How do you manage multi-touch events and touch interception in a custom ViewGroup?

Short answer

You control the touch path through dispatchTouchEvent, the container decides to intercept through onInterceptTouchEvent, while the View handles the actual touch in onTouchEvent.

Explanation

The user's touch begins at the top of the tree and goes down to the children through dispatchTouchEvent. A ViewGroup can inspect the touch and intercept it by returning true from onInterceptTouchEvent (such as the scroll motion in a ScrollView), which sends an ACTION_CANCEL event to the current child and directs the rest of the events to the container's own onTouchEvent. A child can also ask that the touch not be intercepted through requestDisallowInterceptTouchEvent(true).

Code example

override fun onInterceptTouchEvent(ev: MotionEvent): Boolean {
    return when (ev.actionMasked) {
        MotionEvent.ACTION_MOVE -> shouldInterceptScroll(ev) // Intercept scrolling
        else -> false
    }
}

override fun onTouchEvent(event: MotionEvent): Boolean {
    // Run the actual drag animation
    return gestureDetector.onTouchEvent(event)
}

Common mistakes

• Unintentionally returning true on ACTION_DOWN inside onInterceptTouchEvent, which prevents the children from receiving any click events at all.

Follow-up questions

• What are the three basic stages of drawing any View on the screen in Android?

Sources

• Android Developers — Manage touch events in a ViewGroup