129 lines
4.4 KiB
Markdown
129 lines
4.4 KiB
Markdown
---
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layout: post
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title: "BroadcastChannel API"
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date: 2015-04-02 17:13:46
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permalink: broadcastchannel-api
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categories: api
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---
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[BroadcastChannel API](https://developer.mozilla.org/en-US/docs/Web/API/Broadcast_Channel_API)
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is a new API used to communicate between same-origin tabs opened by the same user.
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#Why
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Let's say you open two GitHub tabs, the [rust repository](https://github.com/rust-lang/rust) and [your stars](https://github.com/stars) page. You decide to star the awesome rust repository, but then you have to
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refresh your stars page to see your new star. That's sad. There must be a way for GitHub to refresh
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your stars page in case you star something in another tab, right?
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#Show me something!
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Okay, open another page of my blog in a new tab. Now open up your console and enter this:
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{% highlight javascript %}
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channel.postMessage('Anybody hears me?');
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{% endhighlight %}
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Boom.
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I've put this code in my `<head>`, pretty simple:
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{% highlight javascript %}
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var channel = new BroadcastChannel('egg');
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channel.addEventListener('message', message => {
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alert('Got a message from the other tab:\n' + message.data);
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// Egg
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document.querySelector('header').classList.add('channel');
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});
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{% endhighlight %}
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#How
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BroadcastChannels are pretty easy, here I'm going over the small details.
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##Creating channels
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BroadcastChannels are constructed with a single argument, their name. Browsing contexts should use
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this name to communicate over a specified channel. There's no limit to how many channels you can create.
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In the first sentence of article I said it's used to communicate between tabs, but it's actually "browsing contexts".
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[Browsing contexts](http://www.w3.org/TR/html5/browsers.html#browsing-context) are any environments owning a `Document`, e.g. tabs, windows, iframes, etc.
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{% highlight javascript %}
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var channel = new BroadcastChannel('star');
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{% endhighlight %}
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Channels have only one property, `name`.
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##Methods
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Channels have two methods:
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### #postMessage(data: Anything)
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This is the method used to broadcast a message to everyone subscribed to this channel. `data` can be any kind of Object.
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### #close()
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This method is used to leave a channel, in case you don't want to hear from the channel anymore.
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Try leaving `channel` from my blog pages, and posting messages with others.
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##Events
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Channels inherit from [`EventTarget`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget), so you can use `addEventListener`, `removeEventListener` and `dispatchEvent` methods.
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Channels have only one event:
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###message
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The event object passed to this event is a [`MessageEvent`](https://developer.mozilla.org/en-US/docs/Web/API/MessageEvent) with the `data` property set to the actual message sent using `postMessage`.
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---
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#Another example
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##Let's Fix GitHub
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Okay, let's try something cool, I promise you will love it. Open a browser with [BroadcastChannel support](http://caniuse.com/#feat=broadcastchannel) and Install [GreaseMonkey](http://www.greasespot.net/).
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You have to add two scripts, one for repository pages, and one for the stars page.
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Click "New User Script", and fill the inputs like this:
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In the Script Editor, enter this:
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{% highlight javascript %}
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// join the "Stars" channel
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var channel = new BroadcastChannel('Stars');
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// reload the page when someone from a github.com tab broadcasts a message
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channel.addEventListener('message', message => {
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location.reload();
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});
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{% endhighlight %}
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Now create another script and fill the form like this:
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Use this code:
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{% highlight javascript %}
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// join the "Stars" channel
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var channel = new BroadcastChannel('Stars');
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// once the star/unstar forms are submitted, broadcast a message to
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// all github.com tabs
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$('.starring-container form').submit(e => {
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channel.postMessage('refresh');
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});
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{% endhighlight %}
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Save, refresh your pages and Voila! You can now Star/Unstar any repository while your stars page
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is open, and your stars page will refresh immediately.
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---
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That's it, I really like this API as it's best of both worlds, simple and useful.
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There are a lot of things which can be done with help of BroadcastChannels, I would love to hear your feedback on this API.
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