useCallback

初始化

创建缓存对象

function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

更新

对依赖进行潜比较，如果没有变化，返回缓存对象。

function updateCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;
  if (prevState !== null) {
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        return prevState[0];
      }
    }
  }
  hook.memoizedState = [callback, nextDeps];
  return callback;
}