99 lines
2.8 KiB
Markdown
99 lines
2.8 KiB
Markdown
# ipcMain
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> Communicate asynchronously from the main process to renderer processes.
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Process: [Main](../glossary.md#main-process)
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The `ipcMain` module is an instance of the
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[EventEmitter](https://nodejs.org/api/events.html#events_class_eventemitter) class. When used in the main
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process, it handles asynchronous and synchronous messages sent from a renderer
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process (web page). Messages sent from a renderer will be emitted to this
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module.
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## Sending Messages
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It is also possible to send messages from the main process to the renderer
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process, see [webContents.send][web-contents-send] for more information.
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* When sending a message, the event name is the `channel`.
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* To reply to a synchronous message, you need to set `event.returnValue`.
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* To send an asynchronous message back to the sender, you can use
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`event.sender.send(...)`.
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An example of sending and handling messages between the render and main
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processes:
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```javascript
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// In main process.
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const {ipcMain} = require('electron')
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ipcMain.on('asynchronous-message', (event, arg) => {
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console.log(arg) // prints "ping"
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event.sender.send('asynchronous-reply', 'pong')
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})
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ipcMain.on('synchronous-message', (event, arg) => {
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console.log(arg) // prints "ping"
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event.returnValue = 'pong'
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})
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```
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```javascript
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// In renderer process (web page).
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const {ipcRenderer} = require('electron')
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console.log(ipcRenderer.sendSync('synchronous-message', 'ping')) // prints "pong"
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ipcRenderer.on('asynchronous-reply', (event, arg) => {
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console.log(arg) // prints "pong"
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})
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ipcRenderer.send('asynchronous-message', 'ping')
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```
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## Methods
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The `ipcMain` module has the following method to listen for events:
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### `ipcMain.on(channel, listener)`
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* `channel` String
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* `listener` Function
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Listens to `channel`, when a new message arrives `listener` would be called with
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`listener(event, args...)`.
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### `ipcMain.once(channel, listener)`
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* `channel` String
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* `listener` Function
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Adds a one time `listener` function for the event. This `listener` is invoked
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only the next time a message is sent to `channel`, after which it is removed.
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### `ipcMain.removeListener(channel, listener)`
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* `channel` String
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* `listener` Function
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Removes the specified `listener` from the listener array for the specified
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`channel`.
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### `ipcMain.removeAllListeners([channel])`
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* `channel` String
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Removes listeners of the specified `channel`.
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## Event object
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The `event` object passed to the `callback` has the following methods:
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### `event.returnValue`
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Set this to the value to be returned in a synchronous message.
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### `event.sender`
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Returns the `webContents` that sent the message, you can call
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`event.sender.send` to reply to the asynchronous message, see
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[webContents.send][web-contents-send] for more information.
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[web-contents-send]: web-contents.md#contentssendchannel-arg1-arg2-
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