为什么action会触发所有reducers

Question

利用combineReducers组成的组合reducers，利用action触发，里面的每个reducers等都会触发？

Answer 1

源码里每个reducer都会执行一遍 只会触发匹配到actionType

Answer 2

combineReducers源码大致是这样的：

/**
 * @params reducers 子reducer
 * @return nextState 返回更新之后的state
 **/
function combineReducers(reducers){
    const reducerKeys = Object.keys(reducers);

    return function combination(state = {}, action) {
        // This is the object we are going to return.
        const nextState = {}

        // Loop through all the reducer keys
        for (let i = 0; i < reducerKeys.length; i++) {
            // Get the current key name
            const key = reducerKeys[i];
            // Get the current reducer
            const reducer = reducers[key]
            // Get the the previous state
            const previousStateForKey = state[key]
            // Get the next state by running the reducer
            const nextStateForKey = reducer(previousStateForKey, action)
            // Update the new state for the current reducer
            nextState[key] = nextStateForKey;
        }
        return nextState;
    }
}

初始化combineReducers时，会遍历所有已传入的reduces对象的key，并将reducerKeys作为闭包变量存储在函数中，之后每次更新(dispatch动作)都只会循环这个数组和reducers对象的每个key。因为combination函数接受的参数state是更新后的庞大的对象，为了将每个子reduce返回更新后的state作为值添加并映射到nextState对象中对应key对象上，我们就需要调用每个reducer(previousStateForKey, action)