remote
The remote
module provides a simple way to do inter-process communication
(IPC) between the renderer process (web page) and the main process.
In Electron, GUI-related modules (such as dialog
, menu
etc.) are only
available in the main process, not in the renderer process. In order to use them
from the renderer process, the ipc
module is necessary to send inter-process
messages to the main process. With the remote
module, you can invoke methods
of the main process object without explicitly sending inter-process messages,
similar to Java's RMI. An example of creating a browser window from a
renderer process:
const remote = require('electron').remote;
const BrowserWindow = remote.BrowserWindow;
var win = new BrowserWindow({ width: 800, height: 600 });
win.loadURL('https://github.com');
Note: for the reverse (access the renderer process from the main process), you can use webContents.executeJavascript.
Remote Objects
Each object (including functions) returned by the remote
module represents an
object in the main process (we call it a remote object or remote function).
When you invoke methods of a remote object, call a remote function, or 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 the renderer
process. Instead, it created a BrowserWindow
object in the main process and
returned the corresponding remote object in the renderer process, namely the
win
object.
Lifetime of Remote Objects
Electron makes sure that as long as the remote object in the renderer process lives (in other words, has not been garbage collected), the corresponding object in the main process will not be released. When the remote object has been garbage collected, the corresponding object in the main process will be dereferenced.
If the remote object is leaked in the renderer process (e.g. stored in a map but never freed), the corresponding object in the main process will also be leaked, so you should be very careful not to leak remote objects.
Primary value types like strings and numbers, however, are sent by copy.
Passing callbacks to the main process
Code in the main process can accept callbacks from the renderer - for instance
the remote
module - but you should be extremely careful when using this
feature.
First, in order to avoid deadlocks, the callbacks passed to the main process are called asynchronously. You should not expect the main process to get the return value of the passed callbacks.
For instance you can't use a function from the renderer process in an
Array.map
called in the main process:
// main process mapNumbers.js
exports.withRendererCallback = function(mapper) {
return [1,2,3].map(mapper);
}
exports.withLocalCallback = function() {
return exports.mapNumbers(function(x) {
return x + 1;
});
}
// renderer process
var mapNumbers = require("remote").require("./mapNumbers");
var withRendererCb = mapNumbers.withRendererCallback(function(x) {
return x + 1;
})
var withLocalCb = mapNumbers.withLocalCallback()
console.log(withRendererCb, withLocalCb) // [true, true, true], [2, 3, 4]
As you can see, the renderer callback's synchronous return value was not as expected, and didn't match the return value of an identical callback that lives in the main process.
Second, the callbacks passed to the main process will persist until the main process garbage-collects them.
For example, the following code seems innocent at first glance. It installs a
callback for the close
event on a remote object:
remote.getCurrentWindow().on('close', function() {
// blabla...
});
But remember the callback is referenced by the main process until you explicitly uninstall it. If you do not, each time you reload your window the callback will be installed again, leaking one callback for each restart.
To make things worse, since the context of previously installed callbacks has
been released, exceptions will be raised in the main process when the close
event is emitted.
To avoid this problem, ensure you clean up any references to renderer callbacks passed to the main process. This involves cleaning up event handlers, or ensuring the main process is explicitly told to deference callbacks that came from a renderer process that is exiting.
Accessing built-in modules in the main process
The built-in modules in the main process are added as getters in the remote
module, so you can use them directly like the electron
module.
const app = remote.app;
Methods
The remote
module has the following methods:
remote.require(module)
module
String
Returns the object returned by require(module)
in the main process.
remote.getCurrentWindow()
Returns the BrowserWindow
object to which this web page
belongs.
remote.getCurrentWebContents()
Returns the WebContents
object of this web page.
remote.getGlobal(name)
name
String
Returns the global variable of name
(e.g. global[name]
) in the main
process.
remote.process
Returns the process
object in the main process. This is the same as
remote.getGlobal('process')
but is cached.