Glide源码流程分析（一）

流程：

with

public class Glide {

    ...

    public static RequestManager with(Context context) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(context);
    }

    public static RequestManager with(Activity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }

    public static RequestManager with(FragmentActivity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public static RequestManager with(android.app.Fragment fragment) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(fragment);
    }

    public static RequestManager with(Fragment fragment) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(fragment);
    }
}

1. 初始化RequestManagerRetriever ，中文的意思就是请求管理者的领取人，我们从代码看他就是为了创建RequestManager
2. 全是重载，传入的retriever.get();参数不同得到不同类型的RequestManager
3. 继续探究retriever.get()

public class RequestManagerRetriever implements Handler.Callback {

    private static final RequestManagerRetriever INSTANCE = new RequestManagerRetriever();

    private volatile RequestManager applicationManager;

    ...

    /**
     * Retrieves and returns the RequestManagerRetriever singleton.
     */
    public static RequestManagerRetriever get() {
        return INSTANCE;
    }

    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);
        }
    }

    public RequestManager get(Fragment fragment) {
        if (fragment.getActivity() == null) {
            throw new IllegalArgumentException("You cannot start a load on a fragment before it is attached");
        }
        if (Util.isOnBackgroundThread()) {
            return get(fragment.getActivity().getApplicationContext());
        } else {
            FragmentManager fm = fragment.getChildFragmentManager();
            return supportFragmentGet(fragment.getActivity(), fm);
        }
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public RequestManager get(Activity activity) {
        if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
            return get(activity.getApplicationContext());
        } else {
            assertNotDestroyed(activity);
            android.app.FragmentManager fm = activity.getFragmentManager();
            return fragmentGet(activity, fm);
        }
    }

    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    private static void assertNotDestroyed(Activity activity) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1 && activity.isDestroyed()) {
            throw new IllegalArgumentException("You cannot start a load for a destroyed activity");
        }
    }

    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    public RequestManager get(android.app.Fragment fragment) {
        if (fragment.getActivity() == null) {
            throw new IllegalArgumentException("You cannot start a load on a fragment before it is attached");
        }
        if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
            return get(fragment.getActivity().getApplicationContext());
        } else {
            android.app.FragmentManager fm = fragment.getChildFragmentManager();
            return fragmentGet(fragment.getActivity(), fm);
        }
    }

    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    RequestManagerFragment getRequestManagerFragment(final android.app.FragmentManager fm) {
        RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
        if (current == null) {
            current = pendingRequestManagerFragments.get(fm);
            if (current == null) {
                current = new RequestManagerFragment();
                pendingRequestManagerFragments.put(fm, current);
                fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
                handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
            }
        }
        return current;
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    RequestManager fragmentGet(Context context, android.app.FragmentManager fm) {
        RequestManagerFragment current = getRequestManagerFragment(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;
    }

    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;
    }

    ...
}

1. 主要分为两个类型主要：Application和非Application两种，每个方法的前面都是判断先不管，看后面会发现Application最后是通过getApplicationManager方法来得到requestManager的，而非Application是通过supportFragmentGet或者fragmentGet方法来获得的。
2. 探究getApplicationManager方法：我们new直接创建一个RequestManager对象传入Application的上下文。
3. 而其他情况就探究fragmentGet方法，在这之前我们看见该方法多传入了FragmentMananger，奇怪为什么传入这个东西呢，接着看fragmentGet方法内，不管怎么样执行都会向Activity添加一个隐藏的fragment，为什么要添加一个隐藏的fragment？很简单，因为我们的Glide必须监听Activity的生命流程，如果在加载过程中，我们的Activity死掉了我们的Glide岂不是就没有比较继续执行了，我们的Glide没办法监听到Activity的生死，那就可以通过fragment来监听，因为fragment的生命周期和Activity是绑定的嘛。 至于为什么传入Application就可以直接创建，因为Glide依然都和APP共生死了，我们就没必要关系Glide的销毁问题了。
4. 值得注意一点就是，如果传入的时候，当前线程属于主线程的话，Glide也会按照Application的流程执行

load

前文已经提到了创建了始终是RequestManager 然后我们看看调用了load（）方法这里我们知道如果看过源码都知道load有蛮多的重载方法。所以我们只要URL的load方法：

public class RequestManager implements LifecycleListener {

    ...

