Building Android Apps with Animation in Android Studio

Create Project

This app implements the animation that looks like the ball is rolling. First, create a new project. 

• Start the Android Studio and click [Start a new Android Studio project]
  

• Type "Ball" to [Application name], and click [Next]
  
  
• Click [Next]
  

  
• Select the "Blank Activity", and click [Next]
  

  
• Click [Finish]
  

  

Create the "Ball" Class

Then, create a class of ball rolling in the screen. "Ball" has radius and coordinates that indicate the position in the screen as member variables. In addition, "Ball" also has method to draw itself to the screen.

• Click [1: Project], [app] right-click, [New] and click [Class]
  
  
• Click [Choose By Neighbor Class] tab, and to select [MainActivity], and click [OK]
  

  
• Enter "Ball" to [Name:], and click [OK]
  

  
• Rewrite the "Ball.java" to the source code of the following

package com.example.ball;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.View;

public class Ball extends View {
    int x, y, radius;
    Paint paint;

    public Ball(Context context) {
        super(context);
        radius = 30;
        x = y = 0;
        paint = new Paint();
        paint.setColor(Color.WHITE);
        paint.setStyle(Paint.Style.FILL);
    }
    protected void onDraw(Canvas canvas){
        super.onDraw(canvas);
        canvas.drawCircle(x, y, radius, paint);
    }
}

Implementation of Animation

To achieve the animation will implement the Runnable interface to MainActivity.

• Rewrite the "MainActivity.java" to the source code of the following

package com.example.ball;

import android.app.Activity;
import android.graphics.Color;
import android.graphics.Point;
import android.os.Bundle;
import android.os.Handler;
import android.view.Display;
import android.view.Window;
import android.view.WindowManager;
import android.widget.RelativeLayout;

public class MainActivity extends Activity implements Runnable {
    Ball ball;
    Handler handler;
    int width, height;
    int dx = 10, dy = 10, time = 10;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getWindow().addFlags(
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        RelativeLayout relativeLayout = new RelativeLayout(this);
        relativeLayout.setBackgroundColor(Color.GREEN);
        setContentView(relativeLayout);

        handler = new Handler();
        handler.postDelayed(this, time);

        WindowManager windowManager = 
                (WindowManager)getSystemService(WINDOW_SERVICE);
        Display display = windowManager.getDefaultDisplay();
        Point point = new Point();
        display.getSize(point);
        width = point.x;
        height = point.y;
        ball = new Ball(this);
        ball.x = width / 2;
        ball.y = height / 2;
        relativeLayout.addView(ball);
    }
    @Override
    public void run() {
        ball.x += dx;
        ball.y += dy;

        if (ball.x <= ball.radius) {
            ball.x = ball.radius;
            dx = -dx;
        } else if (ball.x >= width - ball.radius) {
            ball.x = width - ball.radius;
            dx = -dx;
        }

        if (ball.y <= ball.radius) {
            ball.y = ball.radius;
            dy = -dy;
        } else if (ball.y >= height - ball.radius) {
            ball.y = height - ball.radius;
            dy = -dy;
        }

        ball.invalidate();
        handler.postDelayed(this, time);
    }
    public void onDestroy() {
        super.onDestroy();
        handler.removeCallbacks(this);
    }
}

When you run the app, you can find an animation that looks like a golf ball rolling on lawn. Rebounding at the edge of the screen and keep rolling can also be confirmed.