android @id和@+id的问题

Answer 1

As you know, when we define an id for a control in xml, we use @+id/xxx. If it does not exist in R.java, the corresponding xxx variable name and value will be generated as int. If it exists, it will be used. The original int value, so each xxx has a unique identifier.
However, when we define the ids.xml file in the resource file, we can customize the variable name of the id type in the form:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item name="cc" type="id">213</item>
</resources>

So we can quote it as follows in the layout file:

<Button
    android:id="@id/cc"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
    

As for what you said about @id/xxx referencing the same, it is the same as @+id/xxx.

Answer 2

It is also possible that a set of IDs has been customized under the values ​​folder. This can also be referenced directly using @id/xxx
<resources>

<item name="navigation_view" type="id" />

</resources>