Android开发中利用ListView实现一个渐变式的下拉刷新动画

 private Bitmap initialBitmap; private float mCurrentProgress; private Bitmap scaledBitmap; public ibuRefreshFirstStepView(Context context, AttributeSet attrs,         int defStyle) {  super(context, attrs, defStyle);  init(context); } public ibuRefreshFirstStepView(Context context, AttributeSet attrs) {  super(context, attrs);  init(context); } public ibuRefreshFirstStepView(Context context) {  super(context);  init(context); } private void init(Context context) {  //这个就是那个火箭图片  initialBitmap = Bitmap.createBitmap(BitmapFactory.decodeResource(getResources(), R.drawable.img_huojian1)); }  @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {  //根据设置的宽度来计算高度 设置为符合第二阶段娃娃图片的宽高比例  setMeasuredDimension(measureWidth(widthMeasureSpec),measureWidth(widthMeasureSpec)*initialBitmap.getHeight()/initialBitmap.getWidth()); }  private int measureWidth(int widMeasureSpec){  int result = 0;  int size = MeasureSpec.getSize(widMeasureSpec);  int mode = MeasureSpec.getMode(widMeasureSpec);  if (mode == MeasureSpec.EXACTLY){   result = size;  }else{   result = initialBitmap.getWidth();   if (mode == MeasureSpec.AT_MOST){    result = Math.min(result,size);   }  }  return result;  }  @Override protected void onLayout(boolean changed, int left, int top, int right,   int bottom) {  super.onLayout(changed, left, top, right, bottom);  // 给火箭图片进行等比例的缩放  scaledBitmap = Bitmap.createScaledBitmap(initialBitmap,89,110, false); } @Override protected void onDraw(Canvas canvas) {  super.onDraw(canvas);  //这个方法是对画布进行缩放，从而达到椭圆形图片的缩放，第一个参数为宽度缩放比例，第二个参数为高度缩放比例，//  canvas.scale(mCurrentProgress, mCurrentProgress, measuredWidth/2, measuredHeight/2);  //将等比例缩放后的椭圆形画在画布上面  canvas.drawBitmap(scaledBitmap,90,dip2px(getContext(),80*mCurrentProgress),null); }  public static int dip2px(Context context, float dpValue) {  final float scale = context.getResources().getDisplayMetrics().density;  return (int) (dpValue * scale + 0.5f); }  public void setCurrentProgress(float currentProgress){  mCurrentProgress = currentProgress; }}