Why marginBottom isn't working in Spinner?

Question

Could someome explain me why android:layout_marginBottom doesn't work in a Spinner?:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">

    <Spinner
        android:id="@+id/spinner2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginBottom="20dp"/>

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/spinner2" />
</androidx.constraintlayout.widget.ConstraintLayout>

It doesn't matter if I use android:layout_marginBottom or android:layout_margin. However, If I use android:layout_margin it adds top, right and left margin. Why the only margin that does't work is the bottom?

Thanks.

Answer 1

The problem is not about Spinner specifically - if you use two TextViews the behavior stays the same.

Some observations on "why" and "how":

• the Spinner top and start is constrained to the top and start of the parent ViewGroup. Since you don't specify a bottom (or end) constraint, a bottom margin is meaningless

• the TextView on the other hand has a top constraint - if you let it have a top margin, this will have the desired effect.

Now you could say "well, then I'll just add the bottom constraint to the Spinner". Unfortunately this is not enough (and here I really don't know why the ConstraintLayout solver decides to ignore the margin...)

If you want to set a margin to the Spinner, then the two Views have to belong to a complete vertical chain:

parent top <- Spinner <-> TextView -> parent bottom

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <Spinner
        android:id="@+id/spinner2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        android:layout_marginBottom="20dp"
        app:layout_constraintVertical_bias="0"
        app:layout_constraintVertical_chainStyle="packed"/>

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/spinner2"
        app:layout_constraintBottom_toBottomOf="parent"
       />
</androidx.constraintlayout.widget.ConstraintLayout>