從 Mock Service Worker 源碼中學習

// handler.js
// 1. Import the "HttpResponse" class from the library.
import { http, HttpResponse } from 'msw'

export const handlers = [
  http.get('/resource', () => {
    // 2. Return a mocked "Response" instance from the handler.
    return HttpResponse.json({
      msg: 'Hello world!'
    })
  }),
]

// mock.js
import { setupWorker } from 'msw/browser'
import { handlers } from './handlers'

const worker = setupWorker(...handlers)
worker.start() // 實際可根據環境自由選擇是否引入與執行啟動

// main.js(entrypoint of you web app)
import './mock';

(async () => {
  const res = await fetch('/resource');
  const data = await res.json();
  console.log(data);
})();

// 用來儲存 active 的 clientId
const activeClientIds = new Set()

// 定義 service worker 收到 message
self.addEventListener('message', async function (event) {
  const clientId = event.source.id
  const client = await self.clients.get(clientId)
  const allClients = await self.clients.matchAll({
    type: 'window',
  })
  switch (event.data) {
    // keepAlive 避免 service worker 休眠去了
    case 'KEEPALIVE_REQUEST': {
      sendToClient(client, {
        type: 'KEEPALIVE_RESPONSE',
      })
      break
    }
    // 當 web 載入 service worker 時添加 active client
    // （可能同時開很多個 web page 連上同一個 service worker）
    case 'MOCK_ACTIVATE': {
      activeClientIds.add(clientId)
      // 通知 web 端載入並紀錄 clientId 完成
      sendToClient(client, {
        type: 'MOCKING_ENABLED',
        payload: true,
      })
      break
    }
    // 當 web 端中斷連線時須移除 active clientId
    case 'CLIENT_CLOSED': {
      activeClientIds.delete(clientId)

      const remainingClients = allClients.filter((client) => {
        return client.id !== clientId
      })

      // Unregister itself when there are no more clients
      if (remainingClients.length === 0) {
        self.registration.unregister()
      }

      break
    }
  }
})

self.addEventListener('fetch', function (event) {
  const { request } = event
  // 省略一大段...
  const requestId = crypto.randomUUID()
  event.respondWith(handleRequest(event, requestId))
})

async function handleRequest(event, requestId) {
  const client = await resolveMainClient(event)
  const response = await getResponse(event, client, requestId)
  // 省略一小段...
  return response
}

// 取得主要的 client
// 發出請求的 client 並不一定就是註冊 worker 的那個 client
// 在回應請求時應該使用後者（註冊 worker 的那個 client）
async function resolveMainClient(event) {
  const client = await self.clients.get(event.clientId)

  if (client?.frameType === 'top-level') {
    return client
  }

  const allClients = await self.clients.matchAll({
    type: 'window',
  })

  return allClients
    .filter((client) => {
      return client.visibilityState === 'visible'
    })
    .find((client) => {
      return activeClientIds.has(client.id)
    })
}

async function getResponse(event, client, requestId) {
  const { request } = event

  // 複製 request，因為可能已經被使用
  // (i.e. 比如 body 可能已經被送到 client）
  const requestClone = request.clone()

  function passthrough() {
    const headers = Object.fromEntries(requestClone.headers.entries())
    return fetch(requestClone, { headers })
  }

  // 省略一大段 passthrough 判斷...

  // 通知 main client 端請求已被攔截
  // 這裡會等到 client 端處理好整個 response 後繼續執行
  // 主要是透過 sendToClient 中的 MessageChannel 雙向 sync 溝通
  const requestBuffer = await request.arrayBuffer()
  const clientMessage = await sendToClient(
    client,
    {
      type: 'REQUEST',
      payload: {
        id: requestId,
        url: request.url,
        mode: request.mode,
        method: request.method,
        headers: Object.fromEntries(request.headers.entries()),
        cache: request.cache,
        credentials: request.credentials,
        destination: request.destination,
        integrity: request.integrity,
        redirect: request.redirect,
        referrer: request.referrer,
        referrerPolicy: request.referrerPolicy,
        body: requestBuffer,
        keepalive: request.keepalive,
      },
    },
    [requestBuffer],
  )
  // 根據 main client 端回傳的 message 決定是 intercept 或 passthrough
  // respondWithMock 會實際產生一個 HTTP Response object 並丟回給發出請求的 client
  switch (clientMessage.type) {
    case 'MOCK_RESPONSE': {
      return respondWithMock(clientMessage.data)
    }

    case 'MOCK_NOT_FOUND': {
      return passthrough()
    }
  }

  return passthrough()
}

// 透過 MessageChannel 與 client 端進行 sync 雙向溝通
// 把 port2 丟給 client，讓 client 可透過 port2 與 port1(當前 worker) 溝通
// 透過 MessageChannel 可以藉由 promise 讓 function 等待 client 回傳結果
// 而不是讓 client 透過 postMessage 回傳，因為 postMessage 無法讓 worker 直接 await 等待結果
function sendToClient(client, message, transferrables = []) {
  return new Promise((resolve, reject) => {
    const channel = new MessageChannel()

    channel.port1.onmessage = (event) => {
      if (event.data && event.data.error) {
        return reject(event.data.error)
      }
      resolve(event.data)
    }

    client.postMessage(
      message,
      [channel.port2].concat(transferrables.filter(Boolean)),
    )
  })
}

async function respondWithMock(response) {
  // Setting response status code to 0 is a no-op.
  // However, when responding with a "Response.error()", the produced Response
  // instance will have status code set to 0. Since it's not possible to create
  // a Response instance with status code 0, handle that use-case separately.
  if (response.status === 0) {
    return Response.error()
  }
  const mockedResponse = new Response(response.body, response)
  Reflect.defineProperty(mockedResponse, IS_MOCKED_RESPONSE, {
    value: true,
    enumerable: true,
  })
  return mockedResponse
}