首先，在真正开始看源码之前，我们需要有个心里准备，决心是今天我们一定要搞懂它，不然充满好奇的进来，一脸蒙蔽的出去。带大家看了很多的第三方开源库和 Android 源码，又讲了 23 种设计模式基础，将这些结合起来，再去分析一些第三方库应该是 soEasy。只是希望我们自己能够去读懂，因为以后我们还会用到其他一些第三方框架。只有我们自己习得了看源码的能力才是最重要的。
其次，像 Glide 、Picasso、ImageLoader 这些访问网络的框架，其内部实现的原理应该大致差不多，线程池，连接网络，解析图片，缓存图片，显示图片。那像你这么说是不是开发中选择哪个都行？看心情选择第三方框架肯定是不行的。
最后，我们分析源码最好先从最基本的入手，调用简单方法一个方法一个方法走流程，最后再去抓细节。如果 UML 图你还没忘记还会画，那么可以画个 UML 时序图，便于我们走源代码流程。这篇文章的篇幅会很长请调整好呼吸：
1.Glide.with()
Glide.with() 的有很多重载方法，参数的类型都有 Context、Activity、FragmentActivity、android.support.v4.app.Fragment、android.app.Fragment 为什么要重载这么多方法，我们可以猜猜。其次，很多哥们在实际的开发过程中，可能都是传递的上下文 Context ，那么从现在开始最好不要这么传了。我选择其中一种分析一下：
public static RequestManager with(FragmentActivity activity) {
RequestManagerRetriever retriever = RequestManagerRetriever.get();
return retriever.get(activity);
}
public RequestManager get(FragmentActivity activity) {
if (Util.isOnBackgroundThread()) {
return get(activity.getApplicationContext());
} else {
assertNotDestroyed(activity);
FragmentManager fm = activity.getSupportFragmentManager();
return supportFragmentGet(activity, fm);
}
}
RequestManager supportFragmentGet(Context context, FragmentManager fm) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
RequestManager requestManager = current.getRequestManager();
if (requestManager == null) {
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
SupportRequestManagerFragment getSupportRequestManagerFragment(final FragmentManager fm) {
SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
if (current == null) {
current = pendingSupportRequestManagerFragments.get(fm);
if (current == null) {
// 创建一个 Fragment 绑定到 Activity 上面
current = new SupportRequestManagerFragment();
pendingSupportRequestManagerFragments.put(fm, current);
fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
}
}
return current;
}
上面是我挑的一些主要代码，主要的还是在 getSupportRequestManagerFragment 这个方法，这个方法干了啥？其实蛮好理解的，就是去创建一个 Fragment 然后把创建的 Fragment 绑定到 Activity 上面，搞这个干什么？其实是为了绑定监听 Activity 的生命周期，比如 RecyclerView 一个列表都在加载图片，这个时候我退出当前 Activity 应当停止访问网络加载图片，绑定生命周期就有这个好处。到底是怎么做监听的？看下源码：
RequestManager supportFragmentGet(Context context, FragmentManager fm) {
SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
RequestManager requestManager = current.getRequestManager();
if (requestManager == null) {
requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
current.setRequestManager(requestManager);
}
return requestManager;
}
RequestManager(Context context, final Lifecycle lifecycle, RequestManagerTreeNode treeNode,
RequestTracker requestTracker, ConnectivityMonitorFactory factory) {
// If we're the application level request manager, we may be created on a background thread. In that case we
// cannot risk synchronously pausing or resuming requests, so we hack around the issue by delaying adding
// ourselves as a lifecycle listener by posting to the main thread. This should be entirely safe.
if (Util.isOnBackgroundThread()) {
new Handler(Looper.getMainLooper()).post(new Runnable() {
@Override
public void run() {
lifecycle.addListener(RequestManager.this);
}
});
} else {
// 在这里注册为了 this
lifecycle.addListener(this);
}
lifecycle.addListener(connectivityMonitor);
}
LifecycleListener 是个接口有三个方法，onStart() , onStop()，onDestroy() 对应 RequestManager 中的实现分别是：
/**
* Lifecycle callback that registers for connectivity events (if the android.permission.ACCESS_NETWORK_STATE
* permission is present) and restarts failed or paused requests.
