I am new to android and have a question about the styles.xml file.
Each item for example this
<item name="android:background">#color/black</item>
works both with "android:background" and with just "background".
What is the android: prefix there for?
When creating your own styles, you should always extend an existing style from the framework or support library so that you maintain compatibility with platform UI styles. To extend a style, specify the style you want to extend with the parent attribute. You can then override the inherited style attributes and add new ones.
For example, you can inherit the Android platform's default text appearance and modify it as follows:
<style name="GreenText" parent="#android:style/TextAppearance">
<item name="android:textColor">#00FF00</item>
</style>
However, you should always inherit your core app styles from the Android Support Library.
But if you don't inherit them from the android support library then too things will work the same!
Source: Explanation text and code taken from: https://developer.android.com/guide/topics/ui/look-and-feel/themes
You can read more about Styles and Themes from the above doc, from where I took the example to explain you!
I know that
we have not to use parent attribute. We prefix one style to another
style separating by a period(.)
so in this style, does it have a circular inheritance?
<style name="TextAppearance.A" parent="TextAppearance.A.B">
<item name="android:textAlignment">viewStart</item>
<item name="android:gravity">start</item>
</style>
TextAppearance.A.B inherits from TextAppearance.A because of android dots' syntax.
but TextAppearance.A inherits from TextAppearance.A.B because of android paretn syntax.
Is it really a problem?
Technically As per Android Documentation I dont think this is possible,
Because this will lead to duplication of style, If you refer to same as Diamond Problem it will be one of those, also android prevents you from inheriting from more than one style.
Further Imagine if you have one attribute which is defined in style A also in Style B, it will be a problem at compile time that which attribute to choose from both.
For More Details please refer to android documentation
https://developer.android.com/guide/topics/ui/look-and-feel/themes
What's the parent of Theme.AppCompat.Light.NoActionBar? I can't find a "parent" attribute in Android SDK resources. I guess it should be Theme.AppCompat.Light. But I didn't find any document to prove that.
You can inherit styles by using dot instead of using parent (docs). So Theme.AppCompat.Light is parent of Theme.AppCompat.Light.NoActionBar
You can also inherit styles (except those from the platform) by extending a style's name with a dot notation, instead of using the parent attribute. That is, prefix the name of your style with the name of the style you want to inherit, separated by a period.
I'm not sure what is the best way to develop interfaces in Android.
is it better to clean the layout file by moving inline attributes to a style file? As far as I know, in HTML it is better to use classes and ids inside HTML, and use them in style.css file. What about android?
I found this, maybe it helps someone else.
When to use Styles
The first problem we must address is a simple one: when should you use a style instead of inline attributes?
Rule #1: Use styles when multiple Views are semantically identical.
This rule is best illustrated with a few examples:
You're creating a calculator. Each button should look the same, so it makes sense to create a `CalculatorButton` style.
You've got a couple screens with multiple text formats - say, headers, subheaders, and text. You can unify their look by creating `Header`, `Subheader` and `Text` styles.
You've got thumbnails all over your app. You want them all to look the same. The `Thumbnail` style is born.
The common thread in all these examples is that these Views are not just using the same attributes - they play the same role across the app. Now, when you want to tweak the look/feel of any of these Views, you can just edit the style and change them all at once. It saves you time, effort and keeps your Views consistent.
Want to save even more work? Use resource references!
Rule #2: Use references within styles when appropriate.
You could define a style this way:
<style name="MyButton">
<item name="android:minWidth">88dp</item>
<item name="android:minHeight">48dp</item>
</style>
What if you wanted minWidth to vary based on screen size? You could just duplicate the style once per screen size (say, sw600dp and sw900dp), but then you're having to duplicate the minHeight attribute as well. What if you want both attributes to change? Suddenly you've got tons of MyButtons defined everywhere, each one duplicating all other attributes. It's a recipe for disaster; it's so easy to forget to change one attribute in one of the many copies.
Styles are just an alias to a series of attributes. It's a lot easier to just define the style like this:
<style name="MyButton">
<item name="android:minWidth">#dimen/button_min_width</item>
<item name="android:minHeight">#dimen/button_min_height</item>
</style>
Now you can just modify a single attribute for each resource qualifier. It's absurd to think about duplicating a layout just to change, say, the width of one View in portrait vs. landscape. You'd use a dimension for that. The same applies for styles.
