Do Android's R.id need to be unique?

Question

Do Android's R.id need to be unique (as I've kept them so far)?

Or is it OK to reuse an R.id across different views?

Answer 1

ID's are used for finding views inside a view hierarchy. Android uses depth-first search algorithm - meaning it looks to the very bottom of one tree branch, then to another one etc. If you have two views with same ID, then the algorithm will find first view and stop searching further.

There is no strict requirement on uniqueness of ID. For instance, when you have a list view, then each item of that list will be inflated using same layout and in most cases will have same shared ID, which is totally ok.

Keeping that in mind, if you have two (or more) views sharing same ID, you should help Android to pick up the right one. For that you will first need to search for the correct parent of that view, and then for the view itself.

For instance, if you have two views with the same ID in two different fragments, then you should first search for fragment container view, and then for the view with shared ID inside that container.

Answer 2

Yes, it is OK to have the same ID in different layouts. You could find more info here: http://developer.android.com/guide/topics/ui/declaring-layout.html

Any View object may have an integer ID associated with it, to uniquely identify the View within the tree. When the application is compiled, this ID is referenced as an integer, but the ID is typically assigned in the layout XML file as a string, in the id attribute.

In a relative layout, sibling views can define their layout relative to another sibling view, which is referenced by the unique ID.

An ID need not be unique throughout the entire tree, but it should be unique within the part of the tree you are searching (which may often be the entire tree, so it's best to be completely unique when possible).

Answer 3

It is ok to use same id with different layouts.

For eg :

a.java

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.a);

b.java

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.b);

a.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:id="@+id/id_rel"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="4dp"
 >

b.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:id="@+id/id_rel"
android:layout_height="match_parent"
android:orientation="vertical"
android:padding="4dp"
 >

But using same id in one layout can cause exception

Answer 4

For most cases you can reuse id's in different layouts. But you should be aware about possibility of including one layout into another using , tags or using custom compund views as well as list items or fragments added to current view.