Glide源码分析

看了一些Glide源码分析的文章,发觉里面的部分代码实现已经发生变化,而对一些要点也没有深入分析,于是姑且自己总结一下。

※基于Glide 3.7.0。

Glide的简单使用例子:

// For a simple view:
@Override public void onCreate(Bundle savedInstanceState) {
  ...
  ImageView imageView = (ImageView) findViewById(R.id.my_image_view);

  Glide.with(this).load("http://goo.gl/gEgYUd").into(imageView);
}

先看Glide的with方法。
//com.bumptech.glide.Glide:

    /**
     * Begin a load with Glide by passing in a context.
     *
     * <p>
     *     Any requests started using a context will only have the application level options applied and will not be
     *     started or stopped based on lifecycle events. In general, loads should be started at the level the result
     *     will be used in. If the resource will be used in a view in a child fragment,
     *     the load should be started with {@link #with(android.app.Fragment)}} using that child fragment. Similarly,
     *     if the resource will be used in a view in the parent fragment, the load should be started with
     *     {@link #with(android.app.Fragment)} using the parent fragment. In the same vein, if the resource will be used
     *     in a view in an activity, the load should be started with {@link #with(android.app.Activity)}}.
     * </p>
     *
     * <p>
     *     This method is appropriate for resources that will be used outside of the normal fragment or activity
     *     lifecycle (For example in services, or for notification thumbnails).
     * </p>
     *
     * @see #with(android.app.Activity)
     * @see #with(android.app.Fragment)
     * @see #with(android.support.v4.app.Fragment)
     * @see #with(android.support.v4.app.FragmentActivity)
     *
     * @param context Any context, will not be retained.
     * @return A RequestManager for the top level application that can be used to start a load.
     */
    public static RequestManager with(Context context) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(context);
    }

with方法传入的参数可以是Context,Activity,Fragment或FragmentActiviy,这里以Context为例。
//com.bumptech.glide.manager.RequestManagerRetriever:

    private RequestManager getApplicationManager(Context context) {
        // Either an application context or we're on a background thread.
        if (applicationManager == null) {
            synchronized (this) {
                if (applicationManager == null) {
                    // Normally pause/resume is taken care of by the fragment we add to the fragment or activity.
                    // However, in this case since the manager attached to the application will not receive lifecycle
                    // events, we must force the manager to start resumed using ApplicationLifecycle.
                    applicationManager = new RequestManager(context.getApplicationContext(),
                            new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());
                }
            }
        }

        return applicationManager;
    }

    public RequestManager get(Context context) {
        if (context == null) {
            throw new IllegalArgumentException("You cannot start a load on a null Context");
        } else if (Util.isOnMainThread() && !(context instanceof Application)) {
            if (context instanceof FragmentActivity) {
                return get((FragmentActivity) context);
            } else if (context instanceof Activity) {
                return get((Activity) context);
            } else if (context instanceof ContextWrapper) {
                return get(((ContextWrapper) context).getBaseContext());
            }
        }

        return getApplicationManager(context);
    }

    public RequestManager get(FragmentActivity activity) {
        if (Util.isOnBackgroundThread()) {
            return get(activity.getApplicationContext());
        } else {
            assertNotDestroyed(activity);
            FragmentManager fm = activity.getSupportFragmentManager();
            return supportFragmentGet(activity, fm);
        }
    }

    RequestManager supportFragmentGet(Context context, FragmentManager fm) {
        SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm);
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

    SupportRequestManagerFragment getSupportRequestManagerFragment(final FragmentManager fm) {
        SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(


            FRAGMENT_TAG);
        if (current == null) {
            current = pendingSupportRequestManagerFragments.get(fm);
            if (current == null) {
                current = new SupportRequestManagerFragment();
                pendingSupportRequestManagerFragments.put(fm, current);
                fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
                handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
            }
        }
        return current;
    }

