Fragment.getContext()返回使用片段的活动的上下文

细节

由于api 23在Fragment类中引入了mHost字段

// Activity this fragment is attached to.
FragmentHostCallback mHost;

Fragment.getContext()使用它来获取上下文：

/**
 * Return the {@link Context} this fragment is currently associated with.
 */
public Context getContext() {
    return mHost == null ? null : mHost.getContext();
}

在片段的getContext()方法中获取Activity的上下文之前,有几个步骤.

1)在Activity初始化期间,创建FragmentController：

final FragmentController mFragments = FragmentController.createController(new HostCallbacks());

2)它使用HostCallbacks类(内部类的Activity)

class HostCallbacks extends FragmentHostCallback<Activity> {
    public HostCallbacks() {
        super(Activity.this /*activity*/);
    }
...
}

3)您可以看到mFragments保留对活动上下文的引用.

4)当应用程序创建一个片段时,它使用FragmentManager.并且它的实例取自mFragments(自API级别23)

/**
 * Return the FragmentManager for interacting with fragments associated
 * with this activity.
 */
public FragmentManager getFragmentManager() {
    return mFragments.getFragmentManager();
}

5)最后,Fragment.mHost字段设置在FragmentManager.moveToState(Fragment f,int newState,int transit,int transitionStyle,boolean keepActive)方法中.