first pass at Propagator class

demo/event-propagators.html

@@ -41,7 +41,7 @@
       target="#box4"
       trigger="transform"
       expression="y: from.x, 
-rotation: from.x"
+rotation: from.x/10"
     ></folk-event-propagator>
 
     <script type="module">

src/common/Propagator.ts

@@ -0,0 +1,243 @@
+/**
+ * A function that processes the event and updates the target element.
+ * @param {Element} source - The source element that emitted the event
+ * @param {Element} target - The target element that receives propagated changes
+ * @param {Event} event - The event that triggered the propagation
+ * @returns {any} - The result of the propagation
+ */
+export type PropagatorFunction = (source: Element, target: Element, event: Event) => any;
+
+/**
+ * A parser function that converts a string expression into a PropagatorFunction.
+ * @param {string} body - The string expression to parse
+ * @param {Propagator} [propagator] - The Propagator instance (optional)
+ * @returns {PropagatorFunction | null} - The parsed PropagatorFunction or null if parsing fails
+ */
+export type PropagatorParser = (body: string, propagator?: Propagator) => PropagatorFunction | null;
+
+export type PropagatorOptions = {
+  source?: Element | null;
+  target?: Element | null;
+  event?: string | null;
+  handler?: PropagatorFunction | string;
+  parser?: PropagatorParser;
+  onParseSuccess?: (body: string) => void;
+  onParseError?: (error: Error) => void;
+};
+
+/**
+ * A propagator takes a source and target element and listens for events on the source.
+ * When an event is detected, it will execute a handler and update the target element.
+ */
+export class Propagator {
+  #source: Element | null = null;
+  #target: Element | null = null;
+  #eventName: string | null = null;
+  #handler: PropagatorFunction | null = null;
+
+  #parser: PropagatorParser | null = null;
+  #onParse: ((body: string) => void) | null = null;
+  #onError: ((error: Error) => void) | null = null;
+
+  /**
+   * Creates a new Propagator instance.
+   * @param {PropagatorOptions} options - Configuration options for the propagator
+   * @param {Element} [options.source] - Source element that emits events
+   * @param {Element} [options.target] - Target element that receives propagated changes
+   * @param {string} [options.event] - Event name to listen for on the source element
+   * @param {PropagatorFunction|string} [options.handler] - Event handler function or string expression
+   * @param {PropagatorParser} [options.parser] - Custom parser for string handlers
+   * @param {Function} [options.onParse] - Callback fired when a string handler is parsed
+   * @param {Function} [options.onError] - Callback fired when an error occurs during parsing
+   */
+  constructor(options: PropagatorOptions = {}) {
+    const {
+      source = null,
+      target = null,
+      event = null,
+      handler = null,
+      onParseSuccess: onParse = null,
+      onParseError: onError = null,
+      parser = null,
+    } = options;
+
+    this.#onParse = onParse;
+    this.#onError = onError;
+    this.#parser = parser;
+    this.source = source;
+    this.target = target;
+    if (event) this.event = event;
+    if (handler) this.handler = handler;
+  }
+
+  /**
+   * The source element that emits events.
+   * Setting a new source will automatically update event listeners.
+   */
+  get source(): Element | null {
+    return this.#source;
+  }
+
+  set source(element: Element | null) {
+    // Remove listener from old source
+    if (this.#source && this.#eventName) {
+      this.#source.removeEventListener(this.#eventName, this.#handleEvent);
+    }
+
+    this.#source = element;
+
+    // Add listener to new source
+    if (this.#source && this.#eventName) {
+      this.#source.addEventListener(this.#eventName, this.#handleEvent);
+    }
+  }
+
+  /**
+   * The target element that receives propagated changes.
+   */
+  get target(): Element | null {
+    return this.#target;
+  }
+
+  set target(element: Element | null) {
+    this.#target = element;
+  }
+
+  /**
+   * The name of the event to listen for on the source element.
+   * Setting a new event name will automatically update event listeners.
+   */
+  get event(): string | null {
+    return this.#eventName;
+  }
+
+  set event(name: string) {
