Question

I know how bitwise AND works,but I don’t understand how does (sourceDragMask & NSDragOperationGeneric) work here,I don’t get the point.Is there anyone can explain to me?Thanks a lot.

- (NSDragOperation)draggingEntered:(id <NSDraggingInfo>)sender
{
    NSPasteboard *pboard;
    NSDragOperation sourceDragMask;

    sourceDragMask = [sender draggingSourceOperationMask];
    pboard = [sender draggingPasteboard];

    if ( [[pboard types] containsObject:NSColorPboardType] )
    {
        if (sourceDragMask & NSDragOperationGeneric)
        {
            return NSDragOperationGeneric;
        }
    }

    return NSDragOperationNone;
}

Answer 1

I know how bitwise AND works,but I don’t understand how does (sourceDragMask & NSDragOperationGeneric) work here,I don’t get the point.

NSDragOperationGeneric is most likely a power of two, which means it has only one bit set. This is deliberate: Bit masks are almost all defined as powers of two (single bits) to enable bit-mask operations like this one.

The bitwise-AND operation, as you know, evaluates to only those bits that are set in both sides. If one side has only one bit (NSDragOperationGeneric) set, then the operation effectively tests whether that bit is set in the other side.

That’s the point of the operation: To test whether the NSDragOperationGeneric bit is set.

There is one gotcha: As you know, a successful bitwise AND test will evaluate to the tested-for bit mask, not 1. So, for example, if you test for a bit mask that’s defined as 0x100 (1 followed by 8 clear bits), then assign that result to a BOOL (which is a signed char) variable, you’ll assign zero to the variable! This is why you sometimes see code like this:

BOOL supportsCopyOperation = ((dragOperations & NSDragOperationCopy) == NSDragOperationCopy);

or this:

BOOL supportsCopyOperation = ((dragOperations & NSDragOperationCopy) != 0);

or this:

BOOL supportsCopyOperation = !!(dragOperations & NSDragOperationCopy);

Other bit-mask operations include bitwise-OR (|) to set bits in a value (return NSDragOperationCopy | NSDragOperationMove;, for example) and bitwise-NOT (~, a.k.a. two’s complement) to invert the bits of a value, usually for “anything but” tests.