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.
Related
I've created the project from default template Bottom Navigation Activity. I found that the background color for BottomNavigationView is #2D2D2D for dark theme. I defined it in colors.xml, but I'm not sure that it's good solution. Is there any pre-defined colors in system(smth like #android:color/theColorThatINeed)? Where can I see all of them?
the best practice is to use colors.xml, and put there all of your colors,
and then call it like this:
android:background="#color/red"
there are also system constants for colors, which contains this constants:
http://developer.android.com/reference/android/R.color.html
this resources are read only and you can not add to them more constants.
Unfortunately there isn't a single place to find all the 'system colours'. You can either find them through documentation, which is usually easy with the material components (which is where BottomNavigationView is from), or by trawling through the source code. For your particular example the colour you're after is ?attr/colorSurface, which is in the documentation and the source code.
Is there any pre-defined colors in system?
Yes, there is but it's limited. These colors are defined in android.R.color. Here are some of them.
background_dark
background_light
black
darker_gray
holo_blue_bright
holo_blue_dark
holo_blue_light
holo_green_dark
holo_green_light
holo_orange_dark
holo_orange_light
holo_purple
holo_red_dark
holo_red_light
transparent
white
widget_edittext_dark
To use them:
android:background="#android:color/background_dark"/>
In android developer examples, blogs, etc, I have seen multiple prefixes for the values of style attributes. Can someone please describe when to use what? For example, what is the difference between these 3 cases?
parent="#android:style/TextAppearance"
parent="TextAppearance.AppCompat"
parent="#style/TextAppearance.AppCompat"
What is the difference between these 2 cases?
<item name="colorControlNormal">#color/white</item>
<item name="android:colorControlNormal">#color/white</item>
Without any prefix references the style named on the same file.
#Style references your style.xml project file.
#android:style references defined Android API style.
More about Style Resource and Style and Themes.
About colorControlNormal vs android:colorControlNormal is the same explanation. If you use controlColorNormal you are defining the color applied to framework controls in their normal state in you app. If you use android:colorControlNormal you are overwriting the the color default applied to framework controls by the system.
You can think of # as signaling that a named resource is coming up.
#type/name identifies a resource of type type (string, color, layout, style etc) with name name defined in the app is coming up.
#+id/name identifies an id resource called name that will be created if it doesn't already exist (whereas #id simply refers to an id that already exists).
#android:type/name means that the named resource is part of the Android platform is coming up (it's not defined in the app -- it's provided by the device).
For style parents, the #style is optional. You can refer to styles directly by name. It's redundant because you can't derive a style from anything other than another style anyway.
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>
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 have an android application that I'm working on with a custom theme applied to it in the android manifest on the main activity itself. This activity creates a listview, which has the style applied to it.
If I create a custom layout for that listview, including a separate xml file for the rows themselves, and I apply styling directly to those layouts, does this style overwrite the overall style for the application? I'm trying to get a grasp on the hierarchy of events as far as how styling and themeing works.
The way I'm GUESSING it works in my example is: apply style for row, referenced by row layout xml > apply style for overall listview, referenced by listview layout xml > apply style from custom theme, referenced by style xml referenced by android manifest
Am I right? Or am I approaching this incorrectly. (just for confusions sake, by ">" I mean has a greater importance than)
If you've specified the same attributes in multiple places, the list below determines which attributes are ultimately applied. The list is ordered from highest precedence to lowest:
Applying character- or paragraph-level styling via text spans to TextView-derived classes
Applying attributes programmatically
Applying individual attributes directly to a View
Applying a style to a View
Default styling
Applying a theme to a collection of Views, an activity, or your entire app
Applying certain View-specific styling, such as setting a TextAppearance on a TextView
Hope I am understand your question right here...
The styles you define in styles.xml will always overwrite the styles coming from the theme currently used by android.
But this only works for the attributes you overwrite.
If you leave an attribute untouched, android will provide the style for it, and sometimes this comes bite you in the butt :)
This system is best described like this:
A textview requires an attribute example
<item name="android:textColor">#00FF00</item>
Android will first look in the original layout.
If not found, it will look into your custom styles.
If not found, it will look into android styles.
Hope this helps.
The standard themes have lines like which define the ListView style:
<item name="listViewStyle">#android:style/Widget.ListView</item>
In your own theme you can do a
<item name="listViewStyle">#style/MyOwnListView</item>
Something that is not defined in the ListView style (own or default) will be what is defined in the theme if you have defined it there.