Exception when calling canGoBack within back button listener

[NickIliev]

@bastianjoel commented on Thu Jul 04 2019

Environment

• CLI: 5.4.0
• Cross-platform modules: 5.4.3
• Android Runtime: 5.4.0

Describe the bug
Calling canGoBack() of RouterExtensions causes the following exception when using inside back button listener.

An uncaught Exception occurred on "main" thread.
com.tns.NativeScriptException: 
Calling js method onBackPressed failed

TypeError: Cannot read property 'states' of null
File: "file:///data/data/org.nativescript.preview/files/app/tns_modules/nativescript-angular/router/ns-location-strategy.js, line: 236, column: 18

StackTrace: 
Frame: function:'NSLocationStrategy.canGoBack', file:'file:///data/data/org.nativescript.preview/files/app/tns_modules/nativescript-angular/router/ns-location-strategy.js', line: 236, column: 19
Frame: function:'RouterExtensions.canGoBack', file:'file:///data/data/org.nativescript.preview/files/app/tns_modules/nativescript-angular/router/router-extensions.js', line: 51, column: 41
Frame: function:'', file:'file:///data/data/org.nativescript.preview/files/app/app.component.js', line: 26, column: 34
Frame: function:'Observable.notify', file:'file:///data/data/org.nativescript.preview/files/app/tns_modules/tns-core-modules/data/observable/observable.js', line: 110, column: 15
Frame: function:'ActivityCallbacksImplementation.onBackPressed', file:'file:///data/data/org.nativescript.preview/files/app/tns_modules/tns-core-modules/ui/frame/frame.js', line: 888, column: 25
Frame: function:'NativeScriptActivity.onBackPressed', file:'file:///data/data/org.nativescript.preview/files/app/tns_modules/tns-core-modules/ui/frame/activity.js', line: 41, column: 21


at com.tns.Runtime.callJSMethodNative(Native Method)
at com.tns.Runtime.dispatchCallJSMethodNative(Runtime.java:1203)
at com.tns.Runtime.callJSMethodImpl(Runtime.java:1083)
at com.tns.Runtime.callJSMethod(Runtime.java:1070)
at com.tns.Runtime.callJSMethod(Runtime.java:1050)
at com.tns.Runtime.callJSMethod(Runtime.java:1042)
at com.tns.NativeScriptActivity.onBackPressed(NativeScriptActivity.java:59)
at android.app.Activity.onKeyUp(Activity.java:3087)
at android.view.KeyEvent.dispatch(KeyEvent.java:2722)
at android.app.Activity.dispatchKeyEvent(Activity.java:3380)
at android.support.v4.app.SupportActivity.superDispatchKeyEvent(ComponentActivity.java:108)
at android.support.v4.view.KeyEventDispatcher.dispatchKeyEvent(KeyEventDispatcher.java:84)
at android.support.v4.app.SupportActivity.dispatchKeyEvent(ComponentActivity.java:126)
at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:535)
at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:59)
at android.support.v7.app.AppCompatDelegateImpl$AppCompatWindowCallback.dispatchKeyEvent(AppCompatDelegateImpl.java:2533)
at com.android.internal.policy.DecorView.dispatchKeyEvent(DecorView.java:358)
at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:5347)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:5215)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4653)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4801)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4653)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4858)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4653)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4626)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4679)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4645)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4834)
at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:4995)
at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2602)
at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:2112)
at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:2103)
at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:2579)
at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:326)
at android.os.Looper.loop(Looper.java:160)
at android.app.ActivityThread.main(ActivityThread.java:6854)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:860)

To Reproduce
General:

1. Add a android back button listener and define property tries

application.android.on(application.AndroidApplication.activityBackPressedEvent, (args: any) => {
    if (!this.router.canGoBack()) {
        args.cancel = (this.tries++ > 0) ? false : true;

        setTimeout(() => {
            this.tries = 0;
        }, 2000);
    } else {
        args.cancel = false;
    }
});

2. Close app by using the back button twice
3. Reopen app
4. Tap the back button again

Expected behavior

Sample project
https://play.nativescript.org/?template=play-ng&id=AO3iYR&v=343

Additional context

[mayureshjadhav]

I am getting this issue. Anyone has a solution?

[DariosKrimsKrams]

Hello, have the same problem when triggering the back-button on specific pages. (dont understand the original example with triggering twice and reopen. For me, just the "normal" workflow is breaking it.

application.android.on(application.AndroidApplication.activityBackPressedEvent, (args: any) => {
    this.ngZone.run(() => {
        args.cancel = true;
        if (this.router.canGoBack()) {
            this.router.back();
        }
    }
});

[ivanff]

I also have this problem!

[farismohammed]

facing the same problem.

[001123]

I have same issue on IOS :'(

[huukuntp1]

I’ve same this issue. let me know if this is resolve. thanks everyone

[edusperoni]

When you use the back button twice you're not completely exiting the app, but the UI is destroyed (including angular). What's happening is:

1. You call android.on(function_on_instance1)
2. You press back button to exit
3. it calls function_on_instance1
4. it exits and destroys the angular instance
5. open app, new angular instance
6. You call android.on(function_on_instance2)
7. You press back button to exit
8. it calls function_on_instance1 then function_on_instance2

Solution:

ngOnInit() {
application.android.on(application.AndroidApplication.activityBackPressedEvent, handleBackButton, this);
}
ngOnDestroy() {
application.android.off(application.AndroidApplication.activityBackPressedEvent, handleBackButton, this);
}
handleBackButton(args: any) {
    this.ngZone.run(() => {
        args.cancel = true;
        if (this.router.canGoBack()) {
            this.router.back();
        }
    }
}

[flgraveline]

When you use the back button twice you're not completely exiting the app, but the UI is destroyed (including angular). What's happening is:

1. You call android.on(function_on_instance1)
2. You press back button to exit
3. it calls function_on_instance1
4. it exits and destroys the angular instance
5. open app, new angular instance
6. You call android.on(function_on_instance2)
7. You press back button to exit
8. it calls function_on_instance1 then function_on_instance2

Solution:

ngOnInit() {
application.android.on(application.AndroidApplication.activityBackPressedEvent, handleBackButton, this);
}
ngOnDestroy() {
application.android.off(application.AndroidApplication.activityBackPressedEvent, handleBackButton, this);
}
handleBackButton(args: any) {
    this.ngZone.run(() => {
        args.cancel = true;
        if (this.router.canGoBack()) {
            this.router.back();
        }
    }
}

Thanks for this @edusperoni !