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redux-saga介绍

众所周知，react仅仅是作用在View层的前端框架，redux作为前端的“数据库”，完美！但是依旧残留着前端一直以来的诟病=>异步。

所以就少不了有很多的中间件（middleware）来处理这些数据，而redux-saga就是其中之一。

不要把redux-saga（下面统称为saga）想的多么牛逼，其实他就是一个辅助函数，但是荣耀里辅助拿MVP也不少哈~。

Saga最大的特点就是它可以让你用同步的方式写异步的代码！想象下，如果它能够用来监听你的异步action，然后又用同步的方式去处理。那么，你的react-redux是不是就轻松了很多！

官方介绍，请移步redux-saga

saga相当于在redux原有的数据流中多了一层监控，捕获监听到的action，进行处理后，put一个新的action给相应的reducer去处理。

基本用法

1、 使用createSagaMiddleware方法创建saga 的Middleware，然后在创建的redux的store时，使用applyMiddleware函数将创建的saga Middleware实例绑定到store上，最后可以调用saga Middleware的run函数来执行某个或者某些Middleware。
2、 在saga的Middleware中，可以使用takeEvery或者takeLatest等API来监听某个action，当某个action触发后，saga可以使用call、fetch等api发起异步操作，操作完成后使用put函数触发action，同步更新state，从而完成整个State的更新。

saga的优点

下面介绍saga的API，boring~~~所以先来点动力吧

• 流程拆分更细，应用的逻辑和view更加的清晰，分工明确。异步的action和复杂逻辑的action都可以放到saga中去处理。模块更加的干净

• 因为使用了 Generator，redux-saga让你可以用同步的方式写异步代码

• 能容易地测试 Generator 里所有的业务逻辑

• 可以通过监听Action 来进行前端的打点日志记录，减少侵入式打点对代码的侵入程度

• 。。。

走马观花API(安装啥的步骤直接略过)

takeEvery

用来监听action，每个action都触发一次，如果其对应是异步操作的话，每次都发起异步请求，而不论上次的请求是否返回

import { takeEvery } from 'redux-saga/effects'
 
function* watchFetchData() {
  yield takeEvery('FETCH_REQUESTED',fetchData)
}

takeLatest

作用同takeEvery一样，唯一的区别是它只关注最后，也就是最近一次发起的异步请求，如果上次请求还未返回，则会被取消。

function* watchFetchData() {
  yield takeLatest('FETCH_REQUESTED',fetchData)
}

redux Effects

在saga的世界里，所有的任务都通用 yield Effect 来完成，Effect暂时就理解为一个任务单元吧，其实就是一个JavaScript的对象，可以通过sagaMiddleWare进行执行。

重点说明下，在redux-saga的应用中，所有的Effect都必须被yield后才可以被执行。

import {fork,call} from 'redux-saga/effects'

import {getAdDataFlow,getULikeDataFlow} from './components/home/homeSaga'
import {getLocatioFlow} from './components/wrap/wrapSaga'
import {getDetailFolw} from './components/detail/detailSaga'
import {getCitiesFlow} from './components/city/citySaga'

export default function* rootSaga () {
    yield fork(getLocatioFlow);
    yield fork(getAdDataFlow);
    yield fork(getULikeDataFlow);
    yield fork(getDetailFolw);
    yield fork(getCitiesFlow);
}

call

call用来调用异步函数，将异步函数和函数参数作为call函数的参数传入，返回一个js对象。saga引入他的主要作用是方便测试，同时也能让我们的代码更加规范化。

同js原生的call一样，call函数也可以指定this对象，只要把this对象当第一个参数传入call方法就好了

saga同样提供apply函数，作用同call一样，参数形式同js原生apply方法。

export function* getAdData(url) {
    yield put({type:wrapActionTypes.START_FETCH});
    yield  delay(delayTime);//故意的
    try {
        return yield call(get,url);
    } catch (error) {
        yield put({type:wrapActionTypes.FETCH_ERROR})
    }finally {
        yield put({type:wrapActionTypes.FETCH_END})
    }
}

export function* getAdDataFlow() {
    while (true){
        let request = yield take(homeActionTypes.GET_AD);
        let response = yield call(getAdData,request.url);
        yield put({type:homeActionTypes.GET_AD_RESULT_DATA,data:response.data})
    }
}

take

等待 reactjs dispatch 一个匹配的action。take的表现同takeEvery一样，都是监听某个action，但与takeEvery不同的是，他不是每次action触发的时候都相应，而只是在执行顺序执行到take语句时才会相应action。

