200行代码写一个简易的dva在美团实习的时候，第一次接触到dva这样的react框架，回学校的时候，就想有机会自己实现

在美团实习的时候，第一次接触到dva这样的react框架，回学校的时候，就想有机会自己实现一下这样的框架，虽然自己水平有限，但是可以试一试哈。目标是实现dva model的同步和异步 dispatch action。

看看 dva 的构成

let counterModel = {
	 namespace: 'counter',
	 state: {
	    num: 0
	 }
	 
	 reducers: {
	     add(state, action){
	         return {
            		num: state.num + 1
            }	
	     }
	 }
	}

对state的更新

var app = new dva();
app.model(counterModel);
app.start();
app._store.dispatch({
	type: 'counter/add'
});

上述就是 dva 对 state 的更新，通过dispatch {type: A / B} 其中 A 是指 model的 namespace, B 是指 model 中 reducers 具体的reducer方法。

其中 dva 对异步的处理是用 redux-saga 处理的，因为作者并不熟悉redux-saga，拿 redux-thunk 代替了。

好，我们开工了

第一步创建store

  const createStore = (reducer, initialState) => {
  let currentReducer = reducer;
  let currentState = initialState;
  let listener = () => { };

  return {
      getState() {
          return currentState;
      },
      dispatch(action) {
          let {
              type
          } = action;
          currentState = currentReducer(currentState, action);
          listener();
          return action;
      },
      subscribe(newListener) {
          listener = newListener;
      }
  }
  }

store 主要是存储数据，开放state更新接口。

第二步引入中间件 applyMiddleware

 const compose = (...funcs) => {
 if (funcs.length === 0) {
     return arg => arg
 }

 if (funcs.length === 1) {
     return funcs[0]
 }

 const last = funcs[funcs.length - 1]
 const rest = funcs.slice(0, -1)
 return (...args) => rest.reduceRight((composed, f) => f(composed), last(...args));
 }


 const applyMiddleware = (...middlewares) => {
     return (createStore) => (reducer, initialState, enhancer) => {
         var store = createStore(reducer, initialState, enhancer)
         var dispatch = store.dispatch;
         var chain = [];
         var middlewareAPI = {
             getState: store.getState,
             dispatch: (action) => store.dispatch(action)
         }
         chain = middlewares.map(middleware => middleware(middlewareAPI))
         dispatch = compose(...chain)(store.dispatch)
         return {
             ...store,
             dispatch
         }
     }
 }

redux 中间件洋葱模型，修改了dispatch 方法。

引入异步中间件redux-thunk和logger中间件

 	const logger = store => next => action => {
    console.log('prevState', store.getState());
    let result = next(action);
    console.log('nextState', store.getState());
    return result;
	};

	const thunk = ({
	    dispatch,
	    getState
	}) => next => action => {
	    if (typeof action === 'function') {
	        return action(dispatch, getState);
	    }
	    return next(action);
	}

这里引入 redux-thunk 做异步处理。

加入测试model

  let counterModel = {

  namespace: 'counter',

  state: {
      num: 0
  },

  reducers: {
      add(state, action) {
          console.log('reducer add executed');
          return {
              num: state.num + 1
          }
      },
      asyncAdd(state, action) {
          console.log('reducer asyncAdd executed');
          return {
              num: state.num + 1
          }
      },
      test(state, action) {
          console.log('reducer test executed');
          return {
              state
          }
      }
  }
  };

  let userModel = {
  
  namespace: 'user',

  state: {
      name: 'xxxx'
  },


  reducers: {
      modify(state, {
          payload
      }) {
          console.log('reducer modify executed');
          let {
              name
          } = payload
          return {
              name
          }
      }
  }
  };

对不同model下的reducer进行分发

  const combineReducer = (reducers) => (state = {}, action) => {
  let {
      type
  } = action;
  let stateKey = type.split('/')[0];
  let reducer = type.split('/')[1];

  reducers.map((current) => {
      if (current.name === reducer) {
          state[stateKey] = current(state[stateKey], action);
      }
  });

  return state;
  }

这里因为 combineReducer 是 reducer的总入口，在这里根据action 的 type 转发到具体model下的reducer方法

dva 构造函数

  class dva {
      constructor() {
          this._models = [];
          this._reducers = [];
          this._states = {};
      }
      model(model) {
          this._models.push(model);
      }
      start() {
          for (var i = 0; i < this._models.length; i++) {
              this._states[this._models[i].namespace] = {
                  ...this._models[i].state
              };
              Object.keys(this._models[i].reducers).map((key) => {
                  if (this._models[i].reducers.hasOwnProperty(key)) {
                      this._reducers.push(this._models[i].reducers[key]);
                  }
              })
  
          }
          var rootReducer = combineReducer(this._reducers);
          let createStoreWithMiddleware = applyMiddleware(thunk, logger)(createStore);
          this._store = createStoreWithMiddleware(rootReducer, this._states);
          this._store.subscribe(() => {
              console.log(this._store.getState());
          })
      }
  }

dva 构造方法主要工作是缓存model，创建store。

测试数据

var app = new dva();
app.model(counterModel);
app.model(userModel);

app.start();
app._store.dispatch({
    type: 'counter/add'
});

app._store.dispatch({
    type: 'user/modify',
    payload: {
        name: 'shadow'
    }
})

app._store.dispatch((dispatch, getState) => {
    setTimeout(() => {
        dispatch({
            type: 'counter/asyncAdd'
        })
    }, 5000);
})

控制台的输出

一点留言

这个当然是最粗糙的dva部分实现了，因为本身自己并没有去看dva源码，只是看了dva API 蒙头实现下，其中已经有很多很优秀的redux周边生态，例如redux-thunk,logger等。当然也是复习了一下部分redux源码了，当作自己学习的一个阶段学习吧，最后像dva作者陈谦致敬。

最后留个地址吧:

http://oymaq4uai.bkt.clouddn.com/index.js