Android gallery 3D效果

        在看了iOS上面的CoverFlow后，感觉效果真的不错，就想在android上面实现一个，这个程序在网上参考了一此核心的代码，当然我添加了一些其他的东西，废话不多说，先看效果，不然就是无图无真相。

        Demo下载地址：GalleryFlow

其实实现这个效果很简单，下面作一个简单的介绍

一，创建倒影效果

这个基本思路是：

1，创建一个源图一样的图，利用martrix将图片旋转180度。这个倒影图的高是源图的一半。

 Matrix matrix = new Matrix();

 // 1表示放大比例，不放大也不缩小。
 // -1表示在y轴上相反，即旋转180度。
 matrix.preScale(1, -1);

 Bitmap reflectionBitmap = Bitmap.createBitmap(
     srcBitmap,
     0,
     srcBitmap.getHeight() / 2,  // top为源图的一半
     srcBitmap.getWidth(),       // 宽度与源图一样
     srcBitmap.getHeight() / 2,  // 高度与源图的一半
     matrix,
     false);

2，创建一个最终效果的图，即源图 + 间隙 + 倒影。

 final int REFLECTION_GAP = 5;

 Bitmap bitmapWithReflection = Bitmap.createBitmap(
        reflectionWidth,
        srcHeight + reflectionHeight + REFLECTION_GAP,
        Config.ARGB_8888);

3，依次将源图、倒影图绘制在最终的bitmap上面。

 // Prepare the canvas to draw stuff.
 Canvas canvas = new Canvas(bitmapWithReflection);

 // Draw the original bitmap.
 canvas.drawBitmap(srcBitmap, 0, 0, null);

 // Draw the reflection bitmap.
 canvas.drawBitmap(reflectionBitmap, 0, srcHeight + REFLECTION_GAP, null);

4，创建LinearGradient，从而给定一个由上到下的渐变色。

 Paint paint = new Paint();
 paint.setAntiAlias(true);
 LinearGradient shader = new LinearGradient(
         0,
         srcHeight,
         0,
         bitmapWithReflection.getHeight() + REFLECTION_GAP,
         0x70FFFFFF,
         0x00FFFFFF,
         TileMode.MIRROR);
 paint.setShader(shader);
 paint.setXfermode(new PorterDuffXfermode(android.graphics.PorterDuff.Mode.DST_IN));

 // Draw the linear shader.
 canvas.drawRect(
         0,
         srcHeight,
         srcWidth,
         bitmapWithReflection.getHeight() + REFLECTION_GAP,
         paint);

二，扩展Gallery

扩展系统的gallery，我们需要重写两个方法，getChildStaticTransformation()和getChildDrawingOrder()，同时，要使这两个方法能被调用，必须执行如下两行代码，文档上面是有说明的。

 // Enable set transformation.
 this.setStaticTransformationsEnabled(true);
 // Enable set the children drawing order.
 this.setChildrenDrawingOrderEnabled(true);

• getChildDrawingOrder的实现

 @Override
 protected int getChildDrawingOrder(int childCount, int i)
 {
     // Current selected index.
     int selectedIndex = getSelectedItemPosition() - getFirstVisiblePosition();
     if (selectedIndex < 0)
     {
         return i;
     }

     if (i < selectedIndex)
     {
         return i;
     }
     else if (i >= selectedIndex)
     {
         return childCount - 1 - i + selectedIndex;
     }
     else
     {
         return i;
     }
 }

这里为什么要计算drawing order，因为从上图中看到，我们的效果是：中间左边的顺序是 0， 1， 2，右边的child覆盖左边的child，而在中间右边的顺序正好相反，左边的覆盖右边的，所以我们要重写这个方法，而gallery自身的实现，不是这种效果。

• getChildStaticTransformation的实现

 @Override
 protected boolean getChildStaticTransformation(View child, Transformation t)
 {
     super.getChildStaticTransformation(child, t);

     final int childCenter = getCenterOfView(child);
     final int childWidth  = child.getWidth();

     int rotationAngle = 0;
     t.clear();
     t.setTransformationType(Transformation.TYPE_MATRIX);

     // If the child is in the center, we do not rotate it.
     if (childCenter == mCoveflowCenter)
     {
         transformImageBitmap(child, t, 0);
     }
     else
     {
         // Calculate the rotation angle.
         rotationAngle = (int)(((float)(mCoveflowCenter - childCenter) / childWidth) * mMaxRotationAngle);

         // Make the angle is not bigger than maximum.
         if (Math.abs(rotationAngle) > mMaxRotationAngle)
         {
             rotationAngle = (rotationAngle < 0) ? -mMaxRotationAngle : mMaxRotationAngle;
         }

         transformImageBitmap(child, t, rotationAngle);
     }

     return true;
 }

这个方法就是根据child来计算它的transformation（变换），我们需要去修改它里面的matrix，从而达到旋转的效果。根据位置和角度来计算的matrix的方法写在另外一个方法transformImageBitmap中实现。

