Make alt drag work for drop sources that can take either copy or move drag operations

[acusti]

Browsers treat users holding down the alt-key before or during a drag operation as a special case where if the dropEffect is move, the drop event will not fire. This PR makes react-dnd default the dropEffect to copy when the alt-key is pressed, which makes it so that by default, drop events still fire with or without holding alt, and passes its dropEffect to dropResult.

This addresses:

• Support for modifier keys while dragging react-dnd-html5-backend#23
• Support native html5 dropEffect react-dnd-html5-backend#58
• holding ctrl while drag/dropping #394
• Support native html5 dropEffect #657

An easy way to see how this works is to pull down this branch and run react-dnd, then check the examples. All of the examples now work out of the box with users holding down alt, while the specific drop-effect example still works as before (no behavior changes there, because it sets a specific dropEffect, which overrides the default one).

Also, we are using a build of react-dnd-htm5-backend with this PR in place for our drag-and-drop implementation in Brandcast. If anyone wants to see it working in a real app, feel free to email me at andrew@brandcast.com and I will send you a signup invite link (currently a closed beta, but will be opened up to everybody in the next two weeks).

[darthtrevino]

@acusti This is looking pretty good; would you be willing to add a case in the docs for using alt when drag-dropping?

[t1mmen]

@acusti Please contact me when this gets merged/released to claim you bounty, ref react-dnd/react-dnd-html5-backend#23

Thanks for taking the time to review this @darthtrevino!

[darthtrevino]

Minor comment

[darthtrevino]

We're setting this.altKeyPressed twice, is this necessary?

[acusti]

I’m not sure. This instance is for the dragenter handler, while the other is for dragover. From looking at http://stackoverflow.com/questions/32676890/want-to-know-the-difference-of-dragover-and-dragenter-events-of-html5-drag-and-d and https://social.msdn.microsoft.com/Forums/windows/en-US/be07bcde-6c99-4d68-824e-8ad93e820fe1/dragenter-vs-dragover?forum=winforms (ignoring the “Answer” and instead seeing the first reply under “All Replies”), it seems possible that dragenter could get invoked without dragover being called (on immediately entering a drop target), hence why I decided to just set the boolean in both handlers (in case the user dropped before a dragover event could be called).

[acusti]

Thanks for reviewing, @darthtrevino! I’d be happy to add docs or similar for this behavior. Were you imagining a mention of the behavior and use cases in the docs, or a new example illustrating copy vs move in the examples/ folder?

[darthtrevino]

A new example would be helpful for me, that way I can start the doc server and verify the work you've done

[acusti]

@darthtrevino Checking back in on this. Regarding #675 (comment), do you think my response makes sense for setting this.altKeyPressed both on dragenter and dragover? If you’d prefer, I’d be happy to update it to only set on dragover.

Also, have you had a chance to try the example I added? I hope it makes sense. Let me know if there’s anything else you’d like to see here. Thanks!

[darthtrevino]

Thanks for the ping, @acusti, we'll get this one worked out today

[darthtrevino]

@acusti The docs look good on my end; can you add this.altKeyPressed = false, to the constructor, then I'll merge this in

[alvinhui]

This is an important feature, when will it be released?

[darthtrevino]

It was released today, v2.3.0

[cdauth]

It would be really nice if this supported other modifier keys as well. Using Alt seems to be Apple-specific behaviour. On Linux, Alt+Drag is typically not even possible because it moves the current window around.