一般情况下，我们用Glide加载图片如下：
Glide.with(this).load(url).into(imageView);接下来基于Glide4.9.0源码分析图片加载主线流程。
with是Glide提供的有一组静态方法，它有好几个重载方法，如下所示：
public class Glide{ ... @NonNull public static RequestManager with(@NonNull Context context) { return getRetriever(context).get(context); } @NonNull public static RequestManager with(@NonNull Activity activity) { return getRetriever(activity).get(activity); } @NonNull public static RequestManager with(@NonNull FragmentActivity activity) { return getRetriever(activity).get(activity); } @NonNull public static RequestManager with(@NonNull Fragment fragment) { return getRetriever(fragment.getContext()).get(fragment); } @Deprecated @NonNull public static RequestManager with(@NonNull android.app.Fragment fragment) { return getRetriever(fragment.getActivity()).get(fragment); } @NonNull public static RequestManager with(@NonNull View view) { return getRetriever(view.getContext()).get(view); } }可以看到，with方法的重载种类非常多，可以传入Context，FragmentActivity，Activity，Fragment，View等类型。每一个with方法都非常类似，都是先调用一个getRetriever方法然后调用一个get方法。 getRetriever方法返回一个RequestManagerRetriever的类型，如下所示：
@NonNull private static RequestManagerRetriever getRetriever(@Nullable Context context) { // Context could be null for other reasons (ie the user passes in null), but in practice it will // only occur due to errors with the Fragment lifecycle. Preconditions.checkNotNull( context, "You cannot start a load on a not yet attached View or a Fragment where getActivity() " + "returns null (which usually occurs when getActivity() is called before the Fragment " + "is attached or after the Fragment is destroyed)."); return Glide.get(context).getRequestManagerRetriever(); }接下来我们看下RequestManagerRetriever 的get方法
public class RequestManagerRetriever implements Handler.Callback{ ... @NonNull private RequestManager getApplicationManager(@NonNull 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. // TODO(b/27524013): Factor out this Glide.get() call. Glide glide = Glide.get(context.getApplicationContext()); applicationManager = factory.build( glide, new ApplicationLifecycle(), new EmptyRequestManagerTreeNode(), context.getApplicationContext()); } } } return applicationManager; } @NonNull public RequestManager get(@NonNull 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 // Only unwrap a ContextWrapper if the baseContext has a non-null application context. // Context#createPackageContext may return a Context without an Application instance, // in which case a ContextWrapper may be used to attach one. && ((ContextWrapper) context).getBaseContext().getApplicationContext() != null) { return get(((ContextWrapper) context).getBaseContext()); } } return getApplicationManager(context); } @NonNull public RequestManager get(@NonNull FragmentActivity activity) { if (Util.isOnBackgroundThread()) { return get(activity.getApplicationContext()); } else { assertNotDestroyed(activity); FragmentManager fm = activity.getSupportFragmentManager(); return supportFragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity)); } } @NonNull public RequestManager get(@NonNull Fragment fragment) { Preconditions.checkNotNull( fragment.getContext(), "You cannot start a load on a fragment before it is attached or after it is destroyed"); if (Util.isOnBackgroundThread()) { return get(fragment.getContext().getApplicationContext()); } else { FragmentManager fm = fragment.getChildFragmentManager(); return supportFragmentGet(fragment.getContext(), fm, fragment, fragment.isVisible()); } } @SuppressWarnings("deprecation") @NonNull public RequestManager get(@NonNull Activity activity) { if (Util.isOnBackgroundThread()) { return get(activity.getApplicationContext()); } else { assertNotDestroyed(activity); android.app.FragmentManager fm = activity.getFragmentManager(); return fragmentGet(activity, fm, /*parentHint=*/ null, isActivityVisible(activity)); } } @SuppressWarnings("deprecation") @NonNull public RequestManager get(@NonNull View view) { if (Util.isOnBackgroundThread()) { return get(view.getContext().getApplicationContext()); } Preconditions.checkNotNull(view); Preconditions.checkNotNull( view.getContext(), "Unable to obtain a request manager for a view without a Context"); Activity activity = findActivity(view.getContext()); // The view might be somewhere else, like a service. if (activity == null) { return get(view.getContext().getApplicationContext()); } // Support Fragments. // Although the user might have non-support Fragments attached to FragmentActivity, searching // for non-support Fragments is so expensive pre O and that should be rare enough that we // prefer to just fall back to the Activity directly. if (activity instanceof FragmentActivity) { Fragment fragment = findSupportFragment(view, (FragmentActivity) activity); return fragment != null ? get(fragment) : get(activity); } // Standard Fragments. android.app.Fragment fragment = findFragment(view, activity); if (fragment == null) { return get(activity); } return get(fragment); } ... @SuppressWarnings("deprecation") @Deprecated @NonNull RequestManagerFragment getRequestManagerFragment(Activity activity) { return getRequestManagerFragment( activity.getFragmentManager(), /*parentHint=*/ null, isActivityVisible(activity)); } @SuppressWarnings("deprecation") @NonNull private RequestManagerFragment getRequestManagerFragment( @NonNull final android.app.FragmentManager fm, @Nullable android.app.Fragment parentHint, boolean isParentVisible) { RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG); if (current == null) { current = pendingRequestManagerFragments.get(fm); if (current == null) { current = new RequestManagerFragment(); current.setParentFragmentHint(parentHint); if (isParentVisible) { current.getGlideLifecycle().onStart(); } pendingRequestManagerFragments.put(fm, current); fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss(); handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget(); } } return current; } @SuppressWarnings({"deprecation", "DeprecatedIsStillUsed"}) @Deprecated @NonNull private RequestManager fragmentGet( @NonNull Context context, @NonNull android.app.FragmentManager fm, @Nullable android.app.Fragment parentHint, boolean isParentVisible) { RequestManagerFragment current = getRequestManagerFragment(fm, parentHint, isParentVisible); 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.getGlideLifecycle(), current.getRequestManagerTreeNode(), context); current.setRequestManager(requestManager); } return requestManager; } @NonNull SupportRequestManagerFragment getSupportRequestManagerFragment( Context context, FragmentManager fragmentManager) { return getSupportRequestManagerFragment( fragmentManager, /*parentHint=*/ null, isActivityVisible(context)); } private static boolean isActivityVisible(Context context) { // This is a poor heuristic, but it's about all we have. We'd rather err on the side of visible // and start requests than on the side of invisible and ignore valid requests. Activity activity = findActivity(context); return activity == null || !activity.isFinishing(); } @NonNull private SupportRequestManagerFragment getSupportRequestManagerFragment( @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) { SupportRequestManagerFragment current = (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG); if (current == null) { current = pendingSupportRequestManagerFragments.get(fm); if (current == null) { current = new SupportRequestManagerFragment(); current.setParentFragmentHint(parentHint); if (isParentVisible) { current.getGlideLifecycle().onStart(); } pendingSupportRequestManagerFragments.put(fm, current); fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss(); handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget(); } } return current; } @NonNull private RequestManager supportFragmentGet( @NonNull Context context, @NonNull FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) { SupportRequestManagerFragment current = getSupportRequestManagerFragment(fm, parentHint, isParentVisible); 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.getGlideLifecycle(), current.getRequestManagerTreeNode(), context); current.setRequestManager(requestManager); } return requestManager; } }上述代码看上去逻辑有点复杂，梳理下可以发现，RequestManagerRetriever类中的get方法大致分为两种情况，即传入Application类型的参数和非Application类型的参数。
对于传入Application类型的参数，如果在Glide.with()方法中传入的是一个Application对象，会调用getApplicationManager方法来获取一个RequestManager对象，此时，Glide加载的生命周期和Application的生命周期是同步的，Application的生命周期就是应用程序的生命周期，所以如果应用程序关闭的话，Glide的加载也会同时终止。另外，如果是在非主线程中使用的Glide，不管传入的是Fragment还是Activity，都会被强制当成Application来处理。
对于传入非Application类型的情况。不管你Glide.with方法中传入的是Fragment还是Activity，都会向当前的Activity中加入一个隐藏的Fragment，并返回一个RequestManager对象，这个RequestManager对象中保存有隐藏Fragment的生命周期，这样如果加载图片的Activity被销毁了，隐藏Fragmeng可以监听到，这样Glide就可以捕获这个事件从而停止加载图片。
总的来说，with方法根据传入的参数来确定加载图片的生命周期，并且最终都会得到一个RequestManager的对象。
