中[queue addOperation:operation1];[queue addOperation:operation2];[queue addOperation:operation3];[queue addOperation:operation4];[queue addOperation:operation5];
}

- (void)queueAddBlock{//创建全局队列(实现异步并发执行)NSOperationQueue *queue = [[NSOperationQueue alloc] init];//直接向队列中添加任务[queue addOperationWithBlock:^{NSLog(@"下载图片1 --- %@", [NSThread currentThread]);}];[queue addOperationWithBlock:^{NSLog(@"下载图片2 --- %@", [NSThread currentThread]);}];[queue addOperationWithBlock:^{NSLog(@"下载图片3 --- %@", [NSThread currentThread]);}];[queue addOperationWithBlock:^{NSLog(@"下载图片4 --- %@", [NSThread currentThread]);}];[queue addOperationWithBlock:^{NSLog(@"下载图片5 --- %@", [NSThread currentThread]);}];
}