{{DefaultAPISidebar("Channel Messaging API")}}
Channel Messaging API 는 두 개의 독립적인 스크립트(예를 들면, 두 개의 IFrame, 또는 메인 다큐먼트와 IFrame, 또는 {{domxref("SharedWorker")}}에 의한 두 개의 다큐먼트)를 각 포트를 가진 양방향 채널(또는 파이프)을 통해 서로 직접 통신할 수 있도록 해줍니다. 이 문서에서 이 기술을 사용하는 기본내용에 대해 살펴봅시다.
{{AvailableInWorkers}}
Channel messaging is mainly useful in cases where you've got a social site that embeds capabilities from other sites into its main interface via IFrames, such as games, address book, or an audio player with personalized music choices. When these act as standalone units, things are ok, but the difficulty comes when you want interaction between the main site and the IFrames, or the different IFrames. For example, what if you wanted to add a contact to the address book from the main site, add high scores from your game into your main profile, or add new background music choices from the audio player onto the game? Such things are not so easy using conventional web technology, because of the security models the web uses. You have to think about whether the origins trust one another, and how the messages are passed.
Message channels on the other hand can provide a secure channel that a single data item can be sent down, from one browsing context to another, after which the channel is closed. The sending context asks the receiving context for the capability to send a single message. At the receiving end, this message is actioned as appropriate (for example as "add a contact", or "share high scores".)
Note: For more information and ideas, the Ports as the basis of an object-capability model on the Web section of the spec is a useful read.
To get your started, we have published a couple of demos on Github. First up, check out our channel messaging basic demo (run it live too), which shows a really simple single message transfer between a page and an embedded {{htmlelement("iframe")}}. The embedded IFrame then sends a confirmation message back. Second, have a look at our multimessaging demo (run this live), which shows a slightly more complex setup that can send multiple messages between main page and IFrame.
We'll be focusing on the latter example in this article. It looks like so:
In the main page of the demo, we have a paragraph and a simple form with a text input for entering messages to be sent to an {{htmlelement("iframe")}}.
var para = document.querySelector('p');
var textInput = document.querySelector('.message-box');
var button = document.querySelector('button');
var ifr = document.querySelector('iframe');
var otherWindow = ifr.contentWindow;
ifr.addEventListener("load", iframeLoaded, false);
function iframeLoaded() {
button.onclick = function(e) {
e.preventDefault();
var channel = new MessageChannel();
otherWindow.postMessage(textInput.value, '*', [channel.port2]);
channel.port1.onmessage = handleMessage;
function handleMessage(e) {
para.innerHTML = e.data;
textInput.value = '';
}
}
}
When the IFrame has loaded, we run an iframeLoaded() function containing an onclick handler for our button — when the button is clicked, we prevent the form submitting as normal, create a new message channel with the {{domxref("MessageChannel()","MessageChannel.MessageChannel")}} constructor, then send the value entered in our text input to the IFrame via the {{domxref("MessageChannel")}}. Let's explore how the window.postMessage line works in a bit more detail.
For a start, here we are calling the {{domxref("window.postMessage")}} method — we are posting a message to the IFrame's window context. {{domxref("window.postMessage")}} has three arguments, unlike {{domxref("MessagePort.postMessage")}}, which only has two. The three arguments are:
textInput.value.At the bottom of the iframeLoaded() function there is a {{domxref("MessagePort.onmessage")}} handler, but we'll get to that later.
Over in the IFrame, we have the following JavaScript:
var list = document.querySelector('ul');
onmessage = function(e) {
var listItem = document.createElement('li');
listItem.textContent = e.data;
list.appendChild(listItem);
e.ports[0].postMessage('Message received by IFrame: "' + e.data + '"');
}
The entirety of the code is wrapped in a {{domxref("window.onmessage")}} handler, which runs when the message is received from the main page (via its postMessage().) First we create a list item and insert it in the unordered list, setting the {{domxref("textContent","Node.textContent")}} of the list item equal to the event's data attribute (this contains the actual message).
Next, we post a confirmation message back to the main page via the message channel, using e.ports[0].postMessage(). How does this work? Earlier we transferred port2 over to the IFrame — this is accessible in the event's ports attribute (array position [0]). We call {{domxref("MessagePort.postMessage")}} on this port — since port2 is being controlled by the IFrame, and it is joined to port1 by the message channel, the specified message will be sent back to the main page.
Returning to the main page, let's now look at the onmessage handler at the bottom of the iframeLoaded() function:
channel.port1.onmessage = handleMessage;
function handleMessage(e) {
para.innerHTML = e.data;
textInput.value = '';
}
Here we are setting port1's {{domxref("MessagePort.onmessage")}} handler equal to the handleMessage() function — when a message is received back from the IFrame confirming that the original message was received successfully, this simply outputs the confirmation to a paragraph and empties the text input ready for the next message to be sent.
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('HTML WHATWG', 'web-messaging.html#channel-messaging', 'Channel messaging')}} | {{Spec2('HTML WHATWG')}} |
MessageChannel{{Compat("api.MessageChannel", 0)}}
MessagePort{{Compat("api.MessagePort", 0)}}