使用Android自带Gallery组件实现CoverFlow

源代码转载自国外Neil Davies，使用Apache2.0开源协议

实现了一个自定义的ImageAdapter，为图片制作倒影效果。
传入参数为context和程序内drawable中的图片ID数组。之后调用其中的createReflectedImages()方法分别创造每一个图像的倒影效果，生成对应的ImageView数组，最后在getView()中返回。

/*
* Copyright (C) 2010 Neil Davies
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
* 
* This code is base on the Android Gallery widget and was Created 
* by Neil Davies neild001 'at' gmail dot com to be a Coverflow widget
* 
* @author Neil Davies
*/
public class ImageAdapter extends BaseAdapter {
int mGalleryItemBackground;
private Context mContext;

private Integer[] mImageIds ;

private ImageView[] mImages;

public ImageAdapter(Context c, int[] ImageIds) {
mContext = c;
mImageIds = ImageIds;
mImages = new ImageView[mImageIds.length];
}

public boolean createReflectedImages() {
// The gap we want between the reflection and the original image
final int reflectionGap = 4;

int index = 0;
for (int imageId : mImageIds) {
Bitmap originalImage = BitmapFactory.decodeResource(
mContext.getResources(), imageId);
int width = originalImage.getWidth();
int height = originalImage.getHeight();

// This will not scale but will flip on the Y axis
Matrix matrix = new Matrix();
matrix.preScale(1, -1);

// Create a Bitmap with the flip matrix applied to it.
// We only want the bottom half of the image
Bitmap reflectionImage = Bitmap.createBitmap(originalImage, 0,
height / 2, width, height / 2, matrix, false);

// Create a new bitmap with same width but taller to fit reflection
Bitmap bitmapWithReflection = Bitmap.createBitmap(width,
(height + height / 2), Config.ARGB_8888);

// Create a new Canvas with the bitmap that's big enough for the image plus gap plus reflection
Canvas canvas = new Canvas(bitmapWithReflection);
// Draw in the original image
canvas.drawBitmap(originalImage, 0, 0, null);
// Draw in the gap
Paint deafaultPaint = new Paint();
canvas.drawRect(0, height, width, height + reflectionGap,
deafaultPaint);
// Draw in the reflection
canvas.drawBitmap(reflectionImage, 0, height + reflectionGap,
null);

// Create a shader that is a linear gradient that covers the
// reflection
Paint paint = new Paint();
LinearGradient shader = new LinearGradient(0,
originalImage.getHeight(), 0,
bitmapWithReflection.getHeight() + reflectionGap,
0x70ffffff, 0x00ffffff, TileMode.CLAMP);
// Set the paint to use this shader (linear gradient)
paint.setShader(shader);
// Set the Transfer mode to be porter duff and destination in
paint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
// Draw a rectangle using the paint with our linear gradient
canvas.drawRect(0, height, width,
bitmapWithReflection.getHeight() + reflectionGap, paint);

ImageView imageView = new ImageView(mContext);
imageView.setImageBitmap(bitmapWithReflection);
imageView
.setLayoutParams(new GalleryFlow.LayoutParams(160, 240));
// imageView.setScaleType(ScaleType.MATRIX);
mImages[index++] = imageView;

}
return true;
}

public int getCount() {
return mImageIds.length;
}

public Object getItem(int position) {
return position;
}

public long getItemId(int position) {
return position;
}

public View getView(int position, View convertView, ViewGroup parent) {

// Use this code if you want to load from resources
/*
* ImageView i = new ImageView(mContext);
* i.setImageResource(mImageIds[position]); i.setLayoutParams(new
* CoverFlow.LayoutParams(350,350));
* i.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
* 
* //Make sure we set anti-aliasing otherwise we get jaggies
* BitmapDrawable drawable = (BitmapDrawable) i.getDrawable();
* drawable.setAntiAlias(true); return i;
*/

return mImages[position];
}

// Returns the size (0.0f to 1.0f) of the views depending on the 'offset' to the center.
public float getScale(boolean focused, int offset) {
/* Formula: 1 / (2 ^ offset) */
return Math.max(0, 1.0f / (float) Math.pow(2, Math.abs(offset)));
}
}
}

仅仅实现了图片的倒影效果还不够，因为在coverflow中图片切换是有旋转和缩放效果的，而自带的gallery中并没有实现。因此，扩展自带的 gallery，实现自己的galleryflow。在原gallery类中，提供了一个方法 getChildStaticTransformation()以实现对图片的变换。通过覆写这个方法并在其中调用自定义的 transformImageBitmap(“每个图片与gallery中心的距离”)方法，，即可实现每个图片做相应的旋转和缩放。其中使用了 camera和matrix用于视图变换。具体可参考代码注释。

public class GalleryFlow extends Gallery {

        // Graphics Camera used for transforming the matrix of ImageViews
        private Camera mCamera = new Camera();
        //The maximum angle the Child ImageView will be rotated by
        private int mMaxRotationAngle = 60;
       // The maximum zoom on the centre Child
         private int mMaxZoom = -120;
         // The Centre of the Coverflow
         */
        private int mCoveflowCenter;

        public GalleryFlow(Context context) {
                super(context);
                this.setStaticTransformationsEnabled(true);
        }

        public GalleryFlow(Context context, AttributeSet attrs) {
                super(context, attrs);
                this.setStaticTransformationsEnabled(true);
        }

        public GalleryFlow(Context context, AttributeSet attrs, int defStyle) {
                super(context, attrs, defStyle);
                this.setStaticTransformationsEnabled(true);
        }