*/
@Override
public void onStart() {
// onStart might not be called because this object may be created after the fragment/activity's onStart method.
resumeRequests();
}
/**
* Lifecycle callback that unregisters for connectivity events (if the android.permission.ACCESS_NETWORK_STATE
* permission is present) and pauses in progress loads.
*/
@Override
public void onStop() {
pauseRequests();
}
/**
* Lifecycle callback that cancels all in progress requests and clears and recycles resources for all completed
* requests.
*/
@Override
public void onDestroy() {
requestTracker.clearRequests();
}
所以总结一下Glide.with() 这方法返回的是一个 RequestManager 主要用来监听一些生命周期，以此来管理加载请求 Request 。拿 FragmentActivity 来说当 Activity 退出了会调用 onDestroy() -> requestTracker.clearRequests(); 会清理所有的图片加载请求，并且一个 FragmentActivity 只有一个 SupportRequestManagerFragment 和一个 RequestManager ，怎么看出来的？不是有那么多的 if else 判断代码，当然细节我们不用太关注，看下时序图：
Glide.with()
2.Glide.with().load()
public DrawableTypeRequest<String> load(String string) {
return (DrawableTypeRequest<String>) fromString().load(string);
}
public DrawableTypeRequest<String> fromString() {
return loadGeneric(String.class);
}
private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);
ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =
Glide.buildFileDescriptorModelLoader(modelClass, context);
if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {
throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"
+ " which there is a registered ModelLoader, if you are using a custom model, you must first call"
+ " Glide#register with a ModelLoaderFactory for your custom model class");
}
return optionsApplier.apply(
new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
glide, requestTracker, lifecycle, optionsApplier));
}
Glide.buildStreamModelLoader 和 Glide.buildFileDescriptorModelLoader 这两个方法看不太懂先放一边，待会再回来找他，先记住他的变量名 streamModelLoader 和 fileDescriptorModelLoader，接下来创建了一个 DrawableTypeRequest 返回。这个方法比较简单，时序图：
Glide.with().load()
3.Glide.with().load().into()
public Target<TranscodeType> into(ImageView view) {
Util.assertMainThread();
if (view == null) {
throw new IllegalArgumentException("You must pass in a non null View");
}
return into(glide.buildImageViewTarget(view, transcodeClass));
}
public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
if (GlideDrawable.class.isAssignableFrom(clazz)) {
return (Target<Z>) new GlideDrawableImageViewTarget(view);
} else if (Bitmap.class.equals(clazz)) {
return (Target<Z>) new BitmapImageViewTarget(view);
} else if (Drawable.class.isAssignableFrom(clazz)) {
return (Target<Z>) new DrawableImageViewTarget(view);
} else {
throw new IllegalArgumentException("Unhandled class: " + clazz
+ ", try .as*(Class).transcode(ResourceTranscoder)");
}
}
public <Y extends Target<TranscodeType>> Y into(Y target) {
Util.assertMainThread();
// 通过 target 构建一个 Request
Request request = buildRequest(target);
target.setRequest(request);
lifecycle.addListener(target);
// 看样子要执行
requestTracker.runRequest(request);
return target;
}
transcodeClass 是在构造函数里面赋值初始化的，也就是在第二步 load 的时候赋值的是 GlideDrawable.class 那么 buildTarget 方法返回的应该是 GlideDrawableImageViewTarget 对象，通过 Target
构建了一个 Request ，最后调用 requestTracker.runRequest(request); 接下来肯定要分析一下怎么构建 buildRequest 的：
private Request buildRequest(Target<TranscodeType> target) {
if (priority == null) {
priority = Priority.NORMAL;
}
return buildRequestRecursive(target, null);
}
// 这里省略了一个处理缩略图的方法，感兴趣可以自己研究一下
// obtainRequest
private Request obtainRequest(Target<TranscodeType> target, float sizeMultiplier, Priority priority,
RequestCoordinator requestCoordinator) {
// 构建了一个 GenericRequest 参数很多，但我肯定都知道是啥意思
return GenericRequest.obtain(
loadProvider,
model,
signature,
context,
priority,
target,
sizeMultiplier,
placeholderDrawable,
placeholderId,
errorPlaceholder,
errorId,
fallbackDrawable,
fallbackResource,
requestListener,
requestCoordinator,
glide.getEngine(),
transformation,
transcodeClass,
isCacheable,
animationFactory,
overrideWidth,
overrideHeight,
diskCacheStrategy);
}
buildRequest 构建的是一个 GenericRequest 实例，传递的参数很多但是我们肯定都认识，比如 placeholderId，errorId，transformation，diskCacheStrategy 等等，接下来看下真正的 runRequest() 方法：
/**
* Starts tracking the given request.
