More colors

One of the points this example sets out to illustrate is how thinking in terms of very simple abstractions and then building more complex ones on top of them make for code that is simpler to maintain and easier to modify.

As such, we will now update our animation to draw the sine wave in different colors. In this case, we would like to draw the wave in red if the sine of x is negative, and blue otherwise.

We already have the sine value coming through the sine-wave stream, so all we need to do is transform this stream into one that will give us the colors according to the preceding criteria:

(def colour (.pipe sine-wave 
                   (rx-map
                     (fn [{:keys [sin]}] 
                       (if (< sin 0) 
                         "red" 
                         "blue"))))) 

The next step is to add the new stream into the main drawing loop—remember to comment the previous one so that we don't end up with multiple waves being drawn at the same time:

(-> (js/rxjs.zip sine-wave colour) 
    (.pipe (rx-take 700)) 
    (.subscribe (fn [[{:keys [x y]} colour]] 
                  (fill-rect x y colour)))) 

Once we save the file, we should see a new sine wave alternating between red and blue as the sine of x oscillates from -1 to 1.