Why we use ViewTreeObserver#addOnGlobalLayoutListener()

Question

Why do we use ViewTreeObserver, please can anyone explain it?

In below code creditsView is TextView object. By this whole code I understand that "this is to hide some text based on condition", but only thing is why we are using ViewTreeObserver?

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);
            }
        }
    });

Answer 1

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

Answer 2

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.