根据传入Context的类型有不同的实现,这里以FragmentActivity为例(现在常用的MD样式Activity类AppCompatActivity是FragmentActivity的子类)。方法get(FragmentActivity activity)调用了方法supportFragmentGet(activity, fm),后者返回的对象类型是SupportRequestManagerFragment 。SupportRequestManagerFragment 是一个无界面的Fragment类,起到把请求和Activity生命周期同步的作用。
然后看加载的发起点into方法。
//com.bumptech.glide.DrawableRequestBuilder:

    @Override
    public Target<GlideDrawable> into(ImageView view) {
        return super.into(view);
    }

//com.bumptech.glide.GenericRequestBuilder:

    public Target<TranscodeType> 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 extends Target<TranscodeType>> Y into(Y target) {
        Util.assertMainThread();
        if (target == null) {
            throw new IllegalArgumentException("You must pass in a non null Target");
        }
        if (!isModelSet) {
            throw new IllegalArgumentException("You must first set a model (try #load())");
        }

        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;
    }

//com.bumptech.glide.manager.RequestTracker

    /**
     * Starts tracking the given request.
     */
    public void runRequest(Request request) {
        requests.add(request);
        if (!isPaused) {
            request.begin();
        } else {
            pendingRequests.add(request);
        }
    }

//com.bumptech.glide.request.GenericRequest:

    @Override
    public void begin() {
        startTime = LogTime.getLogTime();
        if (model == null) {
            onException(null);
            return;
        }

        status = Status.WAITING_FOR_SIZE;
        if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
            onSizeReady(overrideWidth, overrideHeight);
        } else {
            target.getSize(this);
        }

        if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
            target.onLoadStarted(getPlaceholderDrawable());
        }
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished run method in " + LogTime.getElapsedMillis(startTime));
        }
    }

    /**
     * A callback method that should never be invoked directly.
     */
    @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<A, T> modelLoader = loadProvider.getModelLoader();
        final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: \'" + model + "\'"));
            return;
        }
        ResourceTranscoder<Z, R> 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));
        }
    }

onSizeReady方法中调用了Engine的load方法。
//com.bumptech.glide.load.engine.Engine:

    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();
        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<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(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);
    }

    private EngineResource<?> loadFromActiveResources(Key key, boolean isMemoryCacheable) {
        if (!isMemoryCacheable) {
            return null;
        }

        EngineResource<?> active = null;
        WeakReference<EngineResource<?>> 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;
    }

    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;
    }

先从cache中寻找资源,如果找到则将其从cache中移除并放入activeResources中,否则从activeResources中寻找。cache是LruResourceCache对象,作为资源的LRU缓存;activeResources是以弱引用为值的Map,用于缓存使用中的资源。比一般内存缓存额外多一级缓存的意义在于,当内存不足时清理cache中的资源时,不会对使用中的Bitmap造成影响。
若再次寻找失败,则创建EngineJob对象并调用其start方法。
//com.bumptech.glide.load.engine.EngineJob

    public void start(EngineRunnable engineRunnable) {
        this.engineRunnable = engineRunnable;
        future = diskCacheService.submit(engineRunnable);
    }

diskCacheService的类型是ThreadPoolExecutor的子类FifoPriorityThreadPoolExecutor,也就是说将engineRunnable放入线程池中执行。
//com.bumptech.glide.load.engine.EngineRunnable:

    @Override
    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();
    }

在EngineRunnable的run方法中进行编码,根据缓存策略调用decodeFromCache或者decodeFromSource。
先看decodeFromSource,调用DecodeJob的decodeFromSource方法。
//com.bumptech.glide.load.engine.DecodeJob:

    /**
     * Returns a transformed and transcoded resource decoded from source data, or null if no source data could be
     * obtained or no resource could be decoded.
     *
     * <p>
     *     Depending on the {@link com.bumptech.glide.load.engine.DiskCacheStrategy} used, source data is either decoded
     *     directly or first written to the disk cache and then decoded from the disk cache.
     * </p>
     *
     * @throws Exception
     */
    public Resource<Z> decodeFromSource() throws Exception {
        Resource<T> decoded = decodeSource();
        return transformEncodeAndTranscode(decoded);
    }

    private Resource<Z> transformEncodeAndTranscode(Resource<T> decoded) {
        long startTime = LogTime.getLogTime();
        Resource<T> transformed = transform(decoded);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transformed resource from source", startTime);
        }

        writeTransformedToCache(transformed);

        startTime = LogTime.getLogTime();
        Resource<Z> result = transcode(transformed);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transcoded transformed from source", startTime);
        }
        return result;
    }

    private Resource<T> transform(Resource<T> decoded) {
        if (decoded == null) {
            return null;
        }