    /**
     * Returns a request builder to load the given {@link String}.
     * signature.
     *
     * @see #fromString()
     * @see #load(Object)
     *
     * @param string A file path, or a uri or url handled by {@link com.bumptech.glide.load.model.UriLoader}.
     */
    public DrawableTypeRequest<String> load(String string) {
        return (DrawableTypeRequest<String>) fromString().load(string);
    }

    /**
     * Returns a request builder that loads data from {@link String}s using an empty signature.
     *
     * <p>
     *     Note - this method caches data using only the given String as the cache key. If the data is a Uri outside of
     *     your control, or you otherwise expect the data represented by the given String to change without the String
     *     identifier changing, Consider using
     *     {@link GenericRequestBuilder#signature(Key)} to mixin a signature
     *     you create that identifies the data currently at the given String that will invalidate the cache if that data
     *     changes. Alternatively, using {@link DiskCacheStrategy#NONE} and/or
     *     {@link DrawableRequestBuilder#skipMemoryCache(boolean)} may be appropriate.
     * </p>
     *
     * @see #from(Class)
     * @see #load(String)
     */
    public DrawableTypeRequest<String> fromString() {
        return loadGeneric(String.class);
    }

    private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
        ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);
        ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =
                Glide.buildFileDescriptorModelLoader(modelClass, context);
        if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {
            throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"
                    + " which there is a registered ModelLoader, if you are using a custom model, you must first call"
                    + " Glide#register with a ModelLoaderFactory for your custom model class");
        }
        return optionsApplier.apply(
                new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
                        glide, requestTracker, lifecycle, optionsApplier));
    }

    ...

}


    class OptionsApplier {

        public <A, X extends GenericRequestBuilder<A, ?, ?, ?>> X apply(X builder) {
            if (options != null) {
                options.apply(builder);
            }
            return builder;
        }
    }

• 前面都不需要解释太多，我们只需要看见最后的调用loadGeneric方法即可，传入的是String.class，重点看看这个方法里 ，我们组要创建了两个图片下载器下载器：streamModelLoader和fileDescriptorModelLoader，这个不过多研究，但是我们得知道的是传入的modelClass创建了是一个StreamStringLoader他同样实现ModelLoader的接口。
• 最后我们创建了一个DrawableTypeRequest对象它持有下载器，上下文，生命周期等等的一个请求对象，然后调用optionsApplier.apply(）方法
• 我们查看optionsApplier.apply(）方法，可以得出相关结论，就是传出和传出的对象既然相同，所以问该方法要么是对该对象进行检查，或者是修改或者添加某样属性，这里我们options单词，大胆猜测肯定是和剪切有关的属性。

接着我们查看诉所谓的DrawableTypeRequest

public class DrawableTypeRequest<ModelType> extends DrawableRequestBuilder<ModelType> implements DownloadOptions {
    private final ModelLoader<ModelType, InputStream> streamModelLoader;
    private final ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader;
    private final RequestManager.OptionsApplier optionsApplier;

    private static <A, Z, R> FixedLoadProvider<A, ImageVideoWrapper, Z, R> buildProvider(Glide glide,
            ModelLoader<A, InputStream> streamModelLoader,
            ModelLoader<A, ParcelFileDescriptor> fileDescriptorModelLoader, Class<Z> resourceClass,
            Class<R> transcodedClass,
            ResourceTranscoder<Z, R> transcoder) {
        if (streamModelLoader == null && fileDescriptorModelLoader == null) {
            return null;
        }

        if (transcoder == null) {
            transcoder = glide.buildTranscoder(resourceClass, transcodedClass);
        }
        DataLoadProvider<ImageVideoWrapper, Z> dataLoadProvider = glide.buildDataProvider(ImageVideoWrapper.class,
                resourceClass);
        ImageVideoModelLoader<A> modelLoader = new ImageVideoModelLoader<A>(streamModelLoader,
                fileDescriptorModelLoader);
        return new FixedLoadProvider<A, ImageVideoWrapper, Z, R>(modelLoader, transcoder, dataLoadProvider);
    }

    DrawableTypeRequest(Class<ModelType> modelClass, ModelLoader<ModelType, InputStream> streamModelLoader,
            ModelLoader<ModelType, ParcelFileDescriptor> fileDescriptorModelLoader, Context context, Glide glide,
            RequestTracker requestTracker, Lifecycle lifecycle, RequestManager.OptionsApplier optionsApplier) {
        super(context, modelClass,
                buildProvider(glide, streamModelLoader, fileDescriptorModelLoader, GifBitmapWrapper.class,
                        GlideDrawable.class, null),
                glide, requestTracker, lifecycle);
        this.streamModelLoader = streamModelLoader;
        this.fileDescriptorModelLoader = fileDescriptorModelLoader;
        this.optionsApplier = optionsApplier;
    }