当在genetator中使用take语句等待action时，generator被阻塞，等待action被分发，然后继续往下执行。

takeEvery只是监听每个action，然后执行处理函数。对于何时响应action和 如何响应action，takeEvery并没有控制权。

而take则不一样，我们可以在generator函数中决定何时响应一个action，以及一个action被触发后做什么操作。

最大区别：take只有在执行流达到时才会响应对应的action，而takeEvery则一经注册，都会响应action。

export function* getAdDataFlow() {
    while (true){
        let request = yield take(homeActionTypes.GET_AD);
        let response = yield call(getAdData,data:response.data})
    }
}

put

触发某一个action，类似于react中的dispatch

实例如上

select

作用和 redux thunk 中的 getState 相同。通常会与reselect库配合使用

fork

非阻塞任务调用机制：上面我们介绍过call可以用来发起异步操作，但是相对于generator函数来说，call操作是阻塞的，只有等promise回来后才能继续执行，而fork是非阻塞的 ，当调用fork启动一个任务时，该任务在后台继续执行，从而使得我们的执行流能继续往下执行而不必一定要等待返回。

cancel

cancel的作用是用来取消一个还未返回的fork任务。防止fork的任务等待时间太长或者其他逻辑错误。

all

all提供了一种并行执行异步请求的方式。之前介绍过执行异步请求的api中，大都是阻塞执行，只有当一个call操作放回后，才能执行下一个call操作， call提供了一种类似Promise中的all操作，可以将多个异步操作作为参数参入all函数中，
如果有一个call操作失败或者所有call操作都成功返回，则本次all操作执行完毕。

import { all,call } from 'redux-saga/effects'
 
// correct,effects will get executed in parallel
const [users,repos]  = yield all([
  call(fetch,'/users'),call(fetch,'/repos')
])

race

有时候当我们并行的发起多个异步操作时，我们并不一定需要等待所有操作完成，而只需要有一个操作完成就可以继续执行流。这就是race的用处。
他可以并行的启动多个异步请求，只要有一个 请求返回（resolved或者reject），race操作接受正常返回的请求，并且将剩余的请求取消。

import { race,take,put } from 'redux-saga/effects'
 
function* backgroundTask() {
  while (true) { ... }
}
 
function* watchStartBackgroundTask() {
  while (true) {
    yield take('START_BACKGROUND_TASK')
    yield race({
      task: call(backgroundTask),cancel: take('CANCEL_TASK')
    })
  }
}

actionChannel　　

在之前的操作中，所有的action分发是顺序的，但是对action的响应是由异步任务来完成，也即是说对action的处理是无序的。

如果需要对action的有序处理的话，可以使用actionChannel函数来创建一个action的缓存队列，但一个action的任务流程处理完成后，才可是执行下一个任务流。

import { take,actionChannel,call,... } from 'redux-saga/effects'
 
function* watchRequests() {
  // 1- Create a channel for request actions
  const requestChan = yield actionChannel('REQUEST')
  while (true) {
    // 2- take from the channel
    const {payload} = yield take(requestChan)
    // 3- Note that we're using a blocking call
    yield call(handleRequest,payload)
  }
}
 
function* handleRequest(payload) { ... }

从我写的这个项目可以看到，其实我很多API都是没有用到，常用的基本也就这么些了

从代码中去记忆API

这里我放两个实际项目中代码实例，大家可以看看熟悉下上面说到的API

rootSaga.js

// This file contains the sagas used for async actions in our app. It's divided into
// "effects" that the sagas call (`authorize` and `logout`) and the actual sagas themselves,// which listen for actions.

