Hi,

I'm trying to get my head around the function extend()

What is the purpose of line 1255 in SiteTree.php:

public function MetaTags($includeTitle = true) {
		$tags = "";
		if($includeTitle === true || $includeTitle == 'true') {
			$tags .= "<title>" . Convert::raw2xml(($this->MetaTitle)
				? $this->MetaTitle
				: $this->Title) . "</title>\n";
		}

		$tags .= "<meta name=\"generator\" content=\"SilverStripe - http://silverstripe.org\" />\n";

		$charset = ContentNegotiator::get_encoding();
		$tags .= "<meta http-equiv=\"Content-type\" content=\"text/html; charset=$charset\" />\n";
		if($this->MetaKeywords) {
			$tags .= "<meta name=\"keywords\" content=\"" . Convert::raw2att($this->MetaKeywords) . "\" />\n";
		}
		if($this->MetaDescription) {
			$tags .= "<meta name=\"description\" content=\"" . Convert::raw2att($this->MetaDescription) . "\" />\n";
		}
		if($this->ExtraMeta) { 
			$tags .= $this->ExtraMeta . "\n";
		} 

		$this->extend('MetaTags', $tags);        // Results of this function are ignored... So what is the purpose of this line?  

		return $tags;
	}

Thanks very much.

VWD.

Well like the comment in the code says that line isn't actually being used.
Since $tags is a string the line would have to have been:

$tags = $this->extend('MetaTags', $tags);

Generally though, anywhere you find $this->extend() is a chance for you to hook in with a decorator or extension and make changes or run your own code.

The arguments are passed by reference so that you can modify the variable value. http://php.net/manual/en/language.references.php

So correct me if I'm wrong... if I create an extension/decorator for SiteTree and implement the MetaTags method, then the line of code there (line 1255 of SiteTree.php) will ensure that any time SiteTree->MetaTags(...) is called, my extension/decorator's MetaTag(...) method is also called?

So would that sum up the purpose of Object::extend(...)? It just ensures that the specified function will be called by all associated extensions/decorators if it is implemented by them?

Thanks guys. Just trying to see what it's there for.

Many thanks.
VWD.

Yes that's pretty much correct. Just remember to make the variable a reference in your function definition.
So

function MetaTags($tags) {...

wouldn't actually do anything but

function MetaTags(&$tags) {...

would. As you can see the variable itself is not passed by reference in the code (and that's deprecated since 5.3) but if you make it a reference in your function definition it should all work out.