Custom View with User Interaction

In my previous exercise "Draw a bitmap on View", "Draw something on a Canvas" and even "Android FaceDetector", the application output is drawn on View in onDraw method in application starting once only; without any interaction with user. In this exercise, a custom View with user interaction will be implemented. When user touch on the screen and move, a circle will be drawn.



There is a customized MyView class, extends from View. The onTouchEvent(MotionEvent event) method is used to handle the user touch events; such as ACTION_MOVE, ACTION_DOWN and ACTION_UP. Then it calls invalidate() to force the view to be redraw, onDraw(Canvas canvas) will be called in turn.

AndroidViewUI.java

package com.exercise.AndroidViewUI;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class AndroidViewUI extends Activity {
 public class MyView extends View {

  private Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

  private float initX, initY, radius;
  private boolean drawing = false;

public MyView(Context context) {
 super(context);
 // TODO Auto-generated constructor stub
 init();
}

public MyView(Context context, AttributeSet attrs) {
 super(context, attrs);
 // TODO Auto-generated constructor stub
 init();
}

public MyView(Context context, AttributeSet attrs, int defStyle) {
 super(context, attrs, defStyle);
 // TODO Auto-generated constructor stub
 init();
}

private void init(){
 paint.setStyle(Paint.Style.STROKE);
 paint.setStrokeWidth(3);
 paint.setColor(Color.WHITE);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 // TODO Auto-generated method stub
 setMeasuredDimension(MeasureSpec.getSize(widthMeasureSpec),
   MeasureSpec.getSize(heightMeasureSpec));
}

@Override
protected void onDraw(Canvas canvas) {
 // TODO Auto-generated method stub
 if(drawing){
  canvas.drawCircle(initX, initY, radius, paint);
 }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
 // TODO Auto-generated method stub

 int action = event.getAction();
 if (action==MotionEvent.ACTION_MOVE){
  float x = event.getX();
  float y = event.getY();

  radius = (float) Math.sqrt(Math.pow(x-initX, 2) + Math.pow(y-initY, 2));

 }
 else if (action==MotionEvent.ACTION_DOWN){
  initX = event.getX();
  initY = event.getY();
  radius = 1;
  drawing = true;
 }
 else if (action==MotionEvent.ACTION_UP){
  drawing = false;
 }
 invalidate();
 return true;
}

}



/** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     //setContentView(R.layout.main);
  
     MyView myView = new MyView(this);
     setContentView(myView);
 }


}

Download the file.

Related Article: Instance two object from the same custom view class