Android改进版CoverFlow效果控件

　　最近研究了一下如何在Android上实现CoverFlow效果的控件，其实早在2010年，就有Neil Davies开发并开源出了这个控件，Neil大神的这篇博客地址http://www.inter-fuser.com/2010/02/android-coverflow-widget-v2.html。首先是阅读源码，弄明白核心思路后，自己重新写了一遍这个控件，并加入了详尽的注释以便日后查阅；而后在使用过程中，发现了有两点可以改进：（1）初始图片位于中间，左边空了一半空间，比较难看，可以改为重复滚动地展示、（2）由于图片一开始就需要加载出来，所以对内存开销较大，很容易OOM，需要对图片的内存空间进行压缩。

　　这个自定义控件包括4个部分，用于创建及提供图片对象的ImageAdapter，计算图片旋转角度等的自定义控件GalleryFlow，压缩采样率解析Bitmap的工具类BitmapScaleDownUtil，以及承载自定义控件的Gallery3DActivity。

　　首先是ImageAdapter，代码如下：

package pym.test.gallery3d.widget;

import pym.test.gallery3d.util.BitmapScaleDownUtil;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.LinearGradient;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.PaintFlagsDrawFilter;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;
import android.graphics.Shader.TileMode;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Gallery;
import android.widget.ImageView;

/**
 * @author pengyiming
 * @date 2013-9-30
 * @function GalleryFlow适配器
 */
public class ImageAdapter extends BaseAdapter
{
    /* 数据段begin */
    private final String TAG = "ImageAdapter";
    private Context mContext;
    
    //图片数组
    private int[] mImageIds ;
    //图片控件数组
    private ImageView[] mImages;
    //图片控件LayoutParams
    private GalleryFlow.LayoutParams mImagesLayoutParams;
    /* 数据段end */

    /* 函数段begin */
    public ImageAdapter(Context context, int[] imageIds)
    {
        mContext = context;
        mImageIds = imageIds;
        mImages = new ImageView[mImageIds.length];
        mImagesLayoutParams = new GalleryFlow.LayoutParams(Gallery.LayoutParams.WRAP_CONTENT, Gallery.LayoutParams.WRAP_CONTENT);
    }
    
    /**
     * @function 根据指定宽高创建待绘制的Bitmap，并绘制到ImageView控件上
     * @param imageWidth
     * @param imageHeight
     * @return void
     */
    public void createImages(int imageWidth, int imageHeight)
    {
        // 原图与倒影的间距5px
        final int gapHeight = 5;
        
        int index = 0;
        for (int imageId : mImageIds)
        {
            /* step1 采样方式解析原图并生成倒影 */
            // 解析原图，生成原图Bitmap对象
//            Bitmap originalImage = BitmapFactory.decodeResource(mContext.getResources(), imageId);
            Bitmap originalImage = BitmapScaleDownUtil.decodeSampledBitmapFromResource(mContext.getResources(), imageId, imageWidth, imageHeight);
            int width = originalImage.getWidth();
            int height = originalImage.getHeight();
            
            // Y轴方向反向，实质就是X轴翻转
            Matrix matrix = new Matrix();
            matrix.setScale(1, -1);
            // 且仅取原图下半部分创建倒影Bitmap对象
            Bitmap reflectionImage = Bitmap.createBitmap(originalImage, 0, height / 2, width, height / 2, matrix, false);
            
            /* step2 绘制 */
            // 创建一个可包含原图+间距+倒影的新图Bitmap对象
            Bitmap bitmapWithReflection = Bitmap.createBitmap(width, (height + gapHeight + height / 2), Config.ARGB_8888);
            // 在新图Bitmap对象之上创建画布
            Canvas canvas = new Canvas(bitmapWithReflection);
            // 抗锯齿效果
            canvas.setDrawFilter(new PaintFlagsDrawFilter(0, Paint.ANTI_ALIAS_FLAG));
            // 绘制原图
            canvas.drawBitmap(originalImage, 0, 0, null);
            // 绘制间距
            Paint gapPaint = new Paint();
            gapPaint.setColor(0xFFCCCCCC);
            canvas.drawRect(0, height, width, height + gapHeight, gapPaint);
            // 绘制倒影
            canvas.drawBitmap(reflectionImage, 0, height + gapHeight, null);
            
