How can i center a view in a guideline? - android-constraintlayout

Question

Suppose i have a horizontal GuideLine at 30% of parent and a View (suppose a Button) how can i make that view be centered on the guideline?
Like this:
Update:
The view i'm using here has height based on ratio, and soltuion on answers doesnt works.
Here is current layout:
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<ImageView
android:id="#+id/imgLogo"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintBottom_toBottomOf="#id/guideline_30"
app:layout_constraintDimensionRatio="H,1:1"
app:layout_constraintEnd_toStartOf="#+id/guidelineV_75"
app:layout_constraintStart_toEndOf="#+id/guidelineV_25"
app:layout_constraintTop_toTopOf="#+id/guideline_30"
app:srcCompat="#drawable/ic_launcher_background" />
<android.support.constraint.Guideline
android:id="#+id/guideline_30"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.3" />
<android.support.constraint.Guideline
android:id="#+id/guidelineV_25"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.25" />
<android.support.constraint.Guideline
android:id="#+id/guidelineV_75"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.75" />
</android.support.constraint.ConstraintLayout>

Answer 1

Using fix width or height
Need to use same Guideline for both top and bottom constraint like:
app:layout_constraintTop_toTopOf="#id/guideline_30"
app:layout_constraintBottom_toBottomOf="#id/guideline_30"
xml:
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="#+id/imgLogo"
android:layout_width="150dp"
android:layout_height="150dp"
app:layout_constraintTop_toTopOf="#id/guideline_30"
app:layout_constraintBottom_toBottomOf="#id/guideline_30"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintEnd_toEndOf="parent"
android:src="#drawable/splash_logo" />
<android.support.constraint.Guideline
android:id="#+id/guideline_30"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.30" />
</android.support.constraint.ConstraintLayout>
output:
Using width and height based on the ratio
As you are using both width and height based on ratio so that it's not centered.
But to get your desired result you can also set Guideline = 0.15 (0.3 / 2) and modify constraintTop and constraintBotttom
xml:
<android.support.constraint.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="#+id/imgLogo"
android:layout_width="0dp"
android:layout_height="0dp"
app:layout_constraintDimensionRatio="H,1:1"
app:layout_constraintEnd_toStartOf="#+id/guidelineV_75"
app:layout_constraintStart_toEndOf="#+id/guidelineV_25"
app:layout_constraintTop_toTopOf="#+id/guideline_15"
app:srcCompat="#drawable/ic_launcher_background" />
<android.support.constraint.Guideline
android:id="#+id/guideline_15"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.15" />
<android.support.constraint.Guideline
android:id="#+id/guidelineV_25"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.25" />
<android.support.constraint.Guideline
android:id="#+id/guidelineV_75"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
app:layout_constraintGuide_percent="0.75" />
</android.support.constraint.ConstraintLayout>
output:

Answer 2

You need to constraint both the top and the bottom of the View to the guideline, like so:
<android.support.constraint.Guideline
android:id="#+id/guideline"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal"
app:layout_constraintGuide_percent="0.3" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:layout_constraintTop_toTopOf="#id/guideline"
app:layout_constraintBottom_toBottomOf="#id/guideline" />

Answer 3

In ConstraintLayout, for centering the View over Vertical axis of any other View Component (e.g. Guidelines, TextViews, ImageViews, layouts, etc.), you can simply follow these two steps, in XML:-
1) app:layout_constraintTop_toTopOf="#id/componentId"
2) app:layout_constraintBottom_toBottomOf="#id/componentId"
This will Equally distribute the component's View in the Vertical center.
BONUS:-
For centering the View in the Horizontal axis, use
1) app:layout_constraintLeft_toLeftOf="#id/componentId"
or
app:layout_constraintStart_toStartOf="#id/componentId"
2) app:layout_constraintRight_toRightOf="#id/componentId"
or
app:layout_constraintEnd_toEndeOf="#id/componentId"