在安卓App项目中，经常会遇到点击一个View展开，再次点击收起。
如果选择用 View.VISIBLE 和 View.GONE的话，会显得很突兀，用户体验不流畅，于是我们就想要制作一个平滑展开和收起的View。

效果图如下：

平滑展开收起.gif

在这里，我写了一个工具类，来操作View的展开和收起的效果。
核心代码：

public class ViewAnimationUtils {
    public static void expand(View view, int maxHeight) {
        DropAnim.getInstance().animateOpen(view, maxHeight);
    }

    public static void collapse(View view) {
        DropAnim.getInstance().animateClose(view);
    }
}

其中，DropAnim是View平滑显示和收起的动画。
DropAnim

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.ValueAnimator;
import android.view.View;
import android.view.ViewGroup;


public class DropAnim {

    private static DropAnim dropAnim;

    public static DropAnim getInstance() {
        if (dropAnim == null) {
            dropAnim = new DropAnim();
        }
        return dropAnim;
    }

    public void animateOpen(View v,int mHiddenViewMeasuredHeight) {
        v.setVisibility(View.VISIBLE);
        ValueAnimator animator = createDropAnimator(v, 0,
                mHiddenViewMeasuredHeight);
        animator.start();
    }

    public void animateClose(final View view) {
        int origHeight = view.getHeight();
        ValueAnimator animator = createDropAnimator(view, origHeight, 0);
        animator.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationEnd(Animator animation) {
                view.setVisibility(View.GONE);
            }
        });
        animator.start();

        
    }

    public ValueAnimator createDropAnimator(final View v, int start, int end) {
        ValueAnimator animator = ValueAnimator.ofInt(start, end);
        animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {

            @Override
            public void onAnimationUpdate(ValueAnimator arg0) {
                int value = (int) arg0.getAnimatedValue();
                ViewGroup.LayoutParams layoutParams = v.getLayoutParams();
                layoutParams.height = value;
                v.setLayoutParams(layoutParams);

            }
        });
        return animator;
    }
}