[iOS] UI Frozen When Adding Many Children To a Layout

[endarea]

Description

To best describe this issue the repro project has the main page split in two:
1. On the left I create a StackView to which I add 1000 simple labels, programatically, at the press of the "FORMS" button. While the content is rendered the UI is completely frozen for many seconds, depending on the number of children. The rendering time increases exponentially, 200 children will not take twice as long as 100 children, but much more, reaching 35 seconds for 1000 labels on an iPhone X Simulator (iOS 11.4)

• setting the children in XAML leads to the same poor performance
• surprisingly, the Android version is smoother, though it's still slower than the native approach

2. On the right, at the press of "NATIVE" button I add a ContentView for which I have created some crude renderers (iOS and Android), enough to be able to add the same 1000 labels to the view from the native projects. The difference is huge, as illustrated by the screenshot.

EnsureChildrenOrder, BringSubviewToFront seem to be the bottleneck.

I have found some good discussions on the Forums about this, but no solution/workaround was found:

• https://forums.xamarin.com/discussion/57185/performance-bottleneck-on-android-in-children-add
• https://forums.xamarin.com/discussion/70719/ensurechildorder-in-xamarin-forms-platform-ios-very-slow-when-appending-child-controls
• https://forums.xamarin.com/discussion/71492/major-performance-issues-with-dynamic-layout-changes
• https://forums.xamarin.com/discussion/43869/keeping-ui-thread-from-freezing-when-setting-large-content

Expected Behavior

The UI shouldn't be frozen for so long, on 200 elements the usability is already reduced. A ListView is not always feasible to use, I'm thinking about an usecase where I would want to display a Data Grid to the users, set in a ScrollView with Orientation set to Both

Basic Information

• Version with issue: 3.1.0.637273
• Last known good version: -
• IDE:
• Platform Target Frameworks:
  • iOS: 11.4
  • Android: 8.1
• Affected Devices: tried it on iPad Air 2, iPhone 5s, various simulators (including iPhone X) and Huawei Nexus 6P (API 27)

Reproduction Link

FreezeRepro.zip

[bnSonic]

Phew - it was 2015 when this problem was tracked down (as shown in the links in the original posting)
Now we have August 2019, Xamarin.Forms 4.x
This Problem still exists!

I'm running into this problem because:

• For reasons i cannot use a ListView (don't let me get down that rabbit hole at this place :))
• So i added a ScrollView on my Page
• I create a StackLayout and add a lot Views to it (=this is fast)
• then i do a: MyScrollView.Content = MyStackLayout; (= THIS takes a LONG TIME - as described above in the original posting and analyzed in the links in that posting

I was not able to Patch it with Mono.Cecil (because i'm not aware how that works and the sample from 2015 that you can find in the links doesn't work here on iOS)

It doesn't matter if i'm using a StackLayout or an AbsoluteLayout
The Bindings doesn't matter (i removed them all for measurement - it's a UI thing)

Really: Is there any chance to workaround that performance problem? Any chance to build a more complex or scrolling layout with good performance when you cannot use ListView?

[velocitysystems]

@bnSonic I would recommend looking into the new CollectionView.

[AlleSchonWeg]

Hi,
i have the same issue. I can't use ListView, because i need scrolling in both directions. I can confirm that:
MyScrollView.Content = MyStackLayout;
takes very long. 300 views take 9 seconds with iOS and a with android a few ms.
Somebody knows a workaround?
Thank you.

[AlleSchonWeg]

@PureWeen
Is PR 8232 (#8231) also the fix for this issue?