TYMED_ISTREAM should be used for CFSTR_FILECONTENTS #25776
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When we drop files to a HTML webpage in Chrome that has a dropzone which handles "drop" events, then files are dropped twice in chrome and Edge. Firefox behaves differently, there only one file is dropped. Debugging showed that Chrome internally calls QueryGetData twice, first with tymed=TYMED_HGLOBAL and second with tymed=TYMED_HGLOBAL|TYMED_ISTREAM|TYMED_ISTORAGE. When we allow both, then two files (the same) are available in event.dataTransfer.files. When we only allow one, then only one file is available. MS does not recommend TYMED_HGLOBAL for virtual files. So, an easy solution to this is to check for TYMED_ISTREAM in QueryGetData. I have tested with several other drop targets like Windows Explorer, Outlook, Firefox, Edge, Chrome, ... and the behavior is OK. In my opinion, this is a Chrome issue. However, since wxWidgets should support virtual files in the future, and TYMED_HGLOBAL is less efficient for larger files, this fix and the limitation to TYMED_ISTREAM are acceptable.
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I'm worried by what happens if someone only calls So it looks like it would be safer (if less efficient) to only return data once, for the first to call, if 2 calls are made in quick succession. But I'm not sure if we can define what "quick" really means... Also, you say that Chrome calls |
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I wouldn't want to restrict CFSTR_FILECONTENTS to TYMED_ISTREAM either. Small files can be transferred via global memory. This would be problematic for larger files. I don't think the idea of a "quick" query is right. Unfortunately, due to the abstraction in wxWidgets, you can't access the actual OLE calls and override them to react based on FORMATETC and STGMEDIUM. But fortunately, you can adapt the wxWidgets code yourself to find a solution to the problem. I'll see if I can find the corresponding code in Chrome to report an issue there. |
Sorry, now I'm really confused. Isn't this exactly what this PR does? AFAICS it forces the use of
I can't think of a good way to allow customizing the medium type used for this. But if you can come up with something, I'd be glad to add it, instead of hardcoding any relationships between the format and the medium.
Yes, open source is great like this. |
When we drop files to a HTML webpage in Chrome that has a dropzone which handles "drop" events, then files are dropped twice in chrome and Edge. Firefox behaves differently, there only one file is dropped. Debugging showed that Chrome internally calls QueryGetData twice, first with tymed=TYMED_HGLOBAL and second with tymed=TYMED_HGLOBAL|TYMED_ISTREAM|TYMED_ISTORAGE. When we allow both, then two files (the same) are available in event.dataTransfer.files. When we only allow one, then only one file is available.
MS does not recommend TYMED_HGLOBAL for virtual files. So, an easy solution to this is to check for TYMED_ISTREAM in QueryGetData. I have tested with several other drop targets like Windows Explorer, Outlook, Firefox, Edge, Chrome, ... and the behavior is OK.
In my opinion, this is a Chrome issue. However, since wxWidgets should support virtual files in the future, and TYMED_HGLOBAL is less efficient for larger files, this fix and the limitation to TYMED_ISTREAM are acceptable.