有人可以解释如何在重新选择库的函数中使用输入函数吗？

Question

https://github.com/reduxjs/reselect/blob/master/src/index.js#L89

export function defaultMemoize(func, equalityCheck = defaultEqualityCheck) {
  let lastArgs = null
  let lastResult = null
  // we reference arguments instead of spreading them for performance reasons
  return function () {
    if (!areArgumentsShallowlyEqual(equalityCheck, lastArgs, arguments)) {
      // apply arguments instead of spreading for performance.
      lastResult = func.apply(null, arguments)
    }

    lastArgs = arguments
    return lastResult
  }
}

export function createSelectorCreator(memoize, ...memoizeOptions) {
  return (...funcs) => {
    let recomputations = 0
    const resultFunc = funcs.pop()
    const dependencies = getDependencies(funcs)

    const memoizedResultFunc = memoize(
      function () {
        recomputations++
        // apply arguments instead of spreading for performance.
        return resultFunc.apply(null, arguments)
      },
      ...memoizeOptions
    )
...}
}


export const createSelector = createSelectorCreator(defaultMemoize)

所以如果我创建 createSelector(getUsers, (users) => users) 来做一个简单的例子。运行与上面的代码相比如何？

createSelectorCreator(defaultMemoize) 使用 getUsers, (users) => users 输入调用。现在defaultMemoize也是一个函数，return是一个函数。他们如何与 return 价值互动？

Answer 1

我认为对于重新选择的工作方式更重要的是为什么要使用它。主要原因是可组合性和记忆化：

可组合性

另一种说法是，您编写一次选择器，然后在其他更详细的选择器中重新使用它。假设我有一个这样的状态：{data:{people:[person,person ...]} 然后我可以这样写一个 filterPerson：

const selectData = state => state.data;
const selectDataEntity = createSelector(
  selectData,//re use selectData
  (_, entity) => entity,
  (data, entity) => data[entity]
);
const filterDataEntity = createSelector(
  selectDataEntity,//re use selectDataEntity
  (a, b, filter) => filter,
  (entities, filter) => entities.filter(filter)
);

如果我将数据移动到 state.apiResult，那么我只需要更改 selectData，这会最大限度地重复使用代码并最大限度地减少重复实现。

记忆

记忆化意味着当你用相同的参数多次调用一个函数时，该函数只会执行一次。纯函数 return 无论调用多少次或何时调用，给定相同参数的结果相同。

这意味着当您使用相同的参数再次调用它时不需要执行该函数，因为您已经知道结果。

Memoization 可用于不调用昂贵的函数（如过滤大型数组）。在 React 中，记忆很重要，因为如果 props 发生变化，纯组件将重新渲染：

const mapStateToProps = state => {
  //already assuming where data is and people is not
  //  a constant
  return {
    USPeople: state.data.people.filter(person=>person.address.countey === US)
  }
}

即使 state.data.people 没有改变过滤器函数每次都会 return 一个新数组。

工作原理

下面是对 createSelector 的重新编写，其中包含一些注释。删除了一些安全检查参数并允许您使用函数数组调用 createSelector 的代码。有什么理解上的困难欢迎评论。

const memoize = fn => {
  let lastResult,
    //initial last arguments is not going to be the same
    //  as anything you will pass to the function the first time
    lastArguments = [{}];
  return (...currentArgs) => {//returning memoized function
    //check if currently passed arguments are the same as
    //  arguments passed last time
    const sameArgs =
      currentArgs.length === lastArguments.length &&
      lastArguments.reduce(
        (result, lastArg, index) =>
          result && Object.is(lastArg, currentArgs[index]),
        true
      );
    if (sameArgs) {
      //current arguments are same as last so just
      //  return the last result and don't execute function
      return lastResult;
    }
    //current arguments are not the same as last time
    //  or function called for the first time, execute the
    //  function and set last result
    lastResult = fn.apply(null, currentArgs);
    //set last args to current args
    lastArguments = currentArgs;
    //return result
    return lastResult;
  };
};

const createSelector = (...functions) => {
  //get the last function by popping it off of functions
  //  this mutates functions so functions does not have the
  //  last function on it anymore
  //  also memoize the last function
  const lastFunction = memoize(functions.pop());
  //return a selector function
  return (...args) => {
    //execute all the functions (last was already removed)
    const argsForLastFunction = functions.map(fn =>
      fn.apply(null, args)
    );
    //return the result of a call to lastFunction with the
    //  result of the other functions as arguments
    return lastFunction.apply(null, argsForLastFunction);
  };
};
//selector to get data from state
const selectData = state => state.data;
//select a particular entity from state data
//  has 2 arguments: state and entity where entity
//  is a string (like 'people')
const selectDataEntity = createSelector(
  selectData,
  (_, entity) => entity,
  (data, entity) => data[entity]
);
//select an entity from state data and filter it
//  has 3 arguments: state, entity and filterFunction
//  entity is string (like 'people') filter is a function like:
//  person=>person.address.country === US
const filterDataEntity = createSelector(
  selectDataEntity,
  (a, b, filter) => filter,
  (entities, filter) => entities.filter(filter)
);
//some constants
const US = 'US';
const PEOPLE = 'people';
//the state (like state from redux in connect or useSelector)
const state = {
  data: {
    people: [
      { address: { country: 'US' } },
      { address: { country: 'CA' } },
    ],
  },
};
//the filter function to get people from the US
const filterPeopleUS = person =>
  person.address.country === US;
//get people from the US first time
const peopleInUS1 = filterDataEntity(
  state,
  PEOPLE,
  filterPeopleUS
);
//get people from the US second time
const peopleInUS2 = filterDataEntity(
  state,
  PEOPLE,
  filterPeopleUS
);

console.log('people in the US:', peopleInUS1);
console.log(
  'first and second time is same:',
  peopleInUS1 === peopleInUS2
);

有人可以解释如何在重新选择库的函数中使用输入函数吗？

Can someone explain how input functions are used in functions in reselect library?

reactjs

functional-programming

currying

reselect