chenyuan

// 在js項目中使用，不需要類型
import { create } from "zustand";

const initStateCreateFunc = (set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
});

const useBearStore = create(initStateCreateFunc);

// ts項目，需要類型提示
import { create } from "zustand";

interface BearState {
  bears: number;
  increase: (by: number) => void;
}

const initStateCreateFunc = (set) => ({
  bears: 0,
  increase: (by) => set((state) => ({ bears: state.bears + by })),
});

const useBearStore = create<BearState>()(initStateCreateFunc);

function BearCounter() {
  const bears = useBearStore((state) => state.bears);
  return <h1>{bears} 在這裡...</h1>;
}

function Controls() {
  const increase = useBearStore((state) => state.increase);
  return <button onClick={increase}>增加一隻</button>;
}

// 生成store閉包，並返回api
// createState是使用者在創建store時傳入的一個函數
const createStoreImpl = (createState) => {
  type TState = ReturnType<typeof createState>;
  type Listener = (state: TState, prevState: TState) => void;
  // 這裡的state就是store，是個閉包，通過暴露的api訪問
  let state: TState;
  const listeners: Set<Listener> = new Set();

  // setState的partial參數支持對象和函數，replace指明是全量替換store還是merge
  // 更新是淺比較
  const setState = (partial, replace) => {
    const nextState = typeof partial === "function" ? partial(state) : partial;
    // 只有在相等的時候才更新，然後觸發listener
    if (!Object.is(nextState, state)) {
      const previousState = state;
      state =
        replace ?? typeof nextState !== "object"
          ? (nextState as TState)
          : Object.assign({}, state, nextState);
      listeners.forEach((listener) => listener(state, previousState));
    }
  };

  const getState = () => state;

  const subscribe = (listener) => {
    listeners.add(listener);
    // 取消訂閱
    return () => listeners.delete(listener);
  };

  // destory之後將被去掉，不用看
  const destroy: StoreApi<TState>["destroy"] = () => {
    if (import.meta.env?.MODE !== "production") {
      console.warn(
        "[DEPRECATED] The `destroy` method will be unsupported in a future version. Instead use unsubscribe function returned by subscribe. Everything will be garbage-collected if store is garbage-collected."
      );
    }
    listeners.clear();
  };

  const api = { setState, getState, subscribe, destroy };
  // 這裡就是官方示例裡的set,get,api
  state = createState(setState, getState, api);
  return api as any;
};

// 調用createStore的時候理論上createState函數是一定存在的
// 但是為了ts類型定義，createStore<T>()(()=>{}) 所以會出現手動調用空值的情況
export const createStore = ((createState) =>
  createState ? createStoreImpl(createState) : createStoreImpl) as CreateStore;

export function useStore<TState, StateSlice>(
  api: WithReact<StoreApi<TState>>,
  selector: (state: TState) => StateSlice = api.getState as any,
  equalityFn?: (a: StateSlice, b: StateSlice) => boolean
) {
  const slice = useSyncExternalStoreWithSelector(
    api.subscribe,
    api.getState,
    api.getServerState || api.getState,
    selector,
    equalityFn
  );
  useDebugValue(slice);
  return slice;
}

const createImpl = (createState) => {
  if (
    import.meta.env?.MODE !== "production" &&
    typeof createState !== "function"
  ) {
    console.warn(
      "[DEPRECATED] Passing a vanilla store will be unsupported in a future version. Instead use `import { useStore } from 'zustand'`."
    );
  }
  // 直接注入自定義的store不會注入api，需要自己在注入的store裡自行實現
  const api =
    typeof createState === "function" ? createStore(createState) : createState;

  const useBoundStore: any = (selector?: any, equalityFn?: any) =>
    useStore(api, selector, equalityFn);