        Resource<T> transformed = transformation.transform(decoded, width, height);
        if (!decoded.equals(transformed)) {
            decoded.recycle();
        }
        return transformed;
    }

    private Resource<Z> transcode(Resource<T> transformed) {
        if (transformed == null) {
            return null;
        }
        return transcoder.transcode(transformed);
    }

    private void writeTransformedToCache(Resource<T> transformed) {
        if (transformed == null || !diskCacheStrategy.cacheResult()) {
            return;
        }
        long startTime = LogTime.getLogTime();
        SourceWriter<Resource<T>> writer = new SourceWriter<Resource<T>>(loadProvider.getEncoder(), transformed);
        diskCacheProvider.getDiskCache().put(resultKey, writer);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Wrote transformed from source to cache", startTime);
        }
    }

    private Resource<T> decodeSource() throws Exception {
        Resource<T> decoded = null;
        try {
            long startTime = LogTime.getLogTime();
            final A data = fetcher.loadData(priority);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Fetched data", startTime);
            }
            if (isCancelled) {
                return null;
            }
            decoded = decodeFromSourceData(data);
        } finally {
            fetcher.cleanup();
        }
        return decoded;
    }

    private Resource<T> decodeFromSourceData(A data) throws IOException {
        final Resource<T> decoded;
        if (diskCacheStrategy.cacheSource()) {
            decoded = cacheAndDecodeSourceData(data);
        } else {
            long startTime = LogTime.getLogTime();
            decoded = loadProvider.getSourceDecoder().decode(data, width, height);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Decoded from source", startTime);
            }
        }
        return decoded;
    }

    private Resource<T> cacheAndDecodeSourceData(A data) throws IOException {
        long startTime = LogTime.getLogTime();
        SourceWriter<A> writer = new SourceWriter<A>(loadProvider.getSourceEncoder(), data);
        diskCacheProvider.getDiskCache().put(resultKey.getOriginalKey(), writer);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Wrote source to cache", startTime);
        }

        startTime = LogTime.getLogTime();
        Resource<T> result = loadFromCache(resultKey.getOriginalKey());
        if (Log.isLoggable(TAG, Log.VERBOSE) && result != null) {
            logWithTimeAndKey("Decoded source from cache", startTime);
        }
        return result;
    }

    private Resource<T> loadFromCache(Key key) throws IOException {
        File cacheFile = diskCacheProvider.getDiskCache().get(key);
        if (cacheFile == null) {
            return null;
        }

        Resource<T> result = null;
        try {
            result = loadProvider.getCacheDecoder().decode(cacheFile, width, height);
        } finally {
            if (result == null) {
                diskCacheProvider.getDiskCache().delete(key);
            }
        }
        return result;
    }

编码前需要先通过DataFetcher访问网络获得文件流。接口DataFetcher的实现类根据配置而不同,设置为通过OkHttp3进行网络通信的情况下,该实现类为OkHttpStreamFetcher。
之后根据需要将文件流写入磁盘缓存,再对文件流进行编码。
//com.bumptech.glide.load.resource.bitmap.StreamBitmapDecoder:

    @Override
    public Resource<Bitmap> decode(InputStream source, int width, int height) {
        Bitmap bitmap = downsampler.decode(source, bitmapPool, width, height, decodeFormat);
        return BitmapResource.obtain(bitmap, bitmapPool);
    }

//com.bumptech.glide.load.resource.bitmap.Downsampler:

    @Override
    public Bitmap decode(InputStream is, BitmapPool pool, int outWidth, int outHeight, DecodeFormat decodeFormat) {
        final ByteArrayPool byteArrayPool = ByteArrayPool.get();
        final byte[] bytesForOptions = byteArrayPool.getBytes();
        final byte[] bytesForStream = byteArrayPool.getBytes();
        final BitmapFactory.Options options = getDefaultOptions();

