Universal-Image-Loader源码阅读（34）-ImageDecoder

看名字翻译成图像解析器。

就一个功能接口，就是跟据图片信息，解析出Bitmap来

源码：

/** * Provide decoding image to result {@link Bitmap}. * * @author Sergey Tarasevich (nostra13[at]gmail[dot]com) * @see ImageDecodingInfo * @since 1.8.3 */public interface ImageDecoder {/** * Decodes image to {@link Bitmap} according target size and other parameters. * * @param imageDecodingInfo * @return * @throws IOException */Bitmap decode(ImageDecodingInfo imageDecodingInfo) throws IOException;}

看看ImageDecodingInfo类（图片解析信息）：

从源码就可以看出，完全是图片信息的集合，包括存储key，uri，原来的uri，目标imageSize，image缩放类型，view缩放类型，图片加载工具（及参数），解析参数。

源码：

/** * Contains needed information for decoding image to Bitmap * * @author Sergey Tarasevich (nostra13[at]gmail[dot]com) * @since 1.8.3 */public class ImageDecodingInfo {private final String imageKey;private final String imageUri;private final String originalImageUri;private final ImageSize targetSize;private final ImageScaleType imageScaleType;private final ViewScaleType viewScaleType;private final ImageDownloader downloader;private final Object extraForDownloader;private final boolean considerExifParams;private final Options decodingOptions;public ImageDecodingInfo(String imageKey, String imageUri, String originalImageUri, ImageSize targetSize, ViewScaleType viewScaleType, ImageDownloader downloader, DisplayImageOptions displayOptions) {this.imageKey = imageKey;this.imageUri = imageUri;this.originalImageUri = originalImageUri;this.targetSize = targetSize;this.imageScaleType = displayOptions.getImageScaleType();this.viewScaleType = viewScaleType;this.downloader = downloader;this.extraForDownloader = displayOptions.getExtraForDownloader();considerExifParams = displayOptions.isConsiderExifParams();decodingOptions = new Options();copyOptions(displayOptions.getDecodingOptions(), decodingOptions);}private void copyOptions(Options srcOptions, Options destOptions) {destOptions.inDensity = srcOptions.inDensity;destOptions.inDither = srcOptions.inDither;destOptions.inInputShareable = srcOptions.inInputShareable;destOptions.inJustDecodeBounds = srcOptions.inJustDecodeBounds;destOptions.inPreferredConfig = srcOptions.inPreferredConfig;destOptions.inPurgeable = srcOptions.inPurgeable;destOptions.inSampleSize = srcOptions.inSampleSize;destOptions.inScaled = srcOptions.inScaled;destOptions.inScreenDensity = srcOptions.inScreenDensity;destOptions.inTargetDensity = srcOptions.inTargetDensity;destOptions.inTempStorage = srcOptions.inTempStorage;if (Build.VERSION.SDK_INT >= 10) copyOptions10(srcOptions, destOptions);if (Build.VERSION.SDK_INT >= 11) copyOptions11(srcOptions, destOptions);}@TargetApi(10)private void copyOptions10(Options srcOptions, Options destOptions) {destOptions.inPreferQualityOverSpeed = srcOptions.inPreferQualityOverSpeed;}@TargetApi(11)private void copyOptions11(Options srcOptions, Options destOptions) {destOptions.inBitmap = srcOptions.inBitmap;destOptions.inMutable = srcOptions.inMutable;}/** @return Original {@linkplain com.nostra13.universalimageloader.utils.MemoryCacheUtils#generateKey(String, ImageSize) image key} (used in memory cache). */public String getImageKey() {return imageKey;}/** @return Image URI for decoding (usually image from disk cache) */public String getImageUri() {return imageUri;}/** @return The original image URI which was passed to ImageLoader */public String getOriginalImageUri() {return originalImageUri;}/** * @return Target size for image. Decoded bitmap should close to this size according to {@linkplain ImageScaleType * image scale type} and {@linkplain ViewScaleType view scale type}. */public ImageSize getTargetSize() {return targetSize;}/** * @return {@linkplain ImageScaleType Scale type for image sampling and scaling}. This parameter affects result size * of decoded bitmap. */public ImageScaleType getImageScaleType() {return imageScaleType;}/** @return {@linkplain ViewScaleType View scale type}. This parameter affects result size of decoded bitmap. */public ViewScaleType getViewScaleType() {return viewScaleType;}/** @return Downloader for image loading */public ImageDownloader getDownloader() {return downloader;}/** @return Auxiliary object for downloader */public Object getExtraForDownloader() {return extraForDownloader;}/** @return <b>true</b> - if EXIF params of image should be considered; <b>false</b> - otherwise */public boolean shouldConsiderExifParams() {return considerExifParams;}/** @return Decoding options */public Options getDecodingOptions() {return decodingOptions;}}

