Implement Pinch Zoom in OnTouchListener

Modify the last exercise of "Implement OnTouchListener to handle multi-touch event" to implement our own pinch detection.

package com.exercise.AndroidTouchPinchZoom;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.util.FloatMath;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;
import android.widget.TextView;

public class AndroidTouchPinchZoomActivity extends Activity {
 
 TextView myTouchEvent;
 ImageView myImageView;
 Bitmap bitmap;
 int bmpWidth, bmpHeight;
 
 //Touch event related variables
 int touchState;
 final int IDLE = 0;
 final int TOUCH = 1;
 final int PINCH = 2;
 float dist0, distCurrent;
 
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        myTouchEvent = (TextView)findViewById(R.id.touchevent);
        myImageView = (ImageView)findViewById(R.id.imageview);
        
        bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
        bmpWidth = bitmap.getWidth();
        bmpHeight = bitmap.getHeight();
        
        distCurrent = 1; //Dummy default distance
        dist0 = 1;   //Dummy default distance
        drawMatrix();
        
        myImageView.setOnTouchListener(MyOnTouchListener);
        touchState = IDLE;
    }
    
    private void drawMatrix(){
     float curScale = distCurrent/dist0;
     if (curScale < 0.1){
      curScale = 0.1f; 
     }
     
     Bitmap resizedBitmap;    
     int newHeight = (int) (bmpHeight * curScale);
     int newWidth = (int) (bmpWidth * curScale);
     resizedBitmap = Bitmap.createScaledBitmap(bitmap, newWidth, newHeight, false);
     myImageView.setImageBitmap(resizedBitmap); 
    }
    
    OnTouchListener MyOnTouchListener
    = new OnTouchListener(){

  @Override
  public boolean onTouch(View view, MotionEvent event) {
   // TODO Auto-generated method stub
   
   float distx, disty;
   
   switch(event.getAction() & MotionEvent.ACTION_MASK){
   case MotionEvent.ACTION_DOWN:
    //A pressed gesture has started, the motion contains the initial starting location.
    myTouchEvent.setText("ACTION_DOWN"); 
    touchState = TOUCH;
    break;
   case MotionEvent.ACTION_POINTER_DOWN:
    //A non-primary pointer has gone down.
    myTouchEvent.setText("ACTION_POINTER_DOWN"); 
    touchState = PINCH;
    
    //Get the distance when the second pointer touch
    distx = event.getX(0) - event.getX(1);
    disty = event.getY(0) - event.getY(1);
    dist0 = FloatMath.sqrt(distx * distx + disty * disty);

    break;
   case MotionEvent.ACTION_MOVE:
    //A change has happened during a press gesture (between ACTION_DOWN and ACTION_UP).
    myTouchEvent.setText("ACTION_MOVE");
    
    if(touchState == PINCH){      
     //Get the current distance
     distx = event.getX(0) - event.getX(1);
     disty = event.getY(0) - event.getY(1);
     distCurrent = FloatMath.sqrt(distx * distx + disty * disty);

     drawMatrix();
    }
    
    break;
   case MotionEvent.ACTION_UP:
    //A pressed gesture has finished.
    myTouchEvent.setText("ACTION_UP"); 
    touchState = IDLE;
    break;
   case MotionEvent.ACTION_POINTER_UP:
    //A non-primary pointer has gone up.
    myTouchEvent.setText("ACTION_POINTER_UP"); 
    touchState = TOUCH;
    break;
   }
   
   return true;
  }
     
    };
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />
    <TextView
        android:id="@+id/touchevent"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" />
    <ImageView
        android:id="@+id/imageview" 
        android:layout_gravity="center"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent" 
        android:scaleType="center" />

</LinearLayout>

Download the files.


Related article:
- Scale bitmap according to ScaleGestureDetector