Reduxでのクライアントサイドvalidationをどこでやるべきか？ ref: http://qiita.com/inuscript/items/5bed7812b3c1447b7b60

actions.js

// 値は値でreducerに返す
export const appendTask = (task) => {
  return {type: "TODO_ADD", payload: task}
}

// 別途、エラーがあれば、そちらもactionを生成する
export const checkTaskError = (task) => {
  if(!isValidTodo(task)){
    return {type: "ERROR_MESSAGE", payload: "Input Some word"}
  }
  return {type: "ERROR_MESSAGE", payload: ""}
}

component.js

class InputComponent extends Component{
    :
  handleClick(e){
       :
    this.props.actions.appendTask(currentInput)
    // validation用にもactionを飛ばす
    this.props.actions.checkTaskError(currentInput)
       :

error-message.js

const errorMessage = (state = null, action) => {
  switch(action.type) {
    case "TODO_ADD":
    if(!isValidTodo(action.payload)){
      return "Invalid Task"
    }
  }
  return state
}

file3.js

  handleClick(e){
    let currentInput = this.state.currentInput

    // validationエラーをstateでやる
    if(!isValidTodo(currentInput)){
      this.setState({
        errorMessage: "Input some word"
      })
      return
    }
    this.props.actions.appendTask(currentInput)
    this.setState({
      currentInput: ""
    })
  }

file4.js

export const appendTask = (task) => {
  // エラーかどうかで返すactionを変える
  if(!isValidTodo(task)){
    return {type: "ERROR_MESSAGE", payload: "Input Some word"}
  }
  return {type: "TODO_ADD", payload: task}
}

middleware.js

const doValidate = (action) => {
  let errors = []
  switch(action.type){
    case "TODO_ADD":
      if(!isValidTodo(action.payload)){
        return "Invalid Task"
      }
  }
}

export const validateMiddleware = ({ dispatch, getStore }) => next => action => {
  let error = doValidate(action)
  if(error){
    // errorがあったらerrorMsgのactionだけ発火させる
    dispatch(actions.errorMsg(error))
    return
  }
  let result = next(action)
  return result
}

tasks.js

const tasks = (state = [], action) => {
  switch (action.type) {
  case 'TODO_ADD':
    let task = action.payload
    // validate通った時だけpushする
    if(isValidTodo(task)){
      state.push(task)
    }
    return state.concat()
  case 'TODO_REMOVE':
     :

validate.js

export const isValidTodo = (task) => {
  if(task === ""){
    return false
  }
  return true
}