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HackSeven Canvas上的动画
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当自定义View的时候，可以利用Canvas给View添加一些动画效果。
下面的例子，是在屏幕上绘制一个红色的小方块，这个小方块会在屏幕上面“乱跳”。
知识点
使用到的知识点：
(1) 在View的子类的draw()中调用invalidate()，可以让View对象一直保持重绘状态，从而可以使Canvas一直处于绘画过程中。
(2) Canvas的绘制功能，例如绘制Rect、Circle、Path等。
(3) 小方块碰撞屏幕边缘的算法。
实现
小方块视图。
继承View类，重写onDraw()方法，并提供一些setter和getter方法，用于设置小方块的属性。判断碰撞事件的逻辑在moveTo()方法中。代码如下：
public class Rectangle extends View {
public static final int MAX_SIZE = 40;
private static final int ALPHA = 255;
private int mCoordX = 0;
private int mCoordY = 0;
private int mRealSize = 40;
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;
}
}
外层视图。
小方块是一个独立的视图，这里不直接把小方块显示在Actiity中，在它的外面又“包”了一层。代码如下：
public class DrawView extends View {
private Rectangle mRectangle;
public int width;
public int height;
public DrawView(Context context) {
super(context);
mRectangle = new Rectangle(context, this);
mRectangle.setARGB(255, 255, 0, 0);
mRectangle.setSpeedX(3);
mRectangle.setSpeedY(3);
}
@SuppressLint("WrongCall")
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
invalidate();
mRectangle.move();
mRectangle.onDraw(canvas);
}
}
主界面。
获取屏幕的尺寸，并把相应的尺寸赋值给DrawView对象。最后，显示DrawView对象。代码如下：
public class MainActivity extends Activity {
private DrawView mDrawView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Display display = getWindowManager().getDefaultDisplay();
mDrawView = new DrawView(this);
mDrawView.height = display.getHeight();
mDrawView.width = display.getWidth();
setContentView(mDrawView);
}
}
源码下载：
