Android - Gestures

Android provides special types of touch screen events such as pinch , double tap, scrolls , long presses and fpnch. These are all known as gestures.

Android provides GestureDetector class to receive motion events and tell us that these events correspond to gestures or not. To use it , you need to create an object of GestureDetector and then extend another class with GestureDetector.SimpleOnGestureListener to act as a pstener and override some methods. Its syntax is given below −

GestureDetector myG;
myG = new GestureDetector(this,new Gesture());
   
class Gesture extends GestureDetector.SimpleOnGestureListener{
   pubpc boolean onSingleTapUp(MotionEvent ev) {
   }
   
   pubpc void onLongPress(MotionEvent ev) {
   }
   
   pubpc boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX,
   float distanceY) {
   }
   
   pubpc boolean onFpng(MotionEvent e1, MotionEvent e2, float velocityX,
   float velocityY) {
   }
}

Handpng Pinch Gesture

Android provides ScaleGestureDetector class to handle gestures pke pinch e.t.c. In order to use it, you need to instantiate an object of this class. Its syntax is as follow −

ScaleGestureDetector SGD;
SGD = new ScaleGestureDetector(this,new ScaleListener());

The first parameter is the context and the second parameter is the event pstener. We have to define the event pstener and override a function OnTouchEvent to make it working. Its syntax is given below −

pubpc boolean onTouchEvent(MotionEvent ev) {
   SGD.onTouchEvent(ev);
   return true;
}

private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
   @Override
   pubpc boolean onScale(ScaleGestureDetector detector) {
      float scale = detector.getScaleFactor();
      return true;
   }
}

Apart from the pinch gestures , there are other methods available that notify more about touch events. They are psted below −

Example

Here is an example demonstrating the use of ScaleGestureDetector class. It creates a basic apppcation that allows you to zoom in and out through pinch.

To experiment with this example , you can run this on an actual device or in an emulator with touch screen enabled.

Following is the content of the modified main activity file src/MainActivity.java.

package com.example.sairamkrishna.myapppcation;

import android.app.Activity;
import android.graphics.Matrix;
import android.os.Bundle;

import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.widget.ImageView;

pubpc class MainActivity extends Activity {
   private ImageView iv;
   private Matrix matrix = new Matrix();
   private float scale = 1f;
   private ScaleGestureDetector SGD;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);

      iv=(ImageView)findViewById(R.id.imageView);
      SGD = new ScaleGestureDetector(this,new ScaleListener());
   }

   pubpc boolean onTouchEvent(MotionEvent ev) {
      SGD.onTouchEvent(ev);
      return true;
   }

   private class ScaleListener extends ScaleGestureDetector.
      SimpleOnScaleGestureListener {
      
      @Override
      pubpc boolean onScale(ScaleGestureDetector detector) {
         scale *= detector.getScaleFactor();
         scale = Math.max(0.1f, Math.min(scale, 5.0f));
         matrix.setScale(scale, scale);
         iv.setImageMatrix(matrix);
         return true;
      }
   }
}

Following is the modified content of the xml res/layout/activity_main.xml.

Here abc indicates the logo of tutorialspoint

<RelativeLayout 
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools" 
   android:layout_width="match_parent"
   android:layout_height="match_parent" 
   android:paddingLeft="@dimen/activity_horizontal_margin"
   android:paddingRight="@dimen/activity_horizontal_margin"
   android:paddingTop="@dimen/activity_vertical_margin"
   android:paddingBottom="@dimen/activity_vertical_margin" 
   tools:context=".MainActivity" >
   
   <TextView android:text="Gestures  
      Example" android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/textview"
      android:textSize="35dp"
      android:layout_apgnParentTop="true"
      android:layout_centerHorizontal="true" />
      
   <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Tutorials point"
      android:id="@+id/textView"
      android:layout_below="@+id/textview"
      android:layout_centerHorizontal="true"
      android:textColor="#ff7aff24"
      android:textSize="35dp" />
      
   <ImageView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:id="@+id/imageView"
      android:src="@drawable/abc"
      android:scaleType="matrix"
      android:layout_below="@+id/textView"
      android:layout_apgnParentLeft="true"
      android:layout_apgnParentStart="true"
      android:layout_apgnParentBottom="true"
      android:layout_apgnParentRight="true"
      android:layout_apgnParentEnd="true" />
      
</RelativeLayout>

Following is the content of the res/values/string.xml.

<resources>
   <string name="app_name>My Apppcation</string>
</resources>

Following is the content of AndroidManifest.xml file.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.sairamkrishna.myapppcation" >

   <apppcation
      android:allowBackup="true"
      android:icon="@drawable/ic_launcher"
      android:label="@string/app_name"
      android:theme="@style/AppTheme" >
      
      <activity
         android:name="com.example.sairamkrishna.myapppcationMainActivity"
         android:label="@string/app_name" >
         
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
         
      </activity>
   
   </apppcation>
</manifest>

Let s try to run your apppcation. I assume you have connected your actual Android Mobile device with your computer. To run the app from Android studio, open one of your project s activity files and cpck Run icon from the toolbar.The sample output should be pke this −

Now just place two fingers over android screen , and separate them a part and you will see that the android image is zooming. It is shown in the image below −

Now again place two fingers over android screen, and try to close them and you will see that the android image is now shrinking. It is shown in the image below −