摘要：請(qǐng)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(dāng)同一個(gè)上要加載兩個(gè)資源，不論資源是否一致，將前一個(gè)資源請(qǐng)求取消創(chuàng)建新的資源請(qǐng)求綁定有兩個(gè)列表，用來(lái)存儲(chǔ)所有的和暫停掉的加載圖片的過程開啟資源請(qǐng)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(dāng)同一個(gè)target上要加載兩個(gè)資源，不論資源是否一致，

1).將前一個(gè)資源請(qǐng)求取消;
2).創(chuàng)建新的資源請(qǐng)求,綁定target,

RequestTracker有兩個(gè)request列表，用來(lái)存儲(chǔ)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(qǐng)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(duì)應(yīng)的Key，與width,height相關(guān)
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?。?/p>

        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