• transformImageBitmap()的实现

 private void transformImageBitmap(View child, Transformation t, int rotationAngle)
 {
     mCamera.save();

     final Matrix imageMatrix = t.getMatrix();
     final int imageHeight = child.getHeight();
     final int imageWidth  = child.getWidth();
     final int rotation    = Math.abs(rotationAngle);

     // Zoom on Z axis.
     mCamera.translate(0, 0, mMaxZoom);

     if (rotation < mMaxRotationAngle)
     {
         float zoomAmount = (float)(mMaxZoom + rotation * 1.5f);
         mCamera.translate(0, 0, zoomAmount);
     }

     // Rotate the camera on Y axis.
     mCamera.rotateY(rotationAngle);
     // Get the matrix from the camera, in fact, the matrix is S (scale) transformation.
     mCamera.getMatrix(imageMatrix);

     // The matrix final is T2 * S * T1, first translate the center point to (0, 0),
     // then scale, and then translate the center point to its original point.
     // T * S * T

     // S * T1
     imageMatrix.postTranslate((imageWidth / 2), (imageHeight / 2));
     // (T2 * S) * T1
     imageMatrix.preTranslate(-(imageWidth / 2), -(imageHeight / 2));

     mCamera.restore();
 }

这里，简单说明一个，

        第一，先在Z轴上平称，其实就是得到一个缩放矩阵变换，我这里简写为 S。

        第二，是利用camera这个类来生成matrix，其实mCamera.rotateY就是围绕Y轴旋转。这里生成了一个旋转矩阵，记为 R 。经过这两步，此时调用mCamera.getMatrix(imageMatrix); 从Camera中得到matrix，此时这个矩阵中包含了S * R。

        第三，最关键是下面两句       

 // S * T1
  imageMatrix.postTranslate((imageWidth / 2), (imageHeight / 2));
  // (T2 * S) * T1
  imageMatrix.preTranslate(-(imageWidth / 2), -(imageHeight / 2));

        由于这里涉及到旋转与缩放，缩放操作其实应该是针对Child中点进行了，这里就是作一个平衡操作，我们必须是先平移，再缩放，再平移回原来位置，所以，我们最终的矩阵变换应该是这样的：

        M = T * （S * R） * T1   （这里在T1表示与T相反）。

三，完整代码

GalleryFlow.java

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

 public class GalleryFlow extends Gallery
 {
     /**
      * The camera class is used to 3D transformation matrix.
      */
     private Camera mCamera = new Camera();

     /**
      * The max rotation angle.
      */
     private int mMaxRotationAngle = 60;

     /**
      * The max zoom value (Z axis).
      */
     private int mMaxZoom = -120;

     /**
      * The center of the gallery.
      */
     private int mCoveflowCenter = 0;

     public GalleryFlow(Context context)
     {
         this(context, null);
     }

     public GalleryFlow(Context context, AttributeSet attrs)
     {
         this(context, attrs, 0);
     }

     public GalleryFlow(Context context, AttributeSet attrs, int defStyle)
     {
         super(context, attrs, defStyle);

         // Enable set transformation.
         this.setStaticTransformationsEnabled(true);
         // Enable set the children drawing order.
         this.setChildrenDrawingOrderEnabled(true);
     }

     public int getMaxRotationAngle()
     {
         return mMaxRotationAngle;
     }

     public void setMaxRotationAngle(int maxRotationAngle)
     {
         mMaxRotationAngle = maxRotationAngle;
     }

     public int getMaxZoom()
     {
         return mMaxZoom;
     }

     public void setMaxZoom(int maxZoom)
     {
         mMaxZoom = maxZoom;
     }

     @Override
     protected int getChildDrawingOrder(int childCount, int i)
     {
         // Current selected index.
         int selectedIndex = getSelectedItemPosition() - getFirstVisiblePosition();
         if (selectedIndex < 0)
         {
             return i;
         }

         if (i < selectedIndex)
         {
             return i;
         }
         else if (i >= selectedIndex)
         {
             return childCount - 1 - i + selectedIndex;
         }
         else
         {
             return i;
         }
     }

     @Override
     protected void onSizeChanged(int w, int h, int oldw, int oldh)
     {
         mCoveflowCenter = getCenterOfCoverflow();
         super.onSizeChanged(w, h, oldw, oldh);
     }

     private int getCenterOfView(View view)
     {
         return view.getLeft() + view.getWidth() / 2;
     }

     @Override
     protected boolean getChildStaticTransformation(View child, Transformation t)
     {
         super.getChildStaticTransformation(child, t);

         final int childCenter = getCenterOfView(child);
         final int childWidth  = child.getWidth();

         int rotationAngle = 0;
         t.clear();
         t.setTransformationType(Transformation.TYPE_MATRIX);

