源码阅读--Glide

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1.用法及参考资料

参考资料：http://www.apkbus.com/blog-705730-60158.html
用法：

Glide.with(this).load("https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1494417111222&di=0679f94eacd58059affec7cc9a33c731&imgtype=0&src=http%3A%2F%2Fi.zeze.com%2Fattachment%2Fforum%2F201506%2F28%2F104039ydramgzhbzxarof9.jpg").into(mImageView;

2.重要类的说明

（1）Engine

负责任务创建，发起，回调，资源的管理

  public <R> LoadStatus load(/**   */) {    ......    EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);//----------------------------WeakReference    ......    EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);    //------------------------------------Map<Key, WeakReference<EngineResource<?>>> activeResources，从这个参数里面获取的    //--------------------------------------activeResources在构造函数中赋值，在loadFromCache中添加（如果cache中有的话）    ......    //---------------------------------------------------------------获取相关联的EngineJob    EngineJob<?> current = jobs.get(key);//-----------------------------jobs是一个Map<Key, EngineJob<?>>，在构造函数中被赋值    if (current != null) {//--------------------------------成功获取，添加Callback      current.addCallback(cb);      if (Log.isLoggable(TAG, Log.VERBOSE)) {        logWithTimeAndKey("Added to existing load", startTime, key);      }      return new LoadStatus(cb, current);    }    //-------------------------------------------------------相关联的EngineJob获取失败    EngineJob<R> engineJob = engineJobFactory.build(key, isMemoryCacheable,        useUnlimitedSourceExecutorPool);    DecodeJob<R> decodeJob = decodeJobFactory.build(/....../);    jobs.put(key, engineJob);    engineJob.addCallback(cb);    engineJob.start(decodeJob);//---------------------------------发起新的decodeJob    return new LoadStatus(cb, engineJob);  }

（2）EngineJob

调度 DecodeJob，添加，移除资源回调，并 notify 回调
如果从cache中获取，那就由diskCacheExecutor来执行decodeJob，否则由ActiveSourceExecutor来执行

  public void start(DecodeJob<R> decodeJob) {    this.decodeJob = decodeJob;    GlideExecutor executor = decodeJob.willDecodeFromCache() ? diskCacheExecutor : getActiveSourceExecutor();    executor.execute(decodeJob);  }

（3）DecodeJob

实现了 Runnable 接口，调度任务的核心类，整个请求的繁重工作都在这里完成：处理来自缓存或者原始的资源，应用转换动画以及 transcode
run方法里面调用了runWrapped

private void runWrapped() {     switch (runReason) {      case INITIALIZE:        stage = getNextStage(Stage.INITIALIZE);//-----------------------获取Stage（INITIALIZE，RESOURCE_CACHE，DATA_CACHE，SOURCE，ENCODE，FINISHED）        currentGenerator = getNextGenerator();//-----------------------根据Stage获取对应的CacheGenerator        runGenerators();        break;      case SWITCH_TO_SOURCE_SERVICE:        runGenerators();        break;      case DECODE_DATA:        decodeFromRetrievedData();//---------------------------调用decodeFromData，如果失败也会调用runGenerators        break;    }  }

runGenerators内部会调用reschedule，重新调度

public void reschedule(DecodeJob<?> job) {    if (isCancelled) {      MAIN_THREAD_HANDLER.obtainMessage(MSG_CANCELLED, this).sendToTarget();    } else {      getActiveSourceExecutor().execute(job);    }  }

3.用法解析

（1）with函数

public static RequestManager with(FragmentActivity activity) {        return getRetriever(activity).get(activity);    }

getRetriever获取的是一个RequestManagerRetriever

private static RequestManagerRetriever getRetriever(@Nullable Context context) {        。。。。。。        return Glide.get(context).getRequestManagerRetriever();//-----------------------get函数中有对Glide的初始化initGlide    }    private static void initGlide(Context context) {        。。。。。。        RequestManagerRetriever.RequestManagerFactory factory =                annotationGeneratedModule != null                        ? annotationGeneratedModule.getRequestManagerFactory() : null;        GlideBuilder builder = new GlideBuilder().setRequestManagerFactory(factory);        。。。。。。        glide = builder.build(applicationContext);//-----------------------------建造者模式生成glide        。。。。。。    }

