Bug 1461742, shared method to indicate that a custom element implements one or more interfaces, r=bgrins

toolkit/content/customElements.js

@@ -2,6 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+/* globals MozQueryInterface */
+
 "use strict";
 
 // This is loaded into all XUL windows. Wrap in a block to prevent
@@ -59,6 +61,54 @@ class MozXULElement extends XULElement {
     range.selectNodeContents(doc.querySelector("box"));
     return range.extractContents();
   }
+
+  /**
+   * Indicate that a class defining an element implements one or more
+   * XPCOM interfaces. The custom element getCustomInterface is added
+   * as well as an implementation of QueryInterface.
+   *
+   * The supplied class should implement the properties and methods of
+   * all of the interfaces that are specified.
+   *
+   * @param cls
+   *        The class that implements the interface.
+   * @param names
+   *        Array of interface names
+   */
+  static implementCustomInterface(cls, ifaces) {
+    cls.prototype.QueryInterface = ChromeUtils.generateQI(ifaces);
+    cls.prototype.getCustomInterfaceCallback = function getCustomInterfaceCallback(iface) {
+      if (ifaces.includes(Ci[Components.interfacesByID[iface.number]])) {
+        return getInterfaceProxy(this);
+      }
+      return null;
+    };
+  }
+}
+
+/**
+ * Given an object, add a proxy that reflects interface implementations
+ * onto the object itself.
+ */
+function getInterfaceProxy(obj) {
+  if (!obj._customInterfaceProxy) {
+    obj._customInterfaceProxy = new Proxy(obj, {
+      get(target, prop, receiver) {
+        let propOrMethod = target[prop];
+        if (typeof propOrMethod == "function") {
+          if (propOrMethod instanceof MozQueryInterface) {
+            return Reflect.get(target, prop, receiver);
+          }
+          return function(...args) {
+            return propOrMethod.apply(target, args);
+          };
+        }
+        return propOrMethod;
+      }
+    });
+  }
+
+  return obj._customInterfaceProxy;
 }
 
 // Attach the base class to the window so other scripts can use it: