Android: Updating custom color attribute using selector

Question

I would like to define what color should be used when painting to a canvas depending on a custom state. This is how far I got:

In res/layout/content.xml:

<com.example.package.MyView
    app:primary_color="@drawable/my_selector"
/>

primary_color is a custom attribute defined in res/values/attrs.xml:

<resource>
    <declare-styleable name="MyView">
        <attr name="primary_color" format="reference"/>
    </declare-styleable>
</resource>

my_selector is defined in res/drawable/my_selector.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res/com.example.package">
    <item
        app:state_a="true"
        android:drawable="@drawable/red" />
    <item
        app:state_b="true"
        android:drawable="@drawable/orange" />
    <item
        app:state_c="true"
        android:drawable="@drawable/red" />
</selector>

red, orange and red are defined in res/values/colordrawable.xml:

<resources>
    <drawable name="red">#f00</drawable>
    <drawable name="orange">#fb0</drawable>
    <drawable name="green">#0f0</drawable>
</resources>

In MyView I can get this drawable:

StateListDrawable primaryColor;

public MyView(Context context, AttributeSet attrs) {
    super(context, attrs);

    try{

        primaryColor = (StateListDrawable) a.getDrawable(
            R.styleable.MyView_primary_color);
    }finally {
        a.recycle();
    }
}

primaryColor updates correctly with the different states, I can test this by calling:

setBackground(primaryColor);

But I want to use this color with Paint, like this:

paint.setColor(primaryColor);

But this is obviously not allowed. I've tried converting the primaryColor to a ColorDrawable which has the method getColor(), but I can't figure out how to do this, if it is possible.

Any suggestions on how to get the color that can be used in the the view from a selector would be amazing.

Answer 1

I found ColorStateList which turned out to be exactly what I needed. The following is a simplified version of my current implementation, in case someone else gets in the same rut as I did.

In res/color/my_selector.xml

<?xml version="1.0" encoding="utf-8" ?>
<selector xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"">

  <item app:state_weak="true" android:color="#F00" />
  <item app:state_average="true" android:color="#0F0" />
  <item app:state_strong="true" android:color="#00F" />
  <item android:color="#FA0" />
</selector>

In res/layout/content.xml (This has another layout wrapped around it, but that is not relevant)

<com.example.package.MyView
    android:id="@+id/strMeter"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:primary_color="@color/my_selector"
    />

primary_color is defines as a reference in res/values/attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="MyView">
        <attr name="primary_color" format="reference"/>
    </declare-styleable>
</resources>

I get the reference to the ColorStateList in the constructor of MyView:

ColorStateList primaryColor;

public PasswordStrengthBar(Context context, AttributeSet attrs) {
    super(context, attrs);

    try{
        primaryColor = a.getColorStateList(
            R.styleable.MyView_primary_color);
    }finally {
        a.recycle();
    }
}

When I want to get the color for the current state:

int color = secondaryColor.getColorForState(
              getDrawableState(), primaryColor.getDefaultColor());

If you implement a custom state, like I did, then you will also have to override onCreateDrawableState for the states to actually update, but there is plenty of documentation/posts that cover that.