RequestManager的load方法也有很多重载方法，如下所示：
public class RequestManager implements LifecycleListener, ModelTypes<RequestBuilder<Drawable>>{ ... @NonNull @CheckResult public RequestBuilder<Bitmap> asBitmap() { return as(Bitmap.class).apply(DECODE_TYPE_BITMAP); } @NonNull @CheckResult public RequestBuilder<GifDrawable> asGif() { return as(GifDrawable.class).apply(DECODE_TYPE_GIF); } @NonNull @CheckResult public RequestBuilder<Drawable> asDrawable() { return as(Drawable.class); } @NonNull @CheckResult public <ResourceType> RequestBuilder<ResourceType> as( @NonNull Class<ResourceType> resourceClass) { return new RequestBuilder<>(glide, this, resourceClass, context); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable Bitmap bitmap) { return asDrawable().load(bitmap); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable Drawable drawable) { return asDrawable().load(drawable); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable String string) { return asDrawable().load(string); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable Uri uri) { return asDrawable().load(uri); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable File file) { return asDrawable().load(file); } @SuppressWarnings("deprecation") @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@RawRes @DrawableRes @Nullable Integer resourceId) { return asDrawable().load(resourceId); } @SuppressWarnings("deprecation") @CheckResult @Override @Deprecated public RequestBuilder<Drawable> load(@Nullable URL url) { return asDrawable().load(url); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable byte[] model) { return asDrawable().load(model); } @NonNull @CheckResult @Override public RequestBuilder<Drawable> load(@Nullable Object model) { return asDrawable().load(model); } ... }从上面代码中可以看到，load方法中会先调用一个as函数，像asBitmap，asGif或者asDrawable，as系列函数会返回一个RequestBuilder的泛型对象，然后调用该对象的load函数。
RequestBuilder的load函数如下所示：
@NonNull @CheckResult @SuppressWarnings("unchecked") @Override public RequestBuilder<TranscodeType> load(@Nullable Object model) { return loadGeneric(model); } @NonNull private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) { this.model = model; isModelSet = true; return this; }另外可以看到RequestBuilder继承了BaseRequestOptions，而BaseRequestOptions中封装了我们日常常用一些方法，比如error，placeholder，override，skipMemoryCache，diskCacheStrategy等等方法，这里由于只分析Glide加载图主线流程，就不会这个方法进行深入分析了。
load方法返回RequestBuilder对象之后，接下来我们分析into方法。
into方法是整个Glide加载图片过程中最复杂的一部分。 into方法代码如下：
@NonNull public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) { Util.assertMainThread(); Preconditions.checkNotNull(view); BaseRequestOptions<?> requestOptions = this; if (!requestOptions.isTransformationSet() && requestOptions.isTransformationAllowed() && view.getScaleType() != null) { // Clone in this method so that if we use this RequestBuilder to load into a View and then // into a different target, we don't retain the transformation applied based on the previous // View's scale type. switch (view.getScaleType()) { case CENTER_CROP: requestOptions = requestOptions.clone().optionalCenterCrop(); break; case CENTER_INSIDE: requestOptions = requestOptions.clone().optionalCenterInside(); break; case FIT_CENTER: case FIT_START: case FIT_END: requestOptions = requestOptions.clone().optionalFitCenter(); break; case FIT_XY: requestOptions = requestOptions.clone().optionalCenterInside(); break; case CENTER: case MATRIX: default: // Do nothing. } } return into( glideContext.buildImageViewTarget(view, transcodeClass), /*targetListener=*/ null, requestOptions, Executors.mainThreadExecutor()); }最后一行会调用glideContext.buildImageViewTarget(view, transcodeClass)来构建一个target对象，这个target对象是用来显示图片用的，我们来看下：
@NonNull public <X> ViewTarget<ImageView, X> buildImageViewTarget( @NonNull ImageView imageView, @NonNull Class<X> transcodeClass) { return imageViewTargetFactory.buildTarget(imageView, transcodeClass); }这里会调用ImageViewTargetFactory的buildTarget()方法，代码如下：
public class ImageViewTargetFactory { @NonNull @SuppressWarnings("unchecked") public <Z> ViewTarget<ImageView, Z> buildTarget( @NonNull ImageView view, @NonNull Class<Z> clazz) { if (Bitmap.class.equals(clazz)) { return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view); } else if (Drawable.class.isAssignableFrom(clazz)) { return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view); } else { throw new IllegalArgumentException( "Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)"); } } }在buildTarget()方法中会根据传入的class参数来构建不同的Target对象，那这个class参数从哪里来呢，追溯可知，这个在前面load的时候由调用asBitmap()，asDrawable()等方法，默认情况下是asDrawable，里面的as方法传入的是Drawable.class，如果加载图片时加上了asBitmap，那么里面会调用as方法并传入Bitmap.class，而这个就是最终传递到buildTarget()方法的class参数。所以，如果你在使用Glide加载图片的时候调用了asBitmap()方法，那么这里就会构建出BitmapImageViewTarget对象，否则的话构建的都是DrawableImageViewTarget对象。
