glide流程

Glide.with(this).load(url).into(imageView);

with里面的参数：

   / * @see #with(android.app.Activity)
     * @see #with(android.app.Fragment)
     * @see #with(android.support.v4.app.Fragment)
     * @see #with(android.support.v4.app.FragmentActivity)
     * @see #with(android.app.Application)
     * @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);
    }

为什么可以这么多的类型呢？glide是通过添加一个fragment到布局，这样就可以通过fragment的生命周期监听到页面的生命周期，随时随地的判断这个页面是否还存在，是否可以进行请求，是否还能进行展示！如果这个页面不存在，就会throw各种错误。比如：throw new IllegalArgumentException("You cannot start a load for a destroyed activity");
throw new IllegalArgumentException("You cannot start a load on a fragment before it is attached");

 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;
    }

with方法结束返回一个requestManager。load方面里面可以传入String，ur以及本地的地址等等。

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

load方法返回DrawableTypeRequest，进去看并没有into方法，找到父类DrawableRequestBuilder，几乎所有的方法实现都在这个类里面。找到info方法，看到是继承父类的方法，继续找到父类GenericRequestBuilder。

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

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));
    }