Question

I can follow most of Apple’s WiTap sample, but am sort of stumped on this bit in the send method:

- (void) send:(const uint8_t)message { if (_outStream && [_outStream hasSpaceAvailable])     if([_outStream write:(const uint8_t *)&message maxLength:sizeof(const uint8_t)] == -1)         [self _showAlert:@'Failed sending data to peer'];  }   - (void) activateView:(TapView*)view {     NSLog(@'ACTIVATE TAG: %d', [view tag]);      //[self send:[view tag] | 0x80];     [self send:[view tag]];  }   - (void) deactivateView:(TapView*)view  {     NSLog(@'DEACTIVATE TAG: %d', [view tag]);      //[self send:[view tag] & 0x7f];     [self send:[view tag]];   } 

Note that I have changed the send: argument to just the tag of the views, which are numbered 1-9. Originally the code had the bitwise AND and OR adjustments.

WHY?

I get the fact that the send method needs a uint8_t, but is that why the bitwise stuff is there? To turn a NSInteger into a unint8_t?

The code doesn’t work with my changes above. It will log fine and visually the client will function correctly, but the messages aren’t being sent/received correctly from client to client.

Can someone explain in small words what the bitwise stuff is doing? Or am I correct?

Thanks! This is my first question to SO so please be kind.

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thanks for the response. I am still puzzled a bit. Get it?

Basically, why?

Is this just a geeky way of passing an identifier? Each of those views have a tag #, why not just pass that, and toggle the state (up/down) from the view class?

Is this just a case of ‘this is how the person who wrote it did it’, or am I missing a crucial piece of the puzzle in that this is how I should also be structuring my code.

I would just want to pass a tag # and then have that tag decide what to do in a clearly readable function like toggleUpOrDownState or something.

This bitwise stuff always makes me feel stupid I guess, unless it is necessary, etc. Then I feel stupid but manage to muddle through somehow anyway. : )

Answer 1

Basically, [view tag] | 0x80 is setting the high bit in that value (so 00010011 would become 10010011) and [view tag] & 0x7f is removing it (10010011 -> 00010011).

Take a look at the [AppController stream:handleEvent:] method. You’ll see this code:

        //We received a remote tap update, forward it to the appropriate view     if(b & 0x80)         [(TapView*)[_window viewWithTag:b & 0x7f] touchDown:YES];     else         [(TapView*)[_window viewWithTag:b] touchUp:YES]; 

So, the receiver of the stream is checking for that high bit.