*/
public void runRequest(Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
pendingRequests.add(request);
}
}
@Override
public void begin() {
startTime = LogTime.getLogTime();
if (model == null) {
// 异常报错显示错误图片
onException(null);
return;
}
status = Status.WAITING_FOR_SIZE;
if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
onSizeReady(overrideWidth, overrideHeight);
} else {
target.getSize(this);
}
if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
target.onLoadStarted(getPlaceholderDrawable());
}
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished run method in " + LogTime.getElapsedMillis(startTime));
}
}
@Override
public void onSizeReady(int width, int height) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
if (status != Status.WAITING_FOR_SIZE) {
return;
}
status = Status.RUNNING;
width = Math.round(sizeMultiplier * width);
height = Math.round(sizeMultiplier * height);
ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);
if (dataFetcher == null) {
// 异常报错显示错误图片
onException(new Exception("Failed to load model: \'" + model + "\'"));
return;
}
ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
}
loadedFromMemoryCache = true;
loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
priority, isMemoryCacheable, diskCacheStrategy, this);
loadedFromMemoryCache = resource != null;
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
}
}
渐渐的开始复杂起来了，modelLoader 和 transcoder 都是通过 loadProvider 获取的，那么我们必须要了解一下 loadProvider 是怎么实例化的，他是在 obtainRequest 的时候传递初始化的，是在第二步 load 的时候在 DrawableTypeRequest 的构造方法中调用 buildProvider 构建的，是一个 FixedLoadProvider 。接下来看下 engine.load() 方法：
public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,
DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,
Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {
Util.assertMainThread();
long startTime = LogTime.getLogTime();
final String id = fetcher.getId();
EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
transcoder, loadProvider.getSourceEncoder());
EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
if (cached != null) {
cb.onResourceReady(cached);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from cache", startTime, key);
}
return null;
}
EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
if (active != null) {
cb.onResourceReady(active);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Loaded resource from active resources", startTime, key);
}
return null;
}
EngineJob current = jobs.get(key);
if (current != null) {
current.addCallback(cb);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Added to existing load", startTime, key);
}
return new LoadStatus(cb, current);
}
EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
transcoder, diskCacheProvider, diskCacheStrategy, priority);
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
jobs.put(key, engineJob);
engineJob.addCallback(cb);
engineJob.start(runnable);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
有一部分代码是处理缓存的，我们主要关注看下 DecodeJob 调用 start 方法，让线程池去执行 EngineRunnable 的 run 方法：
@Override
public void run() {
if (isCancelled) {
return;
}
Exception exception = null;
Resource<?> resource = null;
try {
resource = decode();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception decoding", e);
}
exception = e;
}
if (isCancelled) {
// 如果被取消了，回收资源，不在往下执行
if (resource != null) {
resource.recycle();
}
return;
}
if (resource == null) {
// 失败
onLoadFailed(exception);
} else {
// 处理成功
onLoadComplete(resource);
}
}
private Resource<?> decode() throws Exception {
// 判断缓存
if (isDecodingFromCache()) {
return decodeFromCache();
} else {
return decodeFromSource();
}
}
private Resource<?> decodeFromSource() throws Exception {
return decodeJob.decodeFromSource();
}
最终调用的是 decodeJob.decodeFromSource() 方法，走了半天还没找到联网解析流的代码，应该在 decodeJob 里面吧？点进去看下：
public Resource<Z> decodeFromSource() throws Exception {
Resource<T> decoded = decodeSource();
return transformEncodeAndTranscode(decoded);
}
