Properties and dependency-injection

PoP operates in a strictly top-down manner: parent modules know what inner modules they contain, down the line to the last module, and can change those modules’ properties; however, inner modules do not know who is including them, and do not need to care either.

When initializing properties, parent modules take precedence over inner modules: when a module initializes itself, if any one property has already been set by a parent module, then that value will be kept. This way, PoP allows for dependency-injection, where each module can set its default attributes, yet allow for properties to be injected externally in advance (possibly coming from a configuration file).

class GD_EM_Template_Processor_ScrollMapBlocksBase extends GD_Template_Processor_BlocksBase {

	function init_atts($template_id, &$atts) {

		$this->append_att($template_id, $atts, 'class', 'map');
		$this->add_att($template_id, $atts, 'direction', $this->get_map_direction($template_id, $atts));
		$direction = $this->get_att($template_id, $atts, 'direction');

		$mapblock_inner_template = $this->get_block_inner_template($template_id);
		$this->add_att($mapblock_inner_template, $atts, 'direction', $direction);
		return parent::init_atts($template_id, $atts);

	// ...

(Source) A module initializing its own attributes, and also its inner modules’ attributes

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