看到Redux官網上Reducer拆分后，最后生成的state的組合有點懵，主要是對combineReducers()這個工具類不了解，其實該方法是消除了一些樣板代碼。

如todoApp事例：

import { combineReducers } from 'redux'const todoApp = combineReducers({visibilityFilter,todos
})export default todoApp
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上面的寫法跟下面完全等價：

export default function todoApp(state = {}, action) {return {visibilityFilter: visibilityFilter(state.visibilityFilter, action),todos: todos(state.todos, action)}
}
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你也可以給它們設置不同的 key，或者調用不同的函數。下面兩種合成 reducer 方法完全等價：

const reducer = combineReducers({a: doSomethingWithA,b: processB,c: c
})
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function reducer(state = {}, action) {return {a: doSomethingWithA(state.a, action),b: processB(state.b, action),c: c(state.c, action)}
}
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