what is meaning of @ and ? in layout.xml file in the android-CodePudding

what is the meaning of @ and ? in the layout.xml file I am new to android.

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


    <ImageView
        android:src="@drawable/duck"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:scaleType="centerCrop"
        />
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Link"
        android:textSize="?android:textAppearanceLarge"
        />
</LinearLayout>

android:textSize="?android:textAppearanceLarge"

android:background="@android:color/holo_red_dark"

CodePudding user response：

"?": it extracts a specific field/attribute/value from current "context"/object
"@": it refers to a specific entity that exists as a standalone "thing"

"?" is used when you want to refer to a specific Attribute of a (i.e) style but this style could be changed at "any time" (think about Dark or Light theme: the "Accent" color could be light or dark but the name is always ?attr/accentColor" for both themes). So if you refers to "?android:textAppearanceLarge" you want to use the "text appearance Large" version of current applied Theme. But this "text appearance Large" version could be different between two different Themes.

"@" is used when you want to use EXACTLY a specific "thing" that can't change between themes. In your specific example "@android:color/holo_red_dark" refers to a specific color that remain the same at any Theme and it is always "statically resolvable"

CodePudding user response：

The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.

You can prefer this https://stackoverflow.com/a/31129660/19674027