I don't mean to imply you should always use resource references in styles; just that you should use it if you need multiple values switched on resource qualifiers.
This isn't to say that sometimes you won't need to duplicate a style across resource qualifiers, but you can keep it to a minimum. Usually the only reason to do so is because of platform changes (e.g., the change from paddingLeft and paddingRight to paddingStart and paddingEnd).
Multiple Styles
It would be wonderful if you could apply multiple styles to a single View, like CSS.
You can't. Sorry.
But you can, in a couple of cases, get an approximation of multiple styles.
Rule #3: Use themes to tweak default styles.
Themes provide ways of defining the default style of many standard widgets. For example, if you want to define the default button for the app, you could do this:
<style name="MyTheme">
<item name="android:buttonStyle">#style/MyButton</item>
</style>
If you're just tweaking the default style, the only tricky part is figuring out the parent of your style; you want it to match the appropriate theme for the device, but that varies based on OS version.
If you're using an AppCompat theme, you should use their styles as the parent since they handle differences across platforms as well. For example, they have a Spinner style:
<style name="MySpinner" parent="Widget.AppCompat.Spinner" />
If the style doesn't exist in AppCompat (or you're not using it), the problem is a bit trickier, since you need the parent to switch based on the theme. Here's an example of a custom Button style that uses Holo normally, but Material when appropriate.
You'd put this in /values/values.xml:
<style name="ButtonParent" parent="android:Widget.Holo.Button />
<style name="ButtonParent.Mine">
<item name="android:background">#drawable/my_bg</item>
</style>
Then, in /values-v21/values.xml:
<style name="ButtonParent" parent="android:Widget.Material.Button />
Setting up the correct parent will ensure consistency with both your app and the platform.
If you truly want to define all necessary attributes (instead of just tweaking the defaults), you could skip parenting entirely.
Rule #4: Use text appearance when possible.
TextAppearance allows you to merge two styles for some of the most commonly modified text attributes. Take a look at all your styles: how many of them only modify how the text looks? In those cases, you could instead just modify the TextAppearance.
First, you need to define your TextAppearance:
<style name="MyTextAppearance" parent="TextAppearance.AppCompat">
<item name="android:textColor">#0F0</item>
<item name="android:textStyle">italic</item>
</style>
Notice how I've set a parent - text appearances won't merge, so you need to make sure to define all attributes. You can use any appropriate TextAppearance as the parent.
Now you can use it in a TextView:
<TextView
style="#style/MyStyle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="#style/MyTextAppearance" />
Notice that I can still apply a style to this TextView, getting me a whopping TWO styles for one view! Not as good as true multiple styles, but I'll take what I can get.
You can use TextAppearance in any class that extends TextView. That means that EditText, Button, etc. all support text styling.
Common Pitfalls
I've explained all the times when I use styles. Unfortunately, it is easy to abuse styles in ways that will hurt you in the long run. Here's a few anti-patterns to avoid.
Rule #5: Do NOT create a style if it's only going to be used once.
Styles are an extra layer of abstraction. It adds complexity. You have to lookup the style to see the attributes they apply. As such, I see no reason to use them unless you're going to use the style in multiple places.
Which would you rather see when you open up a layout: This?
<TextView style="HelloWorldTextView" />
Or this?
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="#string/hello_world" />
It's so easy to create a style later if you need to do so. Don't plan ahead too much.
Rule #6: DO NOT create a style just because multiple Views use the same attributes.
The main reason to use styles is to reduce the number of repeated attributes, right? Why not just use a style whenever multiple Views use the same attributes?
The problem with this attitude is that those Views, if they are not used in the same context, may eventually want to differ in how they look. And at that point, your base style becomes difficult to edit without unintended side effects.
Think about this scenario: you've got a few TextViews that the same text appearance and background. You think, "hey, I'll create a style, that'll cut down on code duplication." Everything is hunky dory at first, but eventually you want to tweak how some of the TextViews look. The problem is, by now that style is used all over the place, so you can't edit it without some collateral damage.
