I'm creating my first android app and i have a few layouts so far. In one of them i created a TextView and named it "textViewCurrentUserName". For my surprise, when i created a new layout, inserted a new TextView and try to name it "textViewCurrentUserName" i got a error stating that the name already exists!!! This has got to be a bug right?!? i mean, ok to "no repeat" on the same layout (xml), but these are totally different layouts!!!
Am I doing something wrong here?!? Is there a workaround this??? i dont want to keep a totally idiotic and hard-to-read/understand name such as "activity_UserDetails_TextViewCurrentUserName" and "activity_UserExport_TextViewCurrentUserName"
The Android ID field creates a unique ID for each UI element and stores them in the R file. Since fragments/activities and layouts and fairly decoupled, the app itself won't know which layouts and UI elements are available. Thus, you must explicitly state which element you're looking for. The app will throw an exception if the specified UI element is not currently available (or, perhaps, the element will be null).
The only workaround is to develop a naming convention. One might use [fragment-name]_textViewCurrentUser to allow each text view to have a unique ID. Or perhaps you can put a lowercase "t" in front of each UI element that is a text view: [fragment-name]_tCurrentUser.
When working through the tutorial to build my first Android application I reached a section where it states the #+id/ prefix not only references a resource that is defined in the gen/R.java file, but that the + sign also indicates its first encounter with it so it will create it. Consider this code snippet:
<EditText android:id="#+id/edit_message"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:hint="#string/edit_message" />
After reading through the side-bar in the first link, related to resources, and the article it linked to named Providing Resources (at a somewhat cursory level), I'm unable to get a very clear statement from the documentation on the scope of the resource with the #+id/ prefix. I understand that you can have a resource with the same name scoped inside each prefix:
Note: This string resource has the same name as the element ID: edit_message. However, references to resources are always scoped by the resource type (such as id or string), so using the same name does not cause collisions.
but what I'm driving at is this. Based on the documentation it appears that I can't have two controls resourced as edit_message in two different activities because there would be a conflict.
My concern here is that I'd presumably have to prefix my id attributes with the Activity name to keep them unique so I can access these controls from code.
Am I correct in my statement and assumptions here?
You can place that exact block of XML in another layout file and it will work just fine. #+id generates a new id if it is not defined yet. When your app gets packaged, the packaging tool will create the id once and all the others will correctly refer to that id.
As long as you don't end up with two UI components in the same layout with the same id, everything's ok.
In normal, we should use #+id/ to define an id and use #id to reference an id. Today I found #+android:id/title in apps/settings/res/layout/preferenc_progress.xml.
How to understand it and how to use it?
It is used for resources that are shipped with the SDK.
You can take a look at them by browsing to
[PATH TO ANDROID SDK]/platforms/android-[VERSION]/data/res
By using the android in android.R.whatever you just specify the R file to look up. For more information you should read Accessing Platform Resources.
That belongs to the app preferences activity screen definition.
title and summary are standard Android fields of a TextView preference item.
I think it does the same thing. It's just a more formal way of saying it by specifying where the namespace is.
I've never met this way of giving id, but in theory this means adding new id title to android package. So you'll be able to use it in your code like android.R.id.title. But I'm not sure resource compiler will really create any id in android package. I think it can be used only with predefined ids. But I'll give you more precise answer later, when I'll be able to check it.
EDIT: I've checked it and found some differences. Firstly, if you define Android's id using #+android:id/some_id, which is already present in SDK, this id will not be defined in your R.java file. If it's not present in SDK, it will be defined in R.java, but with different kind of value. Secondly, if you'll try to convert id from its string representation to int value, Resources.getIdentifier() method will return 0 in case of #+android:id format.
This means it will create an id in your resource file.
What is the diffirence between the #id/ and #+id/?
In #+id/ the plus symbol + instructs to create a new resource name and add in to the R.java file but what about #id/? From the documentation of ID: when referencing an Android resource ID, you do not need the plus symbol, but must add the android package namespace, like so:
android:id="#android:id/list"
But in the image below Eclipse doesn't suggest any kind of #android:id/.
Are #id/ and #android:id/ the same?
you refer to Android resources , which are already defined in Android system, with #android:id/.. while to access resources that you have defined/created in your project, you use #id/..
More Info
As per your clarifications in the chat, you said you have a problem like this :
If we use android:id="#id/layout_item_id" it doesn't work. Instead #+id/ works so what's the difference here? And that was my original question.
Well, it depends on the context, when you're using the XML attribute of android:id, then you're specifying a new id, and are instructing the parser (or call it the builder) to create a new entry in R.java, thus you have to include a + sign.
