Handle is invoke method of Middleware. Result is the data to be processed. GatherCallback is a callback for processing. GatherCallback accepts Result | undefined.
handle是Middleware类被调用时使用的方法, result为待处理Result类型, 如果传入值为string类型, 会自动转为["undefined", string] GatherCallback为处理完毕时的回调
Finisher is the Middleware that handles the last call to the data, usually at the very end of the next operation. Finisher's handle is very simple, usually do nothing (Noop class), or call the GatherCallback (Gather class) that has been passed down, and pass in the processed data.
Finisher是处理数据最后调用的Middleware, 通常出现在next操作的最末端. Finisher的handle很简单, 通常是什么都不做(Noop类), 或者调用一直传递下来的GatherCallback（Gather类）, 并传入处理好的数据.