android仿微信表情雨下落效果的实现方法

public class ItemEmoje {
 //坐标
 public int x;
 public int y;
 // 横向偏移
 public int offsetX;
 //纵向偏移
 public int offsetY;
 //缩放
 public float scale;
 //图片资源
 public Bitmap bitmap;
}

public class RainView extends View {
 private Paint paint;
 //图片处理
 private Matrix matrix;
 private Random random;
 //判断是否运行的，默认没有
 private boolean isRun;
 //表情包集合
 private List<ItemEmoje> bitmapList;
 //表情图片
 private int imgResId = R.mipmap.dog;

 public RainView(Context context) {
 this(context, null);
 }

 public RainView(Context context, @Nullable AttributeSet attrs) {
 this(context, attrs, 0);
 }

 public RainView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
 super(context, attrs, defStyleAttr);
 init();
 }

 private void init() {
 paint = new Paint();
 paint.setAntiAlias(true);
 paint.setFilterBitmap(true);
 paint.setDither(true);
 matrix = new Matrix();
 random = new Random();
 bitmapList = new ArrayList<>();
 }
}

private void initData() {
 for (int i = 0; i < 20; i++) {
 ItemEmoje itemEmoje = new ItemEmoje();
 itemEmoje.bitmap = BitmapFactory.decodeResource(getResources(), imgResId);
 //起始横坐标在[100,getWidth()-100) 之间
 itemEmoje.x = random.nextInt(getWidth() - 200) + 100;
 //起始纵坐标在(-getHeight(),0] 之间，即一开始位于屏幕上方以外
 itemEmoje.y = -random.nextInt(getHeight());
 //横向偏移[-2,2) ，即左右摇摆区间
 itemEmoje.offsetX = random.nextInt(4) - 2;
 //纵向固定下落12
 itemEmoje.offsetY = 12;
 //缩放比例[0.8,1.2) 之间
 itemEmoje.scale = (float) (random.nextInt(40) + 80) / 100f;
 bitmapList.add(itemEmoje);
 }
}

@Override
protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);
 if (isRun) {
 //用于判断表情下落结束，结束即不再进行重绘
 boolean isInScreen = false;
 for (int i = 0; i < bitmapList.size(); i++) {
  matrix.reset();
  //缩放
  matrix.setScale(bitmapList.get(i).scale, bitmapList.get(i).scale);
  //下落过程坐标
  bitmapList.get(i).x = bitmapList.get(i).x + bitmapList.get(i).offsetX;
  bitmapList.get(i).y = bitmapList.get(i).y + bitmapList.get(i).offsetY;
  if (bitmapList.get(i).y <= getHeight()) {//当表情仍在视图内，则继续重绘
  isInScreen = true;
  }
  //位移
  matrix.postTranslate(bitmapList.get(i).x, bitmapList.get(i).y);
  canvas.drawBitmap(bitmapList.get(i).bitmap, matrix, paint);
 }
 if (isInScreen) {
  postInvalidate();
 }else {
  release();
 }
 }
}

/**
 *释放资源
 */
private void release(){
 if(bitmapList != null && bitmapList.size()>0){
 for(ItemEmoje itemEmoje : bitmapList){
  if(!itemEmoje.bitmap.isRecycled()){
  itemEmoje.bitmap.recycle();
  }
 }
 bitmapList.clear();
 }
}

public void start(boolean isRun) {
 this.isRun = isRun;
 initData();
 postInvalidate();
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent">

 <com.rain.RainView
 android:id="@+id/testView"
 android:layout_width="match_parent"
 android:layout_height="match_parent" />

 <Button
 android:id="@+id/btn_dog"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignParentBottom="true"
 android:text="真·狗头雨·落！" />

 <Button
 android:id="@+id/btn_cake"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignParentBottom="true"
 android:layout_marginLeft="10dp"
 android:layout_toRightOf="@+id/btn_dog"
 android:text="蛋糕雨" />

</RelativeLayout>

btnCake.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 //蛋糕图片
 rainView.setImgResId(R.mipmap.cake);
 rainView.start(true);
 }
});
btnDog.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 //狗头图片
 rainView.setImgResId(R.mipmap.dog);
 rainView.start(true);
 }
});