        // Use to fix the mark limit to avoid allocating buffers that fit entire images.
        RecyclableBufferedInputStream bufferedStream = new RecyclableBufferedInputStream(
                is, bytesForStream);
        // Use to retrieve exceptions thrown while reading.
        // TODO(#126): when the framework no longer returns partially decoded Bitmaps or provides a way to determine
        // if a Bitmap is partially decoded, consider removing.
        ExceptionCatchingInputStream exceptionStream =
                ExceptionCatchingInputStream.obtain(bufferedStream);
        // Use to read data.
        // Ensures that we can always reset after reading an image header so that we can still attempt to decode the
        // full image even when the header decode fails and/or overflows our read buffer. See #283.
        MarkEnforcingInputStream invalidatingStream = new MarkEnforcingInputStream(exceptionStream);
        try {
            exceptionStream.mark(MARK_POSITION);
            int orientation = 0;
            try {
                orientation = new ImageHeaderParser(exceptionStream).getOrientation();
            } catch (IOException e) {
                if (Log.isLoggable(TAG, Log.WARN)) {
                    Log.w(TAG, "Cannot determine the image orientation from header", e);
                }
            } finally {
                try {
                    exceptionStream.reset();
                } catch (IOException e) {
                    if (Log.isLoggable(TAG, Log.WARN)) {
                        Log.w(TAG, "Cannot reset the input stream", e);
                    }
                }
            }

            options.inTempStorage = bytesForOptions;

            final int[] inDimens = getDimensions(invalidatingStream, bufferedStream, options);
            final int inWidth = inDimens[0];
            final int inHeight = inDimens[1];

            final int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation);
            final int sampleSize = getRoundedSampleSize(degreesToRotate, inWidth, inHeight, outWidth, outHeight);

            final Bitmap downsampled =
                    downsampleWithSize(invalidatingStream, bufferedStream, options, pool, inWidth, inHeight, sampleSize,
                            decodeFormat);

            // BitmapFactory swallows exceptions during decodes and in some cases when inBitmap is non null, may catch
            // and log a stack trace but still return a non null bitmap. To avoid displaying partially decoded bitmaps,
            // we catch exceptions reading from the stream in our ExceptionCatchingInputStream and throw them here.
            final Exception streamException = exceptionStream.getException();
            if (streamException != null) {
                throw new RuntimeException(streamException);
            }

            Bitmap rotated = null;
            if (downsampled != null) {
                rotated = TransformationUtils.rotateImageExif(downsampled, pool, orientation);

                if (!downsampled.equals(rotated) && !pool.put(downsampled)) {
                    downsampled.recycle();
                }
            }

            return rotated;
        } finally {
            byteArrayPool.releaseBytes(bytesForOptions);
            byteArrayPool.releaseBytes(bytesForStream);
            exceptionStream.release();
            releaseOptions(options);
        }
    }

编码过程中通过设置采样率缩放图片,降低内存占用,提高加载性能。
编码之后通过transformEncodeAndTranscode方法进行转换处理。transform方法调用了Transformation的transform方法。Transformation是接口;BitmapTransformation实现了该接口但留下了另一个抽象方法transform;CenterCrop和FitCenter两个类继承了BitmapTransformation并实现了抽象方法transform。
//com.bumptech.glide.load.resource.bitmap.BitmapTransformation:

