前言

近期研究了一下Glide的圖片加載框架，在這里和大家分享一下。由于代碼研讀有限，難免有錯(cuò)誤的地方，了解的童鞋還望指正。學(xué)習(xí)小組QQ群: 193765960。

本篇是Glide框架及源碼解析的第四篇，更多文章敬請(qǐng)關(guān)注后續(xù)文章。如果這篇文章對(duì)大家學(xué)習(xí)Glide有幫助，還望大家多多轉(zhuǎn)載。

版權(quán)歸作者所有，如有轉(zhuǎn)發(fā)，請(qǐng)注明文章出處：http://www.itdecent.cn/u/d43d948bef39

相關(guān)文章：

跟著源碼學(xué)設(shè)計(jì)：Glide框架及源碼解析（一）
跟著源碼學(xué)設(shè)計(jì)：Glide框架及源碼解析（二）
跟著源碼學(xué)設(shè)計(jì)：Glide框架及源碼解析（三）
跟著源碼學(xué)設(shè)計(jì)：Glide框架及源碼解析（四）
跟著源碼學(xué)設(shè)計(jì)：Glide框架及源碼解析（五）

在之前的文章中，我們已經(jīng)知道了Glide請(qǐng)求是如何被加載到請(qǐng)求隊(duì)列中并執(zhí)行的，也了解了資源是如何管理的。本篇文章我們將探究一下資源是如何獲取并分發(fā)的。

資源請(qǐng)求和回調(diào)機(jī)制類圖

資源請(qǐng)求和回調(diào)機(jī)制類圖

request開(kāi)啟請(qǐng)求

request被加載到requestTracker中統(tǒng)一管理啟動(dòng)獲取資源，代碼如下：

public void runRequest(Request request) {
  requests.add(request);
  if (!isPaused) {
    request.begin();
  } else {
    pendingRequests.add(request);
  }
}

• request的執(zhí)行從其begin()方法開(kāi)始：

public void runRequest(Request request) {
  requests.add(request);
  if (!isPaused) {
    request.begin();
  } else {
    pendingRequests.add(request);
  }
}

獲取Engin對(duì)象并封裝任務(wù)

• 獲取engine對(duì)象，在load()方法中創(chuàng)建EnginJob任務(wù)
• 為Enginjob創(chuàng)建異步線程EnginRunnable
• 為EnginRunnable初始化資源獲取解析任務(wù)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();
   
  //獲取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;
  }
 
  //任務(wù)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);
  }
  
  //創(chuàng)建EngineJob
  EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);
   
  //創(chuàng)建DecodeJob:注意fetcher（數(shù)據(jù)加載器）
  DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,
  transcoder, diskCacheProvider, diskCacheStrategy, priority);
  
  //創(chuàng)建任務(wù)線程
  EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);
  
  //將任務(wù)加入管理隊(duì)列
  jobs.put(key, engineJob);
  engineJob.addCallback(cb);
  
  //啟動(dòng)任務(wù)
  engineJob.start(runnable);

  if (Log.isLoggable(TAG, Log.VERBOSE)) {
    logWithTimeAndKey("Started new load", startTime, key);
  }
  return new LoadStatus(cb, engineJob);
}

執(zhí)行任務(wù)獲取數(shù)據(jù)并解析

• DecodJob通過(guò)數(shù)據(jù)獲取器DataFetcher獲取數(shù)據(jù)文件（流）
  request被加載到requestTracker中統(tǒng)一管理啟動(dòng)獲取資源，代碼如下：

//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();
 }

• 數(shù)據(jù)解析器對(duì)data數(shù)據(jù)解析生成resource資源對(duì)象

private Resource<T> decodeSource() throws Exception {
    Resource<T> decoded = null;
    try {
        long startTime = LogTime.getLogTime();
        //數(shù)據(jù)加載器獲取數(shù)據(jù)
        final A data = fetcher.loadData(priority);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Fetched data", startTime);
        }
        if (isCancelled) {
            return null;
        }
        
        //data數(shù)據(jù)解析生成resource資源對(duì)象
        decoded = decodeFromSourceData(data);
    } finally {
        fetcher.cleanup();
    }
    return decoded;
}
 
//HttpUrlFetcher：網(wǎng)絡(luò)資源加載器
public InputStream loadData(Priority priority) throws Exception {
    return loadDataWithRedirects(glideUrl.toURL(), 0 /*redirects*/, null /*lastUrl*/, glideUrl.getHeaders());
}

• resource資源進(jìn)一步封裝成為EnginResource對(duì)象

//Runnable的方法，在run()方法中調(diào)用
private void onLoadComplete(Resource resource) {
    manager.onResourceReady(resource);
}
 
//manager.onResourceReady(resource)實(shí)際回調(diào)了Enginjob的接口實(shí)現(xiàn)
public void onResourceReady(final Resource<?> resource) {
  this.resource = resource;
  MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
}

• 通過(guò)監(jiān)聽(tīng)接口回傳EnginResource對(duì)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();
      //回傳engineResource到request中，request獲取到資源后開(kāi)始刷新UI
      cb.onResourceReady(engineResource);
    }
  }
  // Our request is complete, so we can release the resource.
  engineResource.release();
}

（本篇是Glide框架及源碼解析的第四篇，更多文章敬請(qǐng)關(guān)注后續(xù)文章。版權(quán)歸作者所有，如有轉(zhuǎn)發(fā)，請(qǐng)注明文章出處：原文鏈接）