Develop Reference

Android layout reference xml element later in file

How do I reference a later XML element?
Here's a specific use case. Let's say I have a form with a root LinearLayout, containing LinearLayouts for multiple rows, each row having one or more text input areas.
Here's a visual of what I'm going for. First pic is from Venmo's app, second is a rendering of the following XML.
Such a layout could look like this:
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<LinearLayout
android:id="#+id/row_card_number"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal">
<EditText
android:id="#+id/card_number"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:nextFocusDown="#id/month"/>
</LinearLayout>
<LinearLayout
android:id="#+id/row_date"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal">
<EditText
android:id="#+id/month"
android:layout_height="wrap_content"
android:layout_width="100dp"
android:nextFocusDown="#id/year"/>
<EditText
android:id="#+id/year"
android:layout_height="wrap_content"
android:layout_width="match_parent"/>
</LinearLayout>
</LinearLayout>
In this use case, forward referencing is necassary in order to set the next focus element. That way, when you press the next button on the keyboard, it'll go to the correct view. In this sample xml, without the nextFocusDowns, pressing next would go from name to month, and never go to year.
However, if you try to compile this, you'll get an error:
Error:(18, 36) No resource found that matches the given name (at 'nextFocusDown' with value '#id/month').
This is because the id month hasn't yet been initialized when I'm trying to reference it, since that's later in the file. How can I reference an id in xml that appears later in the file?

The simplest solution is just to replace
android:nextFocusDown="#id/month"
with
android:nextFocusDown="#+id/month"
When the compiler is parsing your XML to add the id's to R.java, it just reads top to bottom. When you have #id/month, it searches through the existing id's, and fails to find it.
However, if you do #+id/month, it creates a new id, and links to that. When it gets to android:id=#+id/month in the actual month view, it links it to the same id that we already created.
This brings up the question: If you can replace #id/ with #+id/, and #+id/ will work regardless of the order of elements, why even bother to use #id/?
The reason for this is if the id doesn't exist, #id/ will throw a compiler error, while #+id/ will log a warning at runtime.
Consider this XML:
<EditText
android:id="#+id/month"
android:layout_height="wrap_content"
android:layout_width="100dp"
android:nextFocusDown="#+id/SOME_RANDOM_ID"/>
<EditText
android:id="#+id/year"
android:layout_height="wrap_content"
android:layout_width="match_parent"/>
When this is parsed, a new id element SOME_RANDOM_ID is created. However, when Android tries to apply it at runtime, it can't find an element with that id. If you look at Logcat, you'll see this:
W/View﹕ couldn't find view with id 2131689604
This log message is both hard to find and hard to debug. One small typo in a #+id/ and you'll have a bug that could be incredibly difficult to debug. However, if we had done:
android:nextFocusDown="#id/SOME_RANDOM_ID"
Then we'd get a compiler error, something like:
Error:(18, 36) No resource found that matches the given name (at 'nextFocusDown' with value '#id/SOME_RANDOM_ID').
This is much easier to find and debug.
tl;dr: You can use #+id/ instead of #id/ and you'll be able to forward reference, but note that that can make small typos incredibly difficult to debug.
You might be able to use a RelativeLayout to make all the Views exist in reverse order in the xml, but that seems like overkill to me.

I had the same issue recently and I used #+id/my_new_id the first time I referenced the element and later in the XML in the element definition, I assigned #id/my_new_id to the android:id attribute. It seems it works fine and it's not necessary write #+id with the same id more than one time avoiding possible warnings.
For example:
<LinearLayout
...
android:layout_toLeftOf="#+id/my_new_id"
... >
...
</LinearLayout>
<ImageButton
android:id="#id/my_new_id"
... />

Why declaring #+id is allowed in other than id attributes

So lastly I have had to rebuild not my XML layout file for android app. And I saw constructions like
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<LinearLayout
android:id="#+id/rev_main"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_below="#+id/rev_arrow">
<!-- some stuff in here -->
</LinearLayout>
<ImageView
android:id="#+id/rev_arrow"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="#drawable/arrow"
/>
</RelativeLayout>
It is really annoying when I see that someone using #+id in not android:id attribute. When any developer will want to search this id he will find LinearLayout first instead ImageView.
My question is why google allow it? Is there any particular reason for it and I just didn't know it?
Sorry for my poor English.

