Simple Moving Object with TouchEvents in Android

In this article, we will make an application through which a user can move an object visible on the screen according to his/her finger movements on the screen.  Touch to drag/move objects (like image, button) is very popular in games like Jigsaw Puzzles. Here we will create a simple way to touch and drag an image on a device screen after the user finishes this action, the image will have a new location on the device screen by using view onTouchListener. A sample GIF is given below to get an idea about what we are going to do in this article. Note that we are going to implement this project using the Kotlin language. 

Step by Step Implementation

Step 1: Create a New Project

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Kotlin as the programming language.

Step 2: Set any image object in the layout

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. 

XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout
        android:id="@+id/main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <ImageView
            android:id="@+id/image"
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:contentDescription="moving object"
            android:src="@drawable/gfg" />

    </RelativeLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

Step 3: Working with the MainActivity.kt file

Go to the MainActivity.kt file and refer to the following code. Below is the code for the MainActivity.kt file. Comments are added inside the code to understand the code in more detail.

Kotlin

import android.annotation.SuppressLint
import android.os.Bundle
import android.view.MotionEvent
import android.view.View.OnTouchListener
import android.view.ViewGroup
import android.widget.ImageView
import android.widget.RelativeLayout
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {
    private lateinit var mainLayout: ViewGroup
    private lateinit var image: ImageView

    // default position of image
    private var xDelta = 0
    private var yDelta = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mainLayout = findViewById(R.id.main)
        image = findViewById(R.id.image)

        // returns True if the listener has
        // consumed the event, otherwise False.
        image.setOnTouchListener(onTouchListener())
    }

    @SuppressLint("ClickableViewAccessibility")
    private fun onTouchListener(): OnTouchListener {
        return OnTouchListener { view, event ->
            // position information
            // about the event by the user
            val x = event.rawX.toInt()
            val y = event.rawY.toInt()
            // detecting user actions on moving
            when (event.action and MotionEvent.ACTION_MASK) {
                MotionEvent.ACTION_DOWN -> {
                    val lParams = view.layoutParams as RelativeLayout.LayoutParams
                    xDelta = x - lParams.leftMargin
                    yDelta = y - lParams.topMargin
                }
                MotionEvent.ACTION_UP -> Toast.makeText(this,
                        "new location!", Toast.LENGTH_SHORT)
                        .show()
                MotionEvent.ACTION_MOVE -> {
                    // based on x and y coordinates (when moving image)
                    // and image is placed with it.
                    val layoutParams = view.layoutParams as RelativeLayout.LayoutParams
                    layoutParams.leftMargin = x - xDelta
                    layoutParams.topMargin = y - yDelta
                    layoutParams.rightMargin = 0
                    layoutParams.bottomMargin = 0
                    view.layoutParams = layoutParams
                }
            }
            // reflect the changes on screen
            mainLayout.invalidate()
            true
        }
    }
}

Now, run the app.

Output: