利用NSOperation 与 NSOperationQueue

• 创建队列

NSOperationQueue * queue = [[NSOperationQueue alloc]init];

• 创建三个操作

NSOperation * A = [NSBlockOperation blockOperationWithBlock:^{
    NSLog{@"A"};
}];
NSOperation * B = [NSBlockOperation blockOperationWithBlock:^{
    NSLog{@"B"};
}];
NSOperation * C = [NSBlockOperation blockOperationWithBlock:^{
    NSLog{@"C"};
}];

• 添加依赖

[C addDependency:a];
[C addDependency:b];

• 执行操作

[queue addOperation:a];
[queue addOperation:b];
[queue addOperation:c];