问题
As far as my understanding, FlatViewManager would flat all its children and itself as a single view called FlatViewGroup. But then I found RCTTextManager return RCTText as its shadow nodes and RCTText is not a virtual node. So RCTTextManager will return FlatViewGroup as its viewInstance? And will there be view hierarchy in FlatViewGroup? I am not sure what the meaning of flat is.
回答1:
Your understanding is correct, but there is something you are missing.
FlatViewGroup doesn't do or draw anything by itself. You can notice that every View is actually a FlatViewGroup (Text, Image, View - everything maps to a FlatViewGroup).
But how can a generic FlatViewGroup draw images, text, borders and more? Thing is, it doesn't know about any of that.
The way it works, View, Image and Text prepare content in background thread, optimize it and try to use as little resources (Android Views) as possible. In many cases, it can fit hundreds of elements into a single FlatViewGroup.
But in some cases, more Views is better. For example, it there is an element that you mutate a lot, it is best if that element is moved out into its own View so that when that element changes we can only redraw that element only.
There is flag in every FlatShadowNode calles mMountsToView that controls whether we can flatten the element into a parent.
So when a FlatShadowNode does have mMountsToView flag set to true, which View should we use? Well, this is where the ViewManager.createViewInstance() kicks in.
Now I can answer your question. Yes, RCTViewManager will return FlatViewGroup for an RCTText, but not for every RCTText. Most texts will have mMountsToView set to false and thus get flattened into a parent.
The thing where you got confused is the virtual node vs mounts to a View.
Virtual node is a Flexbox thing, it means that the node is not an independent one, and cannot be measured. This has a side effect that it is always merged with a parent non-virtual node. This is regardless of Flat implementation.
Flat UI Manager takes node flattening to a whole new level. It adds this mMountsToView flag that controls if the node will be flatten further. Set it to true if the node mutates a lot and causes its siblings to redraw for no reason.
To help you understand a little bit more, imagine that you want to optimize RCTText. You noticed that it sometimes mounts to a FlatViewGroup, but since it cannot contain any children, it doesn't have to be a ViewGroup. You could add a FlatView (or even something more specialized, like FlatTextView), copy the contents of FlatViewGroup into it, remove all children management logic, modify RCTTextManager to return your View and it will work. If you rerun your React, you will notice that some texts are now using your new Views, but only those that have mMountsToView flag set to true (you can trigger it by e.g. setting text opacity to 0.99). The rest will be flattened into parent which is likely still using a FlatViewGroup.
Hope that helps.
来源：https://stackoverflow.com/questions/43465546/what-is-the-purpose-of-flatviewmanager-in-react-native-android