  Object.assign(useBoundStore, api);

  return useBoundStore;
};

export const create = (<T>(createState: StateCreator<T, [], []> | undefined) =>
  createState ? createImpl(createState) : createImpl) as Create;

export function useSyncExternalStoreWithSelector<Snapshot, Selection>(
  subscribe: (() => void) => () => void,
  getSnapshot: () => Snapshot,
  getServerSnapshot: void | null | (() => Snapshot),
  selector: (snapshot: Snapshot) => Selection,
  isEqual?: (a: Selection, b: Selection) => boolean,
): Selection {
  // Use this to track the rendered snapshot.
  const instRef = useRef<
    | {
        hasValue: true,
        value: Selection,
      }
    | {
        hasValue: false,
        value: null,
      }
    | null,
  >(null);
  let inst;
  if (instRef.current === null) {
    inst = {
      hasValue: false,
      value: null,
    };
    instRef.current = inst;
  } else {
    inst = instRef.current;
  }

  /**
   * zustand使用的時候採用的是useStore(selector)的形式，每次re-render都會獲得一個新的selector
   * 所以getSelection在re-render後都是新的，但是因為有instRef.current以及isEqual
   * 當isEqual的時候返回instRef.current緩存的值，也就是getSelection的返回值不變
   * 不會再次re-render，減少了re-render的次數
   *  */
  const [getSelection, getServerSelection] = useMemo(() => {
    // Track the memoized state using closure variables that are local to this
    // memoized instance of a getSnapshot function. Intentionally not using a
    // useRef hook, because that state would be shared across all concurrent
    // copies of the hook/component.
    let hasMemo = false;
    let memoizedSnapshot;
    let memoizedSelection: Selection;
    const memoizedSelector = (nextSnapshot: Snapshot) => {
      if (!hasMemo) {
        // The first time the hook is called, there is no memoized result.
        hasMemo = true;
        memoizedSnapshot = nextSnapshot;
        const nextSelection = selector(nextSnapshot);
        if (isEqual !== undefined) {
          // Even if the selector has changed, the currently rendered selection
          // may be equal to the new selection. We should attempt to reuse the
          // current value if possible, to preserve downstream memoizations.
          if (inst.hasValue) {
            const currentSelection = inst.value;
            if (isEqual(currentSelection, nextSelection)) {
              memoizedSelection = currentSelection;
              return currentSelection;
            }
          }
        }
        memoizedSelection = nextSelection;
        return nextSelection;
      }

      // We may be able to reuse the previous invocation's result.
      const prevSnapshot: Snapshot = (memoizedSnapshot: any);
      const prevSelection: Selection = (memoizedSelection: any);

      if (is(prevSnapshot, nextSnapshot)) {
        // The snapshot is the same as last time. Reuse the previous selection.
        return prevSelection;
      }

      // The snapshot has changed, so we need to compute a new selection.
      const nextSelection = selector(nextSnapshot);

      // If a custom isEqual function is provided, use that to check if the data
      // has changed. If it hasn't, return the previous selection. That signals
      // to React that the selections are conceptually equal, and we can bail
      // out of rendering.
      if (isEqual !== undefined && isEqual(prevSelection, nextSelection)) {
        return prevSelection;
      }

      memoizedSnapshot = nextSnapshot;
      memoizedSelection = nextSelection;
      return nextSelection;
    };
    // Assigning this to a constant so that Flow knows it can't change.
    const maybeGetServerSnapshot =
      getServerSnapshot === undefined ? null : getServerSnapshot;
    const getSnapshotWithSelector = () => memoizedSelector(getSnapshot());
    const getServerSnapshotWithSelector =
      maybeGetServerSnapshot === null
        ? undefined
        : () => memoizedSelector(maybeGetServerSnapshot());
    return [getSnapshotWithSelector, getServerSnapshotWithSelector];
  }, [getSnapshot, getServerSnapshot, selector, isEqual]);

  const value = useSyncExternalStore(
    subscribe,
    getSelection,
    getServerSelection,
  );

  useEffect(() => {
    inst.hasValue = true;
    inst.value = value;
  }, [value]);

  useDebugValue(value);
  return value;
}