29 June, 2025

JavaScript BroadcastChannel API 如何做到跨視窗溝通！

這是我最近做出來的一個簡單的 Demo，主要是利用 JavaScript 的 BroadcastChannel API 來達到瀏覽器互相傳遞資訊，進一步溝通來達到有趣互動的效果。

Broadcast Channel API 是可以在同個網域下，瀏覽器的不同窗口、Tab 頁、同一個網站下不同的頁面的 iframe 之間，都可以做到簡單的資料傳遞。

這個影片主要就是利用 BroadcastChannel API 來完成互相溝通的效果，大致上的作法就是先處理拖曳的效果，然後再拖曳的過程中將資訊給送去給另一個視窗。

Step1. 先註冊一個名叫 web_channel 的 Channel

 const channel = new BroadcastChannel("web_channel");

Step2. 在滑鼠拖移移動的時候將當前坐標用 postMessage 給它送到 web_channel 中

window.addEventListener("mousemove", (e) => {
  if (isDragging) {
    const x = e.clientX - offsetX;
    const y = e.clientY - offsetY;
    draggable.style.left = x + "px";
    draggable.style.top = y + "px";
    

    // 送出資料
    channel.postMessage({ left: x, top: y });
  }
});

Step3. 在另外一個頁面中去監聽 web_channel 然後使用 onmessage 來獲取另外一個頁面丟過來的座標

const channel = new BroadcastChannel("web_channel");

channel.onmessage = (e) => {
    draggable.style.left = -window.innerWidth + e.data.left + "px";
    draggable.style.top = e.data.top + "px";
};

這邊附上我完整的 code

A.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>這是一個酷東西</title>
    <style>
      #draggable {
        width: 200px;
        height: 200px;
        background-color: blue;
        position: absolute;
        cursor: pointer;
      }
    </style>
  </head>
  <body>
    <div id="draggable"></div>
  </body>
</html>

<script>
      const draggable = document.getElementById("draggable");

      const channel = new BroadcastChannel("web_channel");

      let offsetX = 0;
      let offsetY = 0;
      let isDragging = false;

      draggable.addEventListener("mousedown", (e) => {
        offsetX = e.clientX - draggable.getBoundingClientRect().left;
        offsetY = e.clientY - draggable.getBoundingClientRect().top;
        isDragging = true;
      });

      window.addEventListener("mousemove", (e) => {
        if (isDragging) {
          const x = e.clientX - offsetX;
          const y = e.clientY - offsetY;
          draggable.style.left = x + "px";
          draggable.style.top = y + "px";

          channel.postMessage({ left: x, top: y });
        }
      });
      draggable.addEventListener("mouseup", () => {
        isDragging = false;
      });
</script>

B.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>這是一個酷東西</title>
    <style>
      #draggable {
        width: 200px;
        height: 200px;
        background-color: red;
        position: absolute;
        top: -10000px;
      }
    </style>
  </head>
  <body>
    <div id="draggable"></div>
  </body>
</html>

<script>
      const draggable = document.getElementById("draggable");

      const channel = new BroadcastChannel("web_channel");

      channel.onmessage = (e) => {
        draggable.style.left = -window.innerWidth + e.data.left + "px";
        draggable.style.top = e.data.top + "px";
      };
</script>

如果想看我怎麼從頭到尾 coding 的可以透過下面直播的這個 timecode (15分的時候 )來開始觀看，這個 API 真的蠻有趣的。

在我的課程中就會有許多這樣的小知識點的教學，想一起學習更多 JavaScript
現在就加入 👉 https://thecodingpro.com/courses/javascript