android 开发技巧(6)--在 Canvas 上显示动画

Android 中的Canvas 可以在屏幕上绘图，定义是这样的：

“可以把 Canvas 视为 Surface 的替身或者接口，图形便是绘制
在 Surface 上的。 Canvas 封装了所有绘图调用。通过 Canvas，绘制
到 Surface 上的内容首先存储到与之关联的 Bitmap 中，该 Bitmap
最终会呈现到窗口上。”

Canvas 类封装了所有绘图调用，可以创建一个 View（视图），重写其 onDraw() 方法，在该方法中便可以绘制基本的图形单元

效果图

步骤
首先创建一个方块类

public class Rectangle extends View {  public static final int MAX_SIZE = 140;  private static final int ALPHA = 255;  private int mCoordX = 0;  private int mCoordY = 0;  private int mRealSize = 140;  private int mSpeedX = 3;  private int mSpeedY = 3;  private boolean goRight = true;  private boolean goDown = true;  private DrawView mDrawView;  private Paint mInnerPaint;  private RectF mDrawRect;  public Rectangle(Context context, DrawView drawView) {    super(context);    mDrawView = drawView;    mInnerPaint = new Paint();    mDrawRect = new RectF();    /* Red is default */    mInnerPaint.setARGB(ALPHA, 255, 0, 0);    mInnerPaint.setAntiAlias(true);  }  public void setARGB(int a, int r, int g, int b) {    mInnerPaint.setARGB(a, r, g, b);  }  public void setX(int newValue) {    mCoordX = newValue;  }  public float getX() {    return mCoordX;  }  public void setY(int newValue) {    mCoordY = newValue;  }  public float getY() {    return mCoordY;  }  public void move() {    moveTo(mSpeedX, mSpeedY);  }  private void moveTo(int goX, int goY) {    // check the borders, and set the direction if a border has reached    if (mCoordX > (mDrawView.width - MAX_SIZE)) {      goRight = false;    }    if (mCoordX < 0) {      goRight = true;    }    if (mCoordY > (mDrawView.height - MAX_SIZE)) {      goDown = false;    }    if (mCoordY < 0) {      goDown = true;    }    // move the x and y    if (goRight) {      mCoordX += goX;    } else {      mCoordX -= goX;    }    if (goDown) {      mCoordY += goY;    } else {      mCoordY -= goY;    }  }  public int getSpeedX() {    return mSpeedX;  }  public void setSpeedX(int speedX) {    mSpeedX = speedX;  }  public int getmSpeedY() {    return mSpeedY;  }  public void setSpeedY(int speedY) {    mSpeedY = speedY;  }  @Override  protected void onDraw(Canvas canvas) {    super.onDraw(canvas);    mDrawRect.set(mCoordX, mCoordY, mCoordX + mRealSize, mCoordY        + mRealSize);    canvas.drawRoundRect(mDrawRect, 0, 0, mInnerPaint);  }  public void setSize(int newSize) {    mRealSize = newSize;  }  public int getSize() {    return mRealSize;  }}

在主界面显示的内容视图

public class DrawView extends View {  private Rectangle mRectangle;  public int width;  public int height;  public DrawView(Context context) {    super(context);    //创建方块对象    mRectangle = new Rectangle(context, this);    mRectangle.setARGB(255, 255, 0, 0);    mRectangle.setSpeedX(10);    mRectangle.setSpeedY(10);  }  @Override  protected void onDraw(Canvas canvas) {    invalidate();//重绘View    mRectangle.move();//变换方块位置    mRectangle.onDraw(canvas);//将方块绘制到Canvas上  }}

主界面

public class Hack07Activity extends Activity {    private DrawView mDrawView;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Display display = getWindowManager().getDefaultDisplay();//获取屏幕的宽和高        mDrawView = new DrawView(this);        mDrawView.height = display.getHeight();        mDrawView.width = display.getWidth();        setContentView(mDrawView);    }}

一点说明：invalidate() 方法本身就是一个小技巧，这个方法强制重绘视图。把这个方法放在 onDraw() 的目的是为了在 View 绘制完自身后，可以立即重新调用 onDraw() 方法。换句话说，通过循环调用Rectangle 的 move() 和 onDraw() 方法实现一个动画效果。

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