是否有可能在减速器本身调度一个动作?我有一个进度条和一个音频元素。目标是在音频元素中的时间更新时更新进度条。但是我不知道在哪里放置ontimeupdate事件处理程序，或者如何在ontimeupdate的回调中分派一个动作来更新进度条。这是我的代码:
//reducer
const initialState = {
audioElement: new AudioElement('test.mp3'),
progress: 0.0
}
initialState.audioElement.audio.ontimeupdate = () => {
console.log('progress', initialState.audioElement.currentTime/initialState.audioElement.duration);
//how to dispatch 'SET_PROGRESS_VALUE' now?
};
const audio = (state=initialState, action) => {
switch(action.type){
case 'SET_PROGRESS_VALUE':
return Object.assign({}, state, {progress: action.progress});
default: return state;
}
}
export default audio;
在减速器完成之前启动另一个分派是一种反模式，因为当减速器完成时，您在减速器开始时接收到的状态将不再是当前应用程序状态。但是从减速器内部调度另一个分派并不是反模式。事实上，这就是Elm语言所做的，正如您所知，Redux是将Elm体系结构引入JavaScript的一种尝试。
下面是一个中间件，它将向所有操作添加asyncDispatch属性。当您的reducer完成并返回新的应用程序状态时，asyncDispatch将触发存储。用你所能做的一切来调度它。
// This middleware will just add the property "async dispatch" to all actions
const asyncDispatchMiddleware = store => next => action => {
let syncActivityFinished = false;
let actionQueue = [];
function flushQueue() {
actionQueue.forEach(a => store.dispatch(a)); // flush queue
actionQueue = [];
}
function asyncDispatch(asyncAction) {
actionQueue = actionQueue.concat([asyncAction]);
if (syncActivityFinished) {
flushQueue();
}
}
const actionWithAsyncDispatch =
Object.assign({}, action, { asyncDispatch });
const res = next(actionWithAsyncDispatch);
syncActivityFinished = true;
flushQueue();
return res;
};
现在你的减速机可以这样做:
function reducer(state, action) {
switch (action.type) {
case "fetch-start":
fetch('wwww.example.com')
.then(r => r.json())
.then(r => action.asyncDispatch({ type: "fetch-response", value: r }))
return state;
case "fetch-response":
return Object.assign({}, state, { whatever: action.value });;
}
}
减速机内部的调度和动作似乎出现bug。
我用useReducer做了一个简单的反例，其中“INCREASE”被分派，“SUB”也被分派。
在这个例子中，我期望“INCREASE”被分派，然后“SUB”也被分派，将cnt设置为-1，然后
继续“INCREASE”操作，将cnt设置为0，但它是-1(“INCREASE”被忽略)
看到这个:
https://codesandbox.io/s/simple-react-context-example-forked-p7po7?file=/src/index.js:144-154
let listener = () => {
console.log("test");
};
const middleware = (action) => {
console.log(action);
if (action.type === "INCREASE") {
listener();
}
};
const counterReducer = (state, action) => {
middleware(action);
switch (action.type) {
case "INCREASE":
return {
...state,
cnt: state.cnt + action.payload
};
case "SUB":
return {
...state,
cnt: state.cnt - action.payload
};
default:
return state;
}
};
const Test = () => {
const { cnt, increase, substract } = useContext(CounterContext);
useEffect(() => {
listener = substract;
});
return (
<button
onClick={() => {
increase();
}}
>
{cnt}
</button>
);
};
{type: "INCREASE", payload: 1}
{type: "SUB", payload: 1}
// expected: cnt: 0
// cnt = -1
