Gesture handling in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Detecting a Single Tap Gesture

What will be the output when the user performs a single tap on the view with the following code?

Android Kotlin

val gestureDetector = GestureDetector(context, object : GestureDetector.SimpleOnGestureListener() {
  override fun onDown(e: MotionEvent?): Boolean {
    return true
  }
  override fun onSingleTapConfirmed(e: MotionEvent?): Boolean {
    println("Single tap detected")
    return true
  }
})

view.setOnTouchListener { _, event ->
  gestureDetector.onTouchEvent(event)
  true
}

A Prints "Single tap detected" once per tap

B Throws a runtime exception

C Does not print anything on tap

D Prints "Single tap detected" multiple times per tap

💡 Hint

Consider how GestureDetector.SimpleOnGestureListener handles tap events.

The onSingleTapConfirmed method is called once per confirmed single tap, so the message prints once per tap.

❓ lifecycle

intermediate

GestureDetector and Activity Lifecycle

What happens if you create a GestureDetector inside onCreate but forget to set the OnTouchListener on the view?

A GestureDetector will detect gestures automatically without OnTouchListener

B GestureDetector will not receive touch events, so no gestures are detected

C App will crash with NullPointerException

D GestureDetector will detect gestures but with delay

💡 Hint

Think about how touch events reach GestureDetector.

GestureDetector needs touch events passed to it via OnTouchListener; without it, it won't detect gestures.

🔧 Debug

advanced

Why does onFling not trigger?

Given this code snippet, why might the onFling gesture not trigger as expected? val gestureDetector = GestureDetector(context, object : GestureDetector.SimpleOnGestureListener() { override fun onDown(e: MotionEvent?): Boolean { return true } override fun onFling(e1: MotionEvent?, e2: MotionEvent?, velocityX: Float, velocityY: Float): Boolean { println("Fling detected") return true } }) view.setOnTouchListener { _, event -> gestureDetector.onTouchEvent(event) true }

A GestureDetector must be recreated on every touch event

B onFling requires overriding onScroll as well

C The velocity thresholds for fling are not met because the swipe is too slow

D The view does not have focus

💡 Hint

Consider what conditions GestureDetector uses to recognize a fling.

onFling triggers only if the swipe velocity exceeds a certain threshold; slow swipes won't trigger it.

🧠 Conceptual

advanced

Difference Between onTouchEvent and OnTouchListener

Which statement correctly describes the difference between overriding View.onTouchEvent and setting an OnTouchListener?

A OnTouchListener is called before onTouchEvent and can consume the event to prevent onTouchEvent from running

B onTouchEvent is a method inside the View class to handle touch events; OnTouchListener is an interface to listen for touch events externally

C Both are the same and interchangeable

D onTouchEvent only handles multi-touch, OnTouchListener only handles single touch

💡 Hint

Think about event propagation and consumption in Android touch system.

OnTouchListener's onTouch method is called before View's onTouchEvent; returning true consumes the event and stops further processing.

❓ navigation

expert

Gesture Navigation Conflict Resolution

In an app with a horizontal ViewPager and a vertical ScrollView inside each page, what is the best way to handle gesture conflicts so both horizontal swipes and vertical scrolls work smoothly?

A Wrap ScrollView in a FrameLayout to block gestures

B Disable scrolling in ScrollView and handle all gestures in ViewPager

C Use GestureDetector only on ScrollView and ignore ViewPager gestures

D Override onInterceptTouchEvent in the parent ViewPager to detect vertical scrolls and avoid intercepting them

💡 Hint

Think about how parent views can decide to intercept touch events.

Overriding onInterceptTouchEvent allows the ViewPager to decide when to intercept horizontal swipes and let vertical scrolls pass to ScrollView.