How to determine when the tiles are all fetched?

[srfoster]

Thanks again for making this package!

I've been using it to generate picts of maps. It's already been enormously helpful!

There's just one thing. I'd like to detect when the tiles are finished downloading so that the pict can be safely generated without missing tiles.

I thought I could maybe do it by checking (get-download-backlog) -- but that doesn't seem to be working.

I've also used just a simple (sleep X) -- which works, but is gross.

Any advice? Here's my code if it helps:

https://github.com/thoughtstem/map-picts/blob/master/map-pict.rkt

[alex-hhh]

There is no easy way to do that -- the map-widget is designed so that the drawing the map does not block the main GUI thread and relies on the fact that a re-draw can be scheduled later, perhaps when more tiles are available. These two assumptions don't work well for picts, which use synchronous drawing and have no opportunity for a re-draw.

Background Info

You are correct hat get-download-backlog is not working for your purpose: it returns the number of tiles that have been requested from the network, but it does not count tiles which have been requested from the local disk cache and it will count tiles which have been found in the disk cache and are drawn on the dc<%>, but are too old and need re-downloading.

Basically a tile will be only drawn if it is found in the memory cache, otherwise:

• a request is made for it from the disk cache and the tile is skipped from drawing (a white block is drawn instead).
• if not found in the disk cache, a request is made for it to be fetched over the network
• if found in the disk cache, but it is too old, it will be placed in a memory cache, but also a request will be made for re-fetching this tile over the network.

Also all this logic is further complicated by the fact that allow-tile-download might prevent downloading the tiles from the network, in which case a map tile might never become available, but other tiles, which are in the disk cache might still be drawn.

Possible solutions

Perhaps the cleanest solution would be to add a "synchronous" parameter to the package and make get-tile-bitmap behave synchronously if it is set to #t

A simpler solution would be to update map-impl%/draw to return #t if the map drawing is complete and #f otherwise (the draw-map-tiles method has a request-redraw? flag, which could be propagated and returned from the draw method. This way, the map-pict could keep calling the draw method until it returns #t, but keep in mind that if there's a network error, it might never return #t.