            /* step3 渲染 */
            // 创建一个线性渐变的渲染器用于渲染倒影
            Paint paint = new Paint();
            LinearGradient shader = new LinearGradient(0, height, 0, (height + gapHeight + height / 2), 0x70ffffff, 0x00ffffff, TileMode.CLAMP);
            // 设置画笔渲染器
            paint.setShader(shader);
            // 设置图片混合模式
            paint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
            // 渲染倒影+间距
            canvas.drawRect(0, height, width, (height + gapHeight + height / 2), paint);
            
            /* step4 在ImageView控件上绘制 */
            ImageView imageView = new ImageView(mContext);
            imageView.setImageBitmap(bitmapWithReflection);
            imageView.setLayoutParams(mImagesLayoutParams);
            // 打log
            imageView.setTag(index);
            
            /* step5 释放heap */
            originalImage.recycle();
            reflectionImage.recycle();
//          bitmapWithReflection.recycle();
            
            mImages[index++] = imageView;
        }
    }

    @Override
    public int getCount()
    {
        return Integer.MAX_VALUE;
    }
    
    @Override
    public Object getItem(int position)
    {
        return mImages[position];
    }
    
    @Override
    public long getItemId(int position)
    {
        return position;
    }
    
    @Override
    public View getView(int position, View convertView, ViewGroup parent)
    {
        return mImages[position % mImages.length];
    }
    /* 函数段end */
}

　　其次是GalleryFlow，代码如下：

package pym.test.gallery3d.widget;

import android.content.Context;
import android.graphics.Camera;
import android.graphics.Matrix;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.animation.Transformation;
import android.widget.Gallery;

/**
 * @author pengyiming
 * @date 2013-9-30
 * @function 自定义控件
 */
public class GalleryFlow extends Gallery
{
    /* 数据段begin */
    private final String TAG = "GalleryFlow";
    
    // 边缘图片最大旋转角度
    private final float MAX_ROTATION_ANGLE = 75;
    // 中心图片最大前置距离
    private final float MAX_TRANSLATE_DISTANCE = -100;
    // GalleryFlow中心X坐标
    private int mGalleryFlowCenterX;
    // 3D变换Camera
    private Camera mCamera = new Camera();
    /* 数据段end */

    /* 函数段begin */
    public GalleryFlow(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        
        // 开启，在滑动过程中，回调getChildStaticTransformation()
        this.setStaticTransformationsEnabled(true);
    }
    
    /**
     * @function 获取GalleryFlow中心X坐标
     * @return
     */
    private int getCenterXOfCoverflow()
    {
        return (getWidth() - getPaddingLeft() - getPaddingRight()) / 2 + getPaddingLeft();
    }
    
    /**
     * @function 获取GalleryFlow子view的中心X坐标
     * @param childView
     * @return
     */
    private int getCenterXOfView(View childView)
    {
        return childView.getLeft() + childView.getWidth() / 2;
    }
    
    /**
     * @note step1 系统调用measure()方法时，回调此方法；表明此时系统正在计算view的大小
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        
        mGalleryFlowCenterX = getCenterXOfCoverflow();
        Log.d(TAG, "onMeasure, mGalleryFlowCenterX = " + mGalleryFlowCenterX);
    }
    
    /**
     * @note step2 系统调用layout()方法时，回调此方法；表明此时系统正在给child view分配空间
     * @note 必定在onMeasure()之后回调，但与onSizeChanged()先后顺序不一定
     */
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b)
    {
        super.onLayout(changed, l, t, r, b);
        
        mGalleryFlowCenterX = getCenterXOfCoverflow();
        Log.d(TAG, "onLayout, mGalleryFlowCenterX = " + mGalleryFlowCenterX);
    }
    
