继上篇说到Glide的缓存流程后，接下来分析下Gilde是如何从网络下载图片并做优化的
由于在上文中从硬盘缓存中已经拿到了对应的data,继而会调用decodeFromData方法
private <Data> Resource<R> decodeFromData(
DataFetcher<?> fetcher, Data data, DataSource dataSource) throws GlideException {
try {
if (data == null) {
return null;
}
long startTime = LogTime.getLogTime();
Resource<R> result = decodeFromFetcher(data, dataSource);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Decoded result " + result, startTime);
}
return result;
} finally {
fetcher.cleanup();
}
}
这里的重点还是decodeFromFetcher(data,dataSource)方法
private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
throws GlideException {
LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
return runLoadPath(data, dataSource, path);
}
这里的LoadPath简单来说就流的解析器，一共是gif,drawable,bitmap三种，间接会调用loadWithExceptionList方法，也就是核心的decode方法，来看下Glide是如何为我们在从流到bitmap做优化的
private Resource<Transcode> loadWithExceptionList(
DataRewinder<Data> rewinder,
@NonNull Options options,
int width,
int height,
DecodePath.DecodeCallback<ResourceType> decodeCallback,
List<Throwable> exceptions)
throws GlideException {
Resource<Transcode> result = null;
//noinspection ForLoopReplaceableByForEach to improve perf
for (int i = 0, size = decodePaths.size(); i < size; i++) {
DecodePath<Data, ResourceType, Transcode> path = decodePaths.get(i);
try {
result = path.decode(rewinder, width, height, options, decodeCallback);
} catch (GlideException e) {
exceptions.add(e);
}
if (result != null) {
break;
}
}
if (result == null) {
throw new GlideException(failureMessage, new ArrayList<>(exceptions));
}
return result;
}
在分别用不同的解析器调用decode时，我们进去看下
public Resource<Transcode> decode(
DataRewinder<DataType> rewinder,
int width,
int height,
@NonNull Options options,
DecodeCallback<ResourceType> callback)
throws GlideException {
Resource<ResourceType> decoded = decodeResource(rewinder, width, height, options);
Resource<ResourceType> transformed = callback.onResourceDecoded(decoded);
return transcoder.transcode(transformed, options);
}
可以说Glide分为原始的ResourceType和transformed的ResourceType，transformed应该就是做些圆角的处理，高斯模糊的操作，先来看下decodeResource方法，很明显我们这里是bitmap,所以会走到ByteBufferBitmapDecoder的decode方法中
@Override
public Resource<Bitmap> decode(
@NonNull ByteBuffer source, int width, int height, @NonNull Options options)
throws IOException {
InputStream is = ByteBufferUtil.toStream(source);
return downsampler.decode(is, width, height, options);
}
再把source转化成常规的InputStream后这里就等同于从网络下拿到了InputStream转化成bitmap的操作了
间接会调用decodeFromWrappedStreams的方法
方法中其实做了这几个操作，由于方法太长，就贴出部分
private static int[] getDimensions(
ImageReader imageReader,
BitmapFactory.Options options,
DecodeCallbacks decodeCallbacks,
BitmapPool bitmapPool)
throws IOException {
options.inJustDecodeBounds = true;
decodeStream(imageReader, options, decodeCallbacks, bitmapPool);
options.inJustDecodeBounds = false;
return new int[] {options.outWidth, options.outHeight};
}
先把inJustDecodeBounds设置为true，这样在native层就不会生成java的byte数组的图片了，只会把宽高返回
而后这里的inBitmap属性就是为了减少开辟空间，而复用bitmap的
而bitmapPool正式存放这个inBitmap的
接下来就是关于部分手机的图片旋转的操作了
int orientation = imageReader.getImageOrientation();
int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
boolean isExifOrientationRequired = TransformationUtils.isExifOrientationRequired(orientation);
int targetWidth =
requestedWidth == Target.SIZE_ORIGINAL
? (isRotationRequired(degreesToRotate) ? sourceHeight : sourceWidth)
: requestedWidth;
int targetHeight =
requestedHeight == Target.SIZE_ORIGINAL
? (isRotationRequired(degreesToRotate) ? sourceWidth : sourceHeight)
: requestedHeight;
最后会根据不同的ImageType去缩放图片，然后根据图片是否是jpg,或者webp,或者png去设置inSampleSize
// Here we mimic framework logic for determining how inSampleSize division is rounded on various
// versions of Android. The logic here has been tested on emulators for Android versions 15-26.
