Merge pull request #147 from atom/remote-docs

Revise the docs on remote module

docs/api/browser/browser-window.md

@@ -309,6 +309,10 @@ visible page.
 You can write received `image` directly to a `.png` file, or you can base64
 encode it and use data URL to embed the image in HTML.
 
+**Note:** Be sure to read documents on remote buffer in
+[remote](../renderer/remote.md) if you are going to use this API in renderer
+process.
+
 ### BrowserWindow.getPageTitle()
 
 Returns the title of web page.

docs/api/renderer/remote.md

@@ -1,12 +1,17 @@
 # remote
 
-It's common that the developers want to use modules in browsers from the
-renderer, like closing current window, opening file dialogs, etc. Instead of
-writing IPC code for every operation you want to do, atom-shell provides the
-`remote` module to let you do RPC call just like using normal javascript
-objects.
+The `remote` module provides a simple way to do inter-process communication
+between renderer process and browser process.
 
-An example of creating a window in renderer:
+In atom-shell, all GUI related modules are only available in the browser
+process, if users want to call an browser side API in the renderer process
+, they usually would have to explicitly send inter-process messages to the
+browser process. But with the `remote` module, users can invoke methods of
+objects living in browser process without sending inter-process messages
+directly, like Java's
+[RMI](http://en.wikipedia.org/wiki/Java_remote_method_invocation).
+
+An example of creating a browser window in renderer process:
 
 ```javascript
 var remote = require('remote');
@@ -15,31 +20,50 @@ var win = new BrowserWindow({ width: 800, height: 600 });
 win.loadUrl('https://github.com');
 ```
 
+## Remote objects
+
+Each object (including function) returned by `remote` module represents an
+object in browser process (we call it remote object or remote function), when
+you invoke methods of a remote object, or call a remote function, or even create
+a new object with the remote constructor (function), you are actually sending
+synchronous inter-process messages.
+
+In the example above, both `BrowserWindow` and `win` were remote objects. And
+`new BrowserWindow` didn't create a `BrowserWindow` object in renderer process,
+instead it created a `BrowserWindow` object in browser process, and returned the
+corresponding remote object in renderer process, namely the `win` object.
+
 ## Lifetime of remote objects
 
-Every object returned by `remote` module represents an object in browser (e.g.
-a remote object), so when you call methods of an object, or call a returned
-function, or even create a object with the returned constructor, you are
-indeed making a synchronous RPC call. And when the renderer releases the last
-reference to the remote object, the browser would release the corresponding
-reference too.
+Atom-shell makes sure that as long as the remote object in renderer process
+lives (in other words, has not been garbage collected), the corresponding object
+in browser process would never be released. And when the remote object has been
+garbage collected, the corresponding object in browser process would be
+dereferenced.
 
-This also means that, if the renderer keeps a reference to an object in
-browser, the object would never be released. So be careful to never leak the
+But it also means that, if the remote object is leaked in renderer process, like
+being stored in a map but never got freed, the corresponding object in browser
+process would also be leaked too. So you should be very careful not to leak
 remote objects.
 
-## Passing callbacks
+Primary value types like strings and numbers, however, are sent by copy.
 
-Many APIs in browser accepts callbacks, so the `remote` module also supports
-passing callbacks when calling remote functions, and the callbacks passed
-would become remote functions in the browser.
+## Passing callbacks to browser
 
-But in order to avoid possible dead locks, the callbacks passed to browser
-would be called asynchronously in browser, so you should never expect the
-browser to get the return value of the passed callback.
+Some APIs in browser process accepts callbacks, and it would be attempting to
+pass callbacks when calling a remote function. Yes `remote` module does support
+doing this, but you should also be extremely careful on this.
 
