Resize CATextLayer to fit text on iOS

问题:

All my research so far seems to indicate it is not possible to do this accurately. The only two options available to me at the outset were:

a) Using a Layout manager for the CATextLayer - not available on iOS as of 4.0

b) Use sizeWithFont:constrainedToSize:lineBreakMode: and adjust the frame of the CATextLayer