基本用法

使用Glide加载图片非常方便，只要简单三步链式调用即可：

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Glide.with(context)
     .load("url")
     .into(imageView);

with()

Glide.with()方法的所有重载方法都是下面的形式，只是传入的参数类型有区别，这里主要分为两步getRetriever()和get()：

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public static RequestManager with(@NonNull Context context) {
		return getRetriever(context).get(context);
}

不管with()方法的参数是啥，其实最终都是通过getRetriever()方法回去到RequestManagerRetriever对象，再通过它获取到RequestManager：

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).");
  //主要的代码
  return Glide.get(context).getRequestManagerRetriever();
}

重点主要是在RequestManagerRetriever.get(context)上，该方法会判断传入的context的实际类型，并返回不用的RequestManager，主要有如下几种特殊类型：

FragmentActivity
Activity
ContextWrapper
ApplicationContext

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) {
      return get(((ContextWrapper) context).getBaseContext());
    }
  }

  return getApplicationManager(context);
}

对于传入的是ApplicationContext或者不是在UI线程执行的情况，Glide只是简单的获取了单例的ApplicationManager对象：

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;
}

如果传入的是FragmentActivity和Activity的情况，采取的方案比较类型，首先判断是否在UI线程，如果不在，则走的逻辑和ApplicationContext的情况相同，否则继续调用supportFragmentGet()或fragmentGet()：

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));
  }
}

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));
  }
}

supportFragmentGet()和fragmentGet()的实现类似，只是一个用的app包下的Fragment，一个用的Support包下的Fragment，以supportFragmentGet()为例，该方法会创建一个没有View的空Fragment，并将它添加到Activity中，Glide会通过它对Activity的生命周期进行监听，达到根据Activity生命周期进行加载的目的：

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;
}

load()

RequestManager.load()方法会先使用asDrawable()方法创建一个RequestBuilder<Drawable>对象，再调用RequestBuilder对象的load()方法，最终调用loadGeneric()方法设置数据源，并设置数据源以设置的标记为：

public RequestBuilder<Drawable> load(@Nullable String string) {
  return asDrawable().load(string);
}

public RequestBuilder<TranscodeType> load(@Nullable String string) {
  return loadGeneric(string);
}

private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
  this.model = model;
  isModelSet = true;
  return this;
}

从名字可以看到，RequestBuilder实际上使用的是Builder模式，在此之上常见的error()、placeHolder()等方法都是基于此。

Glide源码解析

基本用法

with()

load()

into()