private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
long startTime = LogTime.getLogTime();
Resource<T> transformed = transform(decoded);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transformed resource from source", startTime);
}
writeTransformedToCache(transformed);
startTime = LogTime.getLogTime();
Resource<Z> result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from source", startTime);
}
return result;
}
private Resource<T> decodeSource() throws Exception {
Resource<T> decoded = null;
try {
long startTime = LogTime.getLogTime();
final A data = fetcher.loadData(priority);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Fetched data", startTime);
}
if (isCancelled) {
return null;
}
decoded = decodeFromSourceData(data);
} finally {
fetcher.cleanup();
}
return decoded;
}
decodeFromSource()方法，其实它的工作分为两部，第一步是调用decodeSource()方法来获得一个Resource对象，第二步是调用transformEncodeAndTranscode()方法来处理这个Resource对象。在 decodeSource() 方法中调用了 fetcher.loadData(priority) 点击发现是个抽象方法，我们必须要找到实现类，要找到 fetcher 这个实现类那么先得找到 loadProvider 这个实现类，是在第二个 load 步骤中的 ImageVideoModelLoader 这个类，找到 getResourceFetcher() 返回的就是 fetcher 是 ImageVideoFetcher 类，找到 loadData() 方法如下：
@Override
public ImageVideoWrapper loadData(Priority priority) throws Exception {
InputStream is = null;
if (streamFetcher != null) {
try {
is = streamFetcher.loadData(priority);
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception fetching input stream, trying ParcelFileDescriptor", e);
}
if (fileDescriptorFetcher == null) {
throw e;
}
}
}
ParcelFileDescriptor fileDescriptor = null;
if (fileDescriptorFetcher != null) {
try {
fileDescriptor = fileDescriptorFetcher.loadData(priority);
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception fetching ParcelFileDescriptor", e);
}
if (is == null) {
throw e;
}
}
}
return new ImageVideoWrapper(is, fileDescriptor);
}
发现其方法内部又调用了 streamFetcher.loadData() 方法，streamFetcher 又是调用了 streamLoader.getResourceFetcher() 获取的，所以我们必须要找到 streamLoader ， 它其实是第二步中通过 Glide.buildStreamModelLoader() 方法获取的，我们看下：
public synchronized <T, Y> ModelLoader<T, Y> buildModelLoader(Class<T> modelClass, Class<Y> resourceClass) {
ModelLoader<T, Y> result = getCachedLoader(modelClass, resourceClass);
if (result != null) {
// We've already tried to create a model loader and can't with the currently registered set of factories,
// but we can't use null to demonstrate that failure because model loaders that haven't been requested
// yet will be null in the cache. To avoid this, we use a special signal model loader.
if (NULL_MODEL_LOADER.equals(result)) {
return null;
} else {
return result;
}
}
final ModelLoaderFactory<T, Y> factory = getFactory(modelClass, resourceClass);
if (factory != null) {
result = factory.build(context, this);
cacheModelLoader(modelClass, resourceClass, result);
} else {
// We can't generate a model loader for the given arguments with the currently registered set of factories.
cacheNullLoader(modelClass, resourceClass);
}
return result;
}
modelClass 是 String.class，resourceClass 是 InputStream.class 通过 getFactory 方法获取到 ModelLoaderFactory 对象，调用 factory.build() 方法构建了一个 ModelLoader 对象，是一个 StreamStringLoader 对象，过程有点曲折，但是琢磨琢磨还是能找到的。回到上面找 getResourceFetcher() 方法返回的是 HttpUrlFetcher 对象，接下来看他的 loadData() 方法：
@Override
public InputStream loadData(Priority priority) throws Exception {
return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
}
private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl, Map<String, String> headers)
throws IOException {
if (redirects >= MAXIMUM_REDIRECTS) {
throw new IOException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
} else {
// Comparing the URLs using .equals performs additional network I/O and is generally broken.
// See http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html.
try {
if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
throw new IOException("In re-direct loop");
}
} catch (URISyntaxException e) {
// Do nothing, this is best effort.
