Assign width to half available screen width declaratively

Solution 1:

If your widget is a Button:

<LinearLayout android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:weightSum="2"
    android:orientation="horizontal">
    <Button android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="somebutton"/>

    <TextView android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>
</LinearLayout>

I'm assuming you want your widget to take up one half, and another widget to take up the other half. The trick is using a LinearLayout, setting layout_width="fill_parent" on both widgets, and setting layout_weight to the same value on both widgets as well. If there are two widgets, both with the same weight, the LinearLayout will split the width between the two widgets.

Solution 2:

Using constraints layout

1. Add a Guideline
2. Set the percentage to 50%
3. Constrain your view to the Guideline and the parent.

If you are having trouble changing it to a percentage, then see this answer.

XML

<?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:layout_editor_absoluteX="0dp"
    tools:layout_editor_absoluteY="81dp">

    <android.support.constraint.Guideline
        android:id="@+id/guideline8"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.5"/>

    <TextView
        android:id="@+id/textView6"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="TextView"
        app:layout_constraintBottom_toTopOf="@+id/guideline8"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>
    
</android.support.constraint.ConstraintLayout>