// PNG - Always uses floor
// JPEG - Always uses ceiling
// Webp - Prior to N, always uses floor. At and after N, always uses round.
options.inSampleSize = powerOfTwoSampleSize;
int powerOfTwoWidth;
int powerOfTwoHeight;
if (imageType == ImageType.JPEG) {
// libjpegturbo can downsample up to a sample size of 8. libjpegturbo uses ceiling to round.
// After libjpegturbo's native rounding, skia does a secondary scale using floor
// (integer division). Here we replicate that logic.
int nativeScaling = Math.min(powerOfTwoSampleSize, 8);
powerOfTwoWidth = (int) Math.ceil(orientedSourceWidth / (float) nativeScaling);
powerOfTwoHeight = (int) Math.ceil(orientedSourceHeight / (float) nativeScaling);
int secondaryScaling = powerOfTwoSampleSize / 8;
if (secondaryScaling > 0) {
powerOfTwoWidth = powerOfTwoWidth / secondaryScaling;
powerOfTwoHeight = powerOfTwoHeight / secondaryScaling;
}
可以说远比我们自己设置的要靠谱的多，而且在其线程池做操作的，我们要做的就是告诉Glide图片的宽高就行了
最后如果图片解析失败了，Glide还会到bitmapPool中去取图片
int sourceWidth = options.outWidth;
int sourceHeight = options.outHeight;
String outMimeType = options.outMimeType;
final Bitmap result;
TransformationUtils.getBitmapDrawableLock().lock();
try {
result = imageReader.decodeBitmap(options);
} catch (IllegalArgumentException e) {
IOException bitmapAssertionException =
newIoExceptionForInBitmapAssertion(e, sourceWidth, sourceHeight, outMimeType, options);
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(
TAG,
"Failed to decode with inBitmap, trying again without Bitmap re-use",
bitmapAssertionException);
}
if (options.inBitmap != null) {
try {
bitmapPool.put(options.inBitmap);
options.inBitmap = null;
return decodeStream(imageReader, options, callbacks, bitmapPool);
} catch (IOException resetException) {
throw bitmapAssertionException;
}
}
throw bitmapAssertionException;
} finally {
TransformationUtils.getBitmapDrawableLock().unlock();
}
return result;
最后返回了bitmap
Bitmap downsampled = decodeStream(imageReader, options, callbacks, bitmapPool);
callbacks.onDecodeComplete(bitmapPool, downsampled);
最后通过callCallbackOnResourceReady返回给了调用者
@SuppressWarnings("WeakerAccess")
@Synthetic
@GuardedBy("this")
void callCallbackOnResourceReady(ResourceCallback cb) {
try {
// This is overly broad, some Glide code is actually called here, but it's much
// simpler to encapsulate here than to do so at the actual call point in the
// Request implementation.
cb.onResourceReady(engineResource, dataSource);
} catch (Throwable t) {
throw new CallbackException(t);
}
}
在imageView的target中
private void setResourceInternal(@Nullable Z resource) {
// Order matters here. Set the resource first to make sure that the Drawable has a valid and
// non-null Callback before starting it.
setResource(resource);
maybeUpdateAnimatable(resource);
}
设置了imageview。
至此，Glide从网络加载图片的核心部分也就到此结束了，其实还有很多细节没有分析到，以后可能在用的途中又需要的话再去分析吧。
