initWatch

// 入口
// 当 options 中有 watch，并且 watch 不等于有些浏览器上（火狐）自带的 watch，则视为有效的 watch，进行初始化
if (opts.watch && opts.watch !== nativeWatch) {
  initWatch(vm, opts.watch)
}
function initWatch (vm: Component, watch: Object) {
  // 遍历循环，通过 $watch 创建
  for (const key in watch) {
    const handler = watch[key]
    if (Array.isArray(handler)) {
      for (let i = 0; i < handler.length; i++) {
        createWatcher(vm, key, handler[i])
      }
    } else {
      createWatcher(vm, key, handler)
    }
  }
}
function createWatcher (
  vm: Component,
  expOrFn: string | Function,
  handler: any,
  options?: Object
) {
  if (isPlainObject(handler)) {
    options = handler
    handler = handler.handler
  }
  if (typeof handler === 'string') {
    handler = vm[handler]
  }
  return vm.$watch(expOrFn, handler, options)
}