}
}
urlConnection = connectionFactory.build(url);
for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
}
urlConnection.setConnectTimeout(2500);
urlConnection.setReadTimeout(2500);
urlConnection.setUseCaches(false);
urlConnection.setDoInput(true);
// Connect explicitly to avoid errors in decoders if connection fails.
urlConnection.connect();
if (isCancelled) {
return null;
}
final int statusCode = urlConnection.getResponseCode();
if (statusCode / 100 == 2) {
return getStreamForSuccessfulRequest(urlConnection);
} else if (statusCode / 100 == 3) {
String redirectUrlString = urlConnection.getHeaderField("Location");
if (TextUtils.isEmpty(redirectUrlString)) {
throw new IOException("Received empty or null redirect url");
}
URL redirectUrl = new URL(url, redirectUrlString);
return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
} else {
if (statusCode == -1) {
throw new IOException("Unable to retrieve response code from HttpUrlConnection.");
}
throw new IOException("Request failed " + statusCode + ": " + urlConnection.getResponseMessage());
}
}
private InputStream getStreamForSuccessfulRequest(HttpURLConnection urlConnection)
throws IOException {
if (TextUtils.isEmpty(urlConnection.getContentEncoding())) {
int contentLength = urlConnection.getContentLength();
stream = ContentLengthInputStream.obtain(urlConnection.getInputStream(), contentLength);
} else {
if (Log.isLoggable(TAG, Log.DEBUG)) {
Log.d(TAG, "Got non empty content encoding: " + urlConnection.getContentEncoding());
}
stream = urlConnection.getInputStream();
}
return stream;
}
上面这个代码一般都能读懂了，就是通过 urlConnection 联网请求数据，这里返回的是一个 InputStream 并没有做任何的一些其他处理，所以我们才刚刚开始，我其实还是比较关心他怎么处理 InputStream 的，因为开发中经常用到，刚好可以借鉴一下。接着回到上面去看下 decodeFromSourceData 这个方法：
private Resource<T> decodeFromSourceData(A data) throws IOException {
final Resource<T> decoded;
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;
}
调用了 loadProvider.getSourceDecoder().decode() 方法来进行解码。loadProvider 就是刚才在 onSizeReady() 方法中得到的FixedLoadProvider，而 getSourceDecoder() 得到的则是一个 GifBitmapWrapperResourceDecoder 对象，也就是要调用这个对象的 decode() 方法来对图片进行解码。那么我们来看下 GifBitmapWrapperResourceDecoder 的代码：
// @see ResourceDecoder.decode
@Override
public Resource<GifBitmapWrapper> decode(ImageVideoWrapper source, int width, int height) throws IOException {
ByteArrayPool pool = ByteArrayPool.get();
byte[] tempBytes = pool.getBytes();
GifBitmapWrapper wrapper = null;
try {
wrapper = decode(source, width, height, tempBytes);
} finally {
pool.releaseBytes(tempBytes);
}
return wrapper != null ? new GifBitmapWrapperResource(wrapper) : null;
}
private GifBitmapWrapper decode(ImageVideoWrapper source, int width, int height, byte[] bytes) throws IOException {
final GifBitmapWrapper result;
if (source.getStream() != null) {
result = decodeStream(source, width, height, bytes);
} else {
result = decodeBitmapWrapper(source, width, height);
}
return result;
}
private GifBitmapWrapper decodeStream(ImageVideoWrapper source, int width, int height, byte[] bytes)
throws IOException {
InputStream bis = streamFactory.build(source.getStream(), bytes);
bis.mark(MARK_LIMIT_BYTES);
ImageHeaderParser.ImageType type = parser.parse(bis);
bis.reset();
GifBitmapWrapper result = null;
if (type == ImageHeaderParser.ImageType.GIF) {
result = decodeGifWrapper(bis, width, height);
}
// Decoding the gif may fail even if the type matches.
if (result == null) {
// We can only reset the buffered InputStream, so to start from the beginning of the stream, we need to
// pass in a new source containing the buffered stream rather than the original stream.
