MarqueeTextView | A reliable , lightweight , scrolling textview for Android | Android library

1. First I describe the context, i.e. what I'm trying to do

2. Second I explain you in which cases I encounter the bug and what is it (it's the current behavior)

3. Third, I explain you the expected behavior

4. Then I explain you how to reproduce the bug

5. Finally I show you the minimal and executable code that can lead to the bug

I am using the widget AutoScrollTextView available between others in the API https://github.com/ronghao/AutoScrollTextView. AutoScrollTextView is a TextView that can automatically autoscroll vertically and moreover, if the text is too long, horizontally. So an AutoScrollTextView can contain at least one text to be automatically scrolled (see the illustration in the GitHub repository).

1. If I use one AutoScrollTextView in the main activity, I don't see any bug. If I use it in the fragment of the main activity, I think it would be the case too (I didn't test it).
2. If I use one AutoScrollTextView in each of at least two fragments of a ViewPager (2 instances of this same fragment class are used for the Viewpager), with at least one text to be automatically scrolled by each AutoScrollTextView, the app crashes with the following error (NB: this ViewPager belongs to the main activity):

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.myapplication, PID: 2814 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.sendAccessibilityEventUnchecked(android.view.accessibility.AccessibilityEvent)' on a null object reference at android.view.ViewRootImpl$SendWindowContentChangedAccessibilityEvent.run(ViewRootImpl.java:9304) at android.os.Handler.handleCallback(Handler.java:789) at android.os.Handler.dispatchMessage(Handler.java:98) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6944) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

The error, however, is difficult to trigger:

1. Sometimes, it's triggered as soon as the fragment of the main activity is started
2. Sometimes, there is a random delay before it's triggered in the fragment of the main activity
3. Sometimes, maybe it's not triggered (or I close the app before it would)

As you can see, there isn't any indication of the line that causes this crash.

1. I have taken the library, and isolated the minimal part of it that lead to this bug.

2. I have set up the minimal executable app with only the main activity, the ViewPager, and the fragment (2 instances of this same fragment class are used for the Viewpager).

3. I have set up this minimal executable app with the isolated part of the library. You will find the sources below (even if there are "many" files, each one has been minimalized and contains only the strict minimal lines in order to make you able to reproduce the bug). Also: with the sources I give you, it's totally normal that you don't see text scrolling (nor text at all). It's because I've completely isolated the part of the API that triggers the bug, and the scrolling text wasn't related to this error, so it's not part of my isolated sources.

4. I have tried to use Android's Timer instead of an Handler but the error still occurres.

5. MarqueeTextView::scrollTo(currentScrollPos, 0); seems to trigger this error.

Normally, the app of course should not crash. In other words, there shouldn't be any triggered error (especially the one I've quoted above). With the sources I give you, it's totally normal that you don't see text scrolling (nor text at all). It's because I've completely isolated the part of the API that triggers the bug, and the scrolling text wasn't related to this error, so it's not part of my isolated sources.

Below, I provide you all the minimal and executable sources you need (even if there are "many" files, each one has been minimalized and contains only the strict minimal lines in order to make you able to reproduce the bug). After having created the corresponding files and copy/paste these sources in them, run this minimal and executable app several times. Even if the error will or won't be shown for each run, normally you would see it at least one time after 5 runs max. You can use your smartphone to test this app, or maybe an Android Studio's emulator (but I didn't test on an emulator).

NB: with the sources I give you, it's totally normal that you don't see text scrolling (nor text at all). It's because I've completely isolated the part of the API that triggers the bug, and the scrolling text wasn't related to this error, so it's not part of my isolated sources.

Could you tell me why this bug occures (I was unable to debug it) and why it seems so random? How could I correct it?

1. First, I will give you the sources of the isolated part of the library

2. Then, I will give you the sources of the app with the ViewPager, etc.

3. Even if there are "many" files, each one has been minimalized and contains only the strict minimal lines in order to make you able to reproduce the bug. With the sources I give you, it's totally normal that you don't see text scrolling (nor text at all). It's because I've completely isolated the part of the API that triggers the bug, and the scrolling text wasn't related to this error, so it's not part of my isolated sources.