前言
近期研究了一下Glide的图片加载框架，在这里和大家分享一下。由于代码研读有限，难免有错误的地方，了解的童鞋还望指正。学习小组QQ群: 193765960。
本篇是Glide框架及源码解析的第四篇，更多文章敬请关注后续文章。如果这篇文章对大家学习Glide有帮助，还望大家多多转载。
版权归作者所有，如有转发，请注明文章出处：http://www.jianshu.com/u/d43d948bef39
相关文章：
跟着源码学设计：Glide框架及源码解析（一）
跟着源码学设计：Glide框架及源码解析（二）
跟着源码学设计：Glide框架及源码解析（三）
跟着源码学设计：Glide框架及源码解析（四）
跟着源码学设计：Glide框架及源码解析（五）
在之前的文章中，我们已经知道了Glide请求是如何被加载到请求队列中并执行的，也了解了资源是如何管理的。本篇文章我们将探究一下资源是如何获取并分发的。
资源请求和回调机制类图
资源请求和回调机制类图
request开启请求
request被加载到requestTracker中统一管理启动获取资源，代码如下：
public void runRequest(Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
pendingRequests.add(request);
}
}
- request的执行从其begin()方法开始：
public void runRequest(Request request) {
requests.add(request);
if (!isPaused) {
request.begin();
} else {
pendingRequests.add(request);
}
}
获取Engin对象并封装任务
- 获取engine对象，在load()方法中创建EnginJob任务
- 为Enginjob创建异步线程EnginRunnable
- 为EnginRunnable初始化资源获取解析任务DecodJob
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();
//获取key
EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
transcoder, loadProvider.getSourceEncoder());
//查找LruResourceCache
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;
}
//查找ActiveResourceCache
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;
}
//任务排重
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);
}
//创建EngineJob
EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
//创建DecodeJob:注意fetcher（数据加载器）
DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
transcoder, diskCacheProvider, diskCacheStrategy, priority);
//创建任务线程
EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
//将任务加入管理队列
jobs.put(key, engineJob);
engineJob.addCallback(cb);
//启动任务
engineJob.start(runnable);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Started new load", startTime, key);
}
return new LoadStatus(cb, engineJob);
}
执行任务获取数据并解析
- DecodJob通过数据获取器DataFetcher获取数据文件（流）
request被加载到requestTracker中统一管理启动获取资源，代码如下：
//EngineRunnable的run()方法
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);
}
}
private Resource<?> decode() throws Exception {
if (isDecodingFromCache()) {
return decodeFromCache();
} else {
return decodeFromSource();
}
}
private Resource<?> decodeFromSource() throws Exception {
return decodeJob.decodeFromSource();
}
- 数据解析器对data数据解析生成resource资源对象
private Resource<T> decodeSource() throws Exception {
Resource<T> decoded = null;
try {
long startTime = LogTime.getLogTime();
//数据加载器获取数据
final A data = fetcher.loadData(priority);
if (Log.isLoggable(TAG, Log.VERBOSE)) {
logWithTimeAndKey("Fetched data", startTime);
}
if (isCancelled) {
return null;
}
//data数据解析生成resource资源对象
decoded = decodeFromSourceData(data);
} finally {
fetcher.cleanup();
}
return decoded;
}
//HttpUrlFetcher：网络资源加载器
public InputStream loadData(Priority priority) throws Exception {
return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
}
- resource资源进一步封装成为EnginResource对象
//Runnable的方法，在run()方法中调用
private void onLoadComplete(Resource resource) {
manager.onResourceReady(resource);
}
//manager.onResourceReady(resource)实际回调了Enginjob的接口实现
public void onResourceReady(final Resource<?> resource) {
this.resource = resource;
MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
}
- 通过监听接口回传EnginResource对象
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();
//回传engineResource到request中，request获取到资源后开始刷新UI
cb.onResourceReady(engineResource);
}
}
// Our request is complete, so we can release the resource.
engineResource.release();
}
（本篇是Glide框架及源码解析的第四篇，更多文章敬请关注后续文章。版权归作者所有，如有转发，请注明文章出处：原文链接）
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