Add a new Templates::fileModified() hookable method that is called whenever a change is detected to a template file. Plus update the Template and Templates class to make it possible for runtime modification of the templates path.

wire/core/Paths.php

@@ -140,7 +140,7 @@ class Paths extends WireData {
 	 *
 	 */
 	public function set($key, $value) {
-		$value = self::normalizeSeparators($value); 
+		if(DIRECTORY_SEPARATOR != '/') $value = self::normalizeSeparators($value); 
 		if($key == 'root') {
 			$this->_root = $value;
 			return $this;
@@ -159,7 +159,7 @@ class Paths extends WireData {
 	 */
 	public function get($key) {
 		static $_http = null;
-		if($key == 'root') return $this->_root;
+		if($key === 'root') return $this->_root;
 		$http = '';
 		if(is_object($key)) {
 			$key = "$key";
@@ -174,7 +174,7 @@ class Paths extends WireData {
 			$key = substr($key, 4);
 			$key[0] = strtolower($key[0]);
 		}
-		if($key == 'root') {
+		if($key === 'root') {
 			$value = $http . $this->_root;
 		} else {
 			$value = parent::get($key);

wire/core/ProcessWire.php

@@ -424,7 +424,7 @@ class ProcessWire extends Wire {
 		$fieldtypes = $this->wire('fieldtypes', new Fieldtypes(), true);
 		$fields = $this->wire('fields', new Fields(), true);
 		$fieldgroups = $this->wire('fieldgroups', new Fieldgroups(), true);
-		$templates = $this->wire('templates', new Templates($fieldgroups, $config->paths->templates), true); 
+		$templates = $this->wire('templates', new Templates($fieldgroups), true); 
 		$pages = $this->wire('pages', new Pages($this), true);
 
 		$this->initVar('fieldtypes', $fieldtypes);

wire/core/Template.php

@@ -792,9 +792,11 @@ class Template extends WireData implements Saveable, Exportable {
 		if(empty($value)) return; 
 
 		if(strpos($value, '/') === false) {
+			// value is basename
 			$value = $this->config->paths->templates . $value;
 
 		} else if(strpos($value, $this->config->paths->root) !== 0) {
+			// value is path outside of our installation root, which we do not accept
 			$value = $this->config->paths->templates . basename($value); 
 		}
 
@@ -878,35 +880,52 @@ class Template extends WireData implements Saveable, Exportable {
 	 */
 	public function filename() {
 
-		if($this->filename) return $this->filename; 
+		/** @var Config $config */
+		$config = $this->wire('config');
+		$path = $config->paths->templates;
+		$ext = '.' . $config->templateExtension;
+		$altFilename = $this->altFilename;
 
-		if(!$this->settings['name']) throw new WireException("Template must be assigned a name before 'filename' can be accessed"); 
+		if(!$this->settings['name']) {
+			throw new WireException("Template must be assigned a name before 'filename' can be accessed");
+		}
 
-		if($this->altFilename) {
+		if($altFilename) {
-			$altFilename = $this->wire('templates')->path . basename($this->altFilename, "." . $this->config->templateExtension) . "." . $this->config->templateExtension; 
+			$filename = $path . basename($altFilename, $ext) . $ext; 
-			$this->filename = $altFilename; 
 		} else {
-			$this->filename = $this->wire('templates')->path . $this->settings['name'] . '.' . $this->config->templateExtension;
+			$filename = $path . $this->settings['name'] . $ext;
+		}
+	
+		if($filename !== $this->filename) {
+			// first set of filename, or filename/path has been changed
+			$this->filenameExists = null;
+			$this->filename = $filename;
 		}
-
-		$isModified = false;
-		$fileExists = $this->filenameExists();
 		
-		if($fileExists) {
+		if($this->filenameExists === null) { 
-			$modified = filemtime($this->filename);
+			$this->filenameExists = file_exists($filename);
-			if($modified > $this->modified) {
+			if($this->filenameExists) {
-				$isModified = true;
+				// if filename exists, keep track of last modification time
-				$this->modified = $modified;
+				$isModified = false;
-			}
+				$modified = filemtime($filename);
-			if($isModified || !$this->ns) {
+				if($modified > $this->modified) {
-				// determine namespace
+					$isModified = true;
-				$this->ns = $this->wire('files')->getNamespace($this->filename);
+					$this->modified = $modified;
-				// tell it to save the template after the request is finished
+				}
-				$this->addHookAfter('ProcessWire::finished', $this, 'hookFinished'); 
+				if($isModified || !$this->ns) {
+					// determine namespace
+					$files = $this->wire('files');
+					/** @var WireFileTools $files */
+					$templates = $this->wire('templates');
+					/** @var Templates $templates */
+					$this->ns = $files->getNamespace($filename);
+					$templates->fileModified($this);
+				}
 			}
 		}
 		
