What is the scope of android:id? [duplicate]

Question

Possible Duplicate:
Scope of R.id values

Could someone please clarify the scope of resource id's in Android? Recently I was making a custom title bar which contained a RelativeLayout called "header". I was invoking this title bar in the onCreate() handlers of each of my Activities thusly:

requestWindowFeature(Window.FEATURE_CUSTOM_TITLE);
setContentView(R.layout.activity_layout);
getWindow().setFeatureInt(Window.FEATURE_CUSTOM_TITLE, R.layout.title_bar);  

One of my Activities had a TextView in its layout with the id of "header" and in that Activity's onCreate() I do a

TextView tv1 = (TextView)findViewById(R.id.header); 

. . . which blew up at runtime with ClassCastException, apparently because it thought I was trying to cast a RelativeLayout to a TextView. Changing the name of the RelativeLayout in the title bar fixed it.

But I don't really understand this. Are Android resource id's global? If so why wouldn't this collision be caught at build-time?

Note that this question has been asked before on SO: Scope of R.id values . . . Where the OP said "as far as I can tell" R.id's are global but my question is not a duplicate because neither of my questions is really addressed in that one.

Answer 1

R.* values are global to their own type. So you can have a R.string and an R.id with the same name, but not two IDs with the same name. In your case, I feel that doing a build clean would have fixed your problem as well. For example, take a look at the R.java file contents:

    public static final class drawable {
    public static final int icon=0x7f020000;
    public static final int info=0x7f020001;
    public static final int menu=0x7f020002;
    public static final int splash1=0x7f020003;
}
public static final class id {
    public static final int ImageView02=0x7f070008;
    public static final int bestTimeTextView=0x7f070005;
    public static final int infoButton=0x7f070003;
    public static final int infoImageView=0x7f070000;
    public static final int levelSpinner=0x7f070002;
    public static final int menuImageView=0x7f070001;
    public static final int selectLevelTextView=0x7f070004;
    public static final int startButton=0x7f070006;
    public static final int timeTextView=0x7f070007;
}
public static final class layout {
    public static final int info=0x7f030000;
    public static final int menu=0x7f030001;
    public static final int splash=0x7f030002;
}
public static final class raw {
    public static final int down=0x7f040000;
    public static final int up=0x7f040001;
}
public static final class string {
    public static final int appname=0x7f060000;
    public static final int besttime=0x7f060002;
    public static final int help=0x7f060004;
    public static final int selectlevel=0x7f060001;
    public static final int start=0x7f060003;
}

Here I have the info name used in both drawable and layout parts, and the app functions perfectly. However, if I try adding two resources with the same ID, or two drawables with the same name, or two strings with the same name etc. it will not function properly.

Answer 2

Are Android resource id's global?

They are "global" within an activity. The title bar is in the activity. The layout is in the activity. Widgets from both will be in the activity. Widgets with the same ID from both will be in the activity and will represent a point of conflict for findViewById() calls.

If so why wouldn't this collision be caught at build-time?

First, why would it? How is the build system supposed to know that you are are going to take res/layout/title_bar.xml and res/layout/whatever_the_heck_you_called_it.xml and use them both in the same activity?

Second, there is nothing wrong with having duplicate IDs. Your job, as a developer, is to call findViewById() on whatever will give you a unique result. For example, you could have done:

TextView tv1 = (TextView)(findViewById(android.R.id.content).findViewById(R.id.header));

and kept the duplicate IDs. Going the route you did, of having distinct IDs, is a better choice here for maintainability. But there will be plenty of cases where you will intentionally have duplicate IDs, such as every AdapterView you ever implement (e.g., ListView), as their contents will tend to have duplicate IDs (e.g., each row in the ListView is stamped out from the same row layout resource).