ImageVideoWrapper forBitmapDecoder = new ImageVideoWrapper(bis, source.getFileDescriptor());
result = decodeBitmapWrapper(forBitmapDecoder, width, height);
}
return result;
}
private GifBitmapWrapper decodeBitmapWrapper(ImageVideoWrapper toDecode, int width, int height) throws IOException {
GifBitmapWrapper result = null;
Resource<Bitmap> bitmapResource = bitmapDecoder.decode(toDecode, width, height);
if (bitmapResource != null) {
result = new GifBitmapWrapper(bitmapResource, null);
}
return result;
}
decodeStream 会判断是不是 Gif , 如果不是 Gif 会调用 decodeBitmapWrapper 方法，这个时候会调用 bitmapDecoder.decode() 方法，这个方法应该要去处理图片了？赶紧找到实现类 StreamBitmapDecoder 看下：
@Override
public Resource<Bitmap> decode(InputStream source, int width, int height) {
Bitmap bitmap = downsampler.decode(source, bitmapPool, width, height, decodeFormat);
return BitmapResource.obtain(bitmap, bitmapPool);
}
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();
final BitmapFactory.Options options = getDefaultOptions();
// Use to fix the mark limit to avoid allocating buffers that fit entire images.
RecyclableBufferedInputStream bufferedStream = new RecyclableBufferedInputStream(
is, bytesForStream);
// Use to retrieve exceptions thrown while reading.
// TODO(#126): when the framework no longer returns partially decoded Bitmaps or provides a way to determine
// if a Bitmap is partially decoded, consider removing.
ExceptionCatchingInputStream exceptionStream =
ExceptionCatchingInputStream.obtain(bufferedStream);
// Use to read data.
// Ensures that we can always reset after reading an image header so that we can still attempt to decode the
// full image even when the header decode fails and/or overflows our read buffer. See #283.
MarkEnforcingInputStream invalidatingStream = new MarkEnforcingInputStream(exceptionStream);
try {
exceptionStream.mark(MARK_POSITION);
int orientation = 0;
try {
orientation = new ImageHeaderParser(exceptionStream).getOrientation();
} catch (IOException e) {
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Cannot determine the image orientation from header", e);
}
} finally {
try {
exceptionStream.reset();
} catch (IOException e) {
if (Log.isLoggable(TAG, Log.WARN)) {
Log.w(TAG, "Cannot reset the input stream", e);
}
}
}
options.inTempStorage = bytesForOptions;
final int[] inDimens = getDimensions(invalidatingStream, bufferedStream, options);
final int inWidth = inDimens[0];
final int inHeight = inDimens[1];
final int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
final int sampleSize = getRoundedSampleSize(degreesToRotate, inWidth, inHeight, outWidth, outHeight);
final Bitmap downsampled =
downsampleWithSize(invalidatingStream, bufferedStream, options, pool, inWidth, inHeight, sampleSize,
decodeFormat);
// BitmapFactory swallows exceptions during decodes and in some cases when inBitmap is non null, may catch
// and log a stack trace but still return a non null bitmap. To avoid displaying partially decoded bitmaps,
// we catch exceptions reading from the stream in our ExceptionCatchingInputStream and throw them here.
final Exception streamException = exceptionStream.getException();
if (streamException != null) {
throw new RuntimeException(streamException);
}
Bitmap rotated = null;
if (downsampled != null) {
rotated = TransformationUtils.rotateImageExif(downsampled, pool, orientation);
if (!downsampled.equals(rotated) && !pool.put(downsampled)) {
downsampled.recycle();
}
}
return rotated;
} finally {
byteArrayPool.releaseBytes(bytesForOptions);
byteArrayPool.releaseBytes(bytesForStream);
exceptionStream.release();
releaseOptions(options);
}
}
到这里我们就真正开始解析 Bitmap 了，就是从 InputStream 中去获取 Bitmap 对象，这些代码还是比较容易看懂的，Downsampler.decode() 返回的是一个 Bitmap 对象，StreamBitmapDecoder.decode() 对 Bitmap 又进行了一层包裹，返回的是 BitmapResource 。接下来我们回到 GifBitmapWrapperResourceDecoder.decode 中返回的是 GifBitmapWrapperResource , 接着往回走就回到了 DecodeJob.transformEncodeAndTranscode() 中：
private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
