bug: Capacitor listeners failing to be called from Android notifyListeners with foreground service

[corypisano]

Bug Report

Capacitor Version

Latest Dependencies:

@capacitor/cli: 4.6.2
@capacitor/core: 4.6.2
@capacitor/android: 4.6.2
@capacitor/ios: 4.6.2

Installed Dependencies:

@capacitor/cli: 4.6.2
@capacitor/core: 4.6.2
@capacitor/android: 4.6.2
@capacitor/ios: 4.6.2

Platform(s)

• Android

Current Behavior

I'm maintaining a capacitor plugin that emits events for listeners (https://capacitorjs.com/docs/plugins/android#plugin-events). It uses a native Android & iOS package that starts a foreground service (and on Android, adds a notification to status bar). While the app is in the foreground or background, the native plugin code calls notifyListeners, and the javascript listener callback is run as expected.

The issue is that on Android when the app is swiped away, the Android plugin code continues to run as desired due to the foreground service and status bar notification and calls notifyListeners, but the javascript listener callback is never called. In logcat after the AppDestroyed lifecycle event is seen, the Android `"Notifying listeners for event X" is still being output, but the javascript listener callback is never reached (nor is there a "no listeners found for event X").

Expected Behavior

If the android plugin code is running and successfully calls notifyListeners, the javascript listener callback (via addListener) is expected to run.

Code Reproduction

I will create a sample application and update here, but wanted to see if it is a known issue in the meantime

The key components would be on Android to call
startForeground and setup a notification like so https://medium.com/@engineermuse/foreground-services-in-android-e131a863a33d

emit the listener event

JSObject ret = new JSObject();
ret.put("value", "some value");
notifyListeners("myPluginEvent", ret);

and on the javascript side setup a listener callback

import { MyPlugin } from 'my-plugin';

MyPlugin.addListener('myPluginEvent', (info: any) => {
  console.log('myPluginEvent was fired');
});

Notice in the foreground and background the js listener will fire, but on swiping away the app, the Android code will continue to run and notify listeners but javascript listener doesn't stay alive.

---

Update: https://github.com/corypisano/capacitor-listeners-issue

Other Technical Details

npm --version output: 8.1.0

node --version output: v16.13.0

pod --version output (iOS issues only):

Additional Context

[Ionitron]

This issue may need more information before it can be addressed. In particular, it will need a reliable Code Reproduction that demonstrates the issue.

Please see the Contributing Guide for how to create a Code Reproduction.

Thanks!
Ionitron 💙

[corypisano]

Created a sample application and plugin to reproduce: https://github.com/corypisano/capacitor-listeners-issue

the plugin starts a foreground service and outputs logs and a plugin event every 2 seconds on a timer

the app just imports the plugin and adds a listener

Logcat output:

On app start, the foreground service and timer starts

Every two seconds a log is seen from

• BroadcastService.java that is running the timer, and sending a broadcast
• the android plugin that receives the broadcast and calls notifyListeners
• Capacitor/MyPluginPlugin logging that notify listeners was called
• the listener callback in the capacitor app
  

Same behavior continues after app is backgrounded

When app is swiped away, the android code continues and notify listeners is called, but listener callback is never seen

[corypisano]

hi @jcesarmobile any advice here?

[shipley-dcc]

I see similar behaviour with two plugins that I use in an app. Both @capacitor-community/background-geolocation and @transistorsoft/capacitor-background-fetch display this behaviour. When my app is put into the background, the javascript callbacks work for 5 minutes, but then stop being called. You can see that the android code is being called, but the calls to notifyListeners aren't getting processed by javascript. When you bring the app back to the foreground all the queued-up callbacks are processed.

[peitschie]

@shipley-dcc the 5 minute timeout is due to some fairly undocumented WebView optimizations that kick in after the app window moves away from the foreground (even if there's a foreground service).

The easiest way to turn these off is using a plugin like https://bitbucket.org/TheBosZ/cordova-plugin-run-in-background/src/master/ (I haven't confirmed this works with capacitor), specifically:

cordova.plugins.backgroundMode.on('activate', function() {
    cordova.plugins.backgroundMode.disableWebViewOptimizations();
});

[peitschie]

@corypisano swiping away the application generally kills the activity, which in the case of a capacitor-based app will destroy the WebView and kill the JavaScript engine. The native android code continues to function because you're running that in a service which will survive the death of the app activity (for a while, at least).

Generally speaking, in modern Android there's no way to prevent your activity from being killed. If you are not releasing via the Play Store, you might be able to use a technique like automatically restarting your activity on termination (e.g., see cordova-plugin-autostart), but that requires special permissions with current versions of Android, and there's no way to ask the user for them specifically.

If you are trying to make a javascript process that runs independent of the WebView activity and can survive the activity being destroyed... well... that is much much harder, and there's no public plugins capable of doing this that I've found.

A few more relevant links to help can be seen on my comment here: #3032 (comment)

Ideally, Capacitor would provide a mechanism for running a headless WebView so we can execute JS within a service... but... I don't see that being something achieved any time soon 😓

[shipley-dcc]

@shipley-dcc the 5 minute timeout is due to some fairly undocumented WebView optimizations that kick in after the app window moves away from the foreground (even if there's a foreground service).

The easiest way to turn these off is using a plugin like https://bitbucket.org/TheBosZ/cordova-plugin-run-in-background/src/master/ (I haven't confirmed this works with capacitor), specifically:

cordova.plugins.backgroundMode.on('activate', function() {
    cordova.plugins.backgroundMode.disableWebViewOptimizations();
});

@peitschie Thank you for this. It is much appreciated.

[flanger777]

I have a similar issue in my app which uses the @capacitor-community/background-geolocation plugin. After moving the app to the background, javascript callbacks are only executed for 5 minutes and then stop executing. It is worth saying that with capacitor 3.9 everything work as expected. There is also a report of this issue in the repository of the plugin mentioned above, but there is still no solution.

[shipley-dcc]

I followed the advice above but used the capacitor version of the plugin called capacitor-plugin-background-mode. Configure it to disable web view optimizations and then enable it when you start logging location positions. It basically tells the webview that it is still visible so it keeps on processing JavaScript.