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React-router 实现最需要关注的就以下几方面：

1. 路由值到视图的映射规则

2. 对于路由变更的监听

3. 路由操作方法

4. 路由记录存储与操作

1. BrowserRouter

浏览器路由实现，最主要的两个概念是变更路由与监听路由变更。

History 是整个浏览器路由历史记录大对象，其中包含长度等属性 Location 是单一页面所对应的资源定位对象，我们可以理解为当页面跳转时，先生成 Location，然后将 Location push 到 History。

 首先我们要明确几个概念：

1. 历史操作。注意：此操作不会触发 popState

• history.pushState 

• history.replaceState

2. 监听变更

• window.addEventListener("popstate", () => {})

3. 操作。注意：以下操作会触发 popState

• history.back()

• history.forward()

• history.go()

1.1. 创建Router

export function createBrowserRouter(routes: RouteObject[],opts?: DOMRouterOpts
): RemixRouter {return createRouter({basename: opts?.basename,future: {...opts?.future,v7_prependBasename: true,},history: createBrowserHistory({ window: opts?.window }),hydrationData: opts?.hydrationData || parseHydrationData(),routes,mapRouteProperties,}).initialize();
}

1.2. router下承载history

export function createBrowserHistory(options: BrowserHistoryOptions = {}
): BrowserHistory {function createBrowserLocation(window: Window,globalHistory: Window["history"]) {let { pathname, search, hash } = window.location;return createLocation("",{ pathname, search, hash },(globalHistory.state && globalHistory.state.usr) || null,(globalHistory.state && globalHistory.state.key) || "default");}function createBrowserHref(window: Window, to: To) {return typeof to === "string" ? to : createPath(to);}return getUriBasedHistory(createBrowserLocation,createBrowserHref,null,options);
}

1.3. history 确定 location

function getUrlBasedHistory(getLocation: (window: Window, globalHistory: Window["history"]) => Location,createHref: (window: Window, to: To) => string,validateLocation: ((location: Location, to: To) => void) | null,options: UrlHistoryOptions = {}
): UrlHistory {let { window = document.defaultView!, v5Compat = false } = options;let globalHistory = window.history;let action = Action.Pop;let listener: Listener | null = null;let index = getIndex()!if (index == null) {index = 0;globalHistory.replaceState({ ...globalHistory.state, idx: index }, "");}function getIndex(): number {let state = globalHistory.state || { idx: null };return state.idx;}function handlePop() {action = Action.Pop;let nextIndex = getIndex();let delta = nextIndex == null ? null : nextIndex - index;index = nextIndex;if (listener) {listener({ action, location: history.location, delta });}}function push(to: To, state?: any) {action = Action.Push;let location = createLocation(history.location, to, state);if (validateLocation) validateLocation(location, to);index = getIndex() + 1;let historyState = getHistoryState(location, index);let url = history.createHref(location);try {globalHistory.pushState(historyState, "", url);} catch (error) {if (error instanceof DOMException && error.name === "DataCloneError") {throw error;}window.location.assign(url);}if (v5Compat && listener) {listener({ action, location: history.location, delta: 1 });}}function replace(to: To, state?: any) {action = Action.Replace;let location = createLocation(history.location, to, state);if (validateLocation) validateLocation(location, to);index = getIndex();let historyState = getHistoryState(location, index);let url = history.createHref(location);globalHistory.replaceState(historyState, "", url);if (v5Compat && listener) {listener({ action, location: history.location, delta: 0 });}}function createURL(to: To): URL {let base =window.location.origin !== "null"? window.location.origin: window.location.href;let href = typeof to === "string" ? to : createPath(to);invariant(base,`No window.location.(origin|href) available to create URL for href: ${href}`),return new URL(href, base);}let history: History = {get action() {return action;},get location() {return getLocation(window, globalHistory);},listen(fn: Listener) {if (listener) {throw new Error("A history only accepts one active listener");}window.addEventListener(PopStateEventType, handlePop);listener = fn;return () => {window.removeEventListener(PopStateEventType, handlePop);listener = null;};},createHref(to) {return createHref(window, to);},createURL,encodeLocation(to) {let url = createURL(to);return {pathname: url.pathname,search: url.search,hash: url.hash,};},push,replace,go(n) {return globalHistory.go(n);},};return history;
}