// Sagas help us gather all our side effects (network requests in this case) in one place

import {hashSync} from 'bcryptjs'
import genSalt from '../auth/salt'
import {browserHistory} from 'react-router'
import {take,put,fork,race} from 'redux-saga/effects'
import auth from '../auth'

import {
  SENDING_REQUEST,LOGIN_REQUEST,REGISTER_REQUEST,SET_AUTH,logoUT,CHANGE_FORM,REQUEST_ERROR
} from '../actions/constants'

/**
 * Effect to handle authorization
 * @param  {string} username               The username of the user
 * @param  {string} password               The password of the user
 * @param  {object} options                Options
 * @param  {boolean} options.isRegistering Is this a register request?
 */
export function * authorize ({username,password,isRegistering}) {
  // We send an action that tells Redux we're sending a request
  yield put({type: SENDING_REQUEST,sending: true})

  // We then try to register or log in the user,depending on the request
  try {
    let salt = genSalt(username)
    let hash = hashSync(password,salt)
    let response

    // For either log in or registering,we call the proper function in the `auth`
    // module,which is asynchronous. Because we're using generators,we can work
    // as if it's synchronous because we pause execution until the call is done
    // with `yield`!
    if (isRegistering) {
      response = yield call(auth.register,username,hash)
    } else {
      response = yield call(auth.login,hash)
    }

    return response
  } catch (error) {
    console.log('hi')
    // If we get an error we send Redux the appropiate action and return
    yield put({type: REQUEST_ERROR,error: error.message})

    return false
  } finally {
    // When done,we tell Redux we're not in the middle of a request any more
    yield put({type: SENDING_REQUEST,sending: false})
  }
}

/**
 * Effect to handle logging out
 */
export function * logout () {
  // We tell Redux we're in the middle of a request
  yield put({type: SENDING_REQUEST,sending: true})

  // Similar to above,we try to log out by calling the `logout` function in the
  // `auth` module. If we get an error,we send an appropiate action. If we don't,// we return the response.
  try {
    let response = yield call(auth.logout)
    yield put({type: SENDING_REQUEST,sending: false})

    return response
  } catch (error) {
    yield put({type: REQUEST_ERROR,error: error.message})
  }
}

/**
 * Log in saga
 */
export function * loginFlow () {
  // Because sagas are generators,doing `while (true)` doesn't block our program
  // Basically here we say "this saga is always listening for actions"
  while (true) {
    // And we're listening for `LOGIN_REQUEST` actions and destructuring its payload
    let request = yield take(LOGIN_REQUEST)
    let {username,password} = request.data

    // A `logoUT` action may happen while the `authorize` effect is going on,which may
    // lead to a race condition. This is unlikely,but just in case,we call `race` which
    // returns the "winner",i.e. the one that finished first
    let winner = yield race({
      auth: call(authorize,{username,isRegistering: false}),logout: take(logoUT)
    })

    // If `authorize` was the winner...
    if (winner.auth) {
      // ...we send Redux appropiate actions
      yield put({type: SET_AUTH,newAuthState: true}) // User is logged in (authorized)
      yield put({type: CHANGE_FORM,newFormState: {username: '',password: ''}}) // Clear form
      forwardTo('/dashboard') // Go to dashboard page
    }
  }
}

/**
 * Log out saga
 * This is basically the same as the `if (winner.logout)` of above,just written
 * as a saga that is always listening to `logoUT` actions
 */
export function * logoutFlow () {
  while (true) {
    yield take(logoUT)
    yield put({type: SET_AUTH,newAuthState: false})

    yield call(logout)
    forwardTo('/')
  }
}

/**
 * Register saga
 * Very similar to log in saga!
 */
export function * registerFlow () {
  while (true) {
    // We always listen to `REGISTER_REQUEST` actions
    let request = yield take(REGISTER_REQUEST)
    let {username,password} = request.data

