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，废话不多话，先看效果，不然就是无图无真相了。
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; }}