这里我们分析一般情况，也就是说buildImageViewTarget会创建一个DrawableImageViewTarget对象。接着我们继续进入上面into方法最后一行分析：
private <Y extends Target<TranscodeType>> Y into( @NonNull Y target, @Nullable RequestListener<TranscodeType> targetListener, BaseRequestOptions<?> options, Executor callbackExecutor) { Preconditions.checkNotNull(target); if (!isModelSet) { throw new IllegalArgumentException("You must call #load() before calling #into()"); } Request request = buildRequest(target, targetListener, options, callbackExecutor); Request previous = target.getRequest(); if (request.isEquivalentTo(previous) && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) { request.recycle(); // If the request is completed, beginning again will ensure the result is re-delivered, // triggering RequestListeners and Targets. If the request is failed, beginning again will // restart the request, giving it another chance to complete. If the request is already // running, we can let it continue running without interruption. if (!Preconditions.checkNotNull(previous).isRunning()) { // Use the previous request rather than the new one to allow for optimizations like skipping // setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions // that are done in the individual Request. previous.begin(); } return target; } requestManager.clear(target); target.setRequest(request); requestManager.track(target, request); return target; }这里会通过buildRequest方法创建一个request的对象。如果这个target上之前已经创建过一个request对象previous并且和新创建的request相等，就调用previous的begin方法，否则调用requestManager.clear(target)，并设置target的request为新创建的request，然后调用requestManager.track(target, request)方法，我们看看这个track方法。
synchronized void track(@NonNull Target<?> target, @NonNull Request request) { targetTracker.track(target); requestTracker.runRequest(request); }这里继续调用了 requestTracker.runRequest(request);
public void runRequest(@NonNull Request request) { requests.add(request); if (!isPaused) { request.begin(); } else { request.clear(); if (Log.isLoggable(TAG, Log.VERBOSE)) { Log.v(TAG, "Paused, delaying request"); } pendingRequests.add(request); } }从上面可以，最终会调用request.begin()，点进去发现Request是一个接口，那么这个Request接口的具体实现在哪里呢，我们回到上面的buildRequest()方法就可以知道。
private Request buildRequest( Target<TranscodeType> target, @Nullable RequestListener<TranscodeType> targetListener, BaseRequestOptions<?> requestOptions, Executor callbackExecutor) { return buildRequestRecursive( target, targetListener, /*parentCoordinator=*/ null, transitionOptions, requestOptions.getPriority(), requestOptions.getOverrideWidth(), requestOptions.getOverrideHeight(), requestOptions, callbackExecutor); } private Request buildRequestRecursive( Target<TranscodeType> target, @Nullable RequestListener<TranscodeType> targetListener, @Nullable RequestCoordinator parentCoordinator, TransitionOptions<?, ? super TranscodeType> transitionOptions, Priority priority, int overrideWidth, int overrideHeight, BaseRequestOptions<?> requestOptions, Executor callbackExecutor) { // Build the ErrorRequestCoordinator first if necessary so we can update parentCoordinator. ErrorRequestCoordinator errorRequestCoordinator = null; if (errorBuilder != null) { errorRequestCoordinator = new ErrorRequestCoordinator(parentCoordinator); parentCoordinator = errorRequestCoordinator; } Request mainRequest = buildThumbnailRequestRecursive( target, targetListener, parentCoordinator, transitionOptions, priority, overrideWidth, overrideHeight, requestOptions, callbackExecutor); if (errorRequestCoordinator == null) { return mainRequest; } int errorOverrideWidth = errorBuilder.getOverrideWidth(); int errorOverrideHeight = errorBuilder.getOverrideHeight(); if (Util.isValidDimensions(overrideWidth, overrideHeight) && !errorBuilder.isValidOverride()) { errorOverrideWidth = requestOptions.getOverrideWidth(); errorOverrideHeight = requestOptions.getOverrideHeight(); } Request errorRequest = errorBuilder.buildRequestRecursive( target, targetListener, errorRequestCoordinator, errorBuilder.transitionOptions, errorBuilder.getPriority(), errorOverrideWidth, errorOverrideHeight, errorBuilder, callbackExecutor); errorRequestCoordinator.setRequests(mainRequest, errorRequest); return errorRequestCoordinator; }buildRequest()方法会调用buildRequestRecursive()方法，buildThumbnailRequestRecursive方法中又会调用buildThumbnailRequestRecursive()方法，接下来我们看看buildThumbnailRequestRecursive()方法。