         // If the child is in the center, we do not rotate it.
         if (childCenter == mCoveflowCenter)
         {
             transformImageBitmap(child, t, 0);
         }
         else
         {
             // Calculate the rotation angle.
             rotationAngle = (int)(((float)(mCoveflowCenter - childCenter) / childWidth) * mMaxRotationAngle);

             // Make the angle is not bigger than maximum.
             if (Math.abs(rotationAngle) > mMaxRotationAngle)
             {
                 rotationAngle = (rotationAngle < 0) ? -mMaxRotationAngle : mMaxRotationAngle;
             }

             transformImageBitmap(child, t, rotationAngle);
         }

         return true;
     }

     private int getCenterOfCoverflow()
     {
         return (getWidth() - getPaddingLeft() - getPaddingRight()) / 2 + getPaddingLeft();
     }

     private void transformImageBitmap(View child, Transformation t, int rotationAngle)
     {
         mCamera.save();

         final Matrix imageMatrix = t.getMatrix();
         final int imageHeight = child.getHeight();
         final int imageWidth  = child.getWidth();
         final int rotation    = Math.abs(rotationAngle);

         // Zoom on Z axis.
         mCamera.translate(0, 0, mMaxZoom);

         if (rotation < mMaxRotationAngle)
         {
             float zoomAmount = (float)(mMaxZoom + rotation * 1.5f);
             mCamera.translate(0, 0, zoomAmount);
         }

         // Rotate the camera on Y axis.
         mCamera.rotateY(rotationAngle);
         // Get the matrix from the camera, in fact, the matrix is S (scale) transformation.
         mCamera.getMatrix(imageMatrix);

         // The matrix final is T2 * S * T1, first translate the center point to (0, 0),
         // then scale, and then translate the center point to its original point.
         // T * S * T

         // S * T1
         imageMatrix.postTranslate((imageWidth / 2), (imageHeight / 2));
         // (T2 * S) * T1
         imageMatrix.preTranslate(-(imageWidth / 2), -(imageHeight / 2));

         mCamera.restore();
     }
 }

BitmapUtil.java

 package com.lee.gallery3d.utils;

 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.PixelFormat;
 import android.graphics.PorterDuffXfermode;
 import android.graphics.Shader.TileMode;
 import android.graphics.drawable.Drawable;

 public class BitmapUtil
 {
     public static Bitmap createReflectedBitmap(Bitmap srcBitmap)
     {
         if (null == srcBitmap)
         {
             return null;
         }

         // The gap between the reflection bitmap and original bitmap.
         final int REFLECTION_GAP = 4;

         int srcWidth  = srcBitmap.getWidth();
         int srcHeight = srcBitmap.getHeight();
         int reflectionWidth  = srcBitmap.getWidth();
         int reflectionHeight = srcBitmap.getHeight() / 2;

         if (0 == srcWidth || srcHeight == 0)
         {
             return null;
         }

         // The matrix
         Matrix matrix = new Matrix();
         matrix.preScale(1, -1);

         try
         {
             // The reflection bitmap, width is same with original's, height is half of original's.
             Bitmap reflectionBitmap = Bitmap.createBitmap(
                     srcBitmap,
                     0,
                     srcHeight / 2,
                     srcWidth,
                     srcHeight / 2,
                     matrix,
                     false);

             if (null == reflectionBitmap)
             {
                 return null;
             }

             // Create the bitmap which contains original and reflection bitmap.
             Bitmap bitmapWithReflection = Bitmap.createBitmap(
                     reflectionWidth,
                     srcHeight + reflectionHeight + REFLECTION_GAP,
                     Config.ARGB_8888);

             if (null == bitmapWithReflection)
             {
                 return null;
             }

             // Prepare the canvas to draw stuff.
             Canvas canvas = new Canvas(bitmapWithReflection);

             // Draw the original bitmap.
             canvas.drawBitmap(srcBitmap, 0, 0, null);

             // Draw the reflection bitmap.
             canvas.drawBitmap(reflectionBitmap, 0, srcHeight + REFLECTION_GAP, null);

             Paint paint = new Paint();
             paint.setAntiAlias(true);
             LinearGradient shader = new LinearGradient(
                     0,
                     srcHeight,
                     0,
                     bitmapWithReflection.getHeight() + REFLECTION_GAP,
                     0x70FFFFFF,
                     0x00FFFFFF,
                     TileMode.MIRROR);
             paint.setShader(shader);
             paint.setXfermode(new PorterDuffXfermode(android.graphics.PorterDuff.Mode.DST_IN));

             // Draw the linear shader.
             canvas.drawRect(
                     0,
                     srcHeight,
                     srcWidth,
                     bitmapWithReflection.getHeight() + REFLECTION_GAP,
                     paint);

             return bitmapWithReflection;
         }
         catch (Exception e)
         {
             e.printStackTrace();
         }

         return null;
     }
 }