In ConstraintLayout, what is the meaning of android:layout_width="0dp", when there is no `weight` attribute?

Question

I have this layout:

<android.support.constraint.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/ThemeOverlayGoogleMaterialIcons">

  <LinearLayout
      android:layout_height="wrap_content"
      android:layout_width="0dp">

...

What is the meaning of android:layout_width="0dp" not in the child of LinearLayout?

What is the meaning of android:layout_width="0dp" when there is no weight attribute?

Answer 1

for the children of ConstraintLayout if you have set constraints then the 0dp is for match_constraint (take full width, or full height)

Using 0dp, which is the equivalent of "MATCH_CONSTRAINT" https://developer.android.com/reference/android/support/constraint/ConstraintLayout

example

<?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">

    <TextView
        android:id="@+id/tv_1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        tools:text="text1" />

    <TextView
        android:id="@+id/tv_2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toStartOf="@id/tv_3"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/tv_1"
        tools:text="text2" />

    <TextView
        android:id="@+id/tv_3"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@id/tv_2"
        app:layout_constraintTop_toBottomOf="@id/tv_1"
        tools:text="text3" />
</android.support.constraint.ConstraintLayout>

in the above code (and image) you see that text1 TextView width is the width it needs to write the text text1

for text2 and text3 TextView it takes the whole width divided by 2 that is what the constraints say