private Request buildThumbnailRequestRecursive( Target<TranscodeType> target, RequestListener<TranscodeType> targetListener, @Nullable RequestCoordinator parentCoordinator, TransitionOptions<?, ? super TranscodeType> transitionOptions, Priority priority, int overrideWidth, int overrideHeight, BaseRequestOptions<?> requestOptions, Executor callbackExecutor) { if (thumbnailBuilder != null) { // Recursive case: contains a potentially recursive thumbnail request builder. if (isThumbnailBuilt) { throw new IllegalStateException( "You cannot use a request as both the main request and a " + "thumbnail, consider using clone() on the request(s) passed to thumbnail()"); } TransitionOptions<?, ? super TranscodeType> thumbTransitionOptions = thumbnailBuilder.transitionOptions; // Apply our transition by default to thumbnail requests but avoid overriding custom options // that may have been applied on the thumbnail request explicitly. if (thumbnailBuilder.isDefaultTransitionOptionsSet) { thumbTransitionOptions = transitionOptions; } Priority thumbPriority = thumbnailBuilder.isPrioritySet() ? thumbnailBuilder.getPriority() : getThumbnailPriority(priority); int thumbOverrideWidth = thumbnailBuilder.getOverrideWidth(); int thumbOverrideHeight = thumbnailBuilder.getOverrideHeight(); if (Util.isValidDimensions(overrideWidth, overrideHeight) && !thumbnailBuilder.isValidOverride()) { thumbOverrideWidth = requestOptions.getOverrideWidth(); thumbOverrideHeight = requestOptions.getOverrideHeight(); } ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator); Request fullRequest = obtainRequest( target, targetListener, requestOptions, coordinator, transitionOptions, priority, overrideWidth, overrideHeight, callbackExecutor); isThumbnailBuilt = true; // Recursively generate thumbnail requests. Request thumbRequest = thumbnailBuilder.buildRequestRecursive( target, targetListener, coordinator, thumbTransitionOptions, thumbPriority, thumbOverrideWidth, thumbOverrideHeight, thumbnailBuilder, callbackExecutor); isThumbnailBuilt = false; coordinator.setRequests(fullRequest, thumbRequest); return coordinator; } else if (thumbSizeMultiplier != null) { // Base case: thumbnail multiplier generates a thumbnail request, but cannot recurse. ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator); Request fullRequest = obtainRequest( target, targetListener, requestOptions, coordinator, transitionOptions, priority, overrideWidth, overrideHeight, callbackExecutor); BaseRequestOptions<?> thumbnailOptions = requestOptions.clone().sizeMultiplier(thumbSizeMultiplier); Request thumbnailRequest = obtainRequest( target, targetListener, thumbnailOptions, coordinator, transitionOptions, getThumbnailPriority(priority), overrideWidth, overrideHeight, callbackExecutor); coordinator.setRequests(fullRequest, thumbnailRequest); return coordinator; } else { // Base case: no thumbnail. return obtainRequest( target, targetListener, requestOptions, parentCoordinator, transitionOptions, priority, overrideWidth, overrideHeight, callbackExecutor); } } private Request obtainRequest( Target<TranscodeType> target, RequestListener<TranscodeType> targetListener, BaseRequestOptions<?> requestOptions, RequestCoordinator requestCoordinator, TransitionOptions<?, ? super TranscodeType> transitionOptions, Priority priority, int overrideWidth, int overrideHeight, Executor callbackExecutor) { return SingleRequest.obtain( context, glideContext, model, transcodeClass, requestOptions, overrideWidth, overrideHeight, priority, target, targetListener, requestListeners, requestCoordinator, glideContext.getEngine(), transitionOptions.getTransitionFactory(), callbackExecutor); }buildThumbnailRequestRecursive()方法中大部分代码在处理缩略图，只看主要流程的话，这里会调用最后else里面的obtainRequest()方法，在obtainRequest()方法中会继续调用SingleRequest.obtain()来获取Request，我们继续看。
