Referencing a class programatically in PHP

Question

I receive an object during some process and this object needs to figure out its coloring scheme. For example, I have a coloring scheme that is stored like this:

class FirstScheme {
    public static $COLORS = array('1' => 'green', '2' => 'red', ...);
}
class SecondScheme {
    public static $COLORS = array('1' => 'red', '2' => 'green', ...);
}

I know all the coloring schemes names in advance; they can only change when the code changes. But the coloring scheme to be used for each object needs to be determined at run-time by matching the attribute of this object.

And here I don't know what to do. In python I would define a dict holding the mappings of color schemes to names like this:

d = {'attr_value1': FirstScheme, 'attr_value2': SecondScheme, 'attr_value3': FirstScheme, ...}

And then just access the "COLORS" variable, because every class should have it. But in PHP there is not way to reference a class in a such way, so what is the right way to do it? Note that more than one attribute can map to the same coloring scheme.

Answer 1

If every class should have the colors, define the interface that allows to get them:

interface ColorsProvider {
    function getColors();
}
class FirstScheme implements ColorsProvider {
    public static COLORS = array('1' => 'green', '2' => 'red', ...);

    public function getColors() {
        return self::COLORS;
    }
}
class SecondScheme implements ColorsProvider {
    public static COLORS = array('1' => 'red', '2' => 'green', ...);

    public function getColors() {
        return self::COLORS;
    }
}

Then, where you have stack of yout params:

$a = array(
  'attr_value1' => new FirstScheme(),
  'attr_value2' => new SecondScheme(),
);

You can call:

$param = 'attr_value1';

if(!isset($a[$param]))
    throw new Exception("undefined param");

if(!($a[$param] instanceof ColorsProvider))
    throw new Exception("Param should point to ColorsProvider");

$a[$param]->getColors();

---

Please note that it is full-objective. In PHP there are simplier ways to get this effects, but my solution is just elegant.

---

The another point is the interface completely separates the source of colors. There would be from file, database, xml, hardcoded etc.

---

Default implementation might be:

abstract class DefaultColorsProviderImpl implements ColorsProvider             {
    protected static COLORS = array();

    public function getColors() {
        return self::COLORS;
    }
}

class FirstScheme extends DefaultColorsProviderImpl {
    protected static COLORS = array( ... );
}

But still allows to make generic implementation that returns colors from e.x. from file.

Answer 2

Of course you can:

$schemes = [
  'attr_value1' => FirstScheme::$COLORS,
  'attr_value2' => SecondScheme::$COLORS,
  ...
];

Or even at runtime:

$schemes = [
  'attr_value1' => 'FirstScheme',
  'attr_value2' => 'SecondScheme',
  ...
];

And then:

$reflector = new ReflectionClass($schemes['attr_value1']);
$schema = $reflector->getStaticPropertyValue('COLORS');

But this seems not any maintainable at all, and you would like to store such informations in a proper data layer, without hardcoding them as static fields of a class [which is not their purpose].

Answer 3

An alternative to hard-coding the colors in their own classes would be the following approach:

class ColorScheme {

   protected $colors;

   public function __construct(array $colors) {
       $this->colors = $colors;
   }

   public function getColors() {
       return $this->colors;
   }

}

$scheme1 = new ColorScheme(array('red', 'blue', 'green'));
$scheme2 = new ColorScheme(array('yellow', 'pink', 'cyan'));

The equivalent for a python dictionary in PHP is an associative array. So you could do this:

$d = array (
    'values_1' => $scheme1->getColors(),
    'values_2' => $scheme2->getColors()
);