151 lines
4.4 KiB
Markdown
151 lines
4.4 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 [Event Emitter][event-emitter]. 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.reply(...)`. This helper method will automatically handle messages
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coming from frames that aren't the main frame (e.g. iframes) whereas
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`event.sender.send(...)` will always send to the main frame.
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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.reply('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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* `event` IpcMainEvent
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* `...args` any[]
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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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* `event` IpcMainEvent
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* `...args` any[]
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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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* `...args` any[]
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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 (optional)
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Removes listeners of the specified `channel`.
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### `ipcMain.handle(channel, listener)`
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* `channel` String
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* `listener` Function<Promise<void> | any>
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* `event` IpcMainInvokeEvent
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* `...args` any[]
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Adds a handler for an `invoke`able IPC. This handler will be called whenever a
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renderer calls `ipcRenderer.invoke(channel, ...args)`.
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If `listener` returns a Promise, the eventual result of the promise will be
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returned as a reply to the remote caller. Otherwise, the return value of the
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listener will be used as the value of the reply.
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```js
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// Main process
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ipcMain.handle('my-invokable-ipc', async (event, ...args) => {
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const result = await somePromise(...args)
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return result
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})
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// Renderer process
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async () => {
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const result = await ipcRenderer.invoke('my-invokable-ipc', arg1, arg2)
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// ...
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}
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```
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The `event` that is passed as the first argument to the handler is the same as
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that passed to a regular event listener. It includes information about which
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WebContents is the source of the invoke request.
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### `ipcMain.handleOnce(channel, listener)`
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* `channel` String
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* `listener` Function<Promise<void> | any>
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* `event` IpcMainInvokeEvent
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* `...args` any[]
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Handles a single `invoke`able IPC message, then removes the listener. See
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`ipcMain.handle(channel, listener)`.
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### `ipcMain.removeHandler(channel)`
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* `channel` String
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Removes any handler for `channel`, if present.
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## IpcMainEvent object
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The documentation for the `event` object passed to the `callback` can be found
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in the [`ipc-main-event`](structures/ipc-main-event.md) structure docs.
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## IpcMainInvokeEvent object
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The documentation for the `event` object passed to `handle` callbacks can be
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found in the [`ipc-main-invoke-event`](structures/ipc-main-invoke-event.md)
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structure docs.
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[event-emitter]: https://nodejs.org/api/events.html#events_class_eventemitter
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[web-contents-send]: web-contents.md#contentssendchannel-arg1-arg2-
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