android:layout_width and android:layout_height are required attributes, i.e. if you don't specify them for a Layout, Android Studio tells me "Element x doesn't have required attribute android:layout_height."
I'm defining some custom attributes in my own classes and I'd like to require that anyone who uses them in xml specify values just like they have to specify layout_width and layout_height.
How do I do that? Is there something I can add in attrs.xml here:
<declare-styleable name="MyStyleable">
<attr name="myAttribute" format="float"/>
</declare-styleable>
Or maybe in style.xml here:
<style name="MyStyle">
<item name="myAttribute">3</item>
</style>
AFAIK, currently attrs don't have any attributes to mark them mandatory. ("layout_width" and the like are very common mistakes, checking for those is wired into the SDK)
You can, however, throw an RuntimeException from your custom View if an attribute is missing (which you can determine by calling TypedArray.hasValue()) -- TypedArray does that in getLayoutDimension() method.
This is exactly the behavior you get if you compile an app that's missing a "layout_width" somewhere using command line tools (i.e. no Eclipse or Idea to detect the missing atrribute and prevent you from compiling): you run the app and it crashes immediately with a RuntimeException saying you're missing the attribute.
Using Android KTX, there are extension functions that provide getFloatOrThrow() and other similar methods.
Check them out here
TypedArray also has a few methods to cover both attribute getting and exception throwing states. Just use them if you want your attribute to be required (not clearly)
Try to take a look at this link. There is info about how is "wrap_content" and "match_parent" defined. Maybe this will help.
Related
I was reading Ha Duy Trung's tutorial Supporting multiple themes in your Android app and in it he has the following snippet for values/attrs.xml:
<attr name="themedMenuStoryDrawable" format="reference" />
<attr name="themedMenuCommentDrawable" format="reference" />
Where is this <attr> element actually documented? Looking in the Android API Guide Providing Resources, I cannot find any mention of this type of element.
I did find a mention of <attr> elements in Creating a View Class, but there they all appear inside of <declare-styleable> elements, and there is hardly any explanation of how these elements work.
Is there proper reference documentation for the <attr> element? If not, then:
What does it mean to have an <attr> element directly under <resources>?
What is the meaning of the format attribute, and what values can it take?
What other XML attributes can <attr> take?
Can <attr> appear as the child of anything other than <resources> and <declare-styleable>?
I'd specifically like to know how to use <attr> in the context of theming (rather than custom views) though documentation of how it works in general would be even better.
I'm trying to create a custom attribute for my control. Here's my attrs.xml:
<?xml version="1.0" encoding="utf-8" ?>
<resources>
<declare-styleable name="ImageView">
<attr name="testcustom" format="reference"/>
</declare-styleable>
</resources>
In my test app at root, I add this namespace xmlns:TestCustom="http://schemas.android.com/apk/res-auto"
Later in my layout file I have an image view
<ImageView
android:src="#drawable/Icon"
TestCustom:testcustom="#drawable/Icon"/>
The first android:src property is fine, however TestCustom does not work.
The error given is "No resource found that matches the given name (at 'testcustom' with value '#drawable/icon)
So... what's going on here? Anyone have any ideas?
You need to specify your xmlns: to your specific package namespace. For example:
xmlns:custom="http://schemas.android.com/apk/res/com.example.customviews"
Represents custom: for the package namespace of com.example.customviews.
You can read more about this here: https://developer.android.com/training/custom-views/create-view.html#customattr
Note: You may want to consider following the convention of camelCase with regards to your custom attributes, views, etc.
So to recap: The solution here was to use lower case name when referencing the resource via a custom attribute. Apparently Xamarin is doing some work in the background that normally allows resources that start with upper case to be OK, but it probably doesn't address this for custom attributes.
If you are using Xamarin, and custom attributes and you can't reference your resource, try it in lower case!!!
Or as Jon recommends, don't use upper-case stuff at all for resources.
Can anyone explain the question mark means in Android XML attributes?
<TextView
style="?android:attr/windowTitleStyle"
More attributes
/>
The question mark means it's a reference to a resource value in the currently applied theme. See the linuxtopia Android Dev Guide or the android.com Dev Guide for more about it.