    /**
     * @note step2 系统调用measure()方法后，当需要绘制此view时，回调此方法；表明此时系统已计算完view的大小
     * @note 必定在onMeasure()之后回调，但与onSizeChanged()先后顺序不一定
     */
    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh)
    {
        super.onSizeChanged(w, h, oldw, oldh);
        
        mGalleryFlowCenterX = getCenterXOfCoverflow();
        Log.d(TAG, "onSizeChanged, mGalleryFlowCenterX = " + mGalleryFlowCenterX);
    }
    
    @Override
    protected boolean getChildStaticTransformation(View childView, Transformation t)
    {
        // 计算旋转角度
        float rotationAngle = calculateRotationAngle(childView);
        
        // 计算前置距离
        float translateDistance = calculateTranslateDistance(childView);
        
        // 开始3D变换
        transformChildView(childView, t, rotationAngle, translateDistance);
        
        return true;
    }
    
    /**
     * @function 计算GalleryFlow子view的旋转角度
     * @note1 位于Gallery中心的图片不旋转
     * @note2 位于Gallery中心两侧的图片按照离中心点的距离旋转
     * @param childView
     * @return
     */
    private float calculateRotationAngle(View childView)
    {
        final int childCenterX = getCenterXOfView(childView);
        float rotationAngle = 0;
        
        rotationAngle = (mGalleryFlowCenterX - childCenterX) / (float) mGalleryFlowCenterX * MAX_ROTATION_ANGLE;
        
        if (rotationAngle > MAX_ROTATION_ANGLE)
        {
            rotationAngle = MAX_ROTATION_ANGLE;
        }
        else if (rotationAngle < -MAX_ROTATION_ANGLE)
        {
            rotationAngle = -MAX_ROTATION_ANGLE;
        }
        
        return rotationAngle;
    }
    
    /**
     * @function 计算GalleryFlow子view的前置距离
     * @note1 位于Gallery中心的图片前置
     * @note2 位于Gallery中心两侧的图片不前置
     * @param childView
     * @return
     */
    private float calculateTranslateDistance(View childView)
    {
        final int childCenterX = getCenterXOfView(childView);
        float translateDistance = 0;
        
        if (mGalleryFlowCenterX == childCenterX)
        {
            translateDistance = MAX_TRANSLATE_DISTANCE;
        }
        
        return translateDistance;
    }
    
    /**
     * @function 开始变换GalleryFlow子view
     * @param childView
     * @param t
     * @param rotationAngle
     * @param translateDistance
     */
    private void transformChildView(View childView, Transformation t, float rotationAngle, float translateDistance)
    {
        t.clear();
        t.setTransformationType(Transformation.TYPE_MATRIX);
        
        final Matrix imageMatrix = t.getMatrix();
        final int imageWidth = childView.getWidth();
        final int imageHeight = childView.getHeight();
        
        mCamera.save();
        
        /* rotateY */
        // 在Y轴上旋转，位于中心的图片不旋转，中心两侧的图片竖向向里或向外翻转。
        mCamera.rotateY(rotationAngle);
        /* rotateY */
        
        /* translateZ */
        // 在Z轴上前置，位于中心的图片会有放大的效果
        mCamera.translate(0, 0, translateDistance);
        /* translateZ */
        
        // 开始变换（我的理解是：移动Camera，在2D视图上产生3D效果）
        mCamera.getMatrix(imageMatrix);
        imageMatrix.preTranslate(-imageWidth / 2, -imageHeight / 2);
        imageMatrix.postTranslate(imageWidth / 2, imageHeight / 2);
        
        mCamera.restore();
    }
    /* 函数段end */
}

　　Bitmap解析用具BitmapScaleDownUtil，代码如下：

package pym.test.gallery3d.util;

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Display;

/**
 * @author pengyiming
 * @date 2013-9-30
 * @function Bitmap缩放处理工具类
 */
public class BitmapScaleDownUtil
{
    /* 数据段begin */
    private final String TAG = "BitmapScaleDownUtil";
    /* 数据段end */

