How to make ConstraintLayout work with percentage values?

Question

With a Preview 1 of Android Studio 2.2 Google released a new layout in its support library: ConstraintLayout. With ConstraintLayout it is easier to use a Design tool in Android Studio, but I didn't find a way to use relative sizes (percents or 'weights' like in LinearLayout). Is there a way to define the constraints based on percents? E.g. make a View take 40% of a screen, create 20% margin between views, set a View's width to 50% of another View's width?

Answer 1

It may be useful to have a quick reference here.

Placement of views

Use a guideline with app:layout_constraintGuide_percent like this:

<androidx.constraintlayout.widget.Guideline
    android:id="@+id/guideline"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_percent="0.5"/>

And then you can use this guideline as anchor points for other views.

or

Use bias with app:layout_constraintHorizontal_bias and/or app:layout_constraintVertical_bias to modify view location when the available space allows

<Button
    ...
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintHorizontal_bias="0.25"
    ...
    />

Size of views

Another percent based value is height and/or width of elements, with app:layout_constraintHeight_percent and/or app:layout_constraintWidth_percent:

<Button
    ...
    android:layout_width="0dp"
    app:layout_constraintWidth_percent="0.5"
    ...
    />