Yes, there is a reason:
Sometimes, you have to set a View A relative to the position of a View B which is declared later in the XML-file (which is the case in your example. The LinearLayout is "View A" and the ImageView is "View B").
Imagine, the code you've got a problem with
android:layout_below="#+id/rev_arrow"
would look like this instead:
android:layout_above="#+id/rev_arrow"
The android:layout_above would be useless if you couldn't declare an id inside it.
Because it was mentioned in a few comments:
You have to use the "plus"-sign always at that place, where the id is first declared in the layout-file (from top to bottom). It is independet from the attribute, like "id" or "layout_below".

This is a valid use of the ID, as it tells the layout manager that the view identified by id/rev_main view is to be placed below the view identified by id/rev_arrow.
So in places other than android:id, the ids are used as references to views identified by the respective id.

no resource found that matches the given name?

This is driving me mad. My project compiled fine a moment ago. I made some minor change somewhere else, and now I'm getting this error. Here's the error message in full:
no resource found that matches the given name (at 'layout_above' with value '#id/blank_view").
Here's my XML file where the error is occurring:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<ImageButton
android:id="#+id/btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:layout_marginRight="#dimen/margin_right"
android:layout_above="#id/blank_view"
android:src="#drawable/button" />
<View
android:id="#+id/blank_view"
android:layout_width="match_parent"
android:layout_height="#dimen/margin_bottom"
android:layout_alignParentBottom="true" />
</RelativeLayout>
How in the hell could it be failing to find a resource with the name "#id/blank_view" when that resource is just below it in the file?
Btw, the reason I have my layout like this is that I want the ImageButton to be aligned to the bottom of my relative layout, but offset up by a margin. For some reason, those two attributes (layout_alignParentBottom and layout_marginBottom) don't mix well in the same view.
I should also point out that this happened earlier as well, but I just removed the reference that was giving AndroidStudio such a problem, rather than trying to fix it. This, however, is too important to wave away like that.

This happens because the xml is parsed in a linear fashion and the views/objects created in a top to bottom order. So your xml tells the view builder to put your ImageButton above an item that does not exist yet.
Just move it below the blank view and the error should go away.

Try to add the property to your view:
<view
android:id="#+id/blank_view"
android:layout_width="match_parent"
android:layout_height="#dimen/margin_bottom"
android:layout_alignParentBottom="true"
android:layout_below="#id/btn"
/>
And remove:
android:layout_above="#id/blank_view"

Android layout_below not working

I use RelativeLayouts extensively in my app and thought I knew how to specify them, but this has me at a loss. I am basically positioning 4 TextViews in two rows of two each consisting of a label and text that will be supplied. It should look something like:
Born: 23 Aug 1810 Mason Co., Kentucky
Died: 15 Jul 1865 Cincinnati, Hamilton Co., Ohio
This is the relevant portion of the layout:
<TextView android:id="#+id/birth_lbl"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/given_layout"
android:layout_alignLeft="#+id/given_layout"
android:layout_marginTop="6dip"
style="#style/label"
android:text="#string/born"
/>
<TextView android:id="#+id/birth"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/birth_lbl"
android:layout_alignBaseline="#+id/birth_lbl"
android:layout_marginLeft="10dip"
android:layout_marginRight="6dip"
style="#style/main_text"
android:text="dd Mmm yy"
/>
<TextView android:id="#+id/death_lbl"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/birth"
android:layout_alignLeft="#+id/birth_lbl"
android:layout_marginTop="4dip"
style="#style/label"
android:text="#string/died"
/>
<TextView android:id="#+id/death"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_toRightOf="#+id/death_lbl"
android:layout_alignLeft="#+id/birth"
android:layout_alignBaseline="#+id/death_lbl"
android:layout_marginRight="6dip"
style="#style/main_text"
android:text="dd Mmm yy"
/>
For some reason, this displays the death line views ABOVE the birth line views! If I change the spec of the death_lbl view to instead be 'layout_below="#+id/birth_lbl"', the lines are positioned correctly! However, it is possible for the "birth" view to wrap to multiple lines, so I really need to position the 2nd line below "birth", not "birth_lbl".
Anyone know the reason for this behavior? It occurs both in the Graphical Layout editor in Eclipse and at runtime on my tablet running Android 4.0.

