摘要：在的中，可以使用或者等來監聽某個，當某個觸發后，可以使用等發起異步操作，操作完成后使用函數觸發，同步更新，從而完成整個的更新。對于何時響應和如何響應，并沒有控制權。的作用是用來取消一個還未返回的任務。

項目地址

眾所周知，react僅僅是作用在View層的前端框架，redux作為前端的“數據庫”，完美！但是依舊殘留著前端一直以來的詬病=>異步。

所以就少不了有很多的中間件（middleware）來處理這些數據，而redux-saga就是其中之一。

不要把redux-saga（下面統稱為saga）想的多么牛逼，其實他就是一個輔助函數，但是榮耀里輔助拿MVP也不少哈~。

Saga最大的特點就是它可以讓你用同步的方式寫異步的代碼！想象下，如果它能夠用來監聽你的異步action，然后又用同步的方式去處理。那么，你的react-redux是不是就輕松了很多！

官方介紹，請移步redux-saga

saga相當于在redux原有的數據流中多了一層監控，捕獲監聽到的action，進行處理后，put一個新的action給相應的reducer去處理。

1、 使用createSagaMiddleware方法創建saga 的Middleware，然后在創建的redux的store時，使用applyMiddleware函數將創建的saga Middleware實例綁定到store上，最后可以調用saga Middleware的run函數來執行某個或者某些Middleware。
2、 在saga的Middleware中，可以使用takeEvery或者takeLatest等API來監聽某個action，當某個action觸發后，saga可以使用call、fetch等api發起異步操作，操作完成后使用put函數觸發action，同步更新state，從而完成整個State的更新。

下面介紹saga的API，boring~~~所以先來點動力吧

流程拆分更細，應用的邏輯和view更加的清晰，分工明確。異步的action和復雜邏輯的action都可以放到saga中去處理。模塊更加的干凈

因為使用了 Generator，redux-saga讓你可以用同步的方式寫異步代碼

能容易地測試 Generator 里所有的業務邏輯

可以通過監聽Action 來進行前端的打點日志記錄，減少侵入式打點對代碼的侵入程度

。。。

用來監聽action，每個action都觸發一次，如果其對應是異步操作的話，每次都發起異步請求，而不論上次的請求是否返回

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

作用同takeEvery一樣，唯一的區別是它只關注最后，也就是最近一次發起的異步請求，如果上次請求還未返回，則會被取消。

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

在saga的世界里，所有的任務都通用 yield Effect 來完成，Effect暫時就理解為一個任務單元吧，其實就是一個JavaScript的對象，可以通過sagaMiddleWare進行執行。

重點說明下，在redux-saga的應用中，所有的Effect都必須被yield后才可以被執行。

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用來調用異步函數，將異步函數和函數參數作為call函數的參數傳入，返回一個js對象。saga引入他的主要作用是方便測試，同時也能讓我們的代碼更加規范化。

同js原生的call一樣，call函數也可以指定this對象，只要把this對象當第一個參數傳入call方法就好了

saga同樣提供apply函數，作用同call一樣，參數形式同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})
    }
}

等待 reactjs dispatch 一個匹配的action。take的表現同takeEvery一樣，都是監聽某個action，但與takeEvery不同的是，他不是每次action觸發的時候都相應，而只是在執行順序執行到take語句時才會相應action。

當在genetator中使用take語句等待action時，generator被阻塞，等待action被分發，然后繼續往下執行。

takeEvery只是監聽每個action，然后執行處理函數。對于何時響應action和 如何響應action，takeEvery并沒有控制權。

而take則不一樣，我們可以在generator函數中決定何時響應一個action，以及一個action被觸發后做什么操作。

最大區別：take只有在執行流達到時才會響應對應的action，而takeEvery則一經注冊，都會響應action。

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})
    }
}

觸發某一個action，類似于react中的dispatch

實例如上

作用和 redux thunk 中的 getState 相同。通常會與reselect庫配合使用

非阻塞任務調用機制：上面我們介紹過call可以用來發起異步操作，但是相對于generator函數來說，call操作是阻塞的，只有等promise回來后才能繼續執行，而fork是非阻塞的 ，當調用fork啟動一個任務時，該任務在后臺繼續執行，從而使得我們的執行流能繼續往下執行而不必一定要等待返回。

cancel的作用是用來取消一個還未返回的fork任務。防止fork的任務等待時間太長或者其他邏輯錯誤。

all提供了一種并行執行異步請求的方式。之前介紹過執行異步請求的api中，大都是阻塞執行，只有當一個call操作放回后，才能執行下一個call操作， call提供了一種類似Promise中的all操作，可以將多個異步操作作為參數參入all函數中，
如果有一個call操作失敗或者所有call操作都成功返回，則本次all操作執行完畢。

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")
])

有時候當我們并行的發起多個異步操作時，我們并不一定需要等待所有操作完成，而只需要有一個操作完成就可以繼續執行流。這就是race的用處。
他可以并行的啟動多個異步請求，只要有一個 請求返回（resolved或者reject），race操作接受正常返回的請求，并且將剩余的請求取消。

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")
    })
  }
}

在之前的操作中，所有的action分發是順序的，但是對action的響應是由異步任務來完成，也即是說對action的處理是無序的。

如果需要對action的有序處理的話，可以使用actionChannel函數來創建一個action的緩存隊列，但一個action的任務流程處理完成后，才可是執行下一個任務流。

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) { ... }

從我寫的這個項目可以看到，其實我很多API都是沒有用到，常用的基本也就這么些了

這里我放兩個實際項目中代碼實例，大家可以看看熟悉下上面說到的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, call, 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, username, 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, password, 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, {username, password, 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, newFormState: {username: "", 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)
}

另一個demo saga也跟我一樣，拆分了下

簡單看兩個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, put, 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});
    }
}

Node.js技術交流群：209530601

React技術棧：398240621