vue.js 源代码学习笔记 ----- 工具方法 share

/* @flow */

/**
 * Convert a value to a string that is actually rendered.
   { .. } [ .. ] 2 => ''
 */
export function _toString (val: any): string {
  return val == null
    ? ''
    : typeof val === 'object'
      ? JSON.stringify(val, null, 2)  //2 是控制字符串系列化后的空格为2
      : String(val)
}

/**
 * Convert a input value to a number for persistence.
 * If the conversion fails, return original string.
   刻意把 输入框的值转换成数字, 如失败, 则还原
   如 '123abc' 会转换成 123
   这里没有用parseInt 是因为parseInt对浮点型转换不准确, 如
　
　　parseInt(0.0000008,10) => 8  parseFloat(0.0000008,10) => 8e-7

　　而且对于字符串, parFloat只能按十进制来转换, parseInt('0200',8) => 128; parseFloat('0200',8) => 200

*/
export function toNumber (val: string): number | string {
  const n = parseFloat(val)
  return isNaN(n) ? val : n
}

/**
 * Make a map and return a function for checking if a key
 * is in that map.
   输入标签 'div,slot,span'等, div或者slot作为一个对象的key, 返回一个函数 
  检查传入的key是否在map中
  
 */
export function makeMap (
  str: string,
  expectsLowerCase?: boolean  //传入的key还可以不区分大小写
): (key: string) => true | void {
  const map = Object.create(null) //这里没有使用 new Object  或者 = {}; 是为了创建没有 __proto__属性的 真空对象
  const list: Array<string> = str.split(',') //这里用到了闭包, 把map 缓存了起来
  

 for (let i = 0; i < list.length; i++) {
    map[list[i]] = true
  }
  return expectsLowerCase
    ? val => map[val.toLowerCase()]
    : val => map[val]
}

/**
 * Check if a tag is a built-in tag.
   检查一个标签是否是 内置标签
 */
export const isBuiltInTag = makeMap('slot,component', true)

/**
 * Remove an item from an array
   利用indexOf splice 移除一个值
 */
export function remove (arr: Array<any>, item: any): Array<any> | void {
  if (arr.length) {
    const index = arr.indexOf(item)
    if (index > -1) {
      return arr.splice(index, 1)
    }
  }
}

/**
 * Check whether the object has the property.
   缓存原型方法, 避免每次去对象的原型找这个hasOwnProperty方法, 减少一丁点性能损耗
 */
const hasOwnProperty = Object.prototype.hasOwnProperty

export function hasOwn (obj: Object, key: string): boolean {
  return hasOwnProperty.call(obj, key)
}

/**
 * Check if value is primitive
   只有 字符串  数字 才是原始类型
 */
export function isPrimitive (value: any): boolean {
  return typeof value === 'string' || typeof value === 'number'
}

/**
 * Create a cached version of a pure function.   
   利用闭包缓存一个函数执行特定参数的结果,  比如  
   执行 var cFn1 = cached( fn1 ); cFn1(2); 第二次调用 cFn1(2)的时候, 就可以利用第一次的结果,
   适合会多次调用一个函数, 而且参数有可能重复的情况
   由于是利用了缓存, 所以传入的函数应该是纯函数, 就是每次如果参数一样, 结果必须一样, 不能是random这样的函数
 */
export function cached<F: Function> (fn: F): F {
  const cache = Object.create(null)

  return (function cachedFn (str: string) {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  }: any)

}

/**
 * Camelize a hyphen-delimited string.
   把 'ms-box-shadow' 这样的用-分割的连字符, 驼峰化  => msBoxShadow ; 有利于在js中设置样式
 */
const camelizeRE = /-(w)/g
export const camelize = cached((str: string): string => {
  //c 就是(w)这个子项, 前面需要一个-; 所以 -box 会变成 -Box
  return str.replace(camelizeRE, (_, c) => c ? c.toUpperCase() : '')
})

/**
 * Capitalize a string.
   abc 变成 Abc
 */
export const capitalize = cached((str: string): string => {
  return str.charAt(0).toUpperCase() + str.slice(1)
})

