As you can see from the following excerpt of ConstraintLayout's supported attributes, there is no layout_constraintStart_toCenterX attribute. Thus you cannot directly align the left edge of one view to the center of another view.
<attr name="layout_constraintLeft_toLeftOf" format="reference|enum">...</attr>
<attr name="layout_constraintLeft_toRightOf" format="reference|enum">...</attr>
<attr name="layout_constraintRight_toLeftOf" format="reference|enum">...</attr>
<attr name="layout_constraintRight_toRightOf" format="reference|enum">...</attr>
<attr name="layout_constraintTop_toTopOf" format="reference|enum">...</attr>
<attr name="layout_constraintTop_toBottomOf" format="reference|enum">...</attr>
<attr name="layout_constraintBottom_toTopOf" format="reference|enum">...</attr>
<attr name="layout_constraintBottom_toBottomOf" format="reference|enum">...</attr>
<attr name="layout_constraintBaseline_toBaselineOf" format="reference|enum">...</attr>
<attr name="layout_constraintStart_toEndOf" format="reference|enum">...</attr>
<attr name="layout_constraintStart_toStartOf" format="reference|enum">...</attr>
<attr name="layout_constraintEnd_toStartOf" format="reference|enum">...</attr>
<attr name="layout_constraintEnd_toEndOf" format="reference|enum">...</attr>
But ConstraintLayout is flexible enough to get your view aligned to the center of another view without such an attribute. Even though the solutions is a tad hacky 🙂
Say you have view A and want another view B to left-align to the center of A. To achieve this you can add yet another view C. Align both edges of C to the respective edges of A. C is thus centered to A. Now you can align the left edge of B to the right edge of C. Finally set the visibility of the helper view C to View.GONE - and you are done.
Here's the screen showing it before the view is set to View.GONE:
And this is the desired result after setting the helper view to View.GONE:
Have a look at this gist of the actual layout file. It probably makes the steps more obvious.
Keep in mind: You cannot use a view and simply set it's layout_width and layout_height to 0dp - since this is equivalent to stretching within a ConstraintLayout. To make a view have a height and width of zero, you have to set it's visibility to GONE. That's the only way to achieve this!
I think I mentioned it already in my previous ConstraintLayout article. But maybe it's time to repeat myself: I really like what Google's devs are doing with ConstraintLayout. Keep up the good work 🙂
there is a simpler way
you can replace parent by your required view id
i hope you support my rtl bug here
https://code.google.com/p/android/issues/detail?id=220103
Your solution is the best if you want some view to be centered to its parent – but not if it should be left- or right-aligned to the center of another view within the container.
For left- or right-aligning a view to the center of the container, guidelines are the best solution. The solution shown in this post is for arbitrary views. If for example the other view you are aligning to is stretched, but isn’t using the full width of the container (because it is itself bound to another view) there isn’t another solution available right now.
Just wanted to say thanks, this helped me out. Oddly enough, using a Guideline instead of a View breaks this, even though semantically they should be the same. Any thoughts?
Can you post your XML somewhere (gist, pastebin)? How did you even center the
Guidelineto another view? Right now you can only position it relative to the surroundingConstraintLayoutitself.Even if you set the XML params in the XML editor by hand those attributes are not used at all. Have a look at the (decompiled)
ConstraintLayoutsource.Guildelinesare treated very differently.