Can you provide examples of using ViewTreeObserver in…

ViewTreeObserver is used to monitor changes in the view hierarchy. The most common scenarios:

Getting view dimensions before drawing:

view.viewTreeObserver.addOnPreDrawListener(object : ViewTreeObserver.OnPreDrawListener {
    override fun onPreDraw(): Boolean {
        // Get view size
        val width = view.width
        val height = view.height
        // ... do something with sizes

        // Remove listener to avoid repeated calls.
        // If the listener is not removed, onPreDraw will be called on every view change,
        // which can lead to an infinite loop.
        view.viewTreeObserver.removeOnPreDrawListener(this)
        return true // Return true to continue drawing
    }
})

Implementing measurements and layout for custom Views: Although in most cases onMeasure and onLayout methods are used for this, ViewTreeObserver can be useful for more complex scenarios where measurements depend on other elements outside the hierarchy.

Detecting changes in View positioning or structure:

view.viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
    override fun onGlobalLayout() {
        // View and its children have been laid out
        // ... perform actions

        // Deprecated method, use removeOnGlobalLayoutListener
        // view.viewTreeObserver.removeGlobalOnLayoutListener(this); // deprecation notice
        view.viewTreeObserver.removeOnGlobalLayoutListener(this)
    }
})

Tracking focus gain/loss:

view.viewTreeObserver.addOnWindowFocusChangeListener(object : ViewTreeObserver.OnWindowFocusChangeListener {
    override fun onWindowFocusChanged(hasFocus: Boolean) {
        // The window containing the View has gained or lost focus
        // ... perform actions
    }
})

It is always important to remove ViewTreeObserver listeners when they are no longer needed to avoid memory leaks, especially in listeners that can be called multiple times (e.g., addOnGlobalLayoutListener). Use the appropriate remove...Listener methods.

Can you provide examples of using ViewTreeObserver in Android?