/**
 * Hyphenate a camelCase string.
   驼峰转成链接字符 abcAb => abc-ab
 */
const hyphenateRE = /([^-])([A-Z])/g
export const hyphenate = cached((str: string): string => {
  return str
    .replace(hyphenateRE, '$1-$2') //转两次?
    .replace(hyphenateRE, '$1-$2')
    .toLowerCase()
})

/**
 * Simple bind, faster than native
   自定义的bind方法, 比原生快那么一点, 原理?
 */
export function bind (fn: Function, ctx: Object): Function {
  function boundFn (a) {
    const l: number = arguments.length
    return l
      ? l > 1
        ? fn.apply(ctx, arguments)
        : fn.call(ctx, a)
      : fn.call(ctx)
  }
  // record original fn length
  boundFn._length = fn.length
  return boundFn
}

/**
 * Convert an Array-like object to a real Array.
   数组转换类数组, 这里没有用[].slice.call?
 */
export function toArray (list: any, start?: number): Array<any> {
  start = start || 0
  let i = list.length - start

  const ret: Array<any> = new Array(i)
  //这里没使用push

  while (i--) {
    ret[i] = list[i + start]
  }
  return ret
}

/**
 * Mix properties into target object.
   太简单的混入
 */
export function extend (to: Object, _from: ?Object): Object {
  for (const key in _from) {
    to[key] = _from[key]
  }
  return to
}

/**
 * Quick object check - this is primarily used to tell
 * Objects from primitive values when we know the value
 * is a JSON-compliant type.
   简单判断一个对象
 */
export function isObject (obj: mixed): boolean {
  return obj !== null && typeof obj === 'object'
}

/**
 * Strict object type check. Only returns true
 * for plain JavaScript objects.
   确保是一个完全的对象, 而不是数组 null RegExp 之类的东西
 */
const toString = Object.prototype.toString
const OBJECT_STRING = '[object Object]'
export function isPlainObject (obj: any): boolean {
  return toString.call(obj) === OBJECT_STRING
}

/**
 * Merge an Array of Objects into a single Object.
   ['a','b'] => {0:'a',1:'b'}
 */
export function toObject (arr: Array<any>): Object {
  const res = {}
  for (let i = 0; i < arr.length; i++) {
    if (arr[i]) {
      extend(res, arr[i])
    }
  }
  return res
}

/**
 * Perform no operation. 一个空函数
 */
export function noop () {}

/**
 * Always return false. 减少书写?
 */
export const no = () => false

/**
 * Return same value  需要利用函数来返回相同值?
 */
export const identity = (_: any) => _

/**
 * Generate a static keys string from compiler modules.
   把modules的每个statickeys 链接起来 形成一个特定的静态key
 */
export function genStaticKeys (modules: Array<ModuleOptions>): string {
  return modules.reduce((keys, m) => {
    return keys.concat(m.staticKeys || [])
  }, []).join(',')
}

/**
 * Check if two values are loosely equal - that is,
 * if they are plain objects, do they have the same shape?
   判断两个对象 的值是否  相等, 而不用考虑对象地址是否相同
   var a = {}; var b = {};  looseEqual(a,b) => true,

   对于 null 和 0 false是不相等的
 */
export function looseEqual (a: mixed, b: mixed): boolean {
  const isObjectA = isObject(a)
  const isObjectB = isObject(b)

  if (isObjectA && isObjectB) {
    try {
      return JSON.stringify(a) === JSON.stringify(b)
    } catch (e) {
      // possible circular reference
      return a === b
    }
  } else if (!isObjectA && !isObjectB) {
    return String(a) === String(b)
  } else {
    return false
  }

}

//比如 arr是 [{"a":1},...]  looseIndexOf( arr, {"a":1} )  => 0 
export function looseIndexOf (arr: Array<mixed>, val: mixed): number {
  for (let i = 0; i < arr.length; i++) {
    if (looseEqual(arr[i], val)) return i
  }
  return -1
}

/**
 * Ensure a function is called only once. 保证这个函数只调用一次

  var f = once(fn1); f();  f()没执行fn1
 */
export function once (fn: Function): Function {
  let called = false
  return () => {
    if (!called) {
      called = true
      fn()
    }
  }
}