An android animation that collapse or expand a View object.

DropDownAnimation

/**
 * 
 */
package us.liuyong.widget;

import android.view.View;
import android.view.ViewGroup.LayoutParams;
import android.view.animation.Animation;
import android.view.animation.Transformation;

/**
 * @author liuyong
 * 
 */
public class DropDownAnimation extends Animation {
    public static final int DOWN = 0;
    public static final int UP = 1;
    private int mMaxHeight;
    private View mView;
    private int mDirection;

    public DropDownAnimation(View view, int maxHeight, int direction) {
        mView = view;
        mMaxHeight = maxHeight;
        mDirection = direction;
    }

    @Override
    protected void applyTransformation(float interpolatedTime, Transformation t) {
        int newHeight;
        if (mDirection == DOWN) {
            newHeight = (int) (mMaxHeight * interpolatedTime);
        } else {
            newHeight = (int) (mMaxHeight * (1.0f - interpolatedTime));
        }
        LayoutParams params = mView.getLayoutParams();
        params.height = newHeight;
        mView.setLayoutParams(params);
    }
}

The usage is very simple:

DropDownAnimation anim = new DropDownAnimation(targetView, targetView
        .getMeasuredHeight(), DropDownAnimation.UP);
anim.setDuration(300);
targetView.startAnimation(anim);