canDrop:in:at:message; then, if true is answered, performs the drop via
canDrop:in:at:(or its simpler variant,
canDrop:in:) to return true, and also redefine
drop:in:at:(or its simpler variant,
drop:in:) to do the actual drop.
The argument passed is either a dropContext object or a collection of dropObject. (this confusion results from some backward compatibility issues and will be removed in future version - for now, add a check on the arguments type).
If the application is not willing to handle the drop, there is still a chance
that the widget itself knows how to do it (for example, an editor might know
how to drop text, without a need for the application to know about details).
Therefore, after asking the application, the widget tree is asked along the superview chain in a similar manner.
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