Android(安卓)Bitmap移动游戏背景

本文内容摘自《疯狂Android 讲义第3版》
package shortcut.song.com.myapplication;import android.app.Activity;import android.content.Context;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.graphics.Canvas;import android.graphics.Matrix;import android.graphics.drawable.BitmapDrawable;import android.os.Handler;import android.os.Message;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.util.AttributeSet;import android.util.DisplayMetrics;import android.view.Display;import android.view.View;import android.view.WindowManager;import java.util.Timer;import java.util.TimerTask;public class MoveBack extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new MoveBackView(this, null));    }    class MoveBackView extends View{        //背景图的实际高度        final int BACK_HEIGHT = 1700;        //背景图片Bitmap        private Bitmap back;        private Bitmap plane;        //定义图片的宽高        final int WIDTH = 320;  //640         final int HEIGHT = 320; //880        private Matrix matrix = new Matrix();        private int startY = BACK_HEIGHT - HEIGHT;        public MoveBackView(Context context, AttributeSet set)        {            super(context, set);            back = BitmapFactory.decodeResource(context.getResources(), R.drawable.back_img);            //获取窗口管理器            WindowManager windowManager = getWindowManager();            Display display = windowManager.getDefaultDisplay();            DisplayMetrics displayMetrics = new DisplayMetrics();            display.getMetrics(displayMetrics);            //获得屏幕宽度            float screenWidth = displayMetrics.widthPixels;            //获得图片缩放比例            float scale = screenWidth / WIDTH;            matrix.setScale(scale, scale);            plane = BitmapFactory.decodeResource(context.getResources(), R.drawable.plane);            final Handler handler = new Handler(){                @Override                public void handleMessage(Message msg) {                    if (msg.what == 0x123)                    {                        if (startY <=  3)                        {                            startY = BACK_HEIGHT - HEIGHT;                        }                        else                        {                            startY -= 3;                        }                    }                    invalidate();                }            };            new Timer().schedule(new TimerTask() {                @Override                public void run() {                    handler.sendEmptyMessage(0x123);                }            }, 0, 100);        }        @Override        protected void onDraw(Canvas canvas) {            super.onDraw(canvas);            Bitmap bitmap2 = Bitmap.createBitmap(back, 0, startY, WIDTH, HEIGHT, matrix, false);            //绘制新位图            canvas.drawBitmap(bitmap2, 0, 0, null);            //绘制飞机            canvas.drawBitmap(plane, 320, 400, null);        }    }}
运行效果：

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