long startTime = LogTime.getLogTime();
Resource<T> transformed = transform(decoded);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transformed resource from source", startTime);
}
writeTransformedToCache(transformed);
startTime = LogTime.getLogTime();
Resource<Z> result = transcode(transformed);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Transcoded transformed from source", startTime);
}
return result;
}
private Resource<T> transform(Resource<T> decoded) {
if (decoded == null) {
return null;
}
Resource<T> transformed = transformation.transform(decoded, width, height);
if (!decoded.equals(transformed)) {
decoded.recycle();
}
return transformed;
}
private Resource<Z> transcode(Resource<T> transformed) {
if (transformed == null) {
return null;
}
return transcoder.transcode(transformed);
}
上面主要分为三步，transform()，writeTransformedToCache() 写入缓存，transcode() 这里要注意了是把 泛型 T 转成了 Z 。transcode() 方法中又是调用了 transcoder 的 transcode() 方法，发现又找不到代码了，所以我们有得回去找 transcoder 实现类，这也是说我们为什么最好要会画 UML 图，因为代码架构复杂的情况下，我们在里面游几天都上不来。还是回去找找，是在第二步 load 的时候创建的 glide.buildTranscoder(resourceClass, transcodedClass) 是 GifBitmapWrapperDrawableTranscoder ：
@SuppressWarnings("unchecked")
@Override
public Resource<GlideDrawable> transcode(Resource<GifBitmapWrapper> toTranscode) {
GifBitmapWrapper gifBitmap = toTranscode.get();
Resource<Bitmap> bitmapResource = gifBitmap.getBitmapResource();
final Resource<? extends GlideDrawable> result;
if (bitmapResource != null) {
result = bitmapDrawableResourceTranscoder.transcode(bitmapResource);
} else {
result = gifBitmap.getGifResource();
}
// This is unchecked but always safe, anything that extends a Drawable can be safely cast to a Drawable.
return (Resource<GlideDrawable>) result;
}
@Override
public Resource<GlideBitmapDrawable> transcode(Resource<Bitmap> toTranscode) {
GlideBitmapDrawable drawable = new GlideBitmapDrawable(resources, toTranscode.get());
return new GlideBitmapDrawableResource(drawable, bitmapPool);
}
这里主要是把 GifBitmapWrapper 转成了 GlideDrawable ，为什么这么弄肯定是为了后面方便显示，因为 Gif 资源返回的已经是 GifDrawable 了并不需要转换，而图片资源返回的是 Bitmap ，做一次转换之后统一转成 GlideBitmapDrawable 这样方便后面做显示，那么显示我们要回到 EngineRunnable 中的 run 方法了：
@Override
public void run() {
if (isCancelled) {
return;
}
Exception exception = null;
Resource<?> resource = null;
try {
resource = decode();
} catch (Exception e) {
if (Log.isLoggable(TAG, Log.VERBOSE)) {
Log.v(TAG, "Exception decoding", e);
}
exception = e;
}
if (isCancelled) {
if (resource != null) {
resource.recycle();
}
return;
}
if (resource == null) {
onLoadFailed(exception);
} else {
onLoadComplete(resource);
}
}
现在我们来总结一下 decode() 方法都干了啥，首先是 DecodeJob.decodeFromSource() -> fetcher.loadData() 去联网获取 InputStream -> loadProvider.getSourceDecoder().decode() -> bitmapDecoder.decode() 解析 InputStream 压缩旋转等等返回 Bitmap -> DecodeJob.transformEncodeAndTranscode() 去转换保证返回数据一致 GlideDrawable 。接下来只需要看下 onLoadComplete 应该就要完了：
private void onLoadComplete(Resource resource) {
manager.onResourceReady(resource);
}
@Override
public void onResourceReady(final Resource<?> resource) {
this.resource = resource;
MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
}
private static class MainThreadCallback implements Handler.Callback {
@Override
public boolean handleMessage(Message message) {
if (MSG_COMPLETE == message.what || MSG_EXCEPTION == message.what) {
EngineJob job = (EngineJob) message.obj;
if (MSG_COMPLETE == message.what) {
job.handleResultOnMainThread();
} else {
job.handleExceptionOnMainThread();
}
return true;
}
return false;
}
}
private void handleResultOnMainThread() {
if (isCancelled) {
resource.recycle();
return;
} else if (cbs.isEmpty()) {
throw new IllegalStateException("Received a resource without any callbacks to notify");
}
engineResource = engineResourceFactory.build(resource, isCacheable);
hasResource = true;
// Hold on to resource for duration of request so we don't recycle it in the middle of notifying if it
// synchronously released by one of the callbacks.