    @Override
    public final Resource<Bitmap> transform(Resource<Bitmap> resource, int outWidth, int outHeight) {
        if (!Util.isValidDimensions(outWidth, outHeight)) {
            throw new IllegalArgumentException("Cannot apply transformation on width: " + outWidth + " or height: "
                    + outHeight + " less than or equal to zero and not Target.SIZE_ORIGINAL");
        }
        Bitmap toTransform = resource.get();
        int targetWidth = outWidth == Target.SIZE_ORIGINAL ? toTransform.getWidth() : outWidth;
        int targetHeight = outHeight == Target.SIZE_ORIGINAL ? toTransform.getHeight() : outHeight;
        Bitmap transformed = transform(bitmapPool, toTransform, targetWidth, targetHeight);

        final Resource<Bitmap> result;
        if (toTransform.equals(transformed)) {
            result = resource;
        } else {
            result = BitmapResource.obtain(transformed, bitmapPool);
        }

        return result;
    }

//com.bumptech.glide.load.resource.bitmap.CenterCrop:

    protected Bitmap transform(BitmapPool pool, Bitmap toTransform, int outWidth, int outHeight) {
        final Bitmap toReuse = pool.get(outWidth, outHeight, toTransform.getConfig() != null
                ? toTransform.getConfig() : Bitmap.Config.ARGB_8888);
        Bitmap transformed = TransformationUtils.centerCrop(toReuse, toTransform, outWidth, outHeight);
        if (toReuse != null && toReuse != transformed && !pool.put(toReuse)) {
            toReuse.recycle();
        }
        return transformed;
    }

由BitmapPool提供一个Bitmap作为下一步的Canvas载体。BitmapPool的实现类是LruBitmapPool,顾名思义是一个基于LRU方式的Bitmap缓存池,用于Bitmap的复用。
//com.bumptech.glide.load.resource.bitmap.TransformationUtils:

    /**
     * A potentially expensive operation to crop the given Bitmap so that it fills the given dimensions. This operation
     * is significantly less expensive in terms of memory if a mutable Bitmap with the given dimensions is passed in
     * as well.
     *
     * @param recycled A mutable Bitmap with dimensions width and height that we can load the cropped portion of toCrop
     *                 into.
     * @param toCrop The Bitmap to resize.
     * @param width The width in pixels of the final Bitmap.
     * @param height The height in pixels of the final Bitmap.
     * @return The resized Bitmap (will be recycled if recycled is not null).
     */
    public static Bitmap centerCrop(Bitmap recycled, Bitmap toCrop, int width, int height) {
        if (toCrop == null) {
            return null;
        } else if (toCrop.getWidth() == width && toCrop.getHeight() == height) {
            return toCrop;
        }
        // From ImageView/Bitmap.createScaledBitmap.
        final float scale;
        float dx = 0, dy = 0;
        Matrix m = new Matrix();
        if (toCrop.getWidth() * height > width * toCrop.getHeight()) {
            scale = (float) height / (float) toCrop.getHeight();
            dx = (width - toCrop.getWidth() * scale) * 0.5f;
        } else {
            scale = (float) width / (float) toCrop.getWidth();
            dy = (height - toCrop.getHeight() * scale) * 0.5f;
        }

        m.setScale(scale, scale);
        m.postTranslate((int) (dx + 0.5f), (int) (dy + 0.5f));
        final Bitmap result;
        if (recycled != null) {
            result = recycled;
        } else {
            result = Bitmap.createBitmap(width, height, getSafeConfig(toCrop));
        }

        // We don't add or remove alpha, so keep the alpha setting of the Bitmap we were given.
        TransformationUtils.setAlpha(toCrop, result);

        Canvas canvas = new Canvas(result);
        Paint paint = new Paint(PAINT_FLAGS);
        canvas.drawBitmap(toCrop, m, paint);
        return result;
    }

在TransformationUtils的centerCrop方法中,根据目标尺寸调整矩阵并绘制结果。
与上面的流程类似,FitCenter类的transform方法调用TransformationUtils的fitCenter方法调整Bitmap。
继续看DecodeJob的transformEncodeAndTranscode方法。接下来调用writeTransformedToCache将转换结果写入磁盘缓存,再调用transcode方法进行转码。transcode方法中的transcoder的实际类型是GlideBitmapDrawableTranscoder。
//com.bumptech.glide.load.resource.transcode.GlideBitmapDrawableTranscoder:

    @Override
    public Resource<GlideBitmapDrawable> transcode(Resource<Bitmap> toTranscode) {
        GlideBitmapDrawable drawable = new GlideBitmapDrawable(resources, toTranscode.get());
        return new GlideBitmapDrawableResource(drawable, bitmapPool);
    }