\? escapes the question mark.
The ? lets you refer to a style attribute instead of a specific hard-coded resource. See "Referencing style attributes" in the Android Dev Guide for details.
So, how is this actually useful? It makes the most sense when considering multiple themes containing the same custom resource attribute.
Say you have movie-related themes like MyThemeTransformers and MyThemeHobbit, and both have an attribute called movieIcon. And that movieIcon attribute points to a different #drawable resource, say robot.png or hobbit.png, in each theme definition.
You can refer to "?attr/movieIcon" anywhere the theme is in effect (like in a toolbar or dialog or whatever kind of View layout), and it will automatically point to the correct drawable when you switch between themes. You don't need any theme-dependent logic to use the different drawables. You just define the movieIcon attribute for each theme and the Android framework takes care of the rest.
I was looking for how to highlight a selected item in a list when displaying a contextual action bar for the selection, and the solution I found was to set the android:background attribute of my row layout xml to "?android:attr/activatedBackgroundIndicator".
How does setting this work though?
what is the mechanism involved?
what do the syntax elements like "?", "attr", "activatedBackgroundIndicator" mean?
where is the meaning of "activatedBackgroundIndicator" defined?
If you are in a forensic mood here is how to dig and find out what is going on.
android:background="?android:attr/activatedBackgroundIndicator"?
Intuitively this means set the background to some drawable.
But lets decompose this further to see how we get to our mysterious drawable.
To be precise it means "set the background attribute to what the attribute "activatedBackgroundIndicator" refers to in the current theme.
If you understand "refers to in the current theme" part, you have basically understood everything that is going on behind the covers.
Basically, activatedBackgroundIndicator is not an actual drawable but a reference to a drawable. So where is "activateBackgroundIndictor" attribute actually defined?
Its defined in your sdk directory in a file name attrs.xml. For example:
path_to_android_sdk/platforms/android-17/data/res/values/attrs.xml
If you open that file, you will the declaration as follows:
<attr name="activatedBackgroundIndicator" format="reference" />
attrs.xml is where you declare all the attributes that you are later going to use in your view xml. Note we are declaring the attribute and its type and not actually assigning a value here.
The actual value is assigned in themes.xml. This file is located at:
path_to_android_sdk/platforms/android-17/data/res/values/themes.xml
If you open that file, you will see the multiple definitions depending on what theme you are using. For example, here are the definitions for themes name Theme, Theme.Light, Theme.Holo, Theme.Holo.Light respectively:
<item name="activatedBackgroundIndicator">#android:drawable/activated_background</item>
<item name="activatedBackgroundIndicator">#android:drawable/activated_background_light</item>
<item name="activatedBackgroundIndicator">#android:drawable/activated_background_holo_dark</item>
<item name="activatedBackgroundIndicator">#android:drawable/activated_background_holo_light</item>
Now we have our mysterious drawables. If you pick the first one, it is defined in the drawable folder at:
path_to_android_sdk/platforms/android-17/data/res/drawable/activated_background.xml
If you open that file you will see the definition of the drawable which is important to understanding what is going on.
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_activated="true" android:drawable="#android:drawable/list_selector_background_selected" />
<item android:drawable="#color/transparent" />
</selector>
Here we are defining a drawable with two states - default state is just transparent background and if the state is "state_activated" then our drawable is "list_selector_background_selected".
see this link for background information on on drawables and states.
"list_selector_background_selected" is a 9 patch png file that is located in the drawable-hdpi folder.
Now you can see why we defined activatedBackgroundIndicator as a reference rather than linking directly to the drawable file - it allows you to pick the right drawable depending on your theme.
I wondered this as well at one point. A large amount of the Android resources seem to be like a black-box and can't see them directly. I may be missing them someplace, but I can't find them in the SDK source code. Here is what I do know.
android:background will take a drawable.
The syntax is in the style
Must be a reference to another resource, in the form "#[+][package:]type:name" or to a theme attribute in the form "?[package:][type:]name"
In this case the ? signifies to look at a theme in package android and it is of type attr where the name is activatedBackgroundIndicator.
