autohide only horizontal scroller in NSScrollView

Question

I have an NSTableView, created from IB, that I want to only autohide the horizontal scroller on. The main reason I want to do this is because it seems the NSTableView corverView only get's displayed if there is a vertical scroller.

I can't find any method to do this with the base class. So I tried subclassing NSScrollView and observing the hidden key on the horizontal scroller (code below). This works; however, the view tries to reset the current visible options every time the user resizes the window. This makes my implementation somewhat expensive; and it seems inelegant. Any better ideas about how to do this?

Thanks in advance!

Current implementation:

@interface PVScrollView : NSScrollView {
  BOOL autohidesHorizontalScroller;
}

@property(assign) BOOL autohidesHorizontalScroller;

- (void) viewResized:(NSNotification*)notification;

@end

@implementation PVScrollView

@synthesize autohidesHorizontalScroller;

- (void) setAutohidesHorizontalScroller:(BOOL)val
{
  autohidesHorizontalScroller = val;
  [self setAutohidesScrollers:NO];
  [[self horizontalScroller] addObserver:self 
                              forKeyPath:@"hidden"
                                 options:0 
                                 context:nil];

}

- (void) observeValueForKeyPath:(NSString *)keyPath 
                       ofObject:(id)object 
                         change:(NSDictionary *)change 
                        context:(void *)context
{
  if (!([self documentVisibleRect].size.width < [[self documentView] frame].size.width) )
  {
    // remove observer
    [[self horizontalScroller] removeObserver:self
                                   forKeyPath:@"hidden"];
    [[self horizontalScroller] setHidden:YES];
    //[[self horizontalScroller] setNeedsDisplay:YES];
    // add it back
    [[self horizontalScroller] addObserver:self 
                                forKeyPath:@"hidden"
                                   options:0 
                                   context:nil];
  }
}

@end

I'd say don't worry about it till you find out it actually causes performance problems. Premature optimization is bad. — Douwe Maan
I would tend to agree with Douwe, however I'd add that you can still find potential performance problems fairly easily by using Instruments to profile the code. Premature optimization isn't always. Sometimes profiling something while stress-testing it when you know it might perform badly is the only logical thing to do. Ignoring it until it causes a problem when you knew it might to begin with is less logical than "premature" optimization. :-) — Joshua Nozzi
I've already run it through instruments. If the user is actively resizing it can take up to 50% of the cpu as opposed to a fairly constant 25% for the base implementation. I agree about pre-mature optimizations, what bothers me though is the elegance of the solution. I don't like having to reset and re-render every time; for something as simple as just auto-hiding only 1 scrollbar! — Cayden

Nicholas Riley Nicholas Riley · Accepted Answer · 2011-02-19T04:34:47

Give this a shot in your NSScrollView subclass:

- (void)setFrameSize:(NSSize)newSize;
{
    NSSize minFrameSize = [NSScrollView frameSizeForContentSize:[self contentSize] hasHorizontalScroller:NO hasVerticalScroller:YES borderType:[self borderType]];
    BOOL wantScroller = minFrameSize.width > newSize.width;
    [self setHasHorizontalScroller:wantScroller];        
    [super setFrameSize: newSize];
}

You'll need to check "Show Vertical Scroller" and uncheck "Automatically Hide Scrollers" for it to work; I didn't bother making it robust to changes in IB. Also, you'll need to do the same thing when the window is first displayed (in the NSScrollView constructor).

I compared CPU usage with and without this change; it seems to vary at most 1% (19%→20%) in my test application.

autohide only horizontal scroller in NSScrollView

1 Answers