mirror of
https://github.com/facebook/react-native.git
synced 2025-11-01 09:14:26 +00:00
Valentin Shergin
432b434b55
Summary:
This is a very crucial change, everything else in this stack depends on it.
Here is the set of constraints that we have for ShadowNode-based class hierarchy:
* `ShadowNode` is a base class that defines basic operations on the nodes. It does not have virtual methods by design (virtual dispatch hurts performance; we want to limit and centralize that in `ComponentDescriptor`s).
* `ConcreteShadowNode<>` template *statically* wires particular `ShadowNode` and particular `Props` establishing a type-safe relationship between them ensured on compile time.
* Not all `ShadowNode`s are "layoutable", not all "layoutable" `ShadowNode`s use Yoga to do layout.
* Layotability (and YogaLayotability) feature is implemented as two classes `LayoutableShadowNode` and `YogaLayoutableShadowNode`. These classes essentially need to know something about the ShadowNode nature of the object (get the list of children or replace some children).
Before the change, `LayoutableShadowNode` and `YogaLayoutableShadowNode`classes did not inherit `ShadowNode` because we don't want to use *virtual inheritance* for `ShadowNode`: `ConcreteShadowNode<>` already inherits `ShadowNode`, so if we make `LayoutableShadowNode` inherits `ShadowNode`, we will have to somehow flatten this inheritance hierarchy (and the virtual inheritance is an answer to that). (Yes, C++ supports multiple inheritance and virtual inheritance.)
Before this change, we solved this dilemma this way: We have a subclass-template `ConcreteViewShadowNode<>` that inherits `ConcreteShadowNode<>` and `YogaLayoutableShadowNode`. Then, we had a bunch of methods that implement the rerouting of some functionality from `YogaLayoutableShadowNode` to `ShadowNode` and vise-versa. (See the diagram "Before".)
That worked fine, except the caveats:
* That wiring is nasty, complex, hard to reason about and overall limiting.
* There is no way to statically cast `LayoutableShadowNode` to `ShadowNode` (because there is no common base class). That forces us to use dynamic_cast on some perf critical paths (including layout, diffing and so on).
* Adding features that rely on interop between `LayoutableShadowNode` and `ShadowNode` is a nightmare.
It should be a better way to deal with this dilemma, and this diff implements a different approach: We can have a base class of `ConcreteShadowNode<>` as a template parameter. With this approach, we can make `LayoutableShadowNode` inherit `ShadowNode` and when we need to instantiate a `ConcreteShadowNode<>` that needs to be layoutable, we can just specify `YogaLayoutableShadowNode` as a base class. (See the diagram "After".)
This simple change will allow us to simplify a lot of things. The rest of the stack is about getting rid of unnecessary moving parts. Which will finally allow us to build "Inline Views" feature.
```
╭──────────────────────╮
│ ◎ ○ ○ ░░░░░░░░░░░░░░░│
├──────────────────────┤
│ │
│ │
│ Before │
│ │ ┌────────────────────────────┐ ┌────────────────────────────┐
│ │ │ │ │ │
│ │ │ ShadowNode │ │ LayoutableShadowNode │
└──────────────────────┘ │ │ │ │
└────────────────────────────┘ └────────────────────────────┘
▲ ▲
│ │
╔════════════════════════════╗ ┌────────────────────────────┐
║ ║ │ │
┌────────�║ ConcreteShadowNode<> ║ │ YogaLayoutableShadowNode │
│ ║ ║ │ │
│ ╚════════════════════════════╝ └────────────────────────────┘
│ ▲ ▲
│ │ │
│ │ │
│ │ │
│ │ ╔════════════════════════════╗ │
│ │ ║ ║ │
│ └─────║ ConcreteViewShadowNode<> ║─┘
│ ║ ║
│ ╚════════════════════════════╝
│ ▲
│ │
│ ┌──────────────┴───────────────┐
│ │ │
│ │ │
│ │ │
┌────────────────────────────┐ ┌────────────────────────────┐ ┌────────────────────────────┐
│ │ │ │ │ │
│ TextShadowNode │ │ ViewShadowNode │ │ ParagraphShadowNode │
│ │ │ │ │ │
└────────────────────────────┘ └────────────────────────────┘ └────────────────────────────┘
╭──────────────────────╮
│ ◎ ○ ○ ░░░░░░░░░░░░░░░│
├──────────────────────┤
│ │ ┌────────────────────────────┐
│ │ │ │
│ After │ │ ShadowNode │
│ │ │ │
│ │ └────────────────────────────┘
│ │ ▲
└──────────────────────┘ ┌────────────────────────────────────┤
│ │
│ ╔════════════════════════════╗
┌────────────────────────────┐ ║ ConcreteShadowNode ║
│ │ ║ <Base: ShadowNode> ║
│ LayoutableShadowNode │ ║ ║
│ │ ╚════════════════════════════╝
└────────────────────────────┘ ▲
▲ │
│ ┌────────────────────────────┐
┌────────────────────────────┐ │ │
│ │ │ TextShadowNode │
│ YogaLayoutableShadowNode │ │ │
│ │ └────────────────────────────┘
└────────────────────────────┘
▲
│
╔════════════════════════════╗
║ ConcreteShadowNode ║
║ <Base: ║
║ YogaLayoutableShadowNode> ║
╚════════════════════════════╝
▲
│
╔════════════════════════════╗
║ ║
║ ConcreteViewShadowNode<> ║
║ ║
╚════════════════════════════╝
▲
├─────────────────────────────────────┐
│ │
┌────────────────────────────┐ ┌────────────────────────────┐
│ │ │ │
│ ParagraphShadowNode │ │ ViewShadowNode │
│ │ │ │
└────────────────────────────┘ └────────────────────────────┘
```
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D19963353
fbshipit-source-id: b65c8a5064bdb54ab64f08a8e546aa9e2b5a486b
48 lines
1.3 KiB
C++
48 lines
1.3 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*/
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#pragma once
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#include <string>
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#include <react/debug/DebugStringConvertible.h>
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namespace facebook {
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namespace react {
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#if RN_DEBUG_STRING_CONVERTIBLE
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// Trivial implementation of `DebugStringConvertible` abstract class
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// with a stored output; useful for assembling `DebugStringConvertible` values
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// in custom implementations of `getDebugChildren` and `getDebugProps`.
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class DebugStringConvertibleItem : public DebugStringConvertible {
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public:
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DebugStringConvertibleItem(const DebugStringConvertibleItem &item) = default;
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DebugStringConvertibleItem(
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const std::string &name = "",
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const std::string &value = "",
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const SharedDebugStringConvertibleList &props = {},
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const SharedDebugStringConvertibleList &children = {});
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std::string getDebugName() const override;
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std::string getDebugValue() const override;
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SharedDebugStringConvertibleList getDebugChildren() const override;
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SharedDebugStringConvertibleList getDebugProps() const override;
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private:
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std::string name_;
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std::string value_;
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SharedDebugStringConvertibleList debugProps_;
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SharedDebugStringConvertibleList children_;
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};
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#endif
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} // namespace react
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} // namespace facebook
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