1 使用block 可以轻松地绑定各处代码块，使用delete 结构是分散的，不利于变量之间传值，不像block可以随意地获取变量值。
2.使用block可以方便执行异步代码，作为异步处理回调。
In terms of code readability, the block makes it easy to see in one place exactly what will happen before and after the task completes, avoiding the need to trace through delegate methods to find out what’s going to happen.
