为什么 marginBottom 在 Spinner 中不起作用？

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

connect constrain from the button to view or parent then it will affect $ app:layout_constraintBottom_toBottomOf="" `

Answer 2

You need to constraint the spinner to bottom:

1st method: app:layout_constraintBottom_toBottomOf="parent"

2nd method: constraint the spinner to bottom by drag it until the bottom of your root view

Answer 3

The problem is not about Spinner specifically - if you use two TextView s 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 View s 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>