RelativeLayout ignores RelativeToView - xamarin.forms

Question

Are there any rules for which views you can refer to when using RelativeToView on a RelativeLayout?
It ignores it when I use it like this...
RelativeLayout.YConstraint= "{ConstraintExpression ElementName=main Type=RelativeToView, Property=Height, Factor=0.5}"
It seems to default to RelativeToParent.

Answer 1

Are there any rules for which views you can refer to when using
RelativeToView on a RelativeLayout?
which views you can refer to depending on ElementName you set.
In your code:
RelativeLayout.YConstraint= "{ConstraintExpression ElementName=main Type=RelativeToView, Property=Height, Factor=0.5}"
The Y of this view is half of the height of main, that is 0.5 * main.height.
Here I give you an example:
<RelativeLayout>
<BoxView Color="Red" x:Name="redBox"
RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToParent,
Property=Height,Factor=0.15,Constant=0}"
RelativeLayout.WidthConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Width,Factor=1,Constant=0}"
RelativeLayout.HeightConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Height,Factor=.8,Constant=0}" />
<BoxView Color="Blue" x:Name="blueBox"
RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToView,
ElementName=redBox,Property=Height, Factor=.5, Constant=0}"
RelativeLayout.XConstraint="{ConstraintExpression Type=RelativeToView,
ElementName=redBox,Property=X,Factor=1,Constant=20}"
RelativeLayout.WidthConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Width,Factor=.5,Constant=0}"
RelativeLayout.HeightConstraint="{ConstraintExpression
Type=RelativeToParent,Property=Height,Factor=.5,Constant=0}" />
<BoxView Color="Orange"
RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToView,
ElementName=blueBox,Property=Y,Factor=1,Constant=20}"
RelativeLayout.XConstraint="{ConstraintExpression Type=RelativeToView,
ElementName=blueBox,Property=X,Factor=1,Constant=20}"
RelativeLayout.WidthConstraint="{ConstraintExpression ElementName=blueBox,
Type=RelativeToView,Property=Width,Factor=.5,Constant=0}"
RelativeLayout.HeightConstraint="{ConstraintExpression ElementName=blueBox,
Type=RelativeToView,Property=Height,Factor=.5,Constant=0}" />
</RelativeLayout>
You can see that the height of redBox is 0.8 * height(ContentView). Then I set the blueBox:
RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToView,
ElementName=redBox,Property=Height, Factor=.5, Constant=0}"
So, the Y of blueBox is 0.5* 0.8 * height(ContentView).
Note: Both redBox's superView and blueBox's superView is ContentView.
You should calculate the right height of the View you want.
Refer: relative-layout#usage