Pitfalls of Vue dependency detection may cause redundant dependencies

[xiaodemen]

Version

2.4.2

Reproduction link

https://jsfiddle.net/sbmLobvr/9/

Steps to reproduce

click the button，then App component will update twice！

What is expected?

App component updates only once

What is actually happening?

App component updates twice

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When props used in child component initialization，they will be detected as dependencies of parent component, Is there possible to stop dependency detection when entering child component initialization。

[LinusBorg]

Hm, that's indeed odd.

For completeness, here's a version not using the prop in data - it only updates once: https://jsfiddle.net/sbmLobvr/10/

[posva]

@LinusBorg I don't think there's anything wrong going on: the msg prop is used in the template of the root instance, so it's normal for it to render again if it changes (no matter where the update comes from)

[LinusBorg]

Yeah, it's ok to update once, but twice in a row doesn't make sense to me.

[LinusBorg]

Especially since it only updates once if we don't access the prop in data() in the child (see my fiddle)

[posva]

Mmh, at some point it only showed up once or maybe I just didn't see it correctly, my bad 😆

[xiaodemen]

@javoski I don‘t think this would be the best solution，not just data option，but the whole child component initialization.
And I think it should be reactive, but as dependency of child component ?

[javoski]

@xiaodemen Sorry about that there's an ambiguity in the PR's title, component props are reactive, but they shouldn't collect dependencies during initialization of data option.

[xiaodemen]

@javoski I know, but data option is NOT the only case that shouldn't collect dependencies, e.g. component created hook, before mount hook and immediate watch ... so many other cases you should think about, Is it better to pause the dependency collection then restart it in the perspective of lifecycle, rather than simply reseting Dep.target on each case occuring?

[javoski]

@xiaodemen Yes, you are right, didn't aware of that, and will try to figure out a better solution.