Wednesday, October 24, 2012

Smooth moves with Swing

It's about data rendering in Swing. How to make a smooth rendering of real time data...
The secret seems to be in SwingUtilities.invokeAndWait

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Above are two JPanels, displaying, on the left a speed, on the right a heading.
Both are fed by real time data (simulated in this case).
Obviously - and we will not show this code, unless anyone insists - the position of the display (namely the position of the hand) is done when the repaint() is invoked, and its implementation is done by overriding the paintComponent(Graphics) method.
The smoothing is done in a setValue(double) method.
When this method is invoked, the difference between the previous and the next values is splitted in small intervals, between which a repaint() is invoked.
All the difficulty - the problem - is that when a repaint() is invoked, maybe the previous one is not completed yet... This would result in a jerky display, far from being smooth, at the opposite of the expected result.
A solution is to tell Swing to wait, which happens to be shamefully simple... Here is the code for the speed display:
  public void setSpeed(final double d)
    this.speed = d;
    double from = this.prevSpeed; 
    double to = d;
    int sign = (from>to)?-1:1;
    prevSpeed = d;
    // Smooth rotation
    for (double s=from; (sign==1 && s<=to) || (sign==-1 && s>=to); s+=(0.1*sign)) // 0.1 is the damping factor
      final double _s = s;
        SwingUtilities.invokeAndWait(new Runnable()
            public void run()
              speed = _s;
      catch (Exception ex)
The paintComponent(Graphics) method uses the speed member to position the hand on the display.
Notice the SwingUtilities.invokeAndWait method that takes a Runnable as a parameter. This Runnable is the one invoking the repaint() method. This way, Swing will do nothing until the repaint is completed, and it is not even necessary to worry about synchronizing anything...

See also SwingUtilities.invokeLater. It allows Swing to perform demanding tasks asynchronously.

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