docs: Add usage on app packaging

docs/tutorial/application-packaging.md

@@ -1 +1,139 @@
 # Application packaging
+
+To protect your app's resources and source code from the users, you can choose
+to package your app into [asar][asar] archive with little changes to your source
+code.
+
+## Generating `asar` archive
+
+The [asar][asar] archive is a simple tar-like format that concatenates files
+into a single file, atom-shell can read arbitrary file in it without unpacking
+the whole file.
+
+Following is the steps to package your app into `asar` archive:
+
+### 1. Install asar utility
+
+```bash
+$ npm install -g asar
+```
+
+### 2. Package with `asar pack`
+
+```bash
+$ asar pack your-app app.asar
+```
+
+## Using `asar` archives
+
+In atom-shell there are two sets of APIs: Node APIs provided by Node.js, and Web
+APIs provided by Chromium. Both APIs support reading file from `asar` archives.
+
+### Node API
+
+With special patches in atom-shell, Node APIs like `fs.readFile` and `require`
+treat `asar` archives as virtual directories, and the files in it as normal
+files in filesystem.
+
+For example, suppose we have an `example.asar` archive under `/path/to`:
+
+```bash
+$ asar list /path/to/example.asar
+/app.js
+/file.txt
+/dir/module.js
+/static/index.html
+/static/main.css
+/static/jquery.min.js
+```
+
+Read a file in `asar` archive:
+
+```javascript
+var fs = require('fs');
+fs.readFileSync('/path/to/example.asar/file.txt');
+```
+
+List all files under the root of archive:
+
+```javascript
+var fs = require('fs');
+fs.readdirSync('/path/to/example.asar');
+```
+
+Use a module from the archive:
+
+```javascript
+require('/path/to/example.asar/dir/module.js');
+```
+
+### Web API
+
+In web page files in archive can be requests by using the `asar:` protocol,
+like node API, `asar` archives are treated as directories.
+
+For example, to get a file with `$.get`:
+
+```html
+<script>
+var $ = require('./jquery.min.js');
+$.get('asar:/path/to/example.asar/file.txt', function(data) {
+  console.log(data);
+});
+</script>
+```
+
+The `asar:` protocol can also be used to request normal files in filesystem,
+just like the `file:` protocol. But unlike `file:` protocol, there is no slashes
+(`//`) after `asar:`.
+
+You can also display a web page in `asar` archive with `BrowserWindow`:
+
+```javascript
+var BrowserWindow = require('browser-window');
+var win = new BrowserWindow({width: 800, height: 600});
+win.loadUrl('asar:/path/to/example.asar/static/index.html');
+```
+
+## Limitations on Node API
+
+Even though we tried hard to make `asar` archives in Node API work like
+directories as much as possible, there are still limitations due to the
+low-level nature of Node API.
+
+### Archives are read only
+
+The archives can not be modifies so all Node APIs that can modify files will not
+work with `asar` archives.
+
+### Working directory can not be set to directories in archive
+
+Though `asar` archives are treated as directories, there are no actual
+directories in the filesystem, so you can never set working directory to
+directories in `asar` archives, passing them to `cwd` option of some APIs will
+also cause errors.
+
+### Extra unpacking on some APIs
+
+Most `fs` APIs can read file or get file's information from `asar` archives
+without unpacking, but for some APIs that rely on passing the real file path to
+underlying system calls, atom-shell will extract the needed file into a
+temporary file and pass the path of the temporary file to the APIs to make them
+work. This adds a little overhead for those APIs.
+
+APIs that requires extra unpacking are:
+
+* `child_process.execFile`
+* `child_process.fork`
+* `fs.open`
+* `fs.openSync`
+* `process.dlopen` - Used by `require` on native modules
+
+### Fake stat information of `fs.stat`
+
+The `Stats` object returned by `fs.stat` and its friends on files in `asar`
+archives are generated by guessing, because those files do not exist on
+filesystem. So you should not trust the `Stats` object except for getting file
+size and checking file type.
+
+[asar]: https://github.com/atom/asar