DropTest.java

package com.imooc.anim;

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.ValueAnimator;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;

public class DropTest extends Activity {

    private LinearLayout mHiddenView;
    private float mDensity;
    private int mHiddenViewMeasuredHeight;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.drop);
        mHiddenView = (LinearLayout) findViewById(R.id.hidden_view);
        // 获取像素密度
        mDensity = getResources().getDisplayMetrics().density;
        // 获取布局的高度
        mHiddenViewMeasuredHeight = (int) (mDensity * 40 + 0.5);
    }

    public void llClick(View view) {
        if (mHiddenView.getVisibility() == View.GONE) {
            // 打开动画
            animateOpen(mHiddenView);
        } else {
            // 关闭动画
            animateClose(mHiddenView);
        }
    }

    private void animateOpen(final View view) {
        view.setVisibility(View.VISIBLE);
        ValueAnimator animator = createDropAnimator(
                view,
                0,
                mHiddenViewMeasuredHeight);
        animator.start();
    }

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

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

            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                int value = (Integer) valueAnimator.getAnimatedValue();
                ViewGroup.LayoutParams layoutParams =
                        view.getLayoutParams();
                layoutParams.height = value;
                view.setLayoutParams(layoutParams);
            }
        });
        return animator;
    }
}