Polylist widget Lazy Loading images example. [was Re: polylist toys]

[Alex Tweedly]

On 18/08/2022 23:13, Alex Tweedly via use-livecode wrote:
>
> However, the lazy load example still contains bugs due to race conditions

Solution:

replace the card script with the following .... should fix all the race 
conditions I can see.

> local sURLToIndex
>
> on onlineImage pIndex, pURL
>    put  pIndex &CR after sURLToIndex[pURL]
>    load URL pURL  with message "downloadComplete"
> end onlineImage
>
> on downloadComplete pURL, pSatus
>   local tURL
>   repeat for each line L in sURLToIndex[pURL]
>      set the itemPointer of widget "PolyList" to L
>      set the subItemPointer of widget "PolyList" to "url"
>      put the subitemContent of widget "PolyList" into tURL
>      if tURL = pURL then -- still the right URL
>         set the subItemPointer of widget "PolyList" to "image"
>         set the subitemContent of widget "PolyList" to URL pURL
>      end if
>   end repeat
>   put empty into sURLToIndex[pURL]
> end downloadComplete

And in the real world, add in cache aging, limit on number of entries, 
limit on total cache size in bytes, ...

Alex.

>
> 1. if multiple entries reference the same image, then there is a 
> chance the earlier ones never get properly updated (sURLToIndex[pURL] 
> will have been overwritten by the next call to onlineImage before the 
> image value gets set).
>
> 2. if the user (or anything else) resets the dataContent of the widget 
> while there are outstanding "load URL"s, then when the 
> downloadComplete is triggered, it will set the image value for some 
> random new item to the image of the earlier one.
>
> 3. And if the one which was wrongly set in case 2 above also happens 
> to be a URL that occurs multiple time (i.e. case 1 above), then that 
> wrongly set image will remain "forever".
>
>