1.4. 历史记录栈变更监听

事件处理解决后，接下来就是解决监听，我们上面提到监听 popState 以此来处理路由变更。

listen(fn: Listener) {if (listener) {throw new Error("A history only accepts one active listener");}window.addEventListener(PopStateEventType, handlePop);listener = fn;return () => {window.removeEventListener(PopStateEventType, handlePop);listener = null;};
},

1.5. popState 逻辑处理

function handlePop() {action = Action.Pop;let nextIndex = getIndex()let delta = nextIndex == null ? null : nextIndex - indexindex = nextIndexif (listener) {listener({ action, location: history.location, delta })}
}

2. MemoryRouter

内存型路由的上层实现与 BrowserRouter 类似，或者说作者在设计之初就考虑了规范的对外统一接口协议，你会发现在使用 React-router 时，不管你用什么类型记录，API 都是一样的，这就是抽象封装的魅力。

2.1. 创建 router

export function createMemoryRouter(routes: RouteObject[],opts?: {basename?: string;future?: Partial<Omit<RouterFutureConfig, "v7_prependBasename">>;hydrationData?: HydrationState;initialEntries?: InitialEntry[];initialIndex?: number;unstable_dataStrategy?: unstable_DataStrategyFunction;}
): RemixRouter {return createRouter({basename: opts?.basename,future: {...opts?.future,v7_prependBasename: true,},history: createMemoryHistory({initialEntries: opts?.initialEntries,initialIndex: opts?.initialIndex,}),hydrationData: opts?.hydrationData,routes,mapRouteProperties,unstable_dataStrategy: opts?.unstable_dataStrategy,}).initialize();
}

2.2. Router 下承载 history

export function createMemoryHistory(options: MemoryHistoryOptions = {}
): MemoryHistory {let { initialEntries = ["/"], initialIndex, v5Compat = false } = options;let entries: Location[]; // Declare so we can access from createMemoryLocationentries = initialEntries.map((entry, index) =>createMemoryLocation(entry,typeof entry === "string" ? null : entry.state,index === 0 ? "default" : undefined));let index = clampIndex(initialIndex == null ? entries.length - 1 : initialIndex);let action = Action.Pop;let listener: Listener | null = null;function clampIndex(n: number): number {return Math.min(Math.max(n, 0), entries.length - 1);}function getCurrentLocation(): Location {return entries[index];}function createMemoryLocation(to: To,state: any = null,key?: string): Location {let location = createLocation(entries ? getCurrentLocation().pathname : "/",to,state,key);warning(location.pathname.charAt(0) === "/",`relative pathnames are not supported in memory history: ${JSON.stringify(to)}`);return location;}function createHref(to: To) {return typeof to === "string" ? to : createPath(to);}let history: MemoryHistory = {get index() {return index;},get action() {return action;},get location() {return getCurrentLocation();},createHref,createURL(to) {return new URL(createHref(to), "http://localhost");},encodeLocation(to: To) {let path = typeof to === "string" ? parsePath(to) : to;return {pathname: path.pathname || "",search: path.search || "",hash: path.hash || "",};},push(to, state) {action = Action.Push;let nextLocation = createMemoryLocation(to, state);index += 1;entries.splice(index, entries.length, nextLocation);if (v5Compat && listener) {listener({ action, location: nextLocation, delta: 1 });}},replace(to, state) {action = Action.Replace;let nextLocation = createMemoryLocation(to, state);entries[index] = nextLocation;if (v5Compat && listener) {listener({ action, location: nextLocation, delta: 0 });}},go(delta) {action = Action.Pop;let nextIndex = clampIndex(index + delta);let nextLocation = entries[nextIndex];index = nextIndex;if (listener) {listener({ action, location: nextLocation, delta });}},listen(fn: Listener) {listener = fn;return () => {listener = null;};},};return history;
}

因为内存型路由跟浏览器历史记录没有关联，因此相较于 BrowserRouter，没有关于浏览器历史记录栈变更的监听，只有单纯的记录压入和跳转。

重点关注：

push(to, state) {action = Action.Push;let nextLocation = createMemoryLocation(to, state);index += 1;entries.splice(index, entries.length, nextLocation);if (v5Compat && listener) {listener({ action, location: nextLocation, delta: 1 });}
}