摘要：請求處理將要加載的資源放入到其中這段代碼涉及屬性暫且放到一邊，而這段代碼表明當同一個上要加載兩個資源，不論資源是否一致，將前一個資源請求取消創建新的資源請求綁定有兩個列表，用來存儲所有的和暫停掉的加載圖片的過程開啟資源請求加載圖片分幾步

DrawableRequestBuilder.into(ImageView)
RequestManager.load將要加載的資源放入到GenericRequestBuilder
GenericRequestBuilder.java:

    public Target into(ImageView view) {
        Util.assertMainThread();
        if (view == null) {
            throw new IllegalArgumentException("You must pass in a non null View");
        }

        if (!isTransformationSet && view.getScaleType() != null) {
            switch (view.getScaleType()) {
                case CENTER_CROP:
                    applyCenterCrop();
                    break;
                case FIT_CENTER:
                case FIT_START:
                case FIT_END:
                    applyFitCenter();
                    break;
                //$CASES-OMITTED$
                default:
                    // Do nothing.
            }
        }

        return into(glide.buildImageViewTarget(view, transcodeClass));
    }
    
    public > Y into(Y target) {
        Util.assertMainThread();
        ...  ...

        Request previous = target.getRequest();

        if (previous != null) {
            previous.clear();
            requestTracker.removeRequest(previous);
            previous.recycle();
        }

        Request request = buildRequest(target);
        target.setRequest(request);
        lifecycle.addListener(target);
        requestTracker.runRequest(request);

        return target;
    }

其中這段代碼涉及ImageView.scaleType屬性

           switch (view.getScaleType()) {
                case CENTER_CROP:
                    applyCenterCrop();
                    break;
                case FIT_CENTER:
                case FIT_START:
                case FIT_END:
                    applyFitCenter();
                    break;
                //$CASES-OMITTED$
                default:
                    // Do nothing.
            }

暫且放到一邊，

而

        Request previous = target.getRequest();

        if (previous != null) {
            previous.clear();
            requestTracker.removeRequest(previous);
            previous.recycle();
        }

        Request request = buildRequest(target);
        target.setRequest(request);
        lifecycle.addListener(target);
        requestTracker.runRequest(request);

RequestTracker.java:

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

這段代碼表明：
當同一個target上要加載兩個資源，不論資源是否一致，

1).將前一個資源請求取消;
2).創建新的資源請求,綁定target,

RequestTracker有兩個request列表，用來存儲所有的request和暫停掉的request

    private final Set requests = Collections.newSetFromMap(new WeakHashMap());
    private final List pendingRequests = new ArrayList();

開啟資源請求：
GenericRequest.java:

    @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 modelLoader = loadProvider.getModelLoader();
        final DataFetcher dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: "" + model + """));
            return;
        }
        ResourceTranscoder 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));
        }
    }

Engine.java:

     public  LoadStatus load(Key signature, int width, int height, DataFetcher fetcher,
            DataLoadProvider loadProvider, Transformation transformation, ResourceTranscoder 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 decodeJob = new DecodeJob(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);
    }

加載圖片分幾步：
1）.生成資源對應的Key，與width,height相關
相同的資源，不同的width,height，Key不相同

2）.從Cache中獲取

    private final MemoryCache cache;
    
    private EngineResource loadFromCache(Key key, boolean isMemoryCacheable) {
        if (!isMemoryCacheable) {
            return null;
        }

        EngineResource cached = getEngineResourceFromCache(key);
        if (cached != null) {
            cached.acquire();
            activeResources.put(key, new ResourceWeakReference(key, cached, getReferenceQueue()));
        }
        return cached;
    }
    
    @SuppressWarnings("unchecked")
    private EngineResource getEngineResourceFromCache(Key key) {
        Resource cached = cache.remove(key);

        final EngineResource result;
        if (cached == null) {
            result = null;
        } else if (cached instanceof EngineResource) {
            // Save an object allocation if we"ve cached an EngineResource (the typical case).
            result = (EngineResource) cached;
        } else {
            result = new EngineResource(cached, true /*isCacheable*/);
        }
        return result;
    }

3）從常用的中獲取：

        private final Map>> activeResources;
        
        private EngineResource loadFromActiveResources(Key key, boolean isMemoryCacheable) {
        if (!isMemoryCacheable) {
            return null;
        }

        EngineResource active = null;
        WeakReference> activeRef = activeResources.get(key);
        if (activeRef != null) {
            active = activeRef.get();
            if (active != null) {
                active.acquire();
            } else {
                activeResources.remove(key);
            }
        }

        return active;
    }

    private EngineResource loadFromCache(Key key, boolean isMemoryCacheable) {
        if (!isMemoryCacheable) {
            return null;
        }

        EngineResource cached = getEngineResourceFromCache(key);
        if (cached != null) {
            cached.acquire();
            activeResources.put(key, new ResourceWeakReference(key, cached, getReferenceQueue()));
        }
        return cached;
    }

在cache中獲取，放入activeResources