While in the other case, like android:layout_below="#id/myTextView" , you're referring to an id that has already been created, so parser links this to the already created id in R.java.
More Info Again
As you said in your chat, note that android:layout_below="#id/myTextView" won't recognize an element with id myTextViewif it is written after the element you're using it in.
the + sign is a short cut to add the id to your list of resource ids. Otherwise you need to have them in a xml file like this
<?xml version="1.0" encoding="utf-8"?>
<resources>
<item name="my_logo" type="id"/>
</resources>
In Short
android:id="#+id/my_button"
+id Plus sign tells android to add or create a new id in Resources.
while
android:layout_below="#id/my_button"
it just help to refer the already generated id..
Its very simple:
"#+..." - create new
"#..." - link on existing
Source: https://developer.android.com/guide/topics/resources/layout-resource.html#idvalue
The plus sign (+) before the resource type is needed only when you're defining a resource ID for the first time. When you compile the app, the SDK tools use the ID name to create a new resource ID in your project's R.java file that refers to the EditText element. With the resource ID declared once this way, other references to the ID do not need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not needed for concrete resources such as strings or layouts. See the sidebox for more information about resource objects.
From: https://developer.android.com/training/basics/firstapp/building-ui.html
From the Developer Guide:
android:id="#+id/my_button"
The at-symbol (#) at the beginning of the string indicates that the XML parser should parse and expand the rest of the ID string and identify it as an ID resource. The plus-symbol (+) means that this is a new resource name that must be created and added to our resources (in the R.java file). There are a number of other ID resources that are offered by the Android framework. When referencing an Android resource ID, you do not need the plus-symbol, but must add the android package namespace, like so:
android:id="#android:id/empty"
There's a bug with Eclipse where sometimes if you just created a new #+id/.., it won't be added immediately to the R.java file, even after clean-building the project. The solution is to restart Eclipse.
This I think should be solved as soon as possible, because it may (and from experience, will) confuse some developers into thinking that there's something wrong with their syntax, and try to debug it even if there's really nothing to debug.
Android uses some files called resources where values are stored for the XML files.
Now when you use #id/ for an XML object, It is trying to refer to an id which is already registered in the values files. On the other hand, when you use #+id/ it registers a new id in the values files as implied by the '+' symbol.
Hope this helps :).
#id/ and #android:id/ is not the same.
#id/ referencing ID in your application, #android:id/ referencing an item in Android platform.
Eclipse is wrong.
Difference between #+id and #id is:
#+id is used to create an id for a view in R.java file.
#id is used to refer the id created for the view in R.java file.
We use #+id with android:id="", but what if the id is not created and we are referring it before getting created(Forward Referencing).
In that case, we have use #+id to create id and while defining the view we have to refer it.
Please refer the below code:
<RelativeLayout>
<TextView
android:id="#+id/dates"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_toLeftOf="#+id/spinner" />
<Spinner
android:id="#id/spinner"
android:layout_width="96dp"
android:layout_height="wrap_content"
android:layout_below="#id/dates"
android:layout_alignParentRight="true" />
</RelativeLayout>
In the above code,id for Spinner #+id/spinner is created in other view and while defining the spinner we are referring the id created above.
So, we have to create the id if we are using the view before the view has been created.
If the view item performs the same operation, you can use the #+id for each entry in any layout because during the compilation of multiple #+id/foo the R.java file only creates one enumeration. So for example, if I have a save button on each page that performs the same operation, I use android:id="#+id/button_save" in each layout. The R.java file only has one entry for the button_save.
Difference between “#+id/” and “#id/” in Android
The first one is used for to create the ID of the particular ui component and the another one is used for to refer the particular component
The document says "#[+]id/myid" as the definition for android:id, from the notation it indicates that the plus is optional. But nowhere can I find a definition of what the plus sign means or not supplying it would mean.
What does it mean and why is it there?
#+id/foo means you are creating an id named foo in the namespace of
your application. You can refer to it using #id/foo. #android:id/foo
means you are referring to an id defined in the android namespace.
This namespace is the namespace of the framework. In this case, you
need to use #android:id/list and #android:id/empty because these are
the id the framework expects to find (the framework knows only about
the ids in the android namespace.)
Taken from one of the "Android Developer | Google Groups" discussions.
The documentation says,
The plus-symbol (+) means that this is
a new resource name that must be
created and added to our resources (in
the R.java file). There are a number
of other ID resources that are offered
by the Android framework. When
referencing an Android resource ID,
you do not need the plus-symbol, but
must add the android package
namespace, like so:
android:id="#android:id/empty"