Fine, you say - I'll just override the style directly in the layout XML. Problem solved. Then it happens again. And again. Eventually that style is meaningless because you're having to override it everywhere. It ends up adding extra work instead of making life easier.
That's why I specified in rule #1 that you should use styles when the Views are semantically identical. This ensures that when you change a style, you really do want every View using the style to change.
Implicit vs. Explicit Parenting
Styles support parenting, wherein a child style adopts all attributes of a parent style. It would be rather limiting if they did not.
Suppose I want every Button in the app to look the same, so I make a ButtonStyle. Later, I decide half the Buttons should look slightly different - with parenting, I can just create ButtonStyle.Different, getting the base style + the tweaks.
It turns out there are two ways of defining parents, implicitly and explicitly:
<!-- Our parent style -->
<style name="Parent" />
<!-- Implicit parenting, using dot notation -->
<style name="Parent.Child" />
<!-- Explicit parenting, using the parent attribute -->
<style name="Child" parent="Parent" />
Simple enough, right? But what do you think happens here, when we define parents with both methods?
<style name="Parent.Child" parent="AnotherParent" />
If you answered that the style has two parents, you are wrong. It turns out that it only has one parent: AnotherParent.
Each style can only have one parent, even though there are two ways to define it. The explicit parent (using the attribute) takes precedence. This leads me to the next rule:
Rule #7: DO NOT mix implicit and explicit parenting.
Mixing the two is a recipe for confusion. Suppose I have this layout:
<Button
style="#style/MyWidgets.Button.Awesome"
android:layout_width="match_parent"
android:layout_height="match_parent" />
But it turns out that my style is defined thus:
<style name="MyWidgets.Button.Awesome" parent="SomethingElse" />
Even though it looks like my Button is based on MyWidgets.Button, it's not! The style name is misleading and the only way to discover that is to do extra work and dig into your style files.
The common temptation is to keep using dot notation with explicit parenting so that your styles look hierarchically related:
<style name="MyButton" parent="android:Widget.Holo.Button" />
<style name="MyButton.Borderless" parent="android:Widget.Holo.Button.Borderless" />
Object-oriented styles! They look so pretty, right? But looks are all you're getting - an illusion that styles are related when they are not. The deception is that MyButton.Borderless is related to MyButton, but they have nothing in common! Let's remove the confusion by removing the dots from the name:
<style name="MyButton" parent="android:Widget.Holo.Button" />
<style name="MyBorderlessButton" parent="android:Widget.Holo.Button.Borderless" />
I lose out on the hierarchy looking pretty, but I gain a lot of utility in code.
Styles vs. Themes
Styles and themes are two different concepts. While styles apply to a single View, themes are applied to a set of Views (or to a whole Activity).
For example, suppose you are using AppCompat and you want to set the primary color for the screen. For this, you must theme the entire Activity:
<style name="MyTheme">
<style name="colorPrimary">#color/my_primary_color</style>
</style>
Themes use the same data structure as styles - even using the style tag - but they are, in fact, used in totally different circumstances! They don't operate on the same attributes - for example, you can define a textColor on a View, but there is no textColor attribute for a theme. Likewise, there exists colorPrimary in themes, but in styles they go unused. Thus:
Rule #8: DO NOT mix styles and themes.
Two common mistakes I've seen:
Applying a theme (as a style) to a `View`:
It just makes no sense because a `View` can't use any of the theme attributes anyways. Nothing happens.
Combining the themes/styles in your hierarchy via parenting. I've seen this as a result of people trying to maintain the illusion of hierarchy using dot notation:
Stupid! So, stupid! It does not make any sense and sometimes misfires in strange ways. Just don't do it!
As of Lollipop, you can apply themes to a View and all its children2. Even in that circumstance, you shouldn't mix up the two, though you could use them both in parallel:
<View
style="#style/MyView"
android:theme="#style/MyTheme" />
AppCompat has a simulacrum of View theming for the Toolbar, but that's all you'll get for a while until Lollipop is the minimum supported version of your app. In other words - you can have fun with this feature in a couple years. :P
Conclusion
The unifying element of these rules are to be careful and thoughtful when using styles. They can save you time, but only if you know when to use them.
Font: this article
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.