This drawboard is a page where you can draw with your mouse or touch input
(using different colors) and everybody else which has the page open will
immediately see what you are drawing.
If someone opens the page later, they will get the current room image (so they
can see what was already drawn by other people).
It uses asynchronous sending of messages so that it doesn't need separate threads
for each client to send messages (this needs NIO or APR connector to be used).
Each "Room" (where the drawing happens) uses a ReentrantLock to synchronize access
(currently, only a single Room is implemented).
When you open the page, first you will receive a binary websocket message containing
the current room image as PNG image. After that, you will receive string messages
that contain the drawing actions (line from x1,y1 to x2,y2).
Note that it currently only uses simple string messages instead of JSON because
I did not want to introduce a dependency on a JSON lib.
It uses synchronization mechanisms to ensure that the final image will look the same for every user, regardless of what their network latency/speed is â e.g. if two user draw at the same time on the same place, the server will decide which line was the first one, and that will be reflected on every client.