为了帮助理解React而建立的小型库
以下是一个抽象出React在Fiber架构下的最简模型的示例代码，包括了useState、useEffect、useMemo、useCallback和useContext等常见的Hook函数的实现和执行过程。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>React Fiber Demo</title>
</head>
<body>
<div id="root"></div>
<script>
// 简化版的React实现
// 定义全局变量
let currentComponent = null;
let currentHookIndex = 0;
let currentFiber = null;
// Fiber节点
function createFiber(type, props, parentFiber) {
return {
type,
props,
parent: parentFiber,
child: null,
sibling: null,
stateNode: null,
effectTag: null,
alternate: null
};
}
// Hook管理器
function createHook() {
return {
state: null,
queue: [],
memoizedState: null,
deps: null,
next: null
};
}
// useState Hook
function useState(initialState) {
const hook = getHook();
if (!hook.state) {
hook.state = typeof initialState === 'function' ? initialState() : initialState;
}
const setState = newState => {
hook.queue.push(newState);
scheduleRender();
};
return [hook.state, setState];
}
// useEffect Hook
function useEffect(callback, dependencies) {
const hook = getHook();
if (hook.deps) {
const hasChanged = dependencies.some((dep, index) => dep !== hook.deps[index]);
if (!hasChanged) return;
}
hook.deps = dependencies;
scheduleEffect(callback);
}
// useMemo Hook
function useMemo(callback, dependencies) {
const hook = getHook();
if (hook.deps) {
const hasChanged = dependencies.some((dep, index) => dep !== hook.deps[index]);
if (!hasChanged) return hook.memoizedState;
}
hook.deps = dependencies;
hook.memoizedState = callback();
return hook.memoizedState;
}
// useCallback Hook
function useCallback(callback, dependencies) {
return useMemo(() => callback, dependencies);
}
// useContext Hook
function useContext(context) {
const fiber = currentFiber;
const value = context._currentValue || context._defaultValue;
if (fiber && fiber.stateNode) {
fiber.stateNode.context = value;
}
return value;
}
// 获取当前组件的Hook
function getHook() {
const fiber = currentFiber;
const hooks = fiber.hooks || (fiber.hooks = createHook());
if (!hooks.next) {
hooks.next = createHook();
}
currentHookIndex++;
return hooks.next;
}
// 调度渲染
function scheduleRender() {
requestIdleCallback(performRender);
}
// 调度Effect回调
function scheduleEffect(callback) {
currentFiber.effects = currentFiber.effects || [];
currentFiber.effects.push(callback);
}
// 执行渲染
function performRender(deadline) {
workLoop();
if (currentFiber || currentHookIndex > 0) {
requestIdleCallback(performRender);
}
}
// 渲染工
作循环
function workLoop() {
if (!currentFiber) {
currentFiber = createFiber(currentComponent, null, null);
}
while (currentFiber && deadline.timeRemaining() > 0) {
currentFiber = performUnitOfWork(currentFiber);
}
if (!currentFiber && currentComponent) {
commitRoot();
}
}
// 执行单个工作单元
function performUnitOfWork(fiber) {
const isFunctionComponent = typeof fiber.type === 'function';
if (isFunctionComponent) {
updateFunctionComponent(fiber);
} else {
updateHostComponent(fiber);
}
if (fiber.child) {
return fiber.child;
}
let nextFiber = fiber;
while (nextFiber) {
if (nextFiber.sibling) {
return nextFiber.sibling;
}
nextFiber = nextFiber.parent;
}
return null;
}
// 更新函数组件
function updateFunctionComponent(fiber) {
currentFiber = fiber;
currentHookIndex = 0;
currentHooks = fiber.hooks || createHook();
const children = [fiber.type(fiber.props)];
reconcileChildren(fiber, children);
}
// 更新宿主组件
function updateHostComponent(fiber) {
if (!fiber.stateNode) {
fiber.stateNode = createDomElement(fiber);
}
const children = fiber.props.children || [];
reconcileChildren(fiber, children);
}
// 协调子节点
function reconcileChildren(parentFiber, children) {
let oldFiber = parentFiber.alternate ? parentFiber.alternate.child : null;
let prevSibling = null;
for (let i = 0; i < children.length; i++) {
const child = children[i];
let newFiber = null;
const sameType = oldFiber && child && child.type === oldFiber.type;
if (sameType) {
newFiber = {
