What does notations in XML mean for Android UI design-CodePudding

I begin to study Android development. I' confused about the XML meaning for the UI design. Such as the following code, what the keyword xmlns, android, tools, app mean ? Are they some objects? For example, android:layout_width means that the property of the object android.

<?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">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        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：

ns doesn't stand for 'notation' here, It's 'namespace' XMLNS

All attributes of XML belongs to a namespace, for identify/qualify the name of an attribute, we have to declared their namespace's.

CodePudding user response：

As someone answered, ns is a namespace. You can actually call them anything- if you wanted to use "foobar:" instead of "android:" you can, you'd just change xmlns:app to xmls:foobar.

Now there are conventions most people follow, to make code more readable. "android:" is the stuff built into the framework. "app:" are attributes created by your app (or any library you use) and are generated at compile time. "tools:" are meant for things the IDE reads for display, such as default values for preview. Keeping to these names will make it easier for others to read your code.