-		return $this->filename;
+		
+		return $filename;
 	}
 
 	/**
@@ -928,11 +947,11 @@ class Template extends WireData implements Saveable, Exportable {
 	 * 
 	 * #pw-group-files
 	 *
-	 * @return string
+	 * @return bool
 	 *	
 	 */
 	public function filenameExists() {
-		if(!is_null($this->filenameExists)) return $this->filenameExists; 
+		if($this->filenameExists !== null) return $this->filenameExists; 
 		$this->filenameExists = file_exists($this->filename()); 
 		return $this->filenameExists; 
 	}

wire/core/Templates.php

@@ -5,7 +5,7 @@
  *
  * Manages and provides access to all the Template instances
  * 
- * ProcessWire 3.x, Copyright 2016 by Ryan Cramer
+ * ProcessWire 3.x, Copyright 2019 by Ryan Cramer
  * https://processwire.com
  * 
  * #pw-summary Manages and provides access to all the Templates.
@@ -16,6 +16,7 @@
  * @method bool|Saveable|Template clone(Saveable $item, $name = '') #pw-internal
  * @method array getExportData(Template $template) Export Template data for external use. #pw-advanced
  * @method array setImportData(Template $template, array $data) Given an array of Template export data, import it to the given Template. #pw-advanced
+ * @method void fileModified(Template $template) Hook called when a template detects that its file has been modified. #pw-hooker
  *
  */
 class Templates extends WireSaveableItems {
@@ -30,26 +31,26 @@ class Templates extends WireSaveableItems {
 	 * WireArray of all Template instances
 	 *
 	 */
-	protected $templatesArray; 
+	protected $templatesArray;
-
+	
 	/**
-	 * Path where Template files are stored
+	 * Templates that had changed files during this request
+	 *
+	 * @var array Array of Template objects indexed by id
 	 *
 	 */
-	protected $path; 
+	protected $fileModTemplates = array();
 
 	/**
 	 * Construct the Templates
 	 *
 	 * @param Fieldgroups $fieldgroups Reference to the Fieldgroups
-	 * @param string $path Path to where template files are stored
 	 *
 	 */
-	public function __construct(Fieldgroups $fieldgroups, $path) {
+	public function __construct(Fieldgroups $fieldgroups) {
 		$fieldgroups->wire($this);
 		$this->fieldgroups = $fieldgroups; 
 		$this->templatesArray = $this->wire(new TemplatesArray());
-		$this->path = $path;
 	}
 
 	/**
@@ -113,7 +114,7 @@ class Templates extends WireSaveableItems {
 	 *
 	 */
 	public function get($key) {
-		if($key == 'path') return $this->path;
+		if($key == 'path') return $this->wire('config')->paths->templates;
 		$value = $this->templatesArray->get($key); 
 		if(is_null($value)) $value = parent::get($key);
 		return $value; 
@@ -652,6 +653,44 @@ class Templates extends WireSaveableItems {
 		return $updated; 
 	}
 
+	/**
+	 * Hook called when a Template detects that its file has changed
+	 * 
+	 * Note that the hook is not called until something in the system (like a page render) asks for the template’s filename.
+	 * That’s because it would not be efficient for PW to check the file for every template in the system on every request. 
+	 * 
+	 * #pw-hooker
+	 * 
+	 * @param Template $template
+	 * @since 3.0.141
+	 * 
+	 */
+	public function ___fileModified(Template $template) {
+		if(empty($this->fileModTemplates)) {
+			// add hook on first call
+			$this->addHookAfter('ProcessWire::finished', $this, '_hookFinished');
+		}
+		$this->fileModTemplates[$template->id] = $template;
+	}
+	
+	/**
+	 * Saves templates that had modified files to update 'modified' and 'ns' properties after the request is complete
+	 *
+	 * #pw-internal
+	 *
+	 * @param HookEvent $e
+	 * @since 3.0.141
+	 *
+	 */
+	public function _hookFinished(HookEvent $e) {
+		if($e) {}
+		foreach($this->fileModTemplates as $id => $template) {
+			if($template->isChanged('modified') || $template->isChanged('ns')) {
+				$template->save();
+			}
+		}
+		$this->fileModTemplates = array();
+	}
 
 	/**
 	 * FUTURE USE: Is the parent/child relationship allowed?

wire/core/Wire.php

@@ -1682,7 +1682,7 @@ abstract class Wire implements WireTranslatable, WireFuelable, WireTrackable {
 	 */
 	public function wire($name = '', $value = null, $lock = false) {
 
-		if(is_null($this->_wire)) {
+		if($this->_wire === null) {
 			// this object has not yet been wired! use last known current instance as fallback
 			// note this condition is unsafe in multi-instance mode
 			$wire = ProcessWire::getCurrentInstance();