fix #4041, warn overriding Vue's internal methods

[HerringtonDarkholme]

No description provided.

[yyx990803]

Let rephrase this to "Avoid overriding Vue's internaml method ..." and can remove the second line.

[phanan]

@yyx990803 As discussed on Slack, this commit only solves half of the problem, as it doesn't cover the properties, which can be more tricky.

[phanan]

Can't we just use vm instead of importing Vue here?

[HerringtonDarkholme]

Hi, I also considered that approach when coding this. But if we use vm or vm.constructor.prototype, it will break constructor style instance because $emit or so isn't their own properties.

[phanan]

Doesn't a simple key in vm work just fine?

On Saturday, November 5, 2016, (´・ω・｀) notifications@github.com wrote:

@HerringtonDarkholme commented on this pull request.

In src/core/instance/state.js #4111:

       `method "${key}" has an undefined value in the component definition. ` +
       `Did you reference the function correctly?`,
       vm
     )

•    hasOwn(BuiltinVue.prototype, key) && warn(
  

Hi, I also considered that approach when coding this. But if we use vm or
vm.constructor.prototype, it will break constructor style instance
because $emit or so isn't their own properties.

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[HerringtonDarkholme]

I'm sorry that this change introduced more problems than it resolved.
As @phanan pointed out, only method is warned against. Also #4142 is another problem caused by this.

I have reconsidered this. We want to only warn against overriding built-in method. And using key in vm to detect that effectively means:

• vm only has properties inherited from Vue.prototype. This is guaranteed by Vue's core code, where extending is done by option merging.
• Vue.prototype does not have user defined method. This is not controlled by core code, as Internal method overriding detection #4142 showed.

@phanan Could you tell me the tricky part of properties? I'm not on slack.

[phanan]

@HerringtonDarkholme: @yyx990803 should have a better answer because he knows best about the framework's internal obviously, but from what I can tell, not all properties have been registered by the time methods are collected (examples are $el and $slots). This would make a check during initMethods() incomplete and was what kept me from sending a PR over.

[HerringtonDarkholme]

Yes, I just tried to fix this again and found warning is particularly tricky thing. More importantly, a _ and $ prefix have been strong indicators to users. The marginal benefit for this pr doesn't worth... @yyx990803 I'm so sorry make this rushy pull request. Feel free if you want to revert this.

[yyx990803]

I'll just revert this for now - I think this is a very rare case anyway. Or, we can simply change it to warn against any method that starts with $ or _, since this is conventionally reserved for internal and plugin methods/properties.

[phanan]

Agreed. This "improvement" has only been costing us time with no particular benefits to start with :)