如果传入子组件的function是定义在methods中的方法，那么子组件调用此方法时方法内部依然是父组件的this。

initMethods

function initMethods (vm: Component, methods: Object) {
  const props = vm.$options.props
  for (const key in methods) {
    // ....
    // 此处绑定了methods中的函数的this为当前实例
    vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)
  }
}

utils

/**
 * Simple bind polyfill for environments that do not support it,
 * e.g., PhantomJS 1.x. Technically, we don't need this anymore
 * since native bind is now performant enough in most browsers.
 * But removing it would mean breaking code that was able to run in
 * PhantomJS 1.x, so this must be kept for backward compatibility.
 */

/* istanbul ignore next */
function polyfillBind (fn: Function, ctx: Object): Function {
  function boundFn (a) {
    const l = arguments.length
    return l
      ? l > 1
        ? fn.apply(ctx, arguments)
        : fn.call(ctx, a)
      : fn.call(ctx)
  }

  boundFn._length = fn.length
  return boundFn
}

function nativeBind (fn: Function, ctx: Object): Function {
  return fn.bind(ctx)
}

export const bind = Function.prototype.bind
  ? nativeBind
  : polyfillBind