Bug 38564 - ConstraintExpression in Style does not work
Summary: ConstraintExpression in Style does not work
Alias: None
Product: Forms
Classification: Xamarin
Component: Forms ()
Version: 2.3.3
Hardware: PC Windows
: Normal normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2016-02-09 08:09 UTC by Michael Rumpler
Modified: 2016-12-22 04:34 UTC (History)
5 users (show)

Tags: ac
Is this bug a regression?: ---
Last known good build:

repro project (148.64 KB, application/x-zip-compressed)
2016-02-09 08:09 UTC, Michael Rumpler

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Description Michael Rumpler 2016-02-09 08:09:26 UTC
Created attachment 14954 [details]
repro project

I tried moving my RelativeLayout.* properties to a Style, but they do not work. My xaml looks like this:

			<Style x:Key="MiddleLabel" TargetType="Label">
				<Setter Property="Text" Value="This Label should be displayed in the middle and take half of the screen." />
				<Setter Property="BackgroundColor" Value="Red" />
				<Setter Property="RelativeLayout.XConstraint" Value="{ConstraintExpression Type=RelativeToParent, Property=Width, Factor=0.25}" />
				<Setter Property="RelativeLayout.YConstraint" Value="{ConstraintExpression Type=RelativeToParent, Property=Height, Factor=0.25}" />
				<Setter Property="RelativeLayout.WidthConstraint" Value="{ConstraintExpression Type=RelativeToParent, Property=Width, Factor=0.5}" />
				<Setter Property="RelativeLayout.HeightConstraint" Value="{ConstraintExpression Type=RelativeToParent, Property=Height, Factor=0.5}" />


		<Label Style="{DynamicResource MiddleLabel}" />

The Text and BackgroundColor from the Style are applied, but the attached properties for RelativeLayout are not. The Label is displayed at the top left corner and has the size of the text.

The desired result would be, that the ConstraintExpression is parsed when the Style is parsed, but it should only be evaluated during layouting phase. It must not search for the parent too early.
I already had a short discussion with Stephane Delcroix on this in https://forums.xamarin.com/discussion/comment/179494#Comment_179494

I tested with XF 2.1.0 pre3 and pre4 on iOS only. But I guess, this bug affects all platforms.