Android自定义View实现飘动的叶子效果（三）

上一篇对自定义View及一些方法有所了解，下面做一个简单的叶子飘动的例子

主要技术点

1、添加背景图片canvas.drawBitmap()

2、Matrix动画类

3、Matrix添加到画布上

步骤

1、添加黄色背景颜色

public LeafView(Context context, AttributeSet attrs) {
  super(context, attrs);
  bgPaint = new Paint();
  bgPaint.setColor(mResources.getColor(R.color.bg_color));
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  width = w;
  height = h;
 }

 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  bgRect = new RectF(0, 0 , width, height);
  canvas.drawRect(bgRect, bgPaint);
}

2、添加背景图片

 public LeafView(Context context, AttributeSet attrs) {
  super(context, attrs);
  mResources = getResources();
  bgBitmap = ((BitmapDrawable) mResources.getDrawable(R.drawable.leaf_kuang, null)).getBitmap();
  bgPaint = new Paint();
  bgPaint.setColor(mResources.getColor(R.color.bg_color));
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  width = w;
  height = h;
  bgDestRect = new Rect(0, 0 , width, height);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  bgRect = new RectF(0, 0 , width, height);
  canvas.drawRect(bgRect, bgPaint);
  canvas.drawBitmap(bgBitmap, null, bgDestRect, null);
}

canvas.drawBitmap(Bitmap bitmap, Rect src, Rect dst, Paint paint) 添加图片到画布

Rect src：图片剪裁,null图片显示全屏，  RectF dst：图片在Canvas画布区域

3、添加叶子

public LeafView(Context context, AttributeSet attrs) {
  super(context, attrs);
  mResources = getResources();
  bgBitmap = ((BitmapDrawable) mResources.getDrawable(R.drawable.leaf_kuang, null)).getBitmap();
  leafBitmap = ((BitmapDrawable) mResources.getDrawable(R.drawable.leaf, null))).getBitmap();
  bgPaint = new Paint();
  bgPaint.setColor(mResources.getColor(R.color.bg_color));
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  width = w;
  height = h;
  bgDestRect = new Rect(0, 0 , width, height);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  bgRect = new RectF(0, 0 , width, height);
  //添加黄色背景
  canvas.drawRect(bgRect, bgPaint);
  //添加背景图片
  canvas.drawBitmap(bgBitmap, null, bgDestRect, null);
  //添加叶子
  canvas.drawBitmap(leafBitmap, new Matrix(), new Paint());
}

canvas.drawBitmap(Bitmap bitmap, Matrix matrix, Paint paint)添加一个带动画的bitmap到画布

这里matrix什么都没定义，所以默认如下显示，左上角叶子

4、给叶子添加Matrix动画

Matrix作用：

a、translate 平移

b、rotate 旋转

c、scale 缩放

d、skew 倾斜

这里要用到matrix.postTranslate(float x, float y)，以画布左上角为(0,0); xy为平移绝对值

public LeafView(Context context, AttributeSet attrs) {
  super(context, attrs);
  mResources = getResources();
  bgBitmap = ((BitmapDrawable) mResources.getDrawable(R.drawable.leaf_kuang, null)).getBitmap();
  leafBitmap = ((BitmapDrawable) mResources.getDrawable(R.drawable.leaf, null))).getBitmap();
  mLeafHeight = leafBitmap.getWidht();  

  bgPaint = new Paint();
  bgPaint.setColor(mResources.getColor(R.color.bg_color));
 }

 @Override
 protected void onSizeChanged(int w, int h, int oldw, int oldh) {
  super.onSizeChanged(w, h, oldw, oldh);
  width = w;
  height = h;
  bgDestRect = new Rect(0, 0 , width, height);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  super.onDraw(canvas);
  bgRect = new RectF(0, 0 , width, height);
  //添加黄色背景
  canvas.drawRect(bgRect, bgPaint);
  //添加背景图片
  canvas.drawBitmap(bgBitmap, null, bgDestRect, null);
  //添加叶子
  Matrix matrix = new Matrix();
  matrix.postTranslate(getMatriX(), (height-mLeadHeight)/2);
  canvas.drawBitmap(leafBitmap, new Matrix(), new Paint());
  //重复调用onDraw()
  postInvalidate();
 }
 long cycleTime = 5000; //叶子滑动一周的时间5秒
 long startTime = 0;  //叶子滑动开始时间
 private float getMatriX() {
  float betweenTime = startTime - System.currentTimeMillis();
  //周期结束再加一个cycleTime
  if(betweenTime < 0) {
   startTime = System.currentTimeMillis() + cycleTime;
   betweenTime = cycleTime;
  }
  //通过时间差计算出叶子的坐标
  float fraction = (float) betweenTime / cycleTime;
  float x = (int)(width * fraction);
  return x;
 }

好了，看到的效果就是如下，由于Matrix没有设置动画时间长度概念，所以通过时间差来计算出位移值的方式，来设置滑动快慢

完整代码不贴了，就把上面这段代码复制到一个LeafView extends View中，然后在里面定义几个全局变量就好了。

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