Why we use ViewTreeObserver#addOnGlobalLayoutListener()
Solution 1
If you hadn't used ViewTreeObserver
, than mainLayout.getRootView().getHeight()
would simply return 0px, because it hasn't been laid out yet (see getWidth()
and getHeight()
of View
returns 0).
Thus, you are waiting until view is measured, laid out, and then you are fetching width/height values from it. This callback will be fired exactly when the view is going to be laid out on the screen.
Solution 2
Don't know why but this was the first page shown to me when i searched for KOTLIN and after passsing Lamda i was unable to remove listner.
This is how i did in kotlin
tvLoginWith.viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
@RequiresApi(Build.VERSION_CODES.JELLY_BEAN)
override fun onGlobalLayout() {
tvLoginWith.viewTreeObserver.removeOnGlobalLayoutListener(this)
tvLogin.layoutParams.width = tvLoginWith.width
tvLogin.requestLayout()
}
})
Kool way to do this in kotlin (Reusable) create extension like this
fun ViewGroup.addViewObserver(function: () -> Unit) {
val view = this
view.viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
override fun onGlobalLayout() {
view.viewTreeObserver.removeOnGlobalLayoutListener(this)
function.invoke()
}
})
}
And use it from activity like this
listThumb.addViewObserver {
// your code
}
listThumb is recyclerview in this case
Prasanth Yejje
Updated on June 28, 2022Comments
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Prasanth Yejje almost 2 years
Why do we use
ViewTreeObserver
, please can anyone explain it?In below code
creditsView
isTextView
object. By this whole code I understand that "this is to hide some text based on condition", but only thing is why we are usingViewTreeObserver
?mainLayout.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() { @Override public void onGlobalLayout() { int heightDiff = mainLayout.getRootView().getHeight() - mainLayout.getHeight(); if (heightDiff > 100) { Utils.appLogger("MyActivity", "keyboard opened"); creditsView.setVisibility(View.GONE); } if (heightDiff < 100) { Utils.appLogger("MyActivity", "keyboard closed"); creditsView.setVisibility(View.VISIBLE); } } });
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azizbekian about 7 years@PrasanthYejje, as a newcomer, as somebody answers to your question, and you are satisfied with the answer, mark the answer as accepted.
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CoolMind about 5 yearsAgree, with lambda we strangely cannot use and remove listener.
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Dr4ke the b4dass about 3 yearsWhat if I'm using it only in xml, data-binding?
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Aklesh Singh almost 3 years@CoolMind i guess its like how in javascript lamdas functions don't have their own this (context). lexical scopes
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CoolMind almost 3 years@AkleshSingh, yes, it sounds properly.