android - ConstraintLayout: how to have a view be half the screen width and centered?

Question

TL;DR
The view width must be exactly half of the screen, and be centered. Using ConstraintLayout.

Note that the view does not have any inner width.

<View android:background="#ff0000" ... />

Original question
I would like to achieve a layout where a view size is half the screen size, and centered horizontally.

Something like this: |--view--|

I can't find any way using ConstraintLayout. The best i found is by using app:layout_constraintHorizontal_weight="1" on 2 fake views positioned at the full left and full right respectively, and app:layout_constraintHorizontal_weight="1.5" on my view.

Any better way ?

Answer 1

With the beta release you can use percentage widths. If you cannot use the beta release, you can employ two vertical guidelines: one at 25% of the screen width and one at 75% of the width. The view with a width of 0dp would be constrained between these two guidelines. This setup will give you a view that is 1/2 of the screen width and also centered.

The following XML demonstrates both ways; one using the ConstraintLayout beta release and the second using features available in the current production release.

XML Layout

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_main_inference"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <View
        android:id="@+id/viewTop"
        android:layout_width="0dp"
        android:layout_height="100dp"
        android:layout_marginTop="16dp"
        android:background="@android:color/darker_gray"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintWidth_default="percent"
        app:layout_constraintWidth_percent="0.5" />


    <android.support.constraint.Guideline
        android:id="@+id/guidelineLeft"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.25" />

    <View
        android:layout_width="0dp"
        android:layout_height="100dp"
        android:layout_marginTop="16dp"
        android:background="@android:color/darker_gray"
        app:layout_constraintEnd_toStartOf="@id/guidelineRight"
        app:layout_constraintStart_toEndOf="@id/guidelineLeft"
        app:layout_constraintTop_toBottomOf="@id/viewTop" />

    <android.support.constraint.Guideline
        android:id="@+id/guidelineRight"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.75" />

</android.support.constraint.ConstraintLayout>