Develop Reference

xamarin android alignParentRight style

can you tell me how can I add alignParentRight into my xml style?
<style name="My_Style">
<item name="android:layout_width">fill_parent</item>
<item name="android:layout_height">75dp</item>
<item name="android:background">#color/Gray_75</item>
**<item name="android:alignParentRight">true</item>**
</style>

yes but I need to add it programatically for dynamic and I will add only style and thats all.
Generally you can't do that programmatically. What we can do is:
Create two different styles, one with <item name="android:alignParentRight">true</item> and the other one not, apply them to the control when it is needed. Since in which scenario of your styles will be used is not clear in your question, maybe you can look into State List.
I personally think it is more straight if we directly set the Layout Parameters in code behind, for example:
var parameters = btn.LayoutParameters as RelativeLayout.LayoutParams;
parameters.AddRule(LayoutRules.AlignParentRight);
The btn in this code refer to a Button control.

Theming Android App with reference to color

I want to offer my user two themes: light and dark. I want the background color of a certain layout to change based on whichever theme has been chosen.
Essentially, I want to have two themes like this:
<style name="Theme.Light">
<item name="myBackgroundColor">#ffffff</item>
</style>
<style name="Theme.Dark">
<item name="myBackgroundColor">#000000</item>
</style>
And then from my Activity, I would reference the variable like this:
<RelativeLayout
...
android:background="?myBackgroundColor"/>
How do I do that?

Ok I've figured out an answer of sorts. If anyone has a better answer, please post it.
You need to declare the attribute "myBackgroundColor" as styleable. E.G:
<declare-styleable name="TestStyleable">
<attr name="myBackgroundColor" format="color|reference" />
</declare-styleable>
I have no idea what the name refers to, it seems you can set it to anything. Once you have declared this block, you can use "myBackgroundColor" as I have done above.

Themes in android

A create a simple Theme as
<style name='one'>
<item name='android:textColor'>#eea</item>
<item name='android:textSize'>20sp</item>
</style>
However on viewing in the emulator the screen goes black.when i do not apply theme the screen has a white background .
what really happens here.i am just starting with android.
In addition ,if a apply a theme to my activity then the attributes of the theme applies to all components of my activity say button,textfields and edittexts .
why would i then write
android:textSize=?android:textSize
to reference value from the theme for any button in my layout when the same value would already be applying.
is the syntax above the correct way to reference an attribute from my theme to assign to attribute for any view in my layout.
thanks
tejinder

Yeah, so you need to do a little more reading.
Let's start with the basics,
You need to understand the differente betweent an Attribute, a Style, and a Theme.
An Attribute is something that can be styled. For instance: android:textSize is an attribute that can have any value.
A Style is a set of specific attributes that will be applied to a Widget. They are defined
in your /values/styles.xml
For instance:
<style name="normalTextThin" parent="android:Widget.Holo.Light.TextView">
<item name="android:gravity">left|center_vertical</item>
<item name="android:padding">8dp</item>
<item name="android:textColor">#FFFFFF</item>
<item name="android:textSize">16sp</item>
</style>
The styles can be applied either as part of a theme or directly as theme-independent.
Theme-indepentent styling of a widget is like this:
<TextView
android:id="#+id/text"
style="#style/normalTextThin"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
You are then theming only that one TextView.
A Theme is a collection of Styles that can be applied to a part of your UI, such a a whole Activity, or your whole Application.
For instance:
<style name="AppTheme" parent="android:Theme.Light">
<item name="android:editTextStyle">#style/EditTextAppTheme</item>
<item name="android:buttonStyle">#style/ButtonAppTheme</item>
<item name="android:imageButtonStyle">#style/ImageButtonAppTheme</item>
</style>
Here, we are declaring that all EditText in your application will use the style named EditTextAppTheme, and so forth and on. When done like this, in order to actually have the theme be active, you declare it in the manifest:
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme" >
That means that you are not required to declare the style on each widget you create.
<EditText
android:id="#+id/input"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:hint="#string/hint_search">
<requestFocus />
</EditText>
That widget right there would already be styled using EditTextAppTheme without the need of you explicitely declaring so.
I recommend you try to read on what attributes can be styled, how to style them, and so forth and on.
If you don't want to though, it's fine, you can still get a lot done with the following tools for styling:
ActionBarStyleGenerator to help you create styles for the ActionBar.
Android Holo Colors to help you style standard widgets.
Hope that helps.
Additional Info
Let me clarify on the whole ?attr/attributeName
The ? means that the system will choose the specific attributeName value for the current Configuration (not specific to different themes). This should be used only when you want the value to be different on different configurations. For example:
?android:attr/actionBarSize
This line is a dimension, and it will be different not based on the current theme, but on the current device screen size and orientation (values, values-land, values-sw600dp).
It's important to know that specifying ?android: means you are accessing preset Android values, not yours. If you have or want to create and use your own attribute values for specific configurations, you must do the following:
Create a file named attrs.xml on your /values/ folder.
Declare the desired custom attribute:
<?xml version="1.0" encoding="UTF-8"?>
<resources>
<attr name="my_custom_attr" format="reference" />
</resources>
Declare a value for the custom attribute, let's say on your own theme.
<style name="AppTheme" parent="android:Theme.Light">
<item name="my_custom_attr">#resource_type/resource_name</item>
<item name="android:editTextStyle">#style/EditTextAppTheme</item>
<item name="android:buttonStyle">#style/ButtonAppTheme</item>
<item name="android:imageButtonStyle">#style/ImageButtonAppTheme</item>
</style>
And then you can use it on the Widget you'd like:
Hope that clears things out.
EDIT 2
For a better answer to your question, please update your question. And like I said, read more on how to properly create styles.
The Theme named 'one', what do you want to apply it to? An activity, a Widget, the whole Application?
How are you applying the theme? Show the lines of code where you specify the usage of theme 'one'.
Your theme as you specified is simply not a properly constructed theme/style.
<style name='one'>
<item name='android:textColor'>#eea</item>
<item name='android:textSize'>20sp</item>
</style>
This says absolutely nothing, and it is definitely not suitable for an Activity-level theme. The reason you specify a parent is so your theme can inherit all of the attributes from the parent, and then you specifiy which ones to change.
For instance, if you want to use your theme and have a light background, do this:
<style name='one' parent="android:Theme.Holo.Light>
<item name='android:textColor'>#eea</item>
<item name='android:textSize'>20sp</item>
</style>
But even here, despite the fact that it will apply, you don't want to have the same text color and size for the whole application do you? That'd be nonsense, different text color and sizes account for a big part of the user experience, so rather than setting those values from what we can refer to as the main style, we can create substyles and apply them to certain widgets.
I can't really go any more detailed that what I already have, the above explains how to accomplish Widget-specific styling, and activity/application level theming.
For a complete start-up guide, read the Android Developer Site, try the test styles declared there, see how they work, and until then try to create your own, don't try to create something out of nowhere if no reading has been made.

