下弹动画的实现

下弹动画在很多应用都有使用，比如豌豆荚中的应用介绍界面，百度手机助手的应用介绍界面等。

只要熟悉android动画的使用接口，制作动画并不困难。 这里使用开源库nineoldandroids,其实和android3.0 以上直接使用动画接口是一样的。

实现效果

具体可以看youku动画：http://v.youku.com/v_show/id_XNjYyODgzNjQ4.html

PS, 搞了半天GIF 才能播放。原来是最大边不能太大了。原来图片尺寸太大，被默认转成jpg了。现在总算能看到效果了。

DropDownExample.java

package com.buptfarmer.devapp;

import com.nineoldandroids.animation.Animator;
import com.nineoldandroids.animation.AnimatorListenerAdapter;
import com.nineoldandroids.animation.ValueAnimator;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;

public class DropDownExample extends Activity implements OnClickListener {
    private View mHolder;
    private View mHolder2;
//    private static final int DURATION = 2000;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        initView();
    }

    private void initView() {
        setContentView(R.layout.drop_down_example);
        mHolder = findViewById(R.id.holder);
        mHolder2 = findViewById(R.id.holder2);
        mHolder.setOnClickListener(this);
        mHolder2.setOnClickListener(this);
    }

    public static ValueAnimator createHeightAnimator(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);
            }
        });
//        animator.setDuration(DURATION);
        return animator;
    }

    public static void animateExpanding(final View view) {
        view.setVisibility(View.VISIBLE);

        final int widthSpec = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);
        final int heightSpec = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED);
        view.measure(widthSpec, heightSpec);

        ValueAnimator animator = createHeightAnimator(view, 0, view.getMeasuredHeight());
        animator.start();
    }

    public static void animateCollapsing(final View view) {
        int origHeight = view.getHeight();

        ValueAnimator animator = createHeightAnimator(view, origHeight, 0);
        animator.addListener(new AnimatorListenerAdapter() {
            public void onAnimationEnd(Animator animation) {
                view.setVisibility(View.GONE);
            };
        });
        animator.start();
    }

    @Override
    public void onClick(View v) {
        if (v == mHolder) {
            if (View.GONE == mHolder.findViewById(R.id.hiddenview).getVisibility()) {
                animateExpanding(mHolder.findViewById(R.id.hiddenview));
            } else {
                animateCollapsing(mHolder.findViewById(R.id.hiddenview));
            }
        } else if (v == mHolder2) {
            if (View.GONE == mHolder2.findViewById(R.id.hiddenview).getVisibility()) {
                animateExpanding(mHolder2.findViewById(R.id.hiddenview));
            } else {
                animateCollapsing(mHolder2.findViewById(R.id.hiddenview));
            }
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:id="@+id/holder"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@color/greenyellow"
        android:orientation="vertical" >

        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:gravity="center_vertical"
            android:orientation="horizontal"
            android:paddingLeft="1dp" >

            <ImageView
                android:id="@+id/app_icon"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:contentDescription="picture"
                android:minWidth="32dp"
                android:src="@drawable/ic_launcher" />

            <TextView
                android:id="@+id/app_label"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="5dp"
                android:ellipsize="end"
                android:gravity="left"
                android:singleLine="true"
                android:text="what‘s your dream?"
                android:textSize="13sp" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/hiddenview"
            android:layout_width="fill_parent"
            android:layout_height="40dp"
            android:gravity="center_vertical"
            android:orientation="horizontal"
            android:paddingLeft="1dp"
            android:visibility="gone" >

            <TextView
                android:id="@+id/hidden_text"
                android:layout_width="wrap_content"
                android:layout_height="fill_parent"
                android:text="过个好年，马上有房" />
        </LinearLayout>
    </LinearLayout>

    <LinearLayout
        android:id="@+id/holder2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@color/blueviolet"
        android:orientation="vertical" >

        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:gravity="center_vertical"
            android:orientation="horizontal"
            android:paddingLeft="1dp" >

            <ImageView
                android:id="@+id/app_icon"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:contentDescription="picture"
                android:minWidth="32dp"
                android:src="@drawable/ic_launcher" />

            <TextView
                android:id="@+id/app_label"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_marginLeft="5dp"
                android:ellipsize="end"
                android:gravity="left"
                android:singleLine="true"
                android:text="Self introduction"
                android:textSize="13sp" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/hiddenview"
            android:layout_width="fill_parent"
            android:layout_height="40dp"
            android:gravity="center_vertical"
            android:orientation="horizontal"
            android:paddingLeft="1dp"
            android:visibility="gone" >

            <ImageView 
                android:contentDescription="avatar"
                android:src="@drawable/avatar"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"/>
            <TextView
                android:id="@+id/hidden_text"
                android:layout_width="wrap_content"
                android:layout_height="fill_parent"
                android:text="乐山好事，积极乐观" />
        </LinearLayout>
    </LinearLayout>

</LinearLayout>