Dynamic themes and custom styles

Question

I've got an app with two themes (dark and light) that can be selected at runtime. This works. I also have a ListView with rows that can have one of three different layouts, each of which has a style (say, different colors). This also works. But I can't get these two features to work together. I really need six different styles, three for one theme (dark) and three for the other (light), but I can't figure out how to choose a style for a list item based on the current theme, or get that effect any other way by using XML files. My three layouts each point to a custom theme that sets the color, but that overrides whatever theme I've got set. Themes can only contain items that are "styleable", so I can't put my own custom items in there. There may be a way to do this programmatically, but I was hoping to do it declaratively. Any ideas?

Answer 1

Thanks to wingman for the hint. My situation involved colors, which are a bit more complicated, so I'll write up my solution here.

I have two themes (light and dark) which the user can choose from in the Settings screen. I have a ListView which can have two types of rows (plain and note), each with its own styling. Firstly each layout needs to point to a style:

<TextView style="@style/PlainItemText" ... />

(or NoteItemText) and we need to define the styles:

<style name="PlainItemText">
    <item name="android:textSize">@dimen/list_item_font_size</item>
    <item name="android:textStyle">bold</item>
    <item name="android:textColor">?plainTextColor</item>
</style>

The text color can't be fixed because it depends on the selected theme. We must create a custom attribute and refer to it with a question mark, as above. We define the attribute in res/values/attrs.xml:

<!-- Attributes we use to set the text color of the various list items. -->
<attr name="plainTextColor" format="reference|color"/>
<attr name="noteTextColor" format="reference|color"/>

We can then define the various colors. Here we have two styles and two themes, so we need four color state lists, each in its own file under res/color. For example, here's res/color/plain_text_color_dark.xml:

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_window_focused="false" android:color="@android:color/white"/>

    <item android:state_selected="true" android:color="@android:color/black"/>
    <item android:state_focused="true" android:color="@android:color/black"/>
    <item android:state_pressed="true" android:color="@android:color/black"/>

    <item android:color="@android:color/white"/>
</selector>

The selected/focused/pressed colors are the same in all these files because they're over the highlight color. Be careful with the state_window_focused version. It didn't behave as advertised, and I had to set it to the default color (the last line above) in all cases. Now we need to create our themes and bind the attributes to one of the colors. These lines go into res/values/themes.xml:

 <style name="Theme.Dark" parent="android:Theme">
     <item name="plainTextColor">@color/plain_text_color_dark</item>
     <item name="noteTextColor">@color/note_text_color_dark</item>
 </style>
 <style name="Theme.Light" parent="android:Theme.Light">
     <item name="plainTextColor">@color/plain_text_color_light</item>
     <item name="noteTextColor">@color/note_text_color_light</item>
 </style>

Finally we pick a theme at run-time, in an Activity's onCreate() method, before calling super.onCreate():

if (isDarkTheme) {
    activity.setTheme(R.style.Theme_Dark);
} else {
    activity.setTheme(R.style.Theme_Light);
}

Note that I don't take into account newer themes like Holo, so my app looks old on Honeycomb and later. I'll fix that at some point, but it wasn't a regression here.

---

A twist in my case is that some Activities have a larger title bar in order to fit some buttons. In principle I should have created four themes, a light and dark for a narrow title and a light and dark for a fat title. But instead I created a mix-in style:

<!-- Mix-in style for activities. -->
<style name="ButtonTitleBar">
    <item name="android:windowTitleSize">44dp</item> 
</style>

and procedurally add it to whatever theme I'm using. This code goes right after the above setTheme() calls:

if (buttonTitleBar) {
    // Mix in this other style.
    Resources.Theme theme = activity.getTheme();
    theme.applyStyle(R.style.ButtonTitleBar, true);
}

I didn't see this documented anywhere, and I don't know if it's legit, but the code of Activity.getTheme() implies that it should work fine, and it has worked in all my testing. This can help avoid the combinatorial explosion of themes that you can find in the standard Android theme list.

Answer 2

It's a long time ago that Lawrence Kesteloot published his solution in 2012. Now it is six years later, a am new in Android and try to solve the similar problem:

How can I exchange the whole style of the application by just exchanging one theme?

This is a generalisation of Lawrences issue how to organise two exchangeable themes.

I figured out a solution based on Lawrence's and going a step further.

(Not claiming it is the perfect solution, yet an improvement.)

Lawrence figured out the power of user defined attributes to reach this goal. He uses them to address colours depending on the the currently selected theme.

While this is working it still requires to define attributes for each and every property. It doesn't scale well. So why not bundling the properties into styles and themes and using the same mechanism?

This results in a master theme, that is defining child themes and styles.

res/values/attrs.xml

<resources>
     ...
    <attr name="mainViewTheme" format="string"/>
    <attr name="asideViewTheme" format="string"/>
     ...
</resources>

When defining the attribute to set a theme, there is no special format for it. The format string does it.

res/values/styles.xml

<style name="MasterTheme">
     ...
    <item name="mainViewTheme">@style/MainViewTheme</item>
    <item name="asideViewTheme">@style/AsideViewTheme</item>
     ...
</style>

<style name="MainTextTheme">
     ...
</style>

<style name="MainViewTheme">
     ...
</style>

res/layouts/main.xml

<TextView
    android:theme="?mainViewTheme"
    ...

By exchanging the master theme all styles are adjusted. It still requires the definition of a handful of theme attributes and then does a powerful job. Setting up attributes for every property is not required any more.