在Android Studio预览和设备上看不到任何内容

Question

I'm trying to follow a tutorial, and everything works. However, I can't get the view part. When I copy the xml file it should show the placeholder text for the TextView, but nothing happens. I'm not receiving errors, and if i manually drag the textview it appears, but using match parent does not.

<?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"
android:layout_margin="16dp"
tools:context=".MainActivity">


<ScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent">

    <!-- Contains the list of all sensors, generated in onCreate(). -->
    <TextView
        android:id="@+id/sensor_list"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/placeholder_text"
        android:visibility="visible"
        tools:visibility="visible" />
</ScrollView>

Answer 1

我不认为匹配父项有问题，只是您的约束视图没有关闭标签，添加后就可以正常显示了

</androidx.constraintlayout.widget.ConstraintLayout>