How to uniformly scale rich text in an NSTextView?

Works for both iOS and Mac OS

@implementation NSAttributedString (Scale)

- (NSAttributedString *)attributedStringWithScale:(double)scale
{
    if(scale == 1.0)
    {
        return self;
    }

    NSMutableAttributedString *copy = [self mutableCopy];
    [copy beginEditing];

    NSRange fullRange = NSMakeRange(0, copy.length);

    [self enumerateAttribute:NSFontAttributeName inRange:fullRange options:0 usingBlock:^(UIFont *oldFont, NSRange range, BOOL *stop) {
        double currentFontSize = oldFont.pointSize;
        double newFontSize = currentFontSize * scale;

        // don't trust -[UIFont fontWithSize:]
        UIFont *scaledFont = [UIFont fontWithName:oldFont.fontName size:newFontSize];

        [copy removeAttribute:NSFontAttributeName range:range];
        [copy addAttribute:NSFontAttributeName value:scaledFont range:range];
    }];

    [self enumerateAttribute:NSParagraphStyleAttributeName inRange:fullRange options:0 usingBlock:^(NSParagraphStyle *oldParagraphStyle, NSRange range, BOOL *stop) {

        NSMutableParagraphStyle *newParagraphStyle = [oldParagraphStyle mutableCopy];
        newParagraphStyle.lineSpacing *= scale;
        newParagraphStyle.paragraphSpacing *= scale;
        newParagraphStyle.firstLineHeadIndent *= scale;
        newParagraphStyle.headIndent *= scale;
        newParagraphStyle.tailIndent *= scale;
        newParagraphStyle.minimumLineHeight *= scale;
        newParagraphStyle.maximumLineHeight *= scale;
        newParagraphStyle.paragraphSpacing *= scale;
        newParagraphStyle.paragraphSpacingBefore *= scale;

        [copy removeAttribute:NSParagraphStyleAttributeName range:range];
        [copy addAttribute:NSParagraphStyleAttributeName value:newParagraphStyle range:range];
    }];

    [copy endEditing];
    return copy;
}

@end

If the intent is to scale the view (and not actually change the attributes in the string), I would suggest using scaleUnitSquareToSize: method: along with the ScalingScrollView (available with the TextEdit sample code) for the proper scroll bar behavior.

The core piece from the ScalingScrollView is:

- (void)setScaleFactor:(CGFloat)newScaleFactor adjustPopup:(BOOL)flag
{
CGFloat oldScaleFactor = scaleFactor;
    if (scaleFactor != newScaleFactor)
    {
        NSSize curDocFrameSize, newDocBoundsSize;
        NSView *clipView = [[self documentView] superview];

        scaleFactor = newScaleFactor;

        // Get the frame.  The frame must stay the same.
        curDocFrameSize = [clipView frame].size;

        // The new bounds will be frame divided by scale factor
        newDocBoundsSize.width = curDocFrameSize.width / scaleFactor;
        newDocBoundsSize.height = curDocFrameSize.height / scaleFactor;
    }
    scaleFactor = newScaleFactor;
    [scale_delegate scaleChanged:oldScaleFactor newScale:newScaleFactor]; 
}

The scale_delegate is your delegate that can adjust your NSTextView object:

- (void) scaleChanged:(CGFloat)oldScale newScale:(CGFloat)newScale
{
    NSInteger     percent  = lroundf(newScale * 100);

    CGFloat scaler = newScale / oldScale;   
    [textView scaleUnitSquareToSize:NSMakeSize(scaler, scaler)];

    NSLayoutManager* lm = [textView layoutManager];
    NSTextContainer* tc = [textView textContainer];
    [lm ensureLayoutForTextContainer:tc];
}

The scaleUnitSquareToSize: method scales relative to its current state, so you keep track of your scale factor and then convert your absolute scale request (200%) into a relative scale request.

OP here.

I found one solution that kinda works and is not terribly difficult to implement. I'm not sure this is the best/ideal solution however. I'm still interested in finding other solutions. But here's one way:

Manually scale the font point size and line height multiple properties of the NSAttributedString source text before display, and then un-scale the displayed text before storing as source.

The problem with this solution is that while scaled, the system Font Panel will show the actual scaled display point size of selected text (rather than the "real" source point size) while editing. That's not desirable.

---

Here's my implementation of that:

- (void)scaleAttributedString:(NSMutableAttributedString *)str by:(CGFloat)scale {
    if (1.0 == scale) return;

    NSRange r = NSMakeRange(0, [str length]);
    [str enumerateAttribute:NSFontAttributeName inRange:r options:0 usingBlock:^(NSFont *oldFont, NSRange range, BOOL *stop) {
        NSFont *newFont = [NSFont fontWithName:[oldFont familyName] size:[oldFont pointSize] * scale];

        NSParagraphStyle *oldParaStyle = [str attribute:NSParagraphStyleAttributeName atIndex:range.location effectiveRange:NULL];
        NSMutableParagraphStyle *newParaStyle = [[oldParaStyle mutableCopy] autorelease];

        CGFloat oldLineHeight = [oldParaStyle lineHeightMultiple];
        CGFloat newLineHeight = scale * oldLineHeight;
        [newParaStyle setLineHeightMultiple:newLineHeight];

        id newAttrs = @{
            NSParagraphStyleAttributeName: newParaStyle,
            NSFontAttributeName: newFont,
        };
        [str addAttributes:newAttrs range:range];
    }];    
}

This requires scaling the source text before display:

// scale text
CGFloat scale = getCurrentScaleFactor();
[self scaleAttributedString:str by:scale];

And then reverse-scaling the displayed text before storing as source:

// un-scale text
CGFloat scale = 1.0 / getCurrentScaleFactor();
[self scaleAttributedString:str by:scale];