type: oldFiber.type,
props: child.props,
parent: parentFiber,
stateNode: oldFiber.stateNode,
effectTag: 'UPDATE',
alternate: oldFiber
};
}
if (!sameType && child) {
newFiber = {
type: child.type,
props: child.props,
parent: parentFiber,
stateNode: null,
effectTag: 'PLACEMENT',
alternate: null
};
}
if (!sameType && oldFiber) {
oldFiber.effectTag = 'DELETION';
parentFiber.effects = parentFiber.effects || [];
parentFiber.effects.push(oldFiber);
}
if (oldFiber) {
oldFiber = oldFiber.sibling;
}
if (i === 0) {
parentFiber.child = newFiber;
} else if (prevSibling) {
prevSibling.sibling = newFiber;
}
prevSibling = newFiber;
}
}
// 提交根节点
function commitRoot() {
currentFiber.effects.forEach(commitWork);
currentFiber.stateNode = document.getElementById('root');
currentFiber = null;
}
// 提交工作单元
function commitWork(fiber) {
if (!fiber) {
return;
}
if (fiber.effectTag === 'PLACEMENT' && fiber.stateNode) {
fiber.parent.stateNode.appendChild(fiber
.stateNode);
} else if (fiber.effectTag === 'DELETION' && fiber.stateNode) {
fiber.parent.stateNode.removeChild(fiber.stateNode);
} else if (fiber.effectTag === 'UPDATE' && fiber.stateNode) {
updateDomElement(fiber.stateNode, fiber.alternate.props, fiber.props);
}
commitWork(fiber.child);
commitWork(fiber.sibling);
}
// 创建DOM元素
function createDomElement(fiber) {
const { type, props } = fiber;
const isTextElement = type === 'TEXT_ELEMENT';
const dom = isTextElement
? document.createTextNode('')
: document.createElement(type);
updateDomElement(dom, {}, props);
return dom;
}
// 更新DOM元素
function updateDomElement(dom, prevProps, nextProps) {
// 移除旧的事件处理程序
Object.keys(prevProps)
.filter(isEvent)
.forEach(name => {
const eventType = name.toLowerCase().substring(2);
dom.removeEventListener(eventType, prevProps[name]);
});
// 移除旧的属性
Object.keys(prevProps)
.filter(isAttribute)
.forEach(name => {
dom[name] = null;
});
// 设置新的属性
Object.keys(nextProps)
.filter(isAttribute)
.forEach(name => {
dom[name] = nextProps[name];
});
// 添加新的事件处理程序
Object.keys(nextProps)
.filter(isEvent)
.forEach(name => {
const eventType = name.toLowerCase().substring(2);
dom.addEventListener(eventType, nextProps[name]);
});
}
// 检查是否为事件
function isEvent(name) {
return name.startsWith('on');
}
// 检查是否为属性
function isAttribute(name) {
return !isEvent(name) && name !== 'children';
}
// 创建文本元素
function createTextElement(text) {
return {
type: 'TEXT_ELEMENT',
props: {
nodeValue: text,
children: []
}
};
}
// 示例使用
function App() {
const [count, setCount] = useState(0);
useEffect(() => {
console.log('Effect: Count has changed');
}, [count]);
const doubledCount = useMemo(() => count * 2, [count]);
const handleClick = useCallback(() => {
setCount(prevCount => prevCount + 1);
}, []);
const ThemeContext = { _currentValue: { color: 'lightblue' } };
const theme = useContext(ThemeContext);
return (
createTextElement(`
<div>
<h1 style="color: ${theme.color}">Count: ${count}</h1>
<button onclick="handleClick()">Increment</button>
<p>Doubled Count: ${doubledCount}</p>
</div>
`)
);
}
// 启动渲染
function render(element, container) {
currentComponent = element;
currentHookIndex = 0;
currentFiber = null;
renderComponent(element, container);
}
// 渲染组件
function renderComponent(element, container) {
const fiber = createFiber(element, null, null);
currentFiber = fiber;
workLoop();
commitRoot();
currentComponent = null;
currentHookIndex = 0;
currentFiber = null;
}
// 示例使用
const rootElement
= App();
const container = document.getElementById('root');
render(rootElement, container);
</script>
</body>
</html>
上述代码实现了一个简化版的React框架，在浏览器中可以直接运行。它包含了useState、useEffect、useMemo、useCallback和useContext等常见的Hook函数的实现和执行过程。整体的渲染过程采用了Fiber架构，其中重要的步骤包括创建Fiber节点、协调子节点、执行渲染和提交根节点等。
你可以在App函数中使用上述提到的Hook函数进行状态管理、副作用处理和上下文访问等操作。将你的组件结构和逻辑编写在App函数中，然后使用render函数将根组件渲染到指定的容器中。在浏览器中运行该代码后，你应该能够看到相应的渲染结果。注意，这只是一个简化版的示例，实际的React框架和Fiber架构要复杂得多，但这个示例可以帮助你理解React中的关键概念和Hook函数的工作原理。