Try changing android:layout_below="#+id/birth" in death_lbl to android:layout_below="#id/birth", because at this point it is already declared, which the + implies, it could lead to problems when declaring it again.

I was also facing the same problem because I was using constraint layout, but align_below works only in Relative Layout. so check which layout you are using.

If fixing your + signs doesn't help, you could always position your id/death below id/birth, and then put id/death_label toLeftOf id/death.

I was actually able to duplicate this phenomenon by coding up a temporary layout with only those fields, and was able to determine that the problem went away if I did not position the initial birth_lbl view relative to the view above it ("given_layout" in this case).
So I'm not sure if this is classified as a fix, a workaround, or a kludge (is that still a word these days?), but what I did was to position the text views inside their own RelativeLayout and position the RelativeLayout relative to id/given_layout. In any case, it works...

Your problem is that you are calling the properties on the relative layout as if though you are declaring it again.
When you declare an ID for a view then it should be done like this android:id="#+id/button"
and when you want Android to position that particular view above or below any other view in a relative layout you have to call it like this android:layout_below="#id/textview"
This tells android that you declared a button with id button and want it to be positioned below the textview, remember do not use #+id to postion view use #id instead.

LinearLayout in App Widget

I am working with modified version of sample WeatherListWidget to get a better understanding of App Widgets. Things are fine - except when I try to replace the dark_widget_item and light_widget_item layout files with slightly more complex layout files. Here is original layout:
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/widget_item"
android:layout_width="match_parent"
android:layout_height="46dp"
android:paddingLeft="25dp"
android:gravity="center_vertical"
android:background="#drawable/item_bg_light"
android:textColor="#e5e5e1"
android:textSize="24sp" />
I would like to be able to have multiple text lines. But:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/widget_item"
android:background="#drawable/item_bg"
android:layout_width="match_parent"
android:layout_height="46dp"
android:paddingLeft="25dp">
<TextView android:id="#+id/type_string"
android:textColor="#666666"
android:textSize="20sp" />
<TextView android:id="#+id/title_string"
android:textColor="#666666"
android:textSize="18sp" />
</LinearLayout>
fails.
In fact, it results in "Sorry! The application Launcher (process com.android.launcher) has stopped unexpectedly. Please try again. Force close".
Reinstating TextView widget_item.xml fixes this. I suspect that part of the problem is how I reference RemoteViews in WeatherWidgetService.getViewAt() - but I am getting very little help from DDMS or LogCat or anything else.

Thanks guys, I got the notification. (SO requires a username with length > 2, hence the dot)
Answer as per comment:
I don't see any layout_width and layout_height attributes for both of the TextViews in your LinearLayout - they are mandatory. Also, if you want the two TextViews to be above eachother, add android:orientation="vertical" to the LinearLayout. And just to the record, you can break a CharSequence to multiple lines in a single TextView by adding "\n" inbetween the different elements.
If you're going to include an image as well, then you're probably better off with a LinearLayout than a single TextView indeed, although you could potentially use the intrinsic drawable option of the latter. That could get a little messy though, especially if you're planning on using different styles for the different lines of text... Not impossible, but I'd stick with the LinearLayout. ;)