public final class SingleRequest<R> implements Request, SizeReadyCallback, ResourceCallback, FactoryPools.Poolable { ... public static <R> SingleRequest<R> obtain( Context context, GlideContext glideContext, Object model, Class<R> transcodeClass, BaseRequestOptions<?> requestOptions, int overrideWidth, int overrideHeight, Priority priority, Target<R> target, RequestListener<R> targetListener, @Nullable List<RequestListener<R>> requestListeners, RequestCoordinator requestCoordinator, Engine engine, TransitionFactory<? super R> animationFactory, Executor callbackExecutor) { @SuppressWarnings("unchecked") SingleRequest<R> request = (SingleRequest<R>) POOL.acquire(); if (request == null) { request = new SingleRequest<>(); } request.init( context, glideContext, model, transcodeClass, requestOptions, overrideWidth, overrideHeight, priority, target, targetListener, requestListeners, requestCoordinator, engine, animationFactory, callbackExecutor); return request; } ... }可以看到obtain()方法中会创建一个SingleRequest对象,所以前面调用Request的begin方法就是这里SingleRequest的begin()方法，我们继续看。
@Override public synchronized void begin() { assertNotCallingCallbacks(); stateVerifier.throwIfRecycled(); startTime = LogTime.getLogTime(); if (model == null) { if (Util.isValidDimensions(overrideWidth, overrideHeight)) { width = overrideWidth; height = overrideHeight; } // Only log at more verbose log levels if the user has set a fallback drawable, because // fallback Drawables indicate the user expects null models occasionally. int logLevel = getFallbackDrawable() == null ? Log.WARN : Log.DEBUG; onLoadFailed(new GlideException("Received null model"), logLevel); return; } if (status == Status.RUNNING) { throw new IllegalArgumentException("Cannot restart a running request"); } // If we're restarted after we're complete (usually via something like a notifyDataSetChanged // that starts an identical request into the same Target or View), we can simply use the // resource and size we retrieved the last time around and skip obtaining a new size, starting a // new load etc. This does mean that users who want to restart a load because they expect that // the view size has changed will need to explicitly clear the View or Target before starting // the new load. if (status == Status.COMPLETE) { onResourceReady(resource, DataSource.MEMORY_CACHE); return; } // Restarts for requests that are neither complete nor running can be treated as new requests // and can run again from the beginning. status = Status.WAITING_FOR_SIZE; if (Util.isValidDimensions(overrideWidth, overrideHeight)) { onSizeReady(overrideWidth, overrideHeight); } else { target.getSize(this); } if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE) && canNotifyStatusChanged()) { target.onLoadStarted(getPlaceholderDrawable()); } if (IS_VERBOSE_LOGGABLE) { logV("finished run method in " + LogTime.getElapsedMillis(startTime)); } }上面begin方法分两种情况，一种是使用了override() API为图片指定了一个固定的宽高，一种是没有指定。如果指定了的话，就会调用onSizeReady()方法，如果没有指定的话，会调用target.getSize(this)，这里的this是SizeReadyCallback，target.getSize()方法的内部会根据ImageView的layout_width和layout_height的值做一系列的计算，来算出图片应该的宽高，计算完之后回调SizeReadyCallback的onSizeReady()方法，所以，最终都会调用onSizeReady()方法，我们继续分析onSizeReady()方法。
@Override public synchronized void onSizeReady(int width, int height) { stateVerifier.throwIfRecycled(); if (IS_VERBOSE_LOGGABLE) { logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime)); } if (status != Status.WAITING_FOR_SIZE) { return; } status = Status.RUNNING; float sizeMultiplier = requestOptions.getSizeMultiplier(); this.width = maybeApplySizeMultiplier(width, sizeMultiplier); this.height = maybeApplySizeMultiplier(height, sizeMultiplier); if (IS_VERBOSE_LOGGABLE) { logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime)); } loadStatus = engine.load( glideContext, model, requestOptions.getSignature(), this.width, this.height, requestOptions.getResourceClass(), transcodeClass, priority, requestOptions.getDiskCacheStrategy(), requestOptions.getTransformations(), requestOptions.isTransformationRequired(), requestOptions.isScaleOnlyOrNoTransform(), requestOptions.getOptions(), requestOptions.isMemoryCacheable(), requestOptions.getUseUnlimitedSourceGeneratorsPool(), requestOptions.getUseAnimationPool(), requestOptions.getOnlyRetrieveFromCache(), this, callbackExecutor); // This is a hack that's only useful for testing right now where loads complete synchronously // even though under any executor running on any thread but the main thread, the load would // have completed asynchronously. if (status != Status.RUNNING) { loadStatus = null; } if (IS_VERBOSE_LOGGABLE) { logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime)); } }可以看到onSizeReady()方法中最终回到用engine.load()方法，并传入一些列的参数，由于篇幅原因，下一篇Glide加载图片源码分析（二）将继续讲解Glide图片加载流程。
