fix flatlist props being undefined in ios

[dieguezz]

Summary:

When using Flatlist on iOS and Android its failing because props are undefined

The problem is described on #34783 #36828 #42623

Fixed by setting constructor before any statement and removing unnecessary props declaration at the top of the class.

Changelog:

[GENERAL] [FIXED] - Fix undefined props crash in FlatList

Test Plan:

[facebook-github-bot]

Hi @dieguezz!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[robhogan]

Hi @dieguezz - thanks for working on this. Does the fix work without moving the constructor - ie just deleting line 311?

[dieguezz]

Hi @dieguezz - thanks for working on this. Does the fix work without moving the constructor - ie just deleting line 311?

Very good catch @robhogan. In fact you are right. I made the change so its one line following your advice

[facebook-github-bot]

@robhogan has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[robhogan]

Thanks again @dieguezz!

FWIW - maybe to help explain similar cases - what's happening here is that line is intended to be a pure Flow type annotation for the property on the base class, and under the default Babel settings it's stripped out completely by flow-strip-types and has no runtime semantics.

But, when transform-class-properties runs before flow-strip-types (eg, with a custom Babel config), it lifts this line into the constructor and turns it into a runtime initialisation (this.props = void 0;), which overwrites the initialisation performed in super(props), and effectively clobbers the props property.

[dieguezz]

Thanks again @dieguezz!

FWIW - maybe to help explain similar cases - what's happening here is that line is intended to be a pure Flow type annotation for the property on the base class, and under the default Babel settings it's stripped out completely by flow-strip-types and has no runtime semantics.

But, when transform-class-properties runs before flow-strip-types (eg, with a custom Babel config), it lifts this line into the constructor and turns it into a runtime initialisation (this.props = void 0;), which overwrites the initialisation performed in super(props), and effectively clobbers the props property.

yeah that definately makes sense, in fact, i was wondering why this didn't happen before if the code of the component had not changed at all

[facebook-github-bot]

@robhogan merged this pull request in e33767a.