    /* 函数段begin */
    /**
     * @function 获取屏幕大小
     * @param display
     * @return 屏幕宽高
     */
    public static int[] getScreenDimension(Display display)
    {
        int[] dimension = new int[2];
        dimension[0] = display.getWidth();
        dimension[1] = display.getHeight();
        
        return dimension;
    }
    
    /**
     * @function 以取样方式加载Bitmap 
     * @param res
     * @param resId
     * @param reqWidth
     * @param reqHeight
     * @return 取样后的Bitmap
     */
    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight)
    {
        // step1，将inJustDecodeBounds置为true，以解析Bitmap真实尺寸
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);

        // step2，计算Bitmap取样比例
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

        // step3，将inJustDecodeBounds置为false，以取样比列解析Bitmap
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }

    /**
     * @function 计算Bitmap取样比例
     * @param options
     * @param reqWidth
     * @param reqHeight
     * @return 取样比例
     */
    private static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight)
    {
        // 默认取样比例为1:1
        int inSampleSize = 1;

        // Bitmap原始尺寸
        final int width = options.outWidth;
        final int height = options.outHeight;

        // 取最大取样比例
        if (height > reqHeight || width > reqWidth)
        {
            final int widthRatio = Math.round((float) width / (float) reqWidth);
            final int heightRatio = Math.round((float) height / (float) reqHeight);

            // 取样比例为X:1，其中X>=1
            inSampleSize = Math.max(widthRatio, heightRatio);
        }

        return inSampleSize;
    }
    /* 函数段end */
}

　　测试控件的Gallery3DActivity，代码如下：

package pym.test.gallery3d.main;

import pym.test.gallery3d.R;
import pym.test.gallery3d.util.BitmapScaleDownUtil;
import pym.test.gallery3d.widget.GalleryFlow;
import pym.test.gallery3d.widget.ImageAdapter;
import android.app.Activity;
import android.content.Context;
import android.os.Bundle;

/**
 * @author pengyiming
 * @date 2013-9-30
 */
public class Gallery3DActivity extends Activity
{
    /* 数据段begin */
    private final String TAG = "Gallery3DActivity";
    private Context mContext;
    
    // 图片缩放倍率（相对屏幕尺寸的缩小倍率）
    public static final int SCALE_FACTOR = 8;
    
    // 图片间距（控制各图片之间的距离）
    private final int GALLERY_SPACING = -10;
    
    // 控件
    private GalleryFlow mGalleryFlow;
    /* 数据段end */

    /* 函数段begin */
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        mContext = getApplicationContext();
        
        setContentView(R.layout.gallery_3d_activity_layout);
        initGallery();
    }
    
    private void initGallery()
    {
        // 图片ID
        int[] images = {
                R.drawable.picture_1,
                R.drawable.picture_2,
                R.drawable.picture_3,
                R.drawable.picture_4,
                R.drawable.picture_5,
                R.drawable.picture_6,
                R.drawable.picture_7 };

        ImageAdapter adapter = new ImageAdapter(mContext, images);
        // 计算图片的宽高
        int[] dimension = BitmapScaleDownUtil.getScreenDimension(getWindowManager().getDefaultDisplay());
        int imageWidth = dimension[0] / SCALE_FACTOR;
        int imageHeight = dimension[1] / SCALE_FACTOR;
        // 初始化图片
        adapter.createImages(imageWidth, imageHeight);

        // 设置Adapter，显示位置位于控件中间，这样使得左右均可"无限"滑动
        mGalleryFlow = (GalleryFlow) findViewById(R.id.gallery_flow);
        mGalleryFlow.setSpacing(GALLERY_SPACING);
        mGalleryFlow.setAdapter(adapter);
        mGalleryFlow.setSelection(Integer.MAX_VALUE / 2);
    }
    /* 函数段end */
}

　　see效果图~~~