engineResource.acquire();
listener.onEngineJobComplete(key, engineResource);
for (ResourceCallback cb : cbs) {
if (!isInIgnoredCallbacks(cb)) {
engineResource.acquire();
cb.onResourceReady(engineResource);
}
}
// Our request is complete, so we can release the resource.
engineResource.release();
}
这里主要是切换到主线程，然后循环 cbs 调用 cb 的 onResourceReady() 方法，那么 cb 又是啥？其实就是 GenericRequest.this，看下 onResourceReady()：
@Override
public void onResourceReady(Resource<?> resource) {
if (resource == null) {
onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
+ " inside, but instead got null."));
return;
}
Object received = resource.get();
if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
releaseResource(resource);
onException(new Exception("Expected to receive an object of " + transcodeClass
+ " but instead got " + (received != null ? received.getClass() : "") + "{" + received + "}"
+ " inside Resource{" + resource + "}."
+ (received != null ? "" : " "
+ "To indicate failure return a null Resource object, "
+ "rather than a Resource object containing null data.")
));
return;
}
if (!canSetResource()) {
releaseResource(resource);
// We can't set the status to complete before asking canSetResource().
status = Status.COMPLETE;
return;
}
onResourceReady(resource, (R) received);
}
/**
* Internal {@link #onResourceReady(Resource)} where arguments are known to be safe.
*
* @param resource original {@link Resource}, never <code>null</code>
* @param result object returned by {@link Resource#get()}, checked for type and never <code>null</code>
*/
private void onResourceReady(Resource<?> resource, R result) {
// We must call isFirstReadyResource before setting status.
boolean isFirstResource = isFirstReadyResource();
status = Status.COMPLETE;
this.resource = resource;
if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
isFirstResource)) {
GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
target.onResourceReady(result, animation);
}
notifyLoadSuccess();
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
+ (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
}
}
最终调用了 target 的 onResourceReady() 方法，那么 target 又是谁呢？不知道我们是否还记得这代码into(glide.buildImageViewTarget(view, transcodeClass)) 在之前就分析过的 GlideDrawableImageViewTarget :
@Override
public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> animation) {
if (!resource.isAnimated()) {
//TODO: Try to generalize this to other sizes/shapes.
// This is a dirty hack that tries to make loading square thumbnails and then square full images less costly
// by forcing both the smaller thumb and the larger version to have exactly the same intrinsic dimensions.
// If a drawable is replaced in an ImageView by another drawable with different intrinsic dimensions,
// the ImageView requests a layout. Scrolling rapidly while replacing thumbs with larger images triggers
// lots of these calls and causes significant amounts of jank.
float viewRatio = view.getWidth() / (float) view.getHeight();
float drawableRatio = resource.getIntrinsicWidth() / (float) resource.getIntrinsicHeight();
if (Math.abs(viewRatio - 1f) <= SQUARE_RATIO_MARGIN
&& Math.abs(drawableRatio - 1f) <= SQUARE_RATIO_MARGIN) {
resource = new SquaringDrawable(resource, view.getWidth());
}
}
super.onResourceReady(resource, animation);
this.resource = resource;
resource.setLoopCount(maxLoopCount);
resource.start();
}
@Override
public void onResourceReady(Z resource, GlideAnimation<? super Z> glideAnimation) {
if (glideAnimation == null || !glideAnimation.animate(resource, this)) {
setResource(resource);
}
}
@Override
protected void setResource(GlideDrawable resource) {
view.setImageDrawable(resource);
}
Glide.with().load().into()
走下流程其实还是非常简单的，就是需要大家多花些时间多点耐心，这第一步才刚刚开始，如果第一步迈不过去，后面我们就无法分析细节，比如缓存的处理，架构的模式，图片和Gif以及视频的解析处理，等等。源码也不能白看，很多思想是要用到实际项目中的，后面文章我将陆陆续续去分析。还是那句话 Java基础和设计模式基础打牢，自己习得了看源码的能力才是最重要的。
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