Specs: specced TapGesture, added some TODOs around events, and put in a placeholder for ScrollGestu…

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```javascript
   // EVENTS
 
   class Event {
     constructor (String type, Boolean bubbles = true, any data = null); // O(1)
     readonly attribute String type; // O(1)
     readonly attribute Boolean bubbles; // O(1)
     attribute any data; // O(1)
 
     readonly attribute EventTarget target; // O(1)
     attribute Boolean handled; // O(1)
     attribute any result; // O(1)
 
     // TODO(ianh): do events get blocked at scope boundaries, e.g. focus events when both sides are in the scope?
     // TODO(ianh): do events get retargetted, e.g. focus when leaving a custom element?
   }
 
+  // TODO(ianh): decide if we're using this generic Event class and
+  // allowing any arbitrary properties to be set on it, or if we're
+  // going to use subclasses (and drop "type"). If we use subclasses
+  // then how will declarative event handling work in frameworks?
+  // (consider that multiple modules can each have their own FooEvent
+  // class with the same name...)
+  //
+  // The advantage of this would be the ability to enforce (or at
+  // least better catch) incorrect uses of the API, e.g. to make sure
+  // people don't stop on themselves in 'pointer-down' capture.
+
   callback EventListener any (Event event);
     // if the return value is not undefined:
     //   assign it to event.result
     //   set event.handled to true
 
   abstract class EventTarget {
     any dispatchEvent(Event event); // O(N) in total number of listeners for this type in the chain
       // sets event.handled to false and event.result to undefined
       // makes a record of the event target chain by calling getEventDispatchChain()
       // invokes all the handlers on the chain in turn
@@ skipping 15 matching lines @@
       // while (node) {
       //   result.push(node);
       //   node = node.parentNode;
       // }
       // return result;
 
     // you can inherit from this to make your object into an event target
     // or you can inherit from EventTarget and implement your own getEventDispatchChain()
   }
 ```