-Another thing is the lifetime of the remote callbacks in browser, it might be
-very tempting to do things like following:
+First, in order to avoid dead locks, the callbacks passed to browser process
+would be called asynchronously, so you should not expect the browser process to
+get the return value of the passed callbacks.
+
+Second, the callbacks passed to browser process would not get released
+automatically after they were called, instead they would persistent until the
+browser process garbage collected them.
+
+For example, following code seems innocent at first glance, It installed a
+callback for the `close` event on a remote object:
 
 ```javascript
 var remote = require('remote');
@@ -48,35 +72,81 @@ remote.getCurrentWindow().on('close', function() {
 });
 ```
 
-Yes it will work correctly, but when you reload the window, the callback you
-setup on the object in browser will not be erased, resources are leaked and
-there is no magic in javascript to release a referenced object.
+But the callback would be stored in the browser process persistently until you
+explicitly uninstall it! So each time you reload your window, the callback would
+be installed for once and previous callbacks were just leak. To make things
+worse, since the context of previously installed callbacks have been released,
+when `close` event was emitted exceptions would happen in browser process.
 
-So if you really need to keep a reference of callbacks in browser, you should
-write the callback in browser and send messages to renderer. And also make use
-of DOM's events like `unload` and `beforeunload`, they will work perfectly.
+So generally, unless you are clear what you are doing, you should always avoid
+passing callbacks to browser process.
+
+## Remote buffer
+
+An instance of node's `Buffer` is an object, so when you got a `Buffer` from
+browser process, what you got was indeed a remote object (let's call it remote
+buffer), and everything would just follow the rules of remote objects.
+
+However you should remember that though a remote buffer behaves like the real
+`Buffer`, it's not a `Buffer` at all. If you pass a remote buffer to node APIs
+that accepting `Buffer`, you should assume the remote buffer would be treated
+like a normal object, instead of a `Buffer`.
+
+For example you can call `BrowserWindow.capturePage` in renderer process, which
+returns a `Buffer` by calling passed callback:
+
+```javascript
+var remote = require('remote');
+var fs = require('fs');
+remote.getCurrentWindow().capturePage(function(buf) {
+  fs.writeFile('/tmp/screenshot.png', buf, function(err) {
+    console.log(err);
+  });
+});
+```
+
+But you may be surprised to find that the file written was corrupted. This is
+because when you called `fs.writeFile`, you thought `buf` was a `Buffer`, but
+indeed it was a remote buffer, and it would be converted to string before it was
+written to file. Since `buf` contained binary data and could not be represented
+by UTF-8 encoded string, the written file would be corrupted.
+
+The workaround is to write the `buf` in browser process, where it is a real
+`Buffer`:
+
+```javascript
+var remote = require('remote');
+remote.getCurrentWindow().capturePage(function(buf) {
+  remote.require('fs').writeFile('/tmp/screenshot.png', buf, function(err) {
+    console.log(err);
+  });
+});
+```
+
+The same thing could happen for all native types, but usually it would just
+throw a type error. The `Buffer` deserves your special attention because it
+can be converted to string and APIs accepting `Buffer` usually accept string
+too, and data corruption only happens when it contains binary data.
 
 ## remote.require(module)
 
 * `module` String
 
-Return a module in browser.
+Returns the object returned by `require(module)` in the browser process.
 
 ## remote.getCurrentWindow()
 
-Return the `BrowserWindow` object that represents current window.
-
-`Note:` it doesn't return the `window` object which represents the global
-scope, instead it returns an instance of the `BrowserWindow` class which is
-created with `browser-window` module in browser.
+Returns the [BrowserWindow](../browser/browser-window.md) object which
+represents current window.
 
 ## remote.getGlobal(name)
 
 * `name` String
 
-Return the `global[name]` value in browser.
+Returns the global variable of `name` (e.g. `global[name]`) in the browser
+process.
 
 ## remote.process
 
-Getter to return the `process` object in browser, this is the same with
+Returns the `process` object in the browser process, this is the same with
 `remote.getGlobal('process')` but gets cached.