feat: Notify JS-layer when webview crashed

[sandstrom]

Feature Request

It would be great if the JS-layer (Capacitor app) could know that the underlying web view crashed and subsequently restarted. Right now there is nothing to discern these from a regular app startup and users won't be aware that data was lost.

More concretely, an issue that occur frequently is that we take a photograph via an <input capture> element and do on-the-fly resize using JS and canvas, and the web view crashes, causing the page to reload. But for the user this isn't obvious, it's just a short flash when the web view reloads, and they don't know that the photograph failed and they've just lost data.

If we could listen to these events we could show a warning to the user.

This is the code on iOS today:

capacitor/ios/Capacitor/Capacitor/CAPBridgeViewController.swift

Lines 279 to 281 in 0a25cce

	public func webViewWebContentProcessDidTerminate(_ webView: WKWebView) {
	webView.reload()
	}

Platform Support Requested

• Android
• iOS

Describe Preferred Solution

There are multiple solutions, but one could be a listener similar to appRestoredResult , that would let us know that the app restored after a crash.

We'd then have the ability to show a warning to the user.

Additional Context

This related issue about the web view crashing when taking photos: #2265

[jcesarmobile]

Since appRestoredResult is Android only, we could just try to implement it for iOS too, but that won't work for your user case as it passes info from the last plugin call and you are not using a plugin.

[sandstrom]

@jcesarmobile Yeah, I looked at that event too, but seems like it has slightly different semantics/meaning. It's more of a switching thing.

Maybe a separate event for crashes would make more sense, e.g. restartedAfterCrash or similar.

[diachedelic]

Would it be so crazy to crash the app if webViewWebContentProcessDidTerminate was called? I think I would prefer the app to quit immediately and obviously, rather than data disappearing silently. Perhaps it could be a configuration option.

[jcesarmobile]

Yeah, it’s crazy to crash an iOS app, apple will reject apps if they crash

[diachedelic]

In that case I support @sandstrom 's idea of something like a webViewWebContentProcessDidTerminate event. In my case the webview was crashing intermittently due to a memory leak, which was fairly trivial to fix, but painful to diagnose due to the absence of any diagnostic.

[diachedelic]

Just another though – silently restarting the webview can leave plugins in an inconsistent state. For example, if you have called Geolocation.watchPosition, and the webview crashes and reloads, there is no way to halt location updates other than manually killing and restarting the app.

I have been running Capacitor in development with the below modification for months now, and I would love to see this behaviour behind a configuration option.

diff --git a/CAPBridgeViewController.swift b/CAPBridgeViewController.swift
index 431c46ae..fb1c8bdd 100644
--- a/CAPBridgeViewController.swift
+++ b/CAPBridgeViewController.swift
@@ -344,7 +344,7 @@ public class CAPBridgeViewController: UIViewController, CAPBridgeDelegate, WKScr
   }
 
   public func webViewWebContentProcessDidTerminate(_ webView: WKWebView) {
-    webView.reload()
+    fatalError("webViewWebContentProcessDidTerminate")
   }

[dodomui]

Does this work?

public func webViewWebContentProcessDidTerminate(_ webView: WKWebView) { 
   webView.reload() 
 } 

[diachedelic]

I believe that is the default behaviour...

[dodomui]

@diachedelic cannot find this function at Capacitor 3.3.4 @capacitor/ios/Capacitor/Capacitor/CapacitorBridge.swift

[diachedelic]

Oh wow. So what happens now when the webview crashes on iOS?

[KevinKelchen]

Not sure what happens now, but it appears the reload was just added back in #5391

I'm glad it was added back as we've had issues where the OS kills the WebView process and we need to recover (JetSam or what-have-you).

We plan to very soon add logging to Sentry when it occurs so we can learn more about it happening in production and make better informed decisions on our technical approaches. That's why I'm interested in this issue. Not sure exactly what we'll do at the moment to capture the automatic reload event but I have some not-amazing ideas to try out. 🙂

[diachedelic]

We have found the iOS WebView to be very sensitive to memory usage. It crashes quite often on startup, possibly due to this bug: https://bugs.webkit.org/show_bug.cgi?id=212790. If you have any memory leaks in the WebView, it will eventually be terminated. WebKit unfortunately has some nasty memory leaks associated with the <video> and <canvas> elements.

In general, an automatic reload is necessary in production. But I would prefer a crash in development. And ideally, we should be able to override the default behaviour, so we can maintain the integrity of the app.

[KevinKelchen]

Yep, it can be rough.

Using more resource/memory-intensive technologies like WebGL (used in modern map control JS libraries) can exacerbate it as well. We'd like to embrace that technology but are held back.