【Android】使用ViewTreeLifecycleOwner获取Lifecycle

逃离我推掉我的手 2023-07-17 12:37 2阅读 0赞

# 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