        /**
         * Get the max rotational angle of the image * @return the mMaxRotationAngle
         */
        public int getMaxRotationAngle() {
                return mMaxRotationAngle;
        }

        /**
         * Set the max rotational angle of each image * @param maxRotationAngle the mMaxRotationAngle to set
         */
        public void setMaxRotationAngle(int maxRotationAngle) {
                mMaxRotationAngle = maxRotationAngle;
        }

        /**
         * Get the Max zoom of the centre image @return the mMaxZoom
         */
        public int getMaxZoom() {
                return mMaxZoom;
        }

        /**
         * Set the max zoom of the centre image @param maxZoom* the mMaxZoom to set
         */
        public void setMaxZoom(int maxZoom) {
                mMaxZoom = maxZoom;
        }

        /**
         * Get the Centre of the Coverflow
         * 
         * @return The centre of this Coverflow.
         */
        private int getCenterOfCoverflow() {
                return (getWidth() - getPaddingLeft() - getPaddingRight()) / 2
                                + getPaddingLeft();
        }

        /**
         * Get the Centre of the View @return The centre of the given view.
         */
        private static int getCenterOfView(View view) {
                return view.getLeft() + view.getWidth() / 2;
        }

        /**
         * {@inheritDoc} @see #setStaticTransformationsEnabled(boolean)
         */
        protected boolean getChildStaticTransformation(View child, Transformation t) {

                final int childCenter = getCenterOfView(child);
                final int childWidth = child.getWidth();
                int rotationAngle = 0;

                t.clear();
                t.setTransformationType(Transformation.TYPE_MATRIX);

                if (childCenter == mCoveflowCenter) {
                        transformImageBitmap((ImageView) child, t, 0);
                } else {
                        rotationAngle = (int) (((float) (mCoveflowCenter - childCenter) / childWidth) * mMaxRotationAngle);
                        if (Math.abs(rotationAngle) > mMaxRotationAngle) {
                                rotationAngle = (rotationAngle < 0) ? -mMaxRotationAngle
                                                : mMaxRotationAngle;
                        }
                        transformImageBitmap((ImageView) child, t, rotationAngle);
                }

                return true;
        }

        /**
         * This is called during layout when the size of this view has changed. If
         * you were just added to the view hierarchy, you're called with the old
         * values of 0.
         * @param w    Current width of this view.
         * @param h    Current height of this view.
         * @param oldw Old width of this view.
         * @param oldh Old height of this view.
         */
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
                mCoveflowCenter = getCenterOfCoverflow();
                super.onSizeChanged(w, h, oldw, oldh);
        }

        /**
         * Transform the Image Bitmap by the Angle passed
         * @param imageView      ImageView the ImageView whose bitmap we want to rotate
         * @param t              transformation
         * @param rotationAngle  the Angle by which to rotate the Bitmap
         */
        private void transformImageBitmap(ImageView child, Transformation t,
                        int rotationAngle) {
                mCamera.save();
                final Matrix imageMatrix = t.getMatrix();
                final int imageHeight = child.getLayoutParams().height;
                final int imageWidth = child.getLayoutParams().width;
                final int rotation = Math.abs(rotationAngle);

                // 在Z轴上正向移动camera的视角，实际效果为放大图片。
                // 如果在Y轴上移动，则图片上下移动；X轴上对应图片左右移动。
                mCamera.translate(0.0f, 0.0f, 100.0f);

                // As the angle of the view gets less, zoom in
                if (rotation < mMaxRotationAngle) {
                        float zoomAmount = (float) (mMaxZoom + (rotation * 1.5));
                        mCamera.translate(0.0f, 0.0f, zoomAmount);
                }

                // 在Y轴上旋转，对应图片竖向向里翻转。
                // 如果在X轴上旋转，则对应图片横向向里翻转。
                mCamera.rotateY(rotationAngle);
                mCamera.getMatrix(imageMatrix);
                imageMatrix.preTranslate(-(imageWidth / 2), -(imageHeight / 2));
                imageMatrix.postTranslate((imageWidth / 2), (imageHeight / 2));
                mCamera.restore();
        }
}

下面是调用的Activity示例：

public void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                
                
                setContentView(R.layout.layout_gallery);
                
                Integer[] images = { R.drawable.img0001, R.drawable.img0030,
                        R.drawable.img0100, R.drawable.img0130, R.drawable.img0200,
                        R.drawable.img0230, R.drawable.img0300, R.drawable.img0330,
                        R.drawable.img0354 };
                
                ImageAdapter adapter = new ImageAdapter(this, images);
                adapter.createReflectedImages();

                GalleryFlow galleryFlow = (GalleryFlow) findViewById(R.id.gallery_flow);
                galleryFlow.setAdapter(adapter);
                
        }

PS1:
可以看出来这样实现的gallery锯齿问题比较严重。可以在createReflectedImages()使用以下代码：

BitmapDrawable bd = new BitmapDrawable(bitmapWithReflection);
bd.setAntiAlias(true);

然后用iv.setImageDrawable(bd);
代替iv.setImageBitmap(bitmapWithReflection);
即可基本消除锯齿。

PS2:
ImageAdapter有待确定的MemoryLeak问题，貌似的Bitmap的decode方法会造成ML，使用ImageAdapter时多次旋 转屏幕后会出现OOM。目前可以通过将使用完毕的bimap调用recycle()方法和设置null并及时调用system.gc()得到一些改善，但 是问题并不明显。