    /**
     * Attempts to always load the resource as a {@link android.graphics.Bitmap}, even if it could actually be animated.
     *
     * @return A new request builder for loading a {@link android.graphics.Bitmap}
     */
    public BitmapTypeRequest<ModelType> asBitmap() {
        return optionsApplier.apply(new BitmapTypeRequest<ModelType>(this, streamModelLoader,
                fileDescriptorModelLoader, optionsApplier));
    }

    /**
     * Attempts to always load the resource as a {@link com.bumptech.glide.load.resource.gif.GifDrawable}.
     * <p>
     *     If the underlying data is not a GIF, this will fail. As a result, this should only be used if the model
     *     represents an animated GIF and the caller wants to interact with the GIfDrawable directly. Normally using
     *     just an {@link DrawableTypeRequest} is sufficient because it will determine whether or
     *     not the given data represents an animated GIF and return the appropriate animated or not animated
     *     {@link android.graphics.drawable.Drawable} automatically.
     * </p>
     *
     * @return A new request builder for loading a {@link com.bumptech.glide.load.resource.gif.GifDrawable}.
     */
    public GifTypeRequest<ModelType> asGif() {
        return optionsApplier.apply(new GifTypeRequest<ModelType>(this, streamModelLoader, optionsApplier));
    }

    ...
}

上面方法主要提供了两种方法asBitmap()和asGif()这个不用多讲主要是把他转换为动态或者是静态的请求。

最后我们回到起点：

public DrawableTypeRequest<String> load(String string) {
        return (DrawableTypeRequest<String>) fromString().load(string);
    }

中的load方法 ，我们调用的对象是DrawableTypeRequest上面返回的传入的是String即URL，所以继续深入DrawableTypeRequest对象查看：

DrawableTypeRequest#load

  @Override
    public DrawableRequestBuilder<ModelType> load(ModelType model) {
        super.load(model);
        return this;
    }

他调用的是父类DrawableRequestBuilder方法：

public class DrawableRequestBuilder<ModelType>
        extends GenericRequestBuilder<ModelType, ImageVideoWrapper, GifBitmapWrapper, GlideDrawable>
        implements BitmapOptions, DrawableOptions {

    DrawableRequestBuilder(Context context, Class<ModelType> modelClass,
            LoadProvider<ModelType, ImageVideoWrapper, GifBitmapWrapper, GlideDrawable> loadProvider, Glide glide,
            RequestTracker requestTracker, Lifecycle lifecycle) {
        super(context, modelClass, loadProvider, GlideDrawable.class, glide, requestTracker, lifecycle);
        // Default to animating.
        crossFade();
    }

    public DrawableRequestBuilder<ModelType> thumbnail(
            DrawableRequestBuilder<?> thumbnailRequest) {
        super.thumbnail(thumbnailRequest);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> thumbnail(
            GenericRequestBuilder<?, ?, ?, GlideDrawable> thumbnailRequest) {
        super.thumbnail(thumbnailRequest);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> thumbnail(float sizeMultiplier) {
        super.thumbnail(sizeMultiplier);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> sizeMultiplier(float sizeMultiplier) {
        super.sizeMultiplier(sizeMultiplier);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> decoder(ResourceDecoder<ImageVideoWrapper, GifBitmapWrapper> decoder) {
        super.decoder(decoder);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> cacheDecoder(ResourceDecoder<File, GifBitmapWrapper> cacheDecoder) {
        super.cacheDecoder(cacheDecoder);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> encoder(ResourceEncoder<GifBitmapWrapper> encoder) {
        super.encoder(encoder);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> priority(Priority priority) {
        super.priority(priority);
        return this;
    }

    public DrawableRequestBuilder<ModelType> transform(BitmapTransformation... transformations) {
        return bitmapTransform(transformations);
    }

    public DrawableRequestBuilder<ModelType> centerCrop() {
        return transform(glide.getDrawableCenterCrop());
    }

    public DrawableRequestBuilder<ModelType> fitCenter() {
        return transform(glide.getDrawableFitCenter());
    }

    public DrawableRequestBuilder<ModelType> bitmapTransform(Transformation<Bitmap>... bitmapTransformations) {
        GifBitmapWrapperTransformation[] transformations =
                new GifBitmapWrapperTransformation[bitmapTransformations.length];
        for (int i = 0; i < bitmapTransformations.length; i++) {
            transformations[i] = new GifBitmapWrapperTransformation(glide.getBitmapPool(), bitmapTransformations[i]);
        }
        return transform(transformations);
    }