You should be able to access this in the code-behind with android.R.attr.activatedBackgroundIndicator as well.
A list of Android attr properties can be found at R.attr
activatedBackgroundIndicator is a defined drawable in Android 3.0+ as
Drawable used as a background for activated items.
It's basically just a standard item defined in the OS. I can't seem to find in in the Android source, but here is a link to the documentation. activatedBackgroundIndicator
This is a form of attaching a value from a theme. The value is technically not known during resource compilation because the theme values may not be known at that point. Instead the value is resolved at runtime based on the actual theme taken from (most commonly) ContextThemeWrapper.
This provides a way of reusing resource values. I'm not talking performance-wise here, but rather organization and maintenance-wise. The attribute acts as it were a variable with the promise that it will hold an actual value at runtime.
This approach also allows for greater customization - instead of hardcoding the value of e.g. window background drawable it gets the actual drawable from a theme, supplying a chosen attribute as the key. This lets you override the value for that attribute. You simply need to:
Create your own theme (which is just a fancy name for a "style" resource), most commonly deriving from one of default themes.
Supply your own value for the attribute in question.
The platform will automatically use your value provided that you have specified your theme for an activity or application. You do this like described in the question. The general syntax of theme-attribute references is described here: Referencing style attributes. You will also find an example and description of the whole mechanism there.
Edit
One thing that should be noted is the actual attribute names and their existence in various platform versions. It's fairly common for new attributes to be introduced in next platform versions - for example some were added in version 3.0 for the purpose of ActionBar styling.
You should treat attribute names as part of the API - in other words, they are part of the contract you are allowed to use. This is very similar to classes and their signatures - you use LocationManager class for the purpose of obtaining last device location because you know from some source (tutorials, reference, official guides, etc.) what's the purpose of this class. Similarly, the attribute names and their purpose are (sometimes well, sometimes miserably) defined in the Android Platform documentation.
Update: There is a more detailed version available from the API Guide so I'd like to quote it.
A style attribute resource allows you to reference the value of an attribute in the currently-applied theme. Referencing a style attribute allows you to customize the look of UI elements by styling them to match standard variations supplied by the current theme, instead of supplying a hard-coded value. Referencing a style attribute essentially says, "use the style that is defined by this attribute, in the current theme."
To reference a style attribute, the name syntax is almost identical to the normal resource format, but instead of the at-symbol (#), use a question-mark (?), and the resource type portion is optional. For instance:`
Original Answer:
numan salati already offered an perfect answer but it have not addressed the "?" syntax. Here's a quote from API Guide Accessing Resources
To reference a style attribute, the name syntax is almost identical to the normal resource format, but instead of the at-symbol (#), use a question-mark (?), and the resource type portion is optional. For instance:
?[<package_name>:][<resource_type>/]<resource_name>
I need to use a built in value from Android resources. This value is stored com.android.internal.R.attr.listViewStyle. Being unable to get that from within my code, I tried to find the appropriate value I can use insted. Well, I've just found android.R.attr.listViewStyle.
Question 1: Are these values same?
Question 2: Where can I find the XML for com.android.internal.R.attr.listViewStyle? May be I have to create my own style instead that one. In order to find it out I should look at that file.
Sorry if these questions are silly. I'm new to Android development yet.
com.android.internal classes are internal to android, they are only accessible within frameworks.
I think com.android.internal.R.attr.listViewStyle and android.R.attr.listViewStyle are same.
If you want to create your own style you can check here . This contains two listViewStyle. They are used based on the device default theme(Light or dark).
If you want to use this style, then i think you dont need to specify anything in your code, this is default theme, so it is picked automatically, if no attributes are specified.
You can add listViewStyle in values/attr.xml with this code :
<attr name="listViewStyle" format="reference" />
Change com.android.internal.R.attr.listViewStyle in your code to R.attr.listViewStyle
I find it in this
Example for attr.xml
< ?xml version="1.0" encoding="utf-8"?>
< resources>
< attr name="listViewStyle" format="reference" />
</resources>