Android progress bar style syntax

Question

In a progress bar definition the style is defined as

style="?android:attr/progressBarStyleHorizontal"

I'm not clear on how this syntax works. Can someone explain this syntax?

Answer 1

That syntax is used to reference a system style attribute, and it's similar to the syntax used to reference a resource predefined by the Android system: whereas in the general resource case you use e.g. @android:drawable/ic_media_play to reference a Drawable represented by the file ic_media_play.png in the value of an android:src attribute for the ImageButton UI element included in an XML layout, the attribute reference is built using a ? instead of a @, but for the rest is composed following the same rules and works in the same way.

In particular, attributes are usually stored in res/values/attrs.xml, where you can find (looking at the system resources stored in the Android SDK) the following

<attr name="progressBarStyleHorizontal" format="reference" />

indicating that the attribute is a reference to some other resource. The style attribute is defined in res/values/themes.xml in the following fashion:

<item name="progressBarStyleHorizontal">
    @android:style/Widget.ProgressBar.Horizontal
</item>

which is indeed a reference to the style Widget.ProgressBar.Horizontal in the system res/values/styles.xml as already described by another answer.

Note that, in contexts when the system knows to expect a reference to an attribute resource, you may even omit the resource type (i.e. the attr/ part), leading to a syntax such as:

android:textColor="?android:textColorSecondary"

You may find slightly more information on the Android developer guide

Answer 2

android:attr means this style is picked from frameworks of android. So if you browse the source of styles.xml from frameworks http://bit.ly/yBC5pM and search progressBarStyleHorizontal, You can see it inherits from Widget.ProgressBar.Horizontal

<style name="Widget.ProgressBar.Horizontal" parent="@style/Widget.ProgressBar">
    <item name="maxHeight">20.0dip</item>
    <item name="indeterminateOnly">false</item>
    <item name="indeterminateDrawable">@drawable/progress_indeterminate_horizontal</item>
    <item name="progressDrawable">@drawable/progress_horizontal</item>
    <item name="minHeight">20.0dip</item>
</style>

All the above properties + properties from Widget.Progressbar(Since its parent is this) will get inherited to your Progressbar.