在Android应用开发过程中，关于图片加载框架有很多：Glide、Picasso、Fresco、ImageLoader。个人觉得比较好用的还是Glide框架，一行代码就可以搞定图片加载，使用简单，链式调用优雅简洁，且支持Gif、SVG格式，和Activity/Fragment 生命周期相绑定。

一、用法简洁

// 依赖
dependencies {  
    compile 'com.github.bumptech.glide:glide:3.7.0'  
    compile 'com.android.support:support-v4:23.2.1'  
}  
// 用法
Glide.with(context).load(imageUrl).placeholder(phDrawable)
     .thumbnail(0.2f).into(imageView);

二、源码解析

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);
}

接下来在RequestManagerRetriever类的静态方法get

private static final RequestManagerRetriever INSTANCE = 
                new RequestManagerRetriever();

// 公有静态方法实现单例模式
public static RequestManagerRetriever get() {
    return INSTANCE;
}

// 0. 参数为 Application
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);
}

/* Glide加载图片的生命周期和应用的生命周期一致，当退出应用时，图片停止加载。
 **  在getApplicationManager方法中，就是直接创建了一个RequestManager并返回，
 ** 在构建RequestManager对象时，传入了ApplicationLifecycle对象
 */
private RequestManager getApplicationManager(Context context) {
    // Either an application context or we're on a background thread.
    if (applicationManager == null) {
        synchronized (this) {
            if (applicationManager == null) {
                applicationManager = new RequestManager(
                  context.getApplicationContext(),
                  new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());
            }
        }
    }

    return applicationManager;
}

// 1. 参数为Fragment
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);
    }
}

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;
}

// 2. 参数为Activity
@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.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;
}

至此，with方法就介绍结束了，明天接着写，未完待续。。