Change font/typeface when button is pressed

I already how to change the style of an element with selector but I found nothing about the typeface...
Is it possible to do with ? Or is there another way?

Sadly, right now it is not possible.
There is a ticket for that in the Android code repo :
https://code.google.com/p/android/issues/detail?id=8941
Your best option is to manage it yourself in ontouch listeners (and yes this is ugly) or implement these new selectors yourself.

Typeface can easily be an element of your style... If you're using default android styles, then the idea would be to extend whatever style you're implementing and just change the elements you need. like the following style element, taken from the android styles and themes documentation:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="CodeFont" parent="#android:style/TextAppearance.Medium">
<item name="android:layout_width">fill_parent</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textColor">#00FF00</item>
<item name="android:typeface">monospace</item>
</style>
</resources>
Then apply this style in your selector, just like you already know how to do.
The other option is, of course, to do it in code, but the selector is much cleaner

When button is clicked,Use below code to change font of the text(custom typeface).
Put your font under main->assets->fonts directory.
//change font when button pressed
val typeFace = Typeface.createFromAsset(activity!!.assets,"fonts/sf_semi_bold.ttf")
buttonAbout.setTypeface(typeFace)

TextView.setTypeFace() Use Ctrl+Space to show a giant list of classes for TextView or anything in general on.

Different formats for different Textview defined by Custom style

I have a problem figuring out how to do this:
I am currently coding an app that comes with different themes (User can select the complete appereance of the app out of a list of different styles).
Then the list item is selected I want to call setTheme(R.style.Own_App_Style0); to change the complete appearance.
The problem is best explained by an example:
Lets say we have 2 TextView.
Theme1
1. TextView: TextColor should be green and TextSize 15sp.
2. TextView: TextColor should be red and TextSize 10sp.
Theme2
1. TextView: TextColor should be blue and TextSize 10sp.
2. TextView: TextColor should be yellow and TextSize 10sp.
Of course I know that by setting <item name="textViewStyle">#android:style/Widget.TextView</item> I can change the default appearance of TextViews.
But how can it be done to have lets say two (ore more) different types of TextView with different applied styles (and by xml)?

Found a solution (basically in this answer setTextAppearance through code referencing custom attribute). In case anyone else has this problem I shortly explain:
Declare in style.xml a attribute and in the actual style definition asign a value (reference) to this attribute:
<declare-styleable name="CustomTextView">
<attr name="mainTextView" format="reference"/>
</declare-styleable>
<style name="appstyle0" parent="android:style/Theme.Holo.Light">
<item name="#attr/mainTextView">#style/CustomTextViewAppearance1</item>
<item name="android:textViewStyle">#style/CustomTextViewAppearance2</item>
</style>
<style name="appstyle1" parent="android:style/Theme.Holo.Light">
<item name="#attr/mainTextView">#style/CustomTextViewAppearance2</item>
<item name="android:textViewStyle">#style/CustomTextViewAppearance1</item>
</style>
<style name="CustomTextViewAppearance1">
<item name="android:textSize">10dip</item>
</style>
<style name="CustomTextViewAppearance2">
<item name="android:textSize">30dip</item>
</style>
Now in the layout all textViews are like CustomTextViewAppearance2 (because this is set as standard in this style. And the textViews that should use the other style write into the definition:
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="blablabla"
style="?mainButtonTextView"/>
When you now call .setTheme (after restart the activity) the appearance of the textviews switch. Like this method you can define as many different types of View styles and switch between them only by calling .setTheme.

Unfortunately, styles are static once they are defined. To have an entire cascade of styles modified programmatically, you would have to change the definition of the style itself. Instead, all you can do is change the style that is assigned to a TextView (or whatever style-able object), as outlined in the question I linked to in my comment above.