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Refresh Events

The refresh event mechanism is used to synchronise the display of data to the user. For example, when a parameter is changed in a DisplayData, a refresh event should be generated such that all WorldCanvases displaying that DisplayData are redrawn if necessary. Or, if a PixelCanvasColorTable is physically resized, all PixelCanvases using that ColorTable must be cleared and redrawn.

Any class in the Display Library can register its interest in refresh events in one of the following ways:

• it can inherit from the PCRefreshEH or WCRefreshEH classes;
• it can create an instance of a helper class which forwards events to the class itself; or
• it can attach predefined event handlers, or Tools, to itself.

Having taken one of the options above, the class need simply then register itself, or its helper class, as an event handler with all potential sources of the event of interest. A specific example is the WorldCanvas class, which wishes to know when its underlying PixelCanvas is resized, so that it can resize itself accordingly. To this end, the WorldCanvas class creates a helper class, called ``WCOnPCRefreshEH'', an instance of which can be registered as a handler of PixelCanvas refresh events, and whose only function is to add World Coordinate information to the event and pass it on to its parent WorldCanvas. The function called on the WorldCanvas can then look at the type of the refresh, and if it is a size change, will then act accordingly.

There are many possible reasons for a refresh event to be generated, and indeed such an event can be generated by any class which allows other objects to register themselves as handlers of refresh events. The types of refresh event are listed in DisplayEnums.h, and include:

• UserCommand - this is reserved for explicit refresh requests from the user.
• ColorTableChange - the allocation of hardware resources has changed, probably at the level of the PixelCanvasColorTable, and consequently most drawings on PixelCanvases sharing the ColorTable of interest will need to be redrawn.
• ColormapChange - some internal change to a single Colormap has occurred, which may mean that data being drawn with the particular Colormap needs to be redrawn.
• PixelCoordinateChange - the PixelCanvas has changed shape. All WorldCanvases on top of this PixelCanvas will need to erase themselves and redraw to the new geometry.
• LinearCoordinateChange - presently unused, this type of refresh might be required if the WorldCanvas elects to re-negotiate its linear coordinate system.
• WorldCoordinateChange - presently unused, this type of refresh could be used if the WorldCanvas elects somehow to re-negotiate its world coordinate system.
• BackCopiedToFront - a saved image of the PixelCanvas has been restored to the display, and drawings which are not cached on the (back) buffer may need to be redrawn.
• ClearPriorToColorChange - all Pixel and WorldCanvases need to be cleared before a PixelCanvasColorTable change is applied. This can be used to remove drawings before a new Colormap is mapped into the ColorTable, so that invalid images are not left on the screen.