Android实现在屏幕上移动图片的方法

package net.obviam.droidz.model.components;
public class Speed {
  public static final int DIRECTION_RIGHT = 1;
  public static final int DIRECTION_LEFT = -1;
  public static final int DIRECTION_UP  = -1;
  public static final int DIRECTION_DOWN = 1;
  private float xv = 1;  // velocity value on the X axis
  private float yv = 1;  // velocity value on the Y axis
  private int xDirection = DIRECTION_RIGHT;
  private int yDirection = DIRECTION_DOWN;
  public Speed() {
    this.xv = 1;
    this.yv = 1;
  }
  public Speed(float xv, float yv) {
    this.xv = xv;
    this.yv = yv;
  }
  public float getXv() {
    return xv;
  }
  public void setXv(float xv) {
    this.xv = xv;
  }
  public float getYv() {
    return yv;
  }
  public void setYv(float yv) {
    this.yv = yv;
  }
  public int getxDirection() {
    return xDirection;
  }
  public void setxDirection(int xDirection) {
    this.xDirection = xDirection;
  }
  public int getyDirection() {
    return yDirection;
  }
  public void setyDirection(int yDirection) {
    this.yDirection = yDirection;
  }
  // changes the direction on the X axis
  public void toggleXDirection() {
    xDirection = xDirection * -1;
  }
  // changes the direction on the Y axis
  public void toggleYDirection() {
    yDirection = yDirection * -1;
  }
}

public void run() {
  Canvas canvas;
  Log.d(TAG, "Starting game loop");
  while (running) {
    canvas = null;
    // try locking the canvas for exclusive pixel editing
    // in the surface
    try {
      canvas = this.surfaceHolder.lockCanvas();
      synchronized (surfaceHolder) {
        // update game state
        this.gamePanel.update();
        // render state to the screen
        // draws the canvas on the panel
        this.gamePanel.render(canvas);
      }
    } finally {
      // in case of an exception the surface is not left in
      // an inconsistent state
      if (canvas != null) {
        surfaceHolder.unlockCanvasAndPost(canvas);
      }
    }  // end finally
  }
}
public void update() {
  // check collision with right wall if heading right
  if (droid.getSpeed().getxDirection() == Speed.DIRECTION_RIGHT
      && droid.getX() + droid.getBitmap().getWidth() / 2 >= getWidth()) {
    droid.getSpeed().toggleXDirection();
  }
  // check collision with left wall if heading left
  if (droid.getSpeed().getxDirection() == Speed.DIRECTION_LEFT
      && droid.getX() - droid.getBitmap().getWidth() / 2 <= 0) {
    droid.getSpeed().toggleXDirection();
  }
  // check collision with bottom wall if heading down
  if (droid.getSpeed().getyDirection() == Speed.DIRECTION_DOWN
      && droid.getY() + droid.getBitmap().getHeight() / 2 >= getHeight()) {
    droid.getSpeed().toggleYDirection();
  }
  // check collision with top wall if heading up
  if (droid.getSpeed().getyDirection() == Speed.DIRECTION_UP
      && droid.getY() - droid.getBitmap().getHeight() / 2 <= 0) {
    droid.getSpeed().toggleYDirection();
  }
  // Update the lone droid
  droid.update();
}