    @Override
    public DrawableRequestBuilder<ModelType> transform(Transformation<GifBitmapWrapper>... transformation) {
        super.transform(transformation);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> transcoder(
            ResourceTranscoder<GifBitmapWrapper, GlideDrawable> transcoder) {
        super.transcoder(transcoder);
        return this;
    }

    public final DrawableRequestBuilder<ModelType> crossFade() {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>());
        return this;
    }

    public DrawableRequestBuilder<ModelType> crossFade(int duration) {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>(duration));
        return this;
    }

    public DrawableRequestBuilder<ModelType> crossFade(int animationId, int duration) {
        super.animate(new DrawableCrossFadeFactory<GlideDrawable>(context, animationId,
                duration));
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> dontAnimate() {
        super.dontAnimate();
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> animate(ViewPropertyAnimation.Animator animator) {
        super.animate(animator);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> animate(int animationId) {
        super.animate(animationId);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> placeholder(int resourceId) {
        super.placeholder(resourceId);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> placeholder(Drawable drawable) {
        super.placeholder(drawable);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> fallback(Drawable drawable) {
        super.fallback(drawable);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> fallback(int resourceId) {
        super.fallback(resourceId);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> error(int resourceId) {
        super.error(resourceId);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> error(Drawable drawable) {
        super.error(drawable);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> listener(
            RequestListener<? super ModelType, GlideDrawable> requestListener) {
        super.listener(requestListener);
        return this;
    }
    @Override
    public DrawableRequestBuilder<ModelType> diskCacheStrategy(DiskCacheStrategy strategy) {
        super.diskCacheStrategy(strategy);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> skipMemoryCache(boolean skip) {
        super.skipMemoryCache(skip);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> override(int width, int height) {
        super.override(width, height);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> sourceEncoder(Encoder<ImageVideoWrapper> sourceEncoder) {
        super.sourceEncoder(sourceEncoder);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> dontTransform() {
        super.dontTransform();
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> signature(Key signature) {
        super.signature(signature);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> load(ModelType model) {
        super.load(model);
        return this;
    }

    @Override
    public DrawableRequestBuilder<ModelType> clone() {
        return (DrawableRequestBuilder<ModelType>) super.clone();
    }

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

    @Override
    void applyFitCenter() {
        fitCenter();
    }

    @Override
    void applyCenterCrop() {
        centerCrop();
    }
}

我们查看他只不过就将URL添加进request的属性里面而已，当然这有相当多的API是我们使用的时候能接触到的，如placeholder()方法、error()方法、diskCacheStrategy()方法等等，他其实就是传入属性，然后返回对象，这样达到了链式操作。

into

接下来进入重点，into当然和前文一样还是交给父类的into方法那我们来看看into方法吧

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));
}

前面先不管，这个为了 transfrom 而设计的，所以我们直接跳过看最后的return代码，他调用的是glide.buildImageViewTarget 继续查看代码：

Glide.buildImageViewTarget :

<R> Target<R> buildImageViewTarget(ImageView imageView, Class<R> transcodedClass) {
    return imageViewTargetFactory.buildTarget(imageView, transcodedClass);
}

这里我们可以看见通过工厂模式创建Target，Target中文名即为目标这里指的是我们需要展示的目标。接下来继续进入：

ImageViewTargetFactory#buildTarget：

 public class ImageViewTargetFactory {
    @SuppressWarnings("unchecked")
    public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
        if (GlideDrawable.class.isAssignableFrom(clazz)) {
            return (Target<Z>) new GlideDrawableImageViewTarget(view);
        } else if (Bitmap.class.equals(clazz)) {
            return (Target<Z>) new BitmapImageViewTarget(view);
        } else if (Drawable.class.isAssignableFrom(clazz)) {
            return (Target<Z>) new DrawableImageViewTarget(view);
        } else {
            throw new IllegalArgumentException("Unhandled class: " + clazz
                    + ", try .as*(Class).transcode(ResourceTranscoder)");
        }
    }
}