GlideBitmapDrawableTranscoder的transcode方法将Bitmap资源进行封装。到这里就结束了decodeFromSource的流程。
然后看decodeFromCache。
//com.bumptech.glide.load.engine.EngineRunnable:

    private Resource<?> decodeFromCache() throws Exception {
        Resource<?> result = null;
        try {
            result = decodeJob.decodeResultFromCache();
        } catch (Exception e) {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                Log.d(TAG, "Exception decoding result from cache: " + e);
            }
        }

        if (result == null) {
            result = decodeJob.decodeSourceFromCache();
        }
        return result;
    }

先调用DecodeJob的decodeResultFromCache方法获取,获取失败则调用DecodeJob的decodeSourceFromCache方法。
//com.bumptech.glide.load.engine.DecodeJob:

    /**
     * Returns a transcoded resource decoded from transformed resource data in the disk cache, or null if no such
     * resource exists.
     *
     * @throws Exception
     */
    public Resource<Z> decodeResultFromCache() throws Exception {
        if (!diskCacheStrategy.cacheResult()) {
            return null;
        }

        long startTime = LogTime.getLogTime();
        Resource<T> transformed = loadFromCache(resultKey);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Decoded transformed from cache", startTime);
        }
        startTime = LogTime.getLogTime();
        Resource<Z> result = transcode(transformed);
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Transcoded transformed from cache", startTime);
        }
        return result;
    }

    /**
     * Returns a transformed and transcoded resource decoded from source data in the disk cache, or null if no such
     * resource exists.
     *
     * @throws Exception
     */
    public Resource<Z> decodeSourceFromCache() throws Exception {
        if (!diskCacheStrategy.cacheSource()) {
            return null;
        }

        long startTime = LogTime.getLogTime();
        Resource<T> decoded = loadFromCache(resultKey.getOriginalKey());
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logWithTimeAndKey("Decoded source from cache", startTime);
        }
        return transformEncodeAndTranscode(decoded);
    }

decodeResultFromCache方法从磁盘缓存中获取对应Bitmap并将其转码。
decodeSourceFromCache方法从磁盘缓存中获取对应Bitmap并将其转换(因为是原尺寸,需要调整大小)。
到这里结束了EngineRunnable的decode方法的流程。
嗯?好像有什么不对?EngineRunnable的decode是decodeFromCache和decodeFromSource二选一,decodeFromCache也获取失败的话就没有然后了?
//com.bumptech.glide.load.engine.EngineRunnable:

    @Override
    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 void onLoadFailed(Exception e) {
        if (isDecodingFromCache()) {
            stage = Stage.SOURCE;
            manager.submitForSource(this);
        } else {
            manager.onException(e);
        }
    }

//com.bumptech.glide.load.engine.EngineJob:

    @Override
    public void submitForSource(EngineRunnable runnable) {
        future = sourceService.submit(runnable);
    }

缓存中没有结果的情况下会调用onLoadFailed方法,变更缓存策略重新放入线程池中执行,也就是从网络获取。这里的线程池是sourceService而不是上面的diskCacheService,分开两个线程池的用意何在呢?
得到了处理结果,接下来调用EngineRunnable的onLoadComplete方法将结果传入。
//com.bumptech.glide.load.engine.EngineRunnable:

private void onLoadComplete(Resource resource) {
        manager.onResourceReady(resource);
    }

onLoadComplete方法调用了EngineJob的onResourceReady方法。
//com.bumptech.glide.load.engine.EngineJob:

    @Override
    public void onResourceReady(final Resource<?> resource) {
        this.resource = resource;
        MAIN_THREAD_HANDLER.obtainMessage(MSG_COMPLETE, this).sendToTarget();
    }

    private static class MainThreadCallback implements Handler.Callback {

        @Override
        public boolean handleMessage(Message message) {
            if (MSG_COMPLETE == message.what || MSG_EXCEPTION == message.what) {
                EngineJob job = (EngineJob) message.obj;
                if (MSG_COMPLETE == message.what) {
                    job.handleResultOnMainThread();
                } else {
                    job.handleExceptionOnMainThread();
                }
                return true;
            }

            return false;
        }
    }

    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();
                cb.onResourceReady(engineResource);
            }
        }
        // Our request is complete, so we can release the resource.
        engineResource.release();
    }

onResourceReady方法中向Handler传递消息并由MainThreadCallback处理消息 ,也就切换到了主线程。handleResultOnMainThread方法会调用GenericRequest的onResourceReady方法。
//com.bumptech.glide.request.GenericRequest:

