Android Tutorial #4.2: Passing custom attributes via XML resource files

In tutorial #4.1, I mentioned that we passed custom attributes for the text and image variables from the XML resource file to our custom class. This is a critical skill for performing true object-oriented programming and how to do it wasn’t obvious from Google’s Android API Demos.

Luckily I was pointed to the solution myself by an experienced Android programmer in Guatemala by the username of cadlg (thanks again!). If you want to see the official Google Android example though, look at Android’s APIDemos’ custom LabelView example.

So here we go. We’ll use the same code as Tutorial 4.1  to keep this simple.

Setting Up Your Custom Class’s XML Resource Files

We’ll only review the code for the TextOnlyButton as it’s identical in concept to the ImageOnlyButton.

First we’ll create a new file in /res/values called attrs.xml

<?xml version=”1.0″ encoding=”utf-8″?>
<resources>

<declare-styleable name=”TextOnlyButton”>

<attr name=”textColorNotFocused” format=”integer”/>
<attr name=”textColorFocused” format=”integer”/>

</declare-styleable>

</resources>

 

As you see, we first declared a ’styleable’ with the name of our custom Class. Two attributes were then added to contain the values of our focused & unfocused text colors. By default, attributes have values of String, but in our case, we needed integers to represent the resource id’s we’ll declare in our colors.xml file. You can also declare formats such as “boolean” & others if that suits the requirements of your own project.

Next, we declare values for these custom attributes in our layout’s XML file: tutorial4.xml

<LinearLayout xmlns:android=”http://schemas.android.com/apk/res/android”

xmlns:pj=”http://schemas.android.com/apk/res/com.pocketjourney.tutorials”
android:orientation=”vertical”
android:layout_width=”fill_parent”
android:layout_height=”fill_parent”
android:padding=”10px”>

<com.pocketjourney.view.TextOnlyButton

android:id=”@+id/text_only_button”
android:layout_width=”wrap_content”
android:layout_height=”wrap_content”
android:layout_marginTop=”5px”
style=”?android:attr/buttonStyleSmall”
android:text=”Text Button”
pj:textColorNotFocused=”@drawable/white”
pj:textColorFocused=”@drawable/android_orange” />

</LinearLayout> 

 

Referring to our new attributes is actually a two step process. First we declared a new namespace (in our case called ‘pj’ as short for PocketJourney) in the parent layout of our custom class:

xmlns:pj=”http://schemas.android.com/apk/res/com.pocketjourney.tutorials”

Next we specified the values of our new attributes in the XML usage of our TextOnlyButton:

pj:textColorNotFocused=”@drawable/white”
pj:textColorFocused=”@drawable/android_orange”

Now you can see why we specified our format=”integer”. Our custom attributes point to the resource id’s of colors specified in our colors.xml file.

Retrieving Custom Attributes During Class Instantiation

Since our Activity has many constructors, we delegate the attribute parsing to an init() method to keep our code clean.

int notFocusedTextColor, f ocusedTextColor;

private void init(AttributeSet attrs) {

Resources.StyledAttributes a = getContext().obtainStyledAttributes(attrs,R.styleable.TextOnlyButton );
notFocusedTextColor = a.getColor(R.styleable.TextOnlyButton_textColorNotFocused , 0xFF000000);
focusedTextColor = a.getColor(R.styleable.TextOnlyButton_textColorFocused , 0xFF000000);

}

 

By now you’ve undoubtedly seen the AttributeSet that is always passed into an Activity. Well now you get to use it. First we obtain the StyledAttributes instance by requesting just the StyledAttributes for our custom Class. Next, we call the getColor() and pass two variables: the name of the attribute we want along with a default value in case the user did not specify one.

Take note of our styled attribute’s name as it’s a combination of our custom class’s name and the attribute we specified in the attrs.xml file (e.g.TextOnlyButton_textColorNotFocused).

And That’s It

You can now readily pass your own custom attributes and keep your View variables cleanly enclosed in your XML files. You can download the source  to see for yourself. Just look at Tutorial #4.