[Qgis-developer] WMS Design Problem and Mac Crash

[Tom Elwertowski]

Hi all,

I'd like to fix a Mac crash and some redraw problems which involve a 
change extending through several layers of QGIS so I am putting this out 
for review before proceeding. The redraw problem can be seen using a WMS 
layer when resizing the window and sometimes when zooming. Only 0.8 
crashes but the redraw issues are the same in 0.8 and 0.9.

The WMS provider calls processEvents to fetch a remote image while 
inside a draw event. processEvents is needed to receive the network 
event containing the retrieved image, however, calling processEvents 
during a draw event on a Mac can result in incorrect drawing or a crash.

The problem occurs because processEvents may process other draw events 
while waiting for the remote image and, at least on a Mac, draw events 
cannot be interrupted by other draw events.

To a lesser extent, odd redraw artifacts also remain in Ubuntu after 
resizing a WMS layer.

One fix is to add a loadImage routine to the provider so that the canvas 
can notify all layers about what is needed beforehand and not call 
QPainter::begin until all images are locally available.

For local layers, loadImage does nothing. For remote layers, loadImage 
initiates a transfer if the cache is invalid. The canvas then needs to 
wait until all layers signal ready.

Tom