Question

I don’t fully understand the following code. Isn’t NSCommandKeyMask a fixed integer ?

Then isn’t the second part of the condition always true ?
For instance, isn’t the same to: ([theEvent modifiedFlags] & 1)

if ([theEvent modifierFlags] & NSCommandKeyMask) {
    NSLog(@"Alt key Down (again)!");
}

thanks

Answer 1

It absolutely is–but I think you’re confusing the boolean and operator with the bitwise and operator. Here, they’re performing a bitwise operation on modifierFlags. Read more here.

Basically, it performs a calculation on each individual binary digit. In the case of &, it results in 1 if both digits are 1, and 0 otherwise. Example:

    0101 (decimal 5)
AND 0011 (decimal 3)
  = 0001 (decimal 1)

modifierFlags sets each binary place based on some modifier flag, then each mask contains a single one in its appropriate place. Thus, performing this operation results in 0 if that flag is not set, and some non-zero value if that flag is set. That’s why that if statement works.