使用函数分解(Functional Decomposition)和 Reducer 组合(Reducer Composition)重构 Reducer
看看不同类型的 sub-reducer
和如何把他们组合在一起的例子是很有用的。现在让我们看看如何将一个大型的单个 reducer 重构为多个比较小的函数的组合。
注意: 为了说明重构的概念和过程而不是为了编写简洁的代码,这个例子是特意以冗长的风格编写的
初遇 Reducer
让我们看看初始 reducer 长什么样:
const initialState = {
visibilityFilter: "SHOW_ALL",
todos: [],
};
function appReducer(state = initialState, action) {
switch (action.type) {
case "SET_VISIBILITY_FILTER": {
return Object.assign({}, state, {
visibilityFilter: action.filter,
});
}
case "ADD_TODO": {
return Object.assign({}, state, {
todos: state.todos.concat({
id: action.id,
text: action.text,
completed: false,
}),
});
}
case "TOGGLE_TODO": {
return Object.assign({}, state, {
todos: state.todos.map((todo) => {
if (todo.id !== action.id) {
return todo;
}
return Object.assign({}, todo, {
completed: !todo.completed,
});
}),
});
}
case "EDIT_TODO": {
return Object.assign({}, state, {
todos: state.todos.map((todo) => {
if (todo.id !== action.id) {
return todo;
}
return Object.assign({}, todo, {
text: action.text,
});
}),
});
}
default:
return state;
}
}
这个函数非常短,但已经开始变得比较复杂。我们在处理两个不同的区域(filtering 和 todo 列表),嵌套使得更新逻辑难以阅读,并且会让我们不清楚到底是什么跟什么。
提取工具函数(Extracting Utility Functions)
第一步是写一个返回更新了相应区域的新对象。这儿还有一个重复的逻辑是在更新数组中的特定项目,我们也可以将他提成一个函数。
function updateObject(oldObject, newValues) {
// 用空对象作为第一个参数传递给 Object.assign,以确保是复制数据,而不是去改变原来的数据
return Object.assign({}, oldObject, newValues);
}
function updateItemInArray(array, itemId, updateItemCallback) {
const updatedItems = array.map((item) => {
if (item.id !== itemId) {
// 因为我们只想更新一个项目,所以保留所有的其他项目
return item;
}
// 使用提供的回调来创建新的项目
const updatedItem = updateItemCallback(item);
return updatedItem;
});
return updatedItems;
}
function appReducer(state = initialState, action) {
switch (action.type) {
case "SET_VISIBILITY_FILTER": {
return updateObject(state, { visibilityFilter: action.filter });
}
case "ADD_TODO": {
const newTodos = state.todos.concat({
id: action.id,
text: action.text,
completed: false,
});
return updateObject(state, { todos: newTodos });
}
case "TOGGLE_TODO": {
const newTodos = updateItemInArray(state.todos, action.id, (todo) => {
return updateObject(todo, { completed: !todo.completed });
});
return updateObject(state, { todos: newTodos });
}
case "EDIT_TODO": {
const newTodos = updateItemInArray(state.todos, action.id, (todo) => {
return updateObject(todo, { text: action.text });
});
return updateObject(state, { todos: newTodos });
}
default:
return state;
}
}
这样就减少了重复,使得代码的可读性更高。
提取 case reducer
接下来,把特殊逻辑封装成对应的函数:
// 省略了内容
function updateObject(oldObject, newValues) {}
function updateItemInArray(array, itemId, updateItemCallback) {}
function setVisibilityFilter(state, action) {
return updateObject(state, { visibilityFilter: action.filter });
}
function addTodo(state, action) {
const newTodos = state.todos.concat({
id: action.id,
text: action.text,
completed: false,
});
return updateObject(state, { todos: newTodos });
}
function toggleTodo(state, action) {
const newTodos = updateItemInArray(state.todos, action.id, (todo) => {
return updateObject(todo, { completed: !todo.completed });
});
return updateObject(state, { todos: newTodos });
}
function editTodo(state, action) {
const newTodos = updateItemInArray(state.todos, action.id, (todo) => {
return updateObject(todo, { text: action.text });
});
return updateObject(state, { todos: newTodos });
}
function appReducer(state = initialState, action) {
switch (action.type) {
case "SET_VISIBILITY_FILTER":
return setVisibilityFilter(state, action);
case "ADD_TODO":
return addTodo(state, action);
case "TOGGLE_TODO":
return toggleTodo(state, action);
case "EDIT_TODO":
return editTodo(state, action);
default:
return state;
}
}
现在很清楚每个 case
发生了什么。我们也可以看到一些模式的雏形。
按域拆分数据(Separating Data Handling by Domain)
目前的 Reducer 仍然需要关心程序中所有不同的 case。下面尝试把 filter 逻辑和 todo 逻辑分离:
// 省略了内容
function updateObject(oldObject, newValues) {}
function updateItemInArray(array, itemId, updateItemCallback) {}
function setVisibilityFilter(visibilityState, action) {
// 从技术上将,我们甚至不关心之前的状态
return action.filter;
}
function visibilityReducer(visibilityState = "SHOW_ALL", action) {
switch (action.type) {
case "SET_VISIBILITY_FILTER":
return setVisibilityFilter(visibilityState, action);
default:
return visibilityState;
}
}
function addTodo(todosState, action) {
const newTodos = todosState.concat({
