Glide 显示视频缩略图及遇到的坑

实现原理

Glide支持视频格式的文件，但是在3.x里会有些欠缺。其底层是通过 MediaMetadataRetriever实现的。

MediaMetadataRetriever拥有获取视频的第几帧的能力,Glide获取视频里的第n帧的代码如下:

VideoBitmapDecoder.java

@Override

public Bitmap decode(ParcelFileDescriptor resource, BitmapPool bitmapPool, int outWidth, int outHeight,

DecodeFormat decodeFormat)

throws IOException {

MediaMetadataRetriever mediaMetadataRetriever = factory.build();

mediaMetadataRetriever.setDataSource(resource.getFileDescriptor());

Bitmap result;

if (frame >= 0) {

result = mediaMetadataRetriever.getFrameAtTime(frame);

} else {

result = mediaMetadataRetriever.getFrameAtTime();

}

mediaMetadataRetriever.release();

resource.close();

return result;

}

提取核心代码

mediaMetadataRetriever获取Bitmap的代码：

val file = FileInputStream(File(path))

val s = MediaMetadataRetriever()

s.setDataSource(file.fd)

file.close()

val bitmap = s.getFrameAtTime(-1)

imageView.setImageBitmap(bitmap)

s.release()

Glide3.x的一个bug

但3.x的Glide缓存策略不能是Source我们来分析各种缓存策略Glide的内部的逻辑

Result缓存策略

EngineRunnable.java

private Resource> decode() throws Exception {

if (isDecodingFromCache()) {

return decodeFromCache();

} else {

return decodeFromSource();

}

}

EngineRunnable分走两次，第一次走isDecodingFromCache

private Resource> decodeFromCache() throws Exception {

Resource> result = null;

//...ignore code

//先从result缓存里获取

result = decodeJob.decodeResultFromCache();

if (result == null) {

//再从源文件缓存里获取

result = decodeJob.decodeSourceFromCache();

}

return result;

}

第一次加载显示decodeResultFromCache会为空，我们略过,重点看decodeSourceFromCache

public ResourcedecodeSourceFromCache() throws Exception {

//如果不是cacheSource直接返回空

if (!diskCacheStrategy.cacheSource()) {

return null;

}

//...ignore code

}

decode失败以后会把EngineRunnable 扔给另外一个线程

private void onLoadFailed(Exception e) {

if (isDecodingFromCache()) {

stage = Stage.SOURCE;

manager.submitForSource(this);

} else {

manager.onException(e);

}

}

因此它会再一次走到run()方法里的decode，而此次走的是decodeFromSource

DecodeJob.java

public ResourcedecodeFromSource() throws Exception {

Resourcedecoded = decodeSource();

return transformEncodeAndTranscode(decoded);

}

private ResourcedecodeSource() throws Exception {

Resourcedecoded = null;

try {

// 通过fetcher去加载Source

final A data = fetcher.loadData(priority);

//...ignore code

decoded = decodeFromSourceData(data);

} finally {

fetcher.cleanup();

}

return decoded;

}

DecodeJob.java

private ResourcedecodeFromSourceData(A data) {

final Resourcedecoded;

if (diskCacheStrategy.cacheSource()) {

decoded = cacheAndDecodeSourceData(data);

} else {

//因为缓存策略是Result，所以走的是该处

decoded = loadProvider.getSourceDecoder().decode(data, width, height);

//...ignore code

}

return decoded;

}

SourceDecode其实就是ImageVideoBitmapDecoder

public Resourcedecode(ImageVideoWrapper source

, int width, int height){

Resourceresult = null;

InputStream is = source.getStream();

if (is != null) {

//通过streamDecoder去decode图，它是StreamBitmapDecoder

result = streamDecoder.decode(is, width, height);

}

if (result == null) {

ParcelFileDescriptor fileDescriptor = source.getFileDescriptor();

if (fileDescriptor != null) {

//通过fileDescriptorDecoder去decode图，它是FileDescriptorBitmapDecoder

result = fileDescriptorDecoder.decode(fileDescriptor, width, height);