    @Override
    public void onResourceReady(Resource<?> resource) {
        if (resource == null) {
            onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
                    + " inside, but instead got null."));
            return;
        }

        Object received = resource.get();
        if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
            releaseResource(resource);
            onException(new Exception("Expected to receive an object of " + transcodeClass
                    + " but instead got " + (received != null ? received.getClass() : "") + "{" + received + "}"
                    + " inside Resource{" + resource + "}."
                    + (received != null ? "" : " "
                        + "To indicate failure return a null Resource object, "
                        + "rather than a Resource object containing null data.")
            ));
            return;
        }

        if (!canSetResource()) {
            releaseResource(resource);
            // We can't set the status to complete before asking canSetResource().
            status = Status.COMPLETE;
            return;
        }

        onResourceReady(resource, (R) received);
    }

    private void onResourceReady(Resource<?> resource, R result) {
        // We must call isFirstReadyResource before setting status.
        boolean isFirstResource = isFirstReadyResource();
        status = Status.COMPLETE;
        this.resource = resource;

        if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
                isFirstResource)) {
            GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
            target.onResourceReady(result, animation);
        }

        notifyLoadSuccess();

        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
                    + (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
        }
    }

onResourceReady方法调用GlideDrawableImageViewTarget的onResourceReady方法。
//com.bumptech.glide.request.target.GlideDrawableImageViewTarget:

    @Override
    public void onResourceReady(GlideDrawable resource, GlideAnimation<? super GlideDrawable> animation) {
        if (!resource.isAnimated()) {
            //TODO: Try to generalize this to other sizes/shapes.
            // This is a dirty hack that tries to make loading square thumbnails and then square full images less costly
            // by forcing both the smaller thumb and the larger version to have exactly the same intrinsic dimensions.
            // If a drawable is replaced in an ImageView by another drawable with different intrinsic dimensions,
            // the ImageView requests a layout. Scrolling rapidly while replacing thumbs with larger images triggers
            // lots of these calls and causes significant amounts of jank.
            float viewRatio = view.getWidth() / (float) view.getHeight();
            float drawableRatio = resource.getIntrinsicWidth() / (float) resource.getIntrinsicHeight();
            if (Math.abs(viewRatio - 1f) <= SQUARE_RATIO_MARGIN
                    && Math.abs(drawableRatio - 1f) <= SQUARE_RATIO_MARGIN) {
                resource = new SquaringDrawable(resource, view.getWidth());
            }
        }
        super.onResourceReady(resource, animation);
        this.resource = resource;
        resource.setLoopCount(maxLoopCount);
        resource.start();
    }

    /**
     * Sets the drawable on the view using
     * {@link android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
     *
     * @param resource The {@link android.graphics.drawable.Drawable} to display in the view.
     */
    @Override
    protected void setResource(GlideDrawable resource) {
        view.setImageDrawable(resource);
    }

//com.bumptech.glide.request.target.ImageViewTarget:

    @Override
    public void onResourceReady(Z resource, GlideAnimation<? super Z> glideAnimation) {
        if (glideAnimation == null || !glideAnimation.animate(resource, this)) {
            setResource(resource);
        }
    }

    protected abstract void setResource(Z resource);

GlideDrawableImageViewTarget的onResourceReady方法调用其父类ImageViewTarget的onResourceReady方法,而ImageViewTarget的onResourceReady方法中调用的抽象方法setResource在子类GlideDrawableImageViewTarget中实现,该方法中调用了ImageView的setImageDrawable方法设置图像。至此,整个加载流程就完成了。

下图是忽略部分细节的加载流程方法调用示意。

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