+    // Remove old listener
+    if (this.#source && this.#eventName) {
+      this.#source.removeEventListener(this.#eventName, this.#handleEvent);
+    }
+
+    this.#eventName = name;
+
+    // Add new listener
+    if (this.#source && this.#eventName) {
+      this.#source.addEventListener(this.#eventName, this.#handleEvent);
+    }
+  }
+
+  /**
+   * The handler function that processes the event and updates the target.
+   * Can be set using either a function or a string expression.
+   */
+  get handler(): PropagatorFunction | null {
+    return this.#handler;
+  }
+
+  set handler(value: PropagatorFunction | string | null) {
+    if (typeof value === 'string') {
+      try {
+        this.#handler = this.#parser ? this.#parser(value, this) : this.#defaultParser(value);
+      } catch (error) {
+        this.#handler = null;
+      }
+    } else {
+      this.#handler = value;
+    }
+  }
+
+  /**
+   * Manually triggers the propagation with an optional event.
+   * If no event is provided and an event name is set, creates a new event.
+   * @param {Event} [event] - Optional event to propagate
+   */
+  propagate(event?: Event): void {
+    if (!event && this.#eventName) {
+      event = new Event(this.#eventName);
+    }
+    if (!event) return;
+    this.#handleEvent(event);
+  }
+
+  /**
+   * Cleans up the propagator by removing event listeners and clearing references.
+   * Should be called when the propagator is no longer needed.
+   */
+  dispose(): void {
+    if (this.#source && this.#eventName) {
+      this.#source.removeEventListener(this.#eventName, this.#handleEvent);
+    }
+    this.#source = null;
+    this.#target = null;
+    this.#handler = null;
+  }
+
+  #handleEvent = (event: Event) => {
+    if (!this.#source || !this.#target || !this.#handler) return;
+
+    try {
+      this.#handler(this.#source, this.#target, event);
+    } catch (error) {
+      console.error('Error in propagator handler:', error);
+    }
+  };
+
+  // This approach turns object syntax into imperative code.
+  // We could alternatively use a parser (i.e. Babel) to statically analyse the code
+  // and check on keystrokes if the code is valid.
+  // We should try a few different things to figure out how this class could work best.
+  #defaultParser = (body: string): PropagatorFunction | null => {
+    const processedExp = body.trim();
+
+    const codeLines: string[] = [];
+
+    // Split the expression into lines, handling different line endings
+    const lines = processedExp.split(/\r?\n/);
+
+    for (const line of lines) {
+      let line_trimmed = line.trim();
+      if (!line_trimmed) continue;
+
+      // Remove trailing comma if it exists (only if it's at the very end of the line)
+      if (line_trimmed.endsWith(',')) {
+        line_trimmed = line_trimmed.slice(0, -1).trim();
+      }
+
+      // Find the first colon index, which separates the key and value.
+      // Colons can still be used in ternary operators or other expressions,
+      const colonIndex = line_trimmed.indexOf(':');
+      if (colonIndex === -1) {
+        continue;
+      }
+
+      const key = line_trimmed.slice(0, colonIndex).trim();
+      const value = line_trimmed.slice(colonIndex + 1).trim();
+
+      if (key === '()') {
+        // Anonymous function: directly evaluate the value
+        codeLines.push(`${value};`);
+      } else if (key.endsWith('()')) {
+        // If the key is a method, execute it if the condition is true
+        const methodName = key.slice(0, -2);
+        codeLines.push(`
+  if (typeof to.${methodName} !== 'function') throw new Error(\`Method '${methodName}' does not exist on target element.\`);
+  else if (${value}) to.${methodName}();`);
+      } else {
+        // For property assignments, assign the value directly
+        codeLines.push(`
+  if (!('${key}' in to)) throw new Error(\`Property '${key}' does not exist on target element.\`);
+  to.${key} = ${value};`);
+      }
+    }
+
+    const functionBody = codeLines.join('\n');
+
+    try {
+      const handler = new Function('from', 'to', 'event', functionBody) as PropagatorFunction;
+      this.#onParse?.(functionBody);
+      return handler;
+    } catch (error) {
+      this.#onError?.(error as Error);
+      return null;
+    }
+  };
+}