ConstraintLayout 在 Android L 设备上的行为很奇怪

Question

I'm using ConstraintLayout for the layout of a dialog fragment. It's all good except on the device with Android Lollipop. There, the layout adds up a textView(id/title) automatically on the top which is not even declared in the layout. When I run Layout Inspector, I get following results:

• on Android L

• on emulator with Android P

Here I fail to understand why it's happening. Here is my layout's xml code. Appreciate your help.

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:background="@color/black_overlay"
    android:layout_width="325dp"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/tvTitle"
        style="@style/TextAppearance.Black"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="24dp"
        android:layout_marginTop="24dp"
        android:layout_marginEnd="25dp"
        android:maxLines="2"
        android:text="@string/signup_alert_title"
        app:layout_constraintEnd_toStartOf="@+id/ivDismissDialog"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ImageView
        android:id="@+id/ivDismissDialog"
        android:layout_width="18dp"
        android:layout_height="18dp"
        android:layout_marginTop="19dp"
        android:layout_marginEnd="19dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/ic_cancel_deselected" />

    <TextView
        android:id="@+id/tvSignupDetails"
        style="@style/TextAppearance.Regular"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="24dp"
        android:layout_marginTop="16dp"
        android:layout_marginEnd="24dp"
        android:text="@string/signup_alert_details"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tvTitle" />

    <Button
        android:id="@+id/signupButton"
        style="@style/Button.FilledButton"
        android:layout_width="208dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="62dp"
        android:layout_marginTop="24dp"
        android:layout_marginEnd="55dp"
        android:text="@string/signup"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tvSignupDetails" />

    <Button
        android:id="@+id/loginButton"
        style="@style/Button.FlatButton"
        android:layout_width="208dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="62dp"
        android:layout_marginTop="10dp"
        android:layout_marginEnd="55dp"
        android:layout_marginBottom="24dp"
        android:text="@string/login"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/signupButton" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

This is basically theme related. In android the root view in most cases is a FrameLayout and using a theme like Theme.AppCompat.Light.DarkActionBar can add an extra view to the ViewGroup to display an actionbar with title.

The typical default actionBar code looks like below:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:orientation="vertical">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:id="@+id/title"
    android:text="YOUR ACTIVITY TITLE"
    android:textColor="#ffffff"
    android:textSize="24sp" />
</LinearLayout>

This simply means we can access the predefined textview using a reference like below if it exists:

View decor = getWindow().getDecorView();
TextView title = (TextView) decor.findViewById(getResources().getIdentifier("title", "id", "android"));

In other to get read of the action bar or textview overall we use a different app theme that requires no action bar or we add the below options to app's parent/base theme under style:

<item name="android:windowNoTitle">true</item>
<item name="windowActionBar">false</item>