再看具体的实现：

作为ImageDecoder的唯一实现，我们的核心就是decode函数，其它都是辅助函数。

源码：

/** * Decodes images to {@link Bitmap}, scales them to needed size * * @author Sergey Tarasevich (nostra13[at]gmail[dot]com) * @see ImageDecodingInfo * @since 1.8.3 */public class BaseImageDecoder implements ImageDecoder {protected static final String LOG_SUBSAMPLE_IMAGE = "Subsample original image (%1$s) to %2$s (scale = %3$d) [%4$s]";protected static final String LOG_SCALE_IMAGE = "Scale subsampled image (%1$s) to %2$s (scale = %3$.5f) [%4$s]";protected static final String LOG_ROTATE_IMAGE = "Rotate image on %1$d\u00B0 [%2$s]";protected static final String LOG_FLIP_IMAGE = "Flip image horizontally [%s]";protected static final String ERROR_NO_IMAGE_STREAM = "No stream for image [%s]";protected static final String ERROR_CANT_DECODE_IMAGE = "Image can't be decoded [%s]";protected final boolean loggingEnabled;/** * @param loggingEnabled Whether debug logs will be written to LogCat. Usually should match {@link *                       com.nostra13.universalimageloader.core.ImageLoaderConfiguration.Builder#writeDebugLogs() *                       ImageLoaderConfiguration.writeDebugLogs()} */public BaseImageDecoder(boolean loggingEnabled) {this.loggingEnabled = loggingEnabled;}/** * Decodes image from URI into {@link Bitmap}. Image is scaled close to incoming {@linkplain ImageSize target size} * during decoding (depend on incoming parameters). * * @param decodingInfo Needed data for decoding image * @return Decoded bitmap * @throws IOException                   if some I/O exception occurs during image reading * @throws UnsupportedOperationException if image URI has unsupported scheme(protocol) */@Overridepublic Bitmap decode(ImageDecodingInfo decodingInfo) throws IOException {Bitmap decodedBitmap;ImageFileInfo imageInfo;InputStream imageStream = getImageStream(decodingInfo);//先获取数据流if (imageStream == null) {L.e(ERROR_NO_IMAGE_STREAM, decodingInfo.getImageKey());return null;}try {imageInfo = defineImageSizeAndRotation(imageStream, decodingInfo);//获取imageFileInfoimageStream = resetStream(imageStream, decodingInfo);//重新整理数据流Options decodingOptions = prepareDecodingOptions(imageInfo.imageSize, decodingInfo);//获取解析参数decodedBitmap = BitmapFactory.decodeStream(imageStream, null, decodingOptions);//获取最终的bitmap} finally {IoUtils.closeSilently(imageStream);}if (decodedBitmap == null) {L.e(ERROR_CANT_DECODE_IMAGE, decodingInfo.getImageKey());} else {decodedBitmap = considerExactScaleAndOrientatiton(decodedBitmap, decodingInfo, imageInfo.exif.rotation,imageInfo.exif.flipHorizontal);//做最后的拉伸和旋转}return decodedBitmap;}protected InputStream getImageStream(ImageDecodingInfo decodingInfo) throws IOException {return decodingInfo.getDownloader().getStream(decodingInfo.getImageUri(), decodingInfo.getExtraForDownloader());}protected ImageFileInfo defineImageSizeAndRotation(InputStream imageStream, ImageDecodingInfo decodingInfo)throws IOException {Options options = new Options();options.inJustDecodeBounds = true;BitmapFactory.decodeStream(imageStream, null, options);//获取option数据ExifInfo exif;String imageUri = decodingInfo.getImageUri();if (decodingInfo.shouldConsiderExifParams() && canDefineExifParams(imageUri, options.outMimeType)) {//exif信息准备exif = defineExifOrientation(imageUri);} else {exif = new ExifInfo();}return new ImageFileInfo(new ImageSize(options.outWidth, options.outHeight, exif.rotation), exif);}private boolean canDefineExifParams(String imageUri, String mimeType) {return "image/jpeg".equalsIgnoreCase(mimeType) && (Scheme.ofUri(imageUri) == Scheme.FILE);}protected ExifInfo defineExifOrientation(String imageUri) {int rotation = 0;//偏转角度boolean flip = false;//是否颠倒try {ExifInterface exif = new ExifInterface(Scheme.FILE.crop(imageUri));int exifOrientation = exif.getAttributeInt(ExifInterface.TAG_ORIENTATION, ExifInterface.ORIENTATION_NORMAL);switch (exifOrientation) {case ExifInterface.ORIENTATION_FLIP_HORIZONTAL:flip = true;case ExifInterface.ORIENTATION_NORMAL:rotation = 0;break;case ExifInterface.ORIENTATION_TRANSVERSE:flip = true;case ExifInterface.ORIENTATION_ROTATE_90:rotation = 90;break;case ExifInterface.ORIENTATION_FLIP_VERTICAL:flip = true;case ExifInterface.ORIENTATION_ROTATE_180:rotation = 180;break;case ExifInterface.ORIENTATION_TRANSPOSE:flip = true;case ExifInterface.ORIENTATION_ROTATE_270:rotation = 270;break;}} catch (IOException e) {L.w("Can't read EXIF tags from file [%s]", imageUri);}return new ExifInfo(rotation, flip);}protected Options prepareDecodingOptions(ImageSize imageSize, ImageDecodingInfo decodingInfo) {ImageScaleType scaleType = decodingInfo.getImageScaleType();int scale;if (scaleType == ImageScaleType.NONE) {scale = 1;} else if (scaleType == ImageScaleType.NONE_SAFE) {scale = ImageSizeUtils.computeMinImageSampleSize(imageSize);} else {ImageSize targetSize = decodingInfo.getTargetSize();boolean powerOf2 = scaleType == ImageScaleType.IN_SAMPLE_POWER_OF_2;scale = ImageSizeUtils.computeImageSampleSize(imageSize, targetSize, decodingInfo.getViewScaleType(), powerOf2);}if (scale > 1 && loggingEnabled) {L.d(LOG_SUBSAMPLE_IMAGE, imageSize, imageSize.scaleDown(scale), scale, decodingInfo.getImageKey());}Options decodingOptions = decodingInfo.getDecodingOptions();decodingOptions.inSampleSize = scale;return decodingOptions;}protected InputStream resetStream(InputStream imageStream, ImageDecodingInfo decodingInfo) throws IOException {if (imageStream.markSupported()) {try {imageStream.reset();return imageStream;} catch (IOException ignored) {}}IoUtils.closeSilently(imageStream);return getImageStream(decodingInfo);}protected Bitmap considerExactScaleAndOrientatiton(Bitmap subsampledBitmap, ImageDecodingInfo decodingInfo,int rotation, boolean flipHorizontal) {Matrix m = new Matrix();// Scale to exact size if needImageScaleType scaleType = decodingInfo.getImageScaleType();if (scaleType == ImageScaleType.EXACTLY || scaleType == ImageScaleType.EXACTLY_STRETCHED) {ImageSize srcSize = new ImageSize(subsampledBitmap.getWidth(), subsampledBitmap.getHeight(), rotation);float scale = ImageSizeUtils.computeImageScale(srcSize, decodingInfo.getTargetSize(), decodingInfo.getViewScaleType(), scaleType == ImageScaleType.EXACTLY_STRETCHED);if (Float.compare(scale, 1f) != 0) {m.setScale(scale, scale);if (loggingEnabled) {L.d(LOG_SCALE_IMAGE, srcSize, srcSize.scale(scale), scale, decodingInfo.getImageKey());}}}// Flip bitmap if needif (flipHorizontal) {m.postScale(-1, 1);if (loggingEnabled) L.d(LOG_FLIP_IMAGE, decodingInfo.getImageKey());}// Rotate bitmap if needif (rotation != 0) {m.postRotate(rotation);if (loggingEnabled) L.d(LOG_ROTATE_IMAGE, rotation, decodingInfo.getImageKey());}Bitmap finalBitmap = Bitmap.createBitmap(subsampledBitmap, 0, 0, subsampledBitmap.getWidth(), subsampledBitmap.getHeight(), m, true);if (finalBitmap != subsampledBitmap) {subsampledBitmap.recycle();}return finalBitmap;}protected static class ExifInfo {public final int rotation;public final boolean flipHorizontal;protected ExifInfo() {this.rotation = 0;this.flipHorizontal = false;}protected ExifInfo(int rotation, boolean flipHorizontal) {this.rotation = rotation;this.flipHorizontal = flipHorizontal;}}protected static class ImageFileInfo {public final ImageSize imageSize;public final ExifInfo exif;protected ImageFileInfo(ImageSize imageSize, ExifInfo exif) {this.imageSize = imageSize;this.exif = exif;}}}