Web Workers

Chapter 3. Dedicated Workers

Dedicated Web Workers let you run scripts in background threads. Once the Web Worker is running, it can communicate with its web app by posting messages to an event handler registered with the web app that spawned it. Dedicated Web Workers are good for tasks that consume a lot of CPU (e.g., calculating routes, 3D positions, prime numbers, etc.) and are also good for masking the latency in server connections. Having a Worker handle the connections keeps the main user interface thread freer to handle the users’ actions.

A dedicated Web Worker supports two events:

onmessage

Triggered when a message is received. An event object with a data member will be provided with the message.

onerror

Triggered when an error occurs in the Worker thread. The event provides a data member with the error information.

In this example, our web application main page starts a Web Worker to pull data from the server. Once it receives the data, the Web Worker sends it to the parent page so it can save it in the client-side database (or in our example, the localStorage). In the real world we can take this methodology one step further and let our Web Worker handle all of the communication with the server.

Example 3-1 does several things:

1. Starts the Web Worker code (which is in Example 3-2) by calling its constructor: var worker = new Worker("Example-3-2-tweet.js");.

2. Sets itself up to listen for messages (the tweet information) the Web Worker will send, using worker.addEventListener.

3. When ...