Add Iterator supports for v-for

[lyh2668]

添加v-for循环对迭代器(Iterator)的支持

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• [] Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[HerringtonDarkholme]

Iterator looks like new to me. Would you like to give me a link of introduction to Iterator? I might be wrong but Google only gave me Generator.

[lyh2668]

1. Map.prototype.keys() =>return a new Map iterator object.
2. Set.prototype.values() => return a new Iterator object containing the values for each element in the given Set, in insertion order.

[lyh2668]

Iteration_protocols

[HerringtonDarkholme]

Thanks. I have checked the spec and MapIterator's @@toStringTag symbol returns Map Iterator. The similar for other iterators.

GIven this, I suspect if a toString test is legitimate. Checking Symbol.iterator in proper environment is better, I think.

[lyh2668]

Yes, the better way is checking Symbol.iterator. At first I thought I can't use the ES6 grammar.

[lyh2668]

Symbol.toStringTag
A string value used for the default description of an object. Used by Object.prototype.toString().
Symbol

[Gudradain]

Iterator can be infinite no? Would that be a problem?

[blai]

echo @Gudradain's comment, we need to add a limit to this (maybe with a default value), but the same could go for traditional Array with excessive amount of data (so it's virtually infinite)

[mitar]

Related: #5893

[mitar]

So I have been using the following for a while now: https://github.com/vuejs/vue/blob/cb062c50cdd21628dcf758fe54c84d17d75cb2ca/src/core/instance/render-helpers/render-list.js

But I thought that in #5893 it is said that this will not go into core for now?

[mitar]

I think #8179 is a better implementation of this. Better check for iterator support.

[yyx990803]

Thanks for the PR, closing in favor of #8179 due to better support check and tests.