Margin behavior in ConstraintLayout

Question

I am having trouble understanding the behavior of margins (android:layout_margin*) when used in ConstraintLayout. Margins seem to only take effect in certain situations, and I am hoping someone can explain it to me (or confirm that it's a ConstraintLayout bug).

I have the following layout...

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <View
         android:id="@+id/leftView"
         android:layout_width="0dp"
         android:layout_height="100dp"
         android:layout_marginEnd="20dp"
         android:background="@android:color/holo_blue_dark"
         app:layout_constraintEnd_toStartOf="@+id/rightView"
         app:layout_constraintStart_toStartOf="parent"
         app:layout_constraintTop_toTopOf="parent"/>

     <View
         android:id="@+id/rightView"
         android:layout_width="100dp"
         android:layout_height="100dp"
         android:background="@android:color/holo_green_light"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintTop_toTopOf="parent"/>

</android.support.constraint.ConstraintLayout>

... which produces the following output...

However, when I change the margin from the leftView to the rightView...

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <View
        android:id="@+id/leftView"
        android:layout_width="0dp"
        android:layout_height="100dp"
        android:background="@android:color/holo_blue_dark"
        app:layout_constraintEnd_toStartOf="@+id/rightView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>

    <View
        android:id="@+id/rightView"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginStart="20dp"
        android:background="@android:color/holo_green_light"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>

</android.support.constraint.ConstraintLayout> 

... the margin unexpectedly disappears...

Can someone explain whether this is expected behavior and, if so, why that is?

Answer 1

Because leftview is depend on rightview, so you can set margin of leftview to rightview.

When a view have margin, it means the view is giving space TO WHICH VIEW IS DEPENDING ON.

if you write android:layout_marginStart="20dp" in rightview, it will not give space to leftview because rightview is not DEPENDING to leftview.