AnimationUtils.java 文件源码

public static void expand(final View v) {
    v.measure(ActionBar.LayoutParams.MATCH_PARENT, ActionBar.LayoutParams.WRAP_CONTENT);
    final int targetHeight = v.getMeasuredHeight();

    // Older versions of android (pre API 21) cancel animations for views with a height of 0.
    v.getLayoutParams().height = 1;
    v.setVisibility(View.VISIBLE);
    Animation a = new Animation() {
        @Override
        protected void applyTransformation(float interpolatedTime, Transformation t) {
            v.getLayoutParams().height = interpolatedTime == 1
                    ? ActionBar.LayoutParams.WRAP_CONTENT
                    : (int) (targetHeight * interpolatedTime);
            v.requestLayout();
        }

        @Override
        public boolean willChangeBounds() {
            return true;
        }
    };

    // 1dp/ms
    a.setDuration((int) (targetHeight / v.getContext().getResources().getDisplayMetrics().density));
    v.startAnimation(a);
}