Allow preseeding form fields from URL parameters

Form.php

@@ -29,6 +29,14 @@ class Form {
 		}
 	}
 
+	public function preseedFromUrl() {
+		foreach ($this->fields as $name=>$field) {
+			if (isset($_GET[$name])) {
+				$field->setValue($_GET[$name]);
+			}
+		}
+	}
+
 	public function loadFromEnv() {
 		foreach ($this->fields as $name=>$field) {
 			$field->loadFromEnv($name);

test.php

@@ -23,6 +23,7 @@ $form = new Post\Form(array(
 		"select" => new Post\SelectField(array("options"=>array("0"=>"false","1"=>"true")))
 	));
 
+$form->preseedFromUrl();
 $form->denyCsrf();
 
 if ($_SERVER["REQUEST_METHOD"]=="POST") {