}

}

return result;

}

ImageVideoBitmapDecoder里有两种decoder

StreamBitmapDecoder直接将输入流转成Bitmap

FileDescriptorBitmapDecoder能将流通过VideoBitmapDecoder去转Bitmap

而VideoBitmapDecoder底层就是通过MediaMetadataRetriever去获取第一帧

如果源文件是视频，它将先通过StreamBitmapDecoder去decode，结果decode失败result=null

然后通过FileDescriptorBitmapDecoder去decode，可见这里Glide内部并不知道源文件是图片还是视频，所以先用图片的方式解

而图片方式解的时候也是去读它的头文件:

@Override

public Resourcedecode(InputStream source, int width, int height) {

//通过downsampler去decode

Bitmap bitmap = downsampler.decode(source, bitmapPool, width, height, decodeFormat);

return BitmapResource.obtain(bitmap, bitmapPool);

}

@Override

public Bitmap decode(InputStream is, BitmapPool pool, int outWidth,

int outHeight, DecodeFormat decodeFormat) {

final ByteArrayPool byteArrayPool = ByteArrayPool.get();

final byte[] bytesForOptions = byteArrayPool.getBytes();

final byte[] bytesForStream = byteArrayPool.getBytes();

//此处用了Option池，值得学习!

final BitmapFactory.Options options = getDefaultOptions();

//...ignore code

final Bitmap downsampled =

downsampleWithSize(invalidatingStream, bufferedStream,

options, pool, inWidth, inHeight, sampleSize,

decodeFormat);

private static Bitmap decodeStream(MarkEnforcingInputStream is, RecyclableBufferedInputStream bufferedStream,

BitmapFactory.Options options) {

if (options.inJustDecodeBounds) {

is.mark(MARK_POSITION);

} else {

bufferedStream.fixMarkLimit();

}

//如果是视频文件此处无法decode出Bitmap

final Bitmap result = BitmapFactory.decodeStream(is, null, options);

//...ignore code

return result;

}

第一种decode失败会用第二种，也就是最终会走到VideoBitmapDecoder去解析

Source缓存策略

如果缓存策略是Source

private ResourcedecodeSource() throws Exception {

Resourcedecoded = null;

try {

final A data = fetcher.loadData(priority);

//...ignore code

decoded = decodeFromSourceData(data);

} finally {

fetcher.cleanup();

}

return decoded;

}

private ResourcedecodeFromSourceData(A data) {

final Resourcedecoded;

//因为是缓存Source因为走cacheAndDecodeSourceData方法

if (diskCacheStrategy.cacheSource()) {

decoded = cacheAndDecodeSourceData(data);

} else {

long startTime = LogTime.getLogTime();

decoded = loadProvider.getSourceDecoder().decode(data, width, height);

if (Log.isLoggable(TAG, Log.VERBOSE)) {

logWithTimeAndKey("Decoded from source", startTime);

}

}

return decoded;

}

而加载缓存的Decoder是CacheDecoder,非常不幸它不是ImageVideoBitmapDecoder

private ResourceloadFromCache(Key key) {

File cacheFile = diskCacheProvider.getDiskCache().get(key);

if (cacheFile == null) {

return null;

}

Resourceresult = null;

try {

result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);

} finally {

if (result == null) {

diskCacheProvider.getDiskCache().delete(key);

}

}

return result;

}

它是FileToStreamDecoder,以下是截图

而FileToStreamDecoder的Decoder是StreamBitmapDecoder，最终它是把该文件当成图片解析

因此导致它失败，如果result==null,它还会走

diskCacheProvider.getDiskCache().delete(key);

所以如果是视频流，缓存策略是Source，它会很让你失望，不停的copy文件，再解，解失败再删除，一直这样下去

如果该源文件很大，会影响Glide的性能！

为了验证这一猜想,我们可以通过抓包工具看一下发生了什么:

结果它确实把这个视频存下来了，不过发现decode失败就会把它删除

所以，显示视频图的时候，一定切记不要用Source缓存策略，它不仅无法加载成功，而且会给你带来很大隐患

结束!