Develop Reference

Managing multiple button selectors?

My calculator app consists of 30 buttons. I want to provide themes for the calculator keypad. A theme changes button background (gradients, not image backgrounds) and font. Some themes have the same color for all buttons while some have a color for numbers, another color for operators and so on.
The color change is using selectors from res/drawable/*.xml
How do I change the theme via the code?
Hopefully avoiding typing:
button.setBackground(Drawable background);
button.setTypeface(font);
30 times. And if I have 5 themes, then 30 * 5 * 2 = 300 lines of codes!!
I'm new to this and if there is no other way I'll go with the 150 lines.
Also how do I save the user theme selection? Using preferences?

You can create a custom XML theme which will change all of your XML components. After creating a new theme, go into the Android Manifest file and change the theme. For example:
<activity
android:name="com.myapp.MyActivity"
...
android:theme="#style/MyCustomTheme" />
To create the theme, go to res/values/themes.xml and create a new theme with an identifier:
<resources>
...
<style name="MyCustomTheme" parent="android:style/Theme">
<item name="android:textColorPrimary">#ffff0000</item>
</style>
...
</resources>
By using this method, you can create an extensive library of different themes and change to what theme you want.
NOTE: This is not just for changing the background, but it can also be used to change the theme of the buttons. Visit this website for more information:
http://janrain.com/blog/introduction-to-android-theme-customization/
EDIT: As that user commented, it is possible that you can put the function to change the theme of the button in a for() loop.
For your case, I have derived this from the link above. It will change the texture of the buttons in your XML file rather than in Java.
"Using a Custom Nine-Patch With Buttons
A nine-patch drawable is a special kind of image which can be scaled in width and height while maintaining its visual integrity. Nine-patches are the most common way to specify the appearance of Android buttons, though any drawable type can be used.
Example nine-patch PNG.
Notice the one pixel black lines around the edge, they control the scaling of the image.
Save this bitmap as MyApplication/res/drawable/my_nine_patch.9.png
Define a new style (you can define the new style in the same file that you defined your custom theme from Creating a Custom Android Theme above) …:
<resources>
...
<style name="MyCustomButton" parent="android:Widget.Button">
<item name="android:background">#drawable/my_nine_patch</item>
</style>
...
</resources>
Apply the new button style to the buttonStyle attribute of your custom theme:
<resources>
...
<style name="MyCustomTheme" parent=...>
...
<item name="android:buttonStyle">#style/MyCustomButton</item>
</style>
...
</resources>
Now the buttons in the activities your theme is applied to have custom images. However, you may notice that they don’t change appearance when selected. Read Selector Drawables below for an introduction to using multiple drawables to define one drawable that changes based on state."
From here, you can change certain components of the theme (such as the button texture as an image).
After you have a theme that looks good, apply it in the Android Manifest as I mentioned above.
I will FURTHER edit this if it still does not answer your question.

Overriding styles in different values files

I'm using the new fontFamily attribute so that I can control the weight of font used. Because this was only introduced in API level 16 I've created a styles file in values-v16. It seems a bit silly to declare the colour, size etc in both values and values-v16 but it is necessary for all styling to be applied. Is there someway to declare font family in the values-v16 styles file and inherit the colour, size etc from the values version of the file.

Use a style inheritance. You can put all of the common (non-API specific) styles into a parent and inherit this in each of the children.
http://developer.android.com/guide/topics/ui/themes.html
<style name="v16" parent="CommonStyle">
Common file:
<style name="CommonStyle" >

Android Where to place different icons for different themes?

In my application I have different Themes - one dark and one bright theme. Each theme requires his own icon set.
How do I apply it to the theme? Is there something like a special folder like for icons with different sizes ?

The following serves as a hint...
I suppose this may help: http://developer.android.com/reference/android/R.attr.html#icon
Lets say you have the following themes...
<style name="MyTheme.Dark">
...
</style>
<style name="MyTheme.Light">
...
</style>
You can add <item name="android:icon"></item> to each theme and supply each one with their own drawable icon. Then to get the respective drawables in your layout or what have you, you need to make use of ?android:attr/icon.
So your layout may end up looking something like...
<View
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="?android:attr/icon"/>
If you're unsure about what the ?android:attr/icon part exactly means, read http://developer.android.com/guide/topics/resources/accessing-resources.html#ReferencesToThemeAttributes.