    // We call the `authorize` task with the data,telling it that we are registering a user
    // This returns `true` if the registering was successful,`false` if not
    let wasSuccessful = yield call(authorize,isRegistering: true})

    // If we could register a user,we send the appropiate actions
    if (wasSuccessful) {
      yield put({type: SET_AUTH,newAuthState: true}) // User is logged in (authorized) after being registered
      yield put({type: CHANGE_FORM,password: ''}}) // Clear form
      forwardTo('/dashboard') // Go to dashboard page
    }
  }
}

// The root saga is what we actually send to Redux's middleware. In here we fork
// each saga so that they are all "active" and listening.
// Sagas are fired once at the start of an app and can be thought of as processes running
// in the background,watching actions dispatched to the store.
export default function * root () {
  yield fork(loginFlow)
  yield fork(logoutFlow)
  yield fork(registerFlow)
}

// Little helper function to abstract going to different pages
function forwardTo (location) {
  browserHistory.push(location)
}

另一个demo saga也跟我一样，拆分了下

简单看两个demo就好

index.js

import { takeLatest } from 'redux-saga';
import { fork } from 'redux-saga/effects';
import {loadUser} from './loadUser';
import {loadDashboardSequenced} from './loadDashboardSequenced';
import {loadDashboardNonSequenced} from './loadDashboardNonSequenced';
import {loadDashboardNonSequencedNonBlocking,isolatedForecast,isolatedFlight } from './loadDashboardNonSequencedNonBlocking';

function* rootSaga() {
  /*The saga is waiting for a action called LOAD_DASHBOARD to be activated */
  yield [
    fork(loadUser),takeLatest('LOAD_DASHBOARD',loadDashboardSequenced),takeLatest('LOAD_DASHBOARD_NON_SEQUENCED',loadDashboardNonSequenced),takeLatest('LOAD_DASHBOARD_NON_SEQUENCED_NON_BLOCKING',loadDashboardNonSequencedNonBlocking),fork(isolatedForecast),fork(isolatedFlight)
  ];
}

export default rootSaga;

loadDashboardNonSequencedNonBlocking.js

import { call,select,take} from 'redux-saga/effects';
import {loadDeparture,loadFlight,loadForecast } from './apiCalls';

export const getUserFromState = (state) => state.user;

export function* loadDashboardNonSequencedNonBlocking() {
  try {
    //Wait for the user to be loaded
    yield take('FETCH_USER_SUCCESS');

    //Take the user info from the store
    const user = yield select(getUserFromState);

    //Get Departure information
    const departure = yield call(loadDeparture,user);

    //Update the UI
    yield put({type: 'FETCH_DASHBOARD3_SUCCESS',payload: {departure}});

    //trigger actions for Forecast and Flight to start...
    //We can pass and object into the put statement
    yield put({type: 'FETCH_DEPARTURE3_SUCCESS',departure});

  } catch(error) {
    yield put({type: 'FETCH_Failed',error: error.message});
  }
}

export function* isolatedFlight() {
  try {
    /* departure will take the value of the object passed by the put*/
    const departure = yield take('FETCH_DEPARTURE3_SUCCESS');

    //Flight can be called unsequenced /* BUT NON BLOCKING VS FORECAST*/
    const flight = yield call(loadFlight,departure.flightID);
    //Tell the store we are ready to be displayed
    yield put({type: 'FETCH_DASHBOARD3_SUCCESS',payload: {flight}});

  } catch (error) {
    yield put({type: 'FETCH_Failed',error: error.message});
  }
}

export function* isolatedForecast() {
    try {
      /* departure will take the value of the object passed by the put*/
      const departure = yield take('FETCH_DEPARTURE3_SUCCESS');

      const forecast = yield call(loadForecast,departure.date);
      yield put({type: 'FETCH_DASHBOARD3_SUCCESS',payload: { forecast }});

    } catch(error) {
      yield put({type: 'FETCH_Failed',error: error.message});
    }
}

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