这里我们看见我们通过传入的clazz参数不桶从而创建出不同的Target对象，这是一个典型抽象工厂模式。至于clazz我直接在这里简单梳理，除非调用asBitmap()我们创建的是BitmapImageViewTarget对象，一般情况下就是我们不设置的情况下，我们设置的是GlideDrawableImageViewTarget对象，所以我们这里返回的也是他。

所以我们回到上面代码的最后一行调用into方法中

GenericRequestBuilder#into（）：

 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;
    }

这里我们看见他真真正正创建了Request，并且把它传入了target这个目标展示对象，然后target这个目标加入了Glide的生命对象中，然后调用requestTracker.runRequest方法执行这个请求。到这里可能大家有些懵逼，所以我们先拉回buildRequest（）看看他是怎么创建的：

GenericRequestBuilder#buildRequest：

    private Request buildRequest(Target<TranscodeType> target) {
        if (priority == null) {
            priority = Priority.NORMAL;
        }
        return buildRequestRecursive(target, null);
    }

    private Request buildRequestRecursive(Target<TranscodeType> target, ThumbnailRequestCoordinator parentCoordinator) {
        if (thumbnailRequestBuilder != null) {
            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()");
            }
            // Recursive case: contains a potentially recursive thumbnail request builder.
            if (thumbnailRequestBuilder.animationFactory.equals(NoAnimation.getFactory())) {
                thumbnailRequestBuilder.animationFactory = animationFactory;
            }

            if (thumbnailRequestBuilder.priority == null) {
                thumbnailRequestBuilder.priority = getThumbnailPriority();
            }

            if (Util.isValidDimensions(overrideWidth, overrideHeight)
                    && !Util.isValidDimensions(thumbnailRequestBuilder.overrideWidth,
                            thumbnailRequestBuilder.overrideHeight)) {
              thumbnailRequestBuilder.override(overrideWidth, overrideHeight);
            }

            ThumbnailRequestCoordinator coordinator = new ThumbnailRequestCoordinator(parentCoordinator);
            Request fullRequest = obtainRequest(target, sizeMultiplier, priority, coordinator);
            // Guard against infinite recursion.
            isThumbnailBuilt = true;
            // Recursively generate thumbnail requests.
            Request thumbRequest = thumbnailRequestBuilder.buildRequestRecursive(target, coordinator);
            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, sizeMultiplier, priority, coordinator);
            Request thumbnailRequest = obtainRequest(target, thumbSizeMultiplier, getThumbnailPriority(), coordinator);
            coordinator.setRequests(fullRequest, thumbnailRequest);
            return coordinator;
        } else {
            // Base case: no thumbnail.
            return obtainRequest(target, sizeMultiplier, priority, parentCoordinator);
        }
    }
    
       private Request obtainRequest(Target<TranscodeType> target, float sizeMultiplier, Priority priority,
            RequestCoordinator requestCoordinator) {
        return GenericRequest.obtain(
                loadProvider,
                model,
                signature,
                context,
                priority,
                target,
                sizeMultiplier,
                placeholderDrawable,
                placeholderId,
                errorPlaceholder,
                errorId,
                fallbackDrawable,
                fallbackResource,
                requestListener,
                requestCoordinator,
                glide.getEngine(),
                transformation,
                transcodeClass,
                isCacheable,
                animationFactory,
                overrideWidth,
                overrideHeight,
                diskCacheStrategy);
    }
    

• 我们看见先从看的当然是Request是如何创建的这是主线，这里调用的是obtainRequest方法执行的，然后这里大半部分代码都有一个thumbnail 单词，如果你有相关写图库的经历，就知道这个单词与压缩图有关，所以我们大胆猜测与压缩图有关系的代码，接着我们就交给了GenericRequest.obtain代码：