Is there something like a special folder like for icons with different sizes ? (up to my knowledge) NO
no such default folder exist which you can use for Themes.
You need to do it programmatically.
Save the theme info in your Shared Preferences.
and at time of loading the view you can set your desired themed drawable.

Android style descriptions

I tried changing the appearance of a spinner and I partly succeeded. I'm doing this via overriding parts of the theme. I managed to change the text size of the spinner item (i.e. the text size in the drop down button) with my themes.xml and styles.xml:
My themes.xml file looks like this:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="CustomTheme" parent="#android:Theme.Holo.Light">
<item name="android:windowActionBar">false</item>
<item name="android:windowNoTitle">true</item>
<item name="android:spinnerItemStyle">#style/CustomSpinnerItem</item>
</style>
</resources>
My styles.xml file looks like this:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="CustomSpinnerItem" parent="#android:Widget.TextView.SpinnerItem">
<item name="android:textAppearance">#style/CustomTextAppearance</item>
</style>
<style name="CustomTextAppearance">
<item name="android:textSize">30dp</item>
</style>
</resources>
However I cannot find the attributes that are responsible for the text appearance of the items in the dropdown list of the spinner. I tried dropDownItemStyle amongst other things. In my opinion the attribute names are not self-explanatory, so I wanted to know whether there is a documentation of what attribute does what in a style to find out which attributes to override. I found it very cumbersome to trace back all the styles used in a theme via the themes.xml and styles.xml of the platfrom and then try to find the right attributes via trial and error.
I know that one can change the appearance by passing layouts to the adapter, however, this is not really what I was looking for, since (as far as I know), you can only use inheritance in styles and not in layout xml files. If I created a custom layout for the adapter I'd have to create 9-patch images etc., which I think is a bit too time consuming in case I only want to change the text size.
Of course it's possible that I misunderstood the whole concept, since I'm new to Android ;)

You probably have found out the answer since you asked but for others looking at similar questions:
I do not know of a list of attribute names with good explanation of what they do (R.attr's page mostly gives information that is already in the name) but the way I do it is:
Start from the element I give to setDropDownViewResource(), in my case: android.R.layout.simple_spinner_dropdown_item and find.
Find its layout definition in \sdk\platforms\android-17 (specific platform version to avoid redundant results).
Get its style from the layout file. In this case: ?android:attr/spinnerDropDownItemStyle
We now have the attribute name we need.
It's better to do it that way rather than try to guess what attribute to use because you know which attribute the system itself use so it's very likely to be the correct one (unless there's a bug).
If I created a custom layout for the adapter I'd have to create
9-patch images etc.
Well, no, the layout determines what kind of GUI element you would have (a textfield, a spinner, an imagebutton, a custom element...), not how they are styled (nine-patch backgrounds, text colors...), so you still would have to mess with styles to get the right appearance.
For example, for visual consistency I ported the button, checkbox and spinner style from Theme.Holo to Gingerbread, yet I did not mess with layout, all I did was the aforementioned steps plus looking up the result (spinnerDropDownItemStyle in the above example) in themes.xml, which gave me the style name (e.g.: Widget.Holo.DropDownItem.Spinner).
Then I looked that up in styles.xml and imported it (and any parent*) in my project's styles.xml, searching and copying any Holo specific reference in my project and adjusting the namespace accordingly (add android: to attributes and replace ?android:attr with #style for what I copy to my styles.xml file).
So far I haven't had to mess with layouts at all (even the presence of radio buttons in spinner dialogs on Gingerbread is determined by an xml attribute: android:checkMark).
If a style has no parent attribute (like Widget.Holo.DropDownItem.Spinner) then its parent is the same style minus the last element (e.g.: Widget.Holo.DropDownItem)

Multiple parents in Android styles xml

When using android styles, one can specify a parent style like:
<style name="MyStyle" parent="#style/ParentStyle"></style>
Is is possible to specify multiple parents? If so, how?

According to documentation, the syntax for parent attribute value is #[package:]style/style_to_inherit (where package: is optional) so a reference to just a single style resource is allowed.