making a custom centre aligned radio button with a state list drawable

###Custom Widgets These days I have been working extensively on apps for the Android Platform. And one thing keeps coming back to me: Resuse of components and widgets. Although most of the time when you are doing simple apps, you don’t really pay much attention to the type of design and navigation strategies (SINGLE_TOP activities vs SINGLE_TASK blah.. blah) but when you are faced with putting different kinds of components in a single view then you probably have shuddered to think whether you were better off accomplishing this as a separate widget of its own. As can be seen from Romain Guy’s Google IO Talk, developing custom views has its own significant advantages.

Some of the advantages are as follows:

  • A piece of highly reusable component will be at your disposal
  • you will have fine-grained control over the lifecycle of the control/widget that you are developing, (For example, you can decide whether to load images from the network or display a dummy error image in a custom ImageView see volley NetworkImageView for details)
  • Custom views prevent cluttering of code in a single view thereby making abstraction more powerful
  • Other general advantages due to code reuse

So let’s dive into how I managed to make a custom widget for one of the apps.

The Problem

Develop a custom control that behaves like a radio button but has a custom icon instead of the ordinary radio button icon and also has two drawable states to show when in highlighted mode as well as normal mode

The Solution

Once you know how to create custom widgets its very easy.

First start by creating the attibutes your control will have in attrs.xml

 <declare-styleable name="HighlightedImageRadioButton">
        <attr name="focusedDrawable" format="reference"/>
        <attr name="normalDrawable" format="reference" />

Here we have 2 drawables that will function as our highlighted and normal state drawables respectively

Then, create your custom radio button by extending from RadioButton

{% gist 6821592 %}

There’s is a trick though, you need to center align the drawable since the default RadioButton is left aligned and has a text on the right side. To do this you need to override the onDraw method:

     * Fix for putting the drawable in the center
     * notice that we put the background color of the drawable to transparent
     * @param canvas
    protected void onDraw(Canvas canvas) {
        if (buttonDrawable != null) {
            final int verticalGravity = getGravity() & Gravity.VERTICAL_GRAVITY_MASK;
            final int height = buttonDrawable.getIntrinsicHeight();
            int y = 0;
            switch (verticalGravity) {
                case Gravity.BOTTOM:
                    y = getHeight() - height;
                case Gravity.CENTER_VERTICAL:
                    y = (getHeight() - height) / 2;
            int buttonWidth = buttonDrawable.getIntrinsicWidth();
            int buttonLeft = (getWidth() - buttonWidth) / 2;
            buttonDrawable.setBounds(buttonLeft, y, buttonLeft+buttonWidth, y + height);

The above fix was found via this post in Stackoverflow.

And this is how you use it:

<!--Use inside a RadioGroup control-->
                android:layout_width="wrap_content" />

Just like a simple radio button.