Implementing the CakeResponse::etag()

lib/Cake/Network/CakeResponse.php

@@ -875,6 +875,38 @@ public function vary($cacheVariances = null) {
 		return null;
 	}
 
+/**
+ * Sets the response Etag, Etags are a strong indicative that a response
+ * can be cached by a HTTP client. A bad way of generaing Etags is 
+ * creating a hash of teh response output, instead generate a unique 
+ * hash of the unique components that identifies a request, such as a 
+ * modification time, a resource Id, and anything else you consider it 
+ * makes it unique.
+ *
+ * Second parameter is used to instuct clients that the content has 
+ * changed, but sematicallly, it can be used as the same thing. Think 
+ * for instance of a page with a hit counter, two different page views 
+ * are equivalent, but they differ by a few bytes. This leaves off to 
+ * the Client the decision of using or not the cached page.
+ *
+ * If no parameters are passed, current Etag header is returned.
+ *
+ * @param string $hash the unique has that identifies this resposnse
+ * @param boolean $weak whether the response is semantically the same as 
+ * other with th same hash or not
+ * @return string
+ **/
+	public function etag($tag = null, $weak = false) {
+		if ($tag !== null) {
+			$this->_headers['Etag'] = sprintf('%s"%s"', ($weak) ? 'W/' : null, $tag);
+		}
+		if (isset($this->_headers['Etag'])) {
+			return $this->_headers['Etag'];
+		}
+		return null;
+	}
+
+
 /**
  * Returns a DateTime object initialized at the $time param and using UTC
  * as timezone

lib/Cake/Test/Case/Network/CakeResponseTest.php

@@ -784,4 +784,25 @@ public function testVary() {
 		$this->assertEquals(array('Accept-language', 'Accept-encoding'), $response->vary());
 	}
 
+/**
+ * Tests getting/setting the Etag header
+ *
+ * @return void
+ */
+	public function testEtag() {
+		$response = $this->getMock('CakeResponse', array('_sendHeader', '_sendContent'));
+		$response->etag('something');
+		$this->assertEquals('"something"', $response->etag());
+		$response->expects($this->at(1))
+			->method('_sendHeader')->with('Etag', '"something"');
+		$response->send();
+
+		$response = $this->getMock('CakeResponse', array('_sendHeader', '_sendContent'));
+		$response->etag('something', true);
+		$this->assertEquals('W/"something"', $response->etag());
+		$response->expects($this->at(1))
+			->method('_sendHeader')->with('Etag', 'W/"something"');
+		$response->send();
+
+	}
 }