fix(next/iOS): fix blinking during initial render when initialPage !=0 in non-fabric version

[ku8ar]

Summary

Fabric has a trick with -1. Unfortunately here we can't use it. The only place that allows a valid goTo from the initial page is layoutSubviews.

Test Plan

1. render pager with initialPage bigger than zero and many pages inside
2. pager is blinking (for a brief moment)

Compatibility

OS	Implemented
iOS	✅
Android	❌

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I updated the typed files (TS and Flow)

[krozniata]

Hey @ku8ar!
Could you provide video showing blinking issue? I couldn't reproduce it
Also that change introduced a bug causing initial page not to be set correctly once every few render (initialPage set to e.g.7 with 100 pages in BasicPagerViewExample)

[ku8ar]

Thats weird what you say. Maybe you accidentally ran the fabric version? Here is video that demonstrates problem (full-screen video is rendered inside pager-view):

ddd.mov

After this patch, initial page is always rendered correctly when pager-view is loaded:

eee.mov

Screen/pager-view rendering code in our app has not been changed since version 6.x.

[krozniata]

Looks like bug causing initialPage sometimes not to be set correctly can be easily fixed by adding check for contentSize like:

-(BOOL)isScrollViewContentSizePositive {
    return [self isHorizontal] ? _scrollView.contentSize.width > 0 : _scrollView.contentSize.height > 0;
}

and adding it to

-(void)layoutSubviews {
     [super layoutSubviews];
     if (self.isScrollViewContentSizePositive && !_scrolledToInitialPage) {
         _scrolledToInitialPage = YES;
         [self goTo:self.initialPage animated:false];
     }
 }

Could you please add it?