GenericRequest#obtain

从传入的参数印证了Request是请求类的说法，他传入的都是与请求先关的属性。

public static <A, T, Z, R> GenericRequest<A, T, Z, R> obtain(
            LoadProvider<A, T, Z, R> loadProvider,
            A model,
            Key signature,
            Context context,
            Priority priority,
            Target<R> target,
            float sizeMultiplier,
            Drawable placeholderDrawable,
            int placeholderResourceId,
            Drawable errorDrawable,
            int errorResourceId,
            Drawable fallbackDrawable,
            int fallbackResourceId,
            RequestListener<? super A, R> requestListener,
            RequestCoordinator requestCoordinator,
            Engine engine,
            Transformation<Z> transformation,
            Class<R> transcodeClass,
            boolean isMemoryCacheable,
            GlideAnimationFactory<R> animationFactory,
            int overrideWidth,
            int overrideHeight,
            DiskCacheStrategy diskCacheStrategy) {
        @SuppressWarnings("unchecked")
        GenericRequest<A, T, Z, R> request = (GenericRequest<A, T, Z, R>) REQUEST_POOL.poll();
        if (request == null) {
            request = new GenericRequest<A, T, Z, R>();
        }
        request.init(loadProvider,
                model,
                signature,
                context,
                priority,
                target,
                sizeMultiplier,
                placeholderDrawable,
                placeholderResourceId,
                errorDrawable,
                errorResourceId,
                fallbackDrawable,
                fallbackResourceId,
                requestListener,
                requestCoordinator,
                engine,
                transformation,
                transcodeClass,
                isMemoryCacheable,
                animationFactory,
                overrideWidth,
                overrideHeight,
                diskCacheStrategy);
        return request;
    }
    
    
    private void init(
            LoadProvider<A, T, Z, R> loadProvider,
            A model,
            Key signature,
            Context context,
            Priority priority,
            Target<R> target,
            float sizeMultiplier,
            Drawable placeholderDrawable,
            int placeholderResourceId,
            Drawable errorDrawable,
            int errorResourceId,
            Drawable fallbackDrawable,
            int fallbackResourceId,
            RequestListener<? super A, R> requestListener,
            RequestCoordinator requestCoordinator,
            Engine engine,
            Transformation<Z> transformation,
            Class<R> transcodeClass,
            boolean isMemoryCacheable,
            GlideAnimationFactory<R> animationFactory,
            int overrideWidth,
            int overrideHeight,
            DiskCacheStrategy diskCacheStrategy) {
        this.loadProvider = loadProvider;
        this.model = model;
        this.signature = signature;
        this.fallbackDrawable = fallbackDrawable;
        this.fallbackResourceId = fallbackResourceId;
        this.context = context.getApplicationContext();
        this.priority = priority;
        this.target = target;
        this.sizeMultiplier = sizeMultiplier;
        this.placeholderDrawable = placeholderDrawable;
        this.placeholderResourceId = placeholderResourceId;
        this.errorDrawable = errorDrawable;
        this.errorResourceId = errorResourceId;
        this.requestListener = requestListener;
        this.requestCoordinator = requestCoordinator;
        this.engine = engine;
        this.transformation = transformation;
        this.transcodeClass = transcodeClass;
        this.isMemoryCacheable = isMemoryCacheable;
        this.animationFactory = animationFactory;
        this.overrideWidth = overrideWidth;
        this.overrideHeight = overrideHeight;
        this.diskCacheStrategy = diskCacheStrategy;
        status = Status.PENDING;

        // We allow null models by just setting an error drawable. Null models will always have empty providers, we
        // simply skip our sanity checks in that unusual case.
        if (model != null) {
            check("ModelLoader", loadProvider.getModelLoader(), "try .using(ModelLoader)");
            check("Transcoder", loadProvider.getTranscoder(), "try .as*(Class).transcode(ResourceTranscoder)");
            check("Transformation", transformation, "try .transform(UnitTransformation.get())");
            if (diskCacheStrategy.cacheSource()) {
                check("SourceEncoder", loadProvider.getSourceEncoder(),
                        "try .sourceEncoder(Encoder) or .diskCacheStrategy(NONE/RESULT)");
            } else {
                check("SourceDecoder", loadProvider.getSourceDecoder(),
                        "try .decoder/.imageDecoder/.videoDecoder(ResourceDecoder) or .diskCacheStrategy(ALL/SOURCE)");
            }
            if (diskCacheStrategy.cacheSource() || diskCacheStrategy.cacheResult()) {
                // TODO if(resourceClass.isAssignableFrom(InputStream.class) it is possible to wrap sourceDecoder
                // and use it instead of cacheDecoder: new FileToStreamDecoder<Z>(sourceDecoder)
                // in that case this shouldn't throw
                check("CacheDecoder", loadProvider.getCacheDecoder(),
                        "try .cacheDecoder(ResouceDecoder) or .diskCacheStrategy(NONE)");
            }
            if (diskCacheStrategy.cacheResult()) {
                check("Encoder", loadProvider.getEncoder(),
                        "try .encode(ResourceEncoder) or .diskCacheStrategy(NONE/SOURCE)");
            }
        }
    }

可以看到我们是new出来了GenericRequest对象，如果没有从请求池里面拿到的话，然后调用init方法，我们看见只是个request进行属性的复制而已。