R – Determining if a Unicode character is visible

Here's a fully working and clean implementation

@interface GILayoutManager : NSLayoutManager
@end

@implementation GILayoutManager

- (void)drawGlyphsForGlyphRange:(NSRange)range atPoint:(NSPoint)point {
  NSTextStorage* storage = self.textStorage;
  NSString* string = storage.string;
  for (NSUInteger glyphIndex = range.location; glyphIndex < range.location + range.length; glyphIndex++) {
    NSUInteger characterIndex = [self characterIndexForGlyphAtIndex: glyphIndex];
    switch ([string characterAtIndex:characterIndex]) {

      case ' ': {
        NSFont* font = [storage attribute:NSFontAttributeName atIndex:characterIndex effectiveRange:NULL];
        [self replaceGlyphAtIndex:glyphIndex withGlyph:[font glyphWithName:@"periodcentered"]];
        break;
      }

      case '\n': {
        NSFont* font = [storage attribute:NSFontAttributeName atIndex:characterIndex effectiveRange:NULL];
        [self replaceGlyphAtIndex:glyphIndex withGlyph:[font glyphWithName:@"carriagereturn"]];
        break;
      }

    }
  }

  [super drawGlyphsForGlyphRange:range atPoint:point];
}

@end

To install, use:

[myTextView.textContainer replaceLayoutManager:[[GILayoutManager alloc] init]];

To find font glyph names, you have to go to CoreGraphics:

CGFontRef font = CGFontCreateWithFontName(CFSTR("Menlo-Regular"));
for (size_t i = 0; i < CGFontGetNumberOfGlyphs(font); ++i) {
  printf("%s\n", [CFBridgingRelease(CGFontCopyGlyphNameForGlyph(font, i)) UTF8String]);
}

As others have said, you could use

:set list

which will, in combination with

:set listchars=...

display invisible characters.
Now, there isn't an explicit option which you can use to show whitespace, but in listchars, you could set a character to show for everything BUT whitespace. For example, mine looks like this

:set listchars=eol:$,tab:>-,trail:~,extends:>,precedes:<

so, now, after you use

:set list

everything that isn't explicitly shown as something else, is then, really, a plain old whitespace.

As usual, to understand how listchars works, use the help. It provides great information about what chars can be displayed (like trailing space, for instance) and how to do it:

:help listchars

It might be helpful to add a toggle to it so you can see the changes mid editing easily (source: VIM :set list! as a toggle in .vimrc):

noremap <F5> :set list!<CR>
inoremap <F5> <C-o>:set list!<CR>
cnoremap <F5> <C-c>:set list!<CR>