id: action.id,
text: action.text,
completed: false,
});
return newTodos;
}
function toggleTodo(todosState, action) {
const newTodos = updateItemInArray(todosState, action.id, (todo) => {
return updateObject(todo, { completed: !todo.completed });
});
return newTodos;
}
function editTodo(todosState, action) {
const newTodos = updateItemInArray(todosState, action.id, (todo) => {
return updateObject(todo, { text: action.text });
});
return newTodos;
}
function todosReducer(todosState = [], action) {
switch (action.type) {
case "ADD_TODO":
return addTodo(todosState, action);
case "TOGGLE_TODO":
return toggleTodo(todosState, action);
case "EDIT_TODO":
return editTodo(todosState, action);
default:
return todosState;
}
}
function appReducer(state = initialState, action) {
return {
todos: todosReducer(state.todos, action),
visibilityFilter: visibilityReducer(state.visibilityFilter, action),
};
}
我们注意到,两个 reducer 分别关心 state 中的不同的部分。都只需要把自身关心的数据作为参数,不再需要返回复杂的嵌套型 state 对象了,代码变得更简单。
减少样板代码
马上就大功告成了。因为很多人不喜欢使用 switch 这种语法结构,创建一个 action 到 case 查找表示非常通用的做法。可以使用 缩减样板代码 中提到的 createReducer
函数减少样板代码。
// 省略了内容
function updateObject(oldObject, newValues) {}
function updateItemInArray(array, itemId, updateItemCallback) {}
function createReducer(initialState, handlers) {
return function reducer(state = initialState, action) {
if (handlers.hasOwnProperty(action.type)) {
return handlers[action.type](state, action);
} else {
return state;
}
};
}
// 省略了内容
function setVisibilityFilter(visibilityState, action) {}
const visibilityReducer = createReducer("SHOW_ALL", {
SET_VISIBILITY_FILTER: setVisibilityFilter,
});
// 省略了内容
function addTodo(todosState, action) {}
function toggleTodo(todosState, action) {}
function editTodo(todosState, action) {}
const todosReducer = createReducer([], {
ADD_TODO: addTodo,
TOGGLE_TODO: toggleTodo,
EDIT_TODO: editTodo,
});
function appReducer(state = initialState, action) {
return {
todos: todosReducer(state.todos, action),
visibilityFilter: visibilityReducer(state.visibilityFilter, action),
};
}
通过切片组合 Reducer(Combining Reducers by Slice)
最后一步了,使用 Redux 中 combineReducers
这个工具函数去把管理每个 state 切片的逻辑组合起来,形成顶层的 reducer。最终变成这样:
// 可重用的工具函数
function updateObject(oldObject, newValues) {
// 将空对象作为第一个参数传递给 Object.assign,以确保只是复制数据,而不是去改变数据
return Object.assign({}, oldObject, newValues);
}
function updateItemInArray(array, itemId, updateItemCallback) {
const updatedItems = array.map((item) => {
if (item.id !== itemId) {
// 因为我们只想更新一个项目,所以保留所有的其他项目
return item;
}
// 使用提供的回调来创建新的项目
const updatedItem = updateItemCallback(item);
return updatedItem;
});
return updatedItems;
}
function createReducer(initialState, handlers) {
return function reducer(state = initialState, action) {
if (handlers.hasOwnProperty(action.type)) {
return handlers[action.type](state, action);
} else {
return state;
}
};
}
// 处理特殊 case 的 Handler ("case reducer")
function setVisibilityFilter(visibilityState, action) {
// 从技术上将,我们甚至不关心之前的状态
return action.filter;
}
// 处理整个 state 切片的 Handler ("slice reducer")
const visibilityReducer = createReducer("SHOW_ALL", {
SET_VISIBILITY_FILTER: setVisibilityFilter,
});
// Case reducer
function addTodo(todosState, action) {
const newTodos = todosState.concat({
id: action.id,
text: action.text,
completed: false,
});
return newTodos;
}
// Case reducer
function toggleTodo(todosState, action) {
const newTodos = updateItemInArray(todosState, action.id, (todo) => {
return updateObject(todo, { completed: !todo.completed });
});
return newTodos;
}
// Case reducer
function editTodo(todosState, action) {
const newTodos = updateItemInArray(todosState, action.id, (todo) => {
return updateObject(todo, { text: action.text });
});
return newTodos;
}
// Slice reducer
const todosReducer = createReducer([], {
ADD_TODO: addTodo,
TOGGLE_TODO: toggleTodo,
EDIT_TODO: editTodo,
});
// 顶层 reducer
const appReducer = combineReducers({
visibilityFilter: visibilityReducer,
todos: todosReducer,
});
现在我们有了分离集中 reducer 的例子:像 updateObject
和 createReducer
一样的工具函数,像 setVisibilityFilter
和 addTodo
一样的处理器(Handler),像 visibilityReducer
和 todosReducer
一样的处理单个切片数据的 Handler。appReducer
可以被当作是顶层 reducer。
这个例子中最后的结果看上去比原始的版本更长,这主要是因为工具函数的提取,注释的添加和一些为了清楚起见的故意冗长(比如单独的 return 语句)。单独的看每个功能,他们承担的责任更小,意图也更加清楚。在真正的应用中,这些函数将会分到单独的文件中,比如:reducerUtilities.js
,visibilityReducer.js
,todosReudcer.js
和 rootReducer.js
。