Glide源码学习

源码学习先看用法：Glide.with(this).load(url).into(imageView);

第一步：Glide.with(this)得到的是一个RequestManager.

//可以看到with返回的就是一个RequestManager对象
public class Glide{

...
   public static RequestManager with(Context context) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        //通过调用RequestManagerRetriever的get方法得到
        return retriever.get(context);
    }
....
}

public class RequestManagerRetriever implements Handler.Callback {
   ...
//俄汉式实现单利
 private static final RequestManagerRetriever INSTANCE = new RequestManagerRetriever();
   ...
 
    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);
    }
}

第二步：Glide.with(context).load=>RequestManager.load();返回的是一个DrawableTypeRequest<String>.

public class RequestManager implements LifecycleListener {
  ...
 
  public DrawableTypeRequest<String> load(String string) {
        return (DrawableTypeRequest<String>) fromString().load(string);
    }

  public DrawableTypeRequest<String> fromString() {
        return loadGeneric(String.class);
    }

//这个方法就创建了streamModelLoader和fileDescriptorModelLoader两个ModelLoader.
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));
    }
  ...
}

ModelLoader对象是用于加载图片的，而我们给load()方法传入不同类型的参数，这里也会得到不同的ModelLoader对象.

那么DrawableTypeRequest对象的作用是什么？

可以看到，最主要的就是它提供了asBitmap()和asGif()这两个方法。这两个方法分别是用于强制指定加载静态图片和动态图片。而从源码中可以看出，它们分别又创建了一个BitmapTypeRequest和GifTypeRequest，如果没有进行强制指定的话，那默认就是使用DrawableTypeRequest。

刚才分析道fromstring()返回的是DrawableTypeRequest，然后调用了DrawableTypeRequest的load()方法，返回的依然是DrawableTypeRequest。这里面用到了构造者模式，实现了链式编程。我们看一下DrawableTypeRequest的load()方法，没有这个方法，那么就很快想到是父类DrawableRequestBuilder中实现的。

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

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

   ....


}

DrawableRequestBuilder中有很多个方法，这些方法其实就是Glide绝大多数的API了。里面有不少我们在上篇文章中已经用过了，比如说placeholder()方法、error()方法、diskCacheStrategy()方法、override()方法等.

第三步：Glide.with(context).load(url).into=>DrawableTypeRequest(DrawableRequestBuilder).into();

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

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

  ...

}

调用了父类的into()方法。父类GenericRequestBuilder的into（）如下：

 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));
    }

 这个方法会构建出一个Target对象，Target对象则是用来最终展示图片用的.

未完待续...