6.831 | Spring 2011 | Graduate
User Interface Design and Implementation
In-Class Activities


The purpose of this activity is to learn more about how user interfaces handle mouse and keyboard input. You’ll need a modern standards-compliant web browser like Firefox, Opera, or Safari; don’t use an old version of Internet Explorer. The exercises below use an in-browser Javascript environment that you’ll find here (link will open in a new window or tab):

Event Dispatch and Propagation

The workbench is initially configured with an event listener listening for clicks on A. Which objects can you click on to fire this listener?

Add a click listener to object C (keeping the listener on A as well). Which listeners fire when you click on C? In what order do they fire? How about when you click on B?

Change your listener on C so that it consumes the event. In standard browsers, this is done by calling event.stopPropagation(). Now which listeners fire when you click on C? How about when you click on B?

Finally, change your listener for A from a bubbling-phase listener to a capturing listener. Now which listeners fire when you click on C? In what order?

Event Translation

Javascript’s events for mouse clicking are click, mousedown, mouseup, and dblclick. Attach listeners for these events to one of the objects in the workbench to discover the sequence in which these events occur when the user double-clicks on that object. Which of these events are raw, and which are translated?

Mouse Movements

Javascript’s event for mouse movement is mousemove. Add a mouse move listener to A, and display the mouse coordinates using print(event.clientX + “,” + event.clientY). What origin are these (x,y) positions using? How is this different from the coordinate system used by Java Swing’s mouse events? (See MouseEvent in the Java 1.5 API documentation.)

Now slow down your mouse move listener. (The workbench defines a function sleep(milliseconds) for this purpose; e.g., sleep(500) waits for 500 milliseconds before it returns.) Does the browser coalesce mouse move events? How do you know? Does it coalesce mouse click events?

Mouse Capture

Using your mousemove listener on A, determine whether the web browser provides mouse capture by default: i.e., if you press the mouse button over A, does A continue receiving mouse events until the mouse button is released, regardless of where the mouse pointer is?

Try attaching the mousemove listener to the window object instead of A. Does mouse capture work now? Note that you have to move the mouse outside the whole window to really answer this question.

Would you be able to implement a scrollbar using just HTML and Javascript? Why or why not?

Course Info
As Taught In
Spring 2011