本例要实现的目的：

1.图片背景渐变的切换，例如渐变的从红色切换成绿色。

2.代码中进行图层叠加，即把多个Drawable叠加在一起显示在一个组件之上。

效果图：

代码很简单：

（1）布局文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:ignore="ContentDescription"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/color_iv"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:layout_centerHorizontal="true"
        android:src=http://www.mamicode.com/"@drawable/image_bg_2">（2）Activity代码：
package com.sinatj.colorgradientanim;

import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.LayerDrawable;
import android.graphics.drawable.TransitionDrawable;
import android.os.Build;
import android.os.Handler;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;

public class MainActivity extends ActionBarActivity {
    private ImageView imageView;
    private Drawable oldBackground = null;

    private Drawable bgDrawable;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageView = (ImageView) findViewById(R.id.color_iv);
        bgDrawable = getResources().getDrawable(R.drawable.image_bg_1);

        //初始颜色
        changeColor(Color.parseColor("#6696AA39"));
    }

    private void changeColor(int newColor) {
        Drawable colorDrawable = new ColorDrawable(newColor);
        //图层叠加
        LayerDrawable ld = new LayerDrawable(new Drawable[]{bgDrawable, colorDrawable});

        if (oldBackground == null) {
            imageView.setBackgroundDrawable(ld);
        } else {
            //渐变切换
            TransitionDrawable td = new TransitionDrawable(new Drawable[]{oldBackground, ld});
            imageView.setBackgroundDrawable(td);
            td.startTransition(300);
        }

        oldBackground = ld;
    }

    public void onColorClicked(View v) {
        int color = Color.parseColor(v.getTag().toString());
        changeColor(color);
    }
}



Android：图片背景渐变切换与图层叠加的实现