Why viewBinding makes XML root full screen

Question

I am wondering why adding viewBinding to my app, makes the XML root display in full screen when running the app. The root doesn't display in full screen in the designer view inside Android Studio.

Due to the root displaying in full screen, I had to add android:gravity="center" to vertically align the children in the center. While I don't think this should be a problem, I'm still interested in knowing why this is the case.

I have the following code;

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:gravity="center"
    android:layout_gravity="center_vertical"
    android:background="@color/colorAccent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="1"
        android:textSize="30sp" />

    <Button
        android:id="@+id/roll_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:text="@string/roll"/>

</LinearLayout>

package com.example.diceroller

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.example.diceroller.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val binding = ActivityMainBinding.inflate(layoutInflater)

        binding.rollButton.text = "testBind"
        setContentView(binding.root)
    }
}

Answer 1

I am not sure but I think that when You execute this setContentView(binding.root) You are setting a linear layout as fullscreen.

setContentView Android Docs

Set the activity content to an explicit view. This view is placed directly into the activity's view hierarchy. It can itself be a complex view hierarchy. When calling this method, the layout parameters of the specified view are ignored. Both the width and the height of the view are set by default to ViewGroup.LayoutParams#MATCH_PARENT. To use your own layout parameters, invoke setContentView(android.view.View, android.view.ViewGroup.LayoutParams) instead.

To do make it looks like in the preview You can write this code. XML:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <data>

    </data>


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

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="center_vertical"
            android:background="@color/colorAccent"
            android:gravity="center"
            android:orientation="vertical"
            tools:context=".MainActivity">

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:text="1"
                android:textSize="30sp" />

            <Button
                android:id="@+id/roll_button"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:text="Roll" />

        </LinearLayout>

    </FrameLayout>

</layout>

MainActivity.kt:

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        binding = ActivityMainBinding.inflate(layoutInflater)

        binding.rollButton.text = "Test Roll"
        setContentView(binding.root)
    }
}

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Tip: To convert fast Your layout to data binding layout use: Alt + Enter ➡ Convert to data binding layout