【Android】使用ViewTreeLifecycleOwner获取Lifecycle

ViewTreeLifecycleOwner是什么？

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ViewTreeLifecycleOwner是Lifecycle KTX中提供的View的一个扩展方法，可以快速地获取一个最近的Fragment或者Activity的LifecycleOwner。

view.findViewTreeLifecycleOwner()

实现原理？

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ComponentActivity

通过ViewTreeLifecycleOwner.set将当前lifecycle设置到decorView

@Override
public void setContentView(@LayoutRes int layoutResID) {
    // Set the VTLO before setting the content view so that the inflation process
    // and attach listeners will see it already present
    ViewTreeLifecycleOwner.set(getWindow().getDecorView(), this);
    super.setContentView(layoutResID);
}

Fragment

通过ViewTreeLifecycleOwner.set将mViewLifecycleOwner设置到mView

  void performCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
            @Nullable Bundle savedInstanceState) {
...
            ViewTreeLifecycleOwner.set(mView, mViewLifecycleOwner);

ViewTreeLifecycleOwner

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ViewTreeLifecycleOwner.set 的实现很简单，仅仅是使用setTag进行设置

  public static void set(@NonNull View view, @Nullable LifecycleOwner lifecycleOwner) {
        view.setTag(R.id.view_tree_lifecycle_owner, lifecycleOwner);
    }

然后通过不断循环遍历parent获取最近的lifecycleOwner：

  public static LifecycleOwner get(@NonNull View view) {
        LifecycleOwner found = (LifecycleOwner) view.getTag(R.id.view_tree_lifecycle_owner);
        if (found != null) return found;
        ViewParent parent = view.getParent();
        while (found == null && parent instanceof View) {
            final View parentView = (View) parent;
            found = (LifecycleOwner) parentView.getTag(R.id.view_tree_lifecycle_owner);
            parent = parentView.getParent();
        }
        return found;
    }
}

总结

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随着MVVM的普及，我们可能需要在View内部基于lifecycle进行livedata订阅等工作，当View嵌套比较深时，使用ViewTreeLifecycleOwner扩展方法可以避免lifecycle的层层传递，简化代码