再来看一下getRetriever之后的get函数做了什么

public RequestManager get(FragmentActivity activity) {        if (Util.isOnBackgroundThread()) {            return get(activity.getApplicationContext());        } else {            assertNotDestroyed(activity);            FragmentManager fm = activity.getSupportFragmentManager();            return supportFragmentGet(activity, fm, null /*parentHint*/);        }    }    public RequestManager get(Context context) {        。。。。。。        return getApplicationManager(context);    }    private RequestManager getApplicationManager(Context context) {        if (applicationManager == null) {            synchronized (this) {                if (applicationManager == null) {                    。。。。。。                    Glide glide = Glide.get(context);                    applicationManager = factory.build(glide, new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());                }            }        }        return applicationManager;    }

applicationManager就是RequestManager
上面是isOnBackgroundThread，那么就是在主线程的流程————-说白了也就是为了获得RequestManager

private RequestManager supportFragmentGet(Context context, FragmentManager fm,                                              Fragment parentHint) {        SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint);        RequestManager requestManager = current.getRequestManager();        if (requestManager == null) {            // TODO(b/27524013): Factor out this Glide.get() call.            Glide glide = Glide.get(context);            requestManager =                    factory.build(glide, current.getLifecycle(), current.getRequestManagerTreeNode());            current.setRequestManager(requestManager);        }        return requestManager;    }

综上，整个with函数就为了获得RequestManager。
它里面有个RequestTracker是用来保存request的，用的是WeakHashMap

//----------------------------------------------删除不再正常使用的entry    private void expungeStaleEntries() {        for (Object x; (x = queue.poll()) != null; ) {//-------------------------------这里的queue是ReferenceQueue（Reference queue for cleared WeakEntries）            synchronized (queue) {                Entry<K,V> e = (Entry<K,V>) x;                int i = indexFor(e.hash, table.length);                Entry<K,V> prev = table[i];                Entry<K,V> p = prev;                while (p != null) {                    Entry<K,V> next = p.next;                    if (p == e) {                        if (prev == e)                            table[i] = next;                        else                            prev.next = next;                        // Must not null out e.next;                        // stale entries may be in use by a HashIterator                        e.value = null; // Help GC                        size--;                        break;                    }                    prev = p;                    p = next;                }            }        }    }

这个“弱键”的原理呢？大致上就是，通过WeakReference和ReferenceQueue实现的。 WeakHashMap的key是“弱键”，即是WeakReference类型的；ReferenceQueue是一个队列，它会保存被GC回收的“弱键”。实现步骤是：
(01) 新建WeakHashMap，将“键值对”添加到WeakHashMap中。实际上，WeakHashMap是通过数组table保存Entry(键值对)；每一个Entry实际上是一个单向链表，即Entry是键值对链表。
(02) 当某“弱键”不再被其它对象引用，并被GC回收时。在GC回收该“弱键”时，这个“弱键”也同时会被添加到ReferenceQueue(queue)队列中。
(03) 当下一次我们需要操作WeakHashMap时，会先同步table和queue。table中保存了全部的键值对，而queue中保存被GC回收的键值对；同步它们，就是删除table中被GC回收的键值对。
这就是“弱键”如何被自动从WeakHashMap中删除的步骤了。

（2）load函数

跟踪进去就是配置了点参数

（3）into函数

public <Y extends Target<TranscodeType>> Y into(@NonNull Y target) {    Util.assertMainThread();    Preconditions.checkNotNull(target);    if (!isModelSet) {      throw new IllegalArgumentException("You must call #load() before calling #into()");    }    Request previous = target.getRequest();    if (previous != null) {      requestManager.clear(target);    }    requestOptions.lock();    Request request = buildRequest(target);    target.setRequest(request);    requestManager.track(target, request);//-----------------------获取了Request以后就开始runRequest------request.begin();    return target;  }  @Override  public void begin() {    ......    status = Status.WAITING_FOR_SIZE;    if (Util.isValidDimensions(overrideWidth, overrideHeight)) {      onSizeReady(overrideWidth, overrideHeight);//----------------------------------------获取数据并解析    } else {      target.getSize(this);    }    if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)        && canNotifyStatusChanged()) {      target.onLoadStarted(getPlaceholderDrawable());//------------------------------------把图片加载进控件中    }    if (Log.isLoggable(TAG, Log.VERBOSE)) {      logV("finished run method in " + LogTime.getElapsedMillis(startTime));    }  }  public void onSizeReady(int width, int height) {    。。。。。。    status = Status.RUNNING;    float sizeMultiplier = requestOptions.getSizeMultiplier();    this.width = maybeApplySizeMultiplier(width, sizeMultiplier);    this.height = maybeApplySizeMultiplier(height, sizeMultiplier);    loadStatus = engine.load(/*****/);//------------------------------------------之前讨论过的engine.load方法    。。。。。。  }

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