Constraint Layout 0dp (match_constraint)-CodePudding

I can´t understand constraints behaviour. Take a look to this layout

<?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"
    tools:context=".MainActivity">

    <Button
        android:id="@ id/button"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@color/purple_200"
        android:text="whatever"

        app:layout_constraintDimensionRatio="w, 1:1"

        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

It is supposed 0dp mean all available space. This button has both dimensions 0dp and a 1:1 ratio. And it is aligned left and top with parent so I guess it will be showing a square with a parent width as a side. But it is showing something unexpected for me, just a point on the upper left corner (even it is having some text):

It is clear I am not understanding this layout properly. Please enlighten me

CodePudding user response：

You need to add to more constraints in order to work

app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintBottom_toBottomOf="parent"

If you are using 0dp for height, you need to constraint both top and bottom and vice versa

CodePudding user response：

Copy and past this

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
    <Button
        android:id="@ id/btn"
        android:layout_width="126dp"
        android:layout_height="53dp"
        android:text="Button"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

CodePudding user response：

i think you need to add more Constraint!

add this :

app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"

for this case, it will must like this :

<Button
        android:id="@ id/button"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@color/purple_200"
        android:text="whatever"
        app:layout_constraintDimensionRatio="w, 1:1"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

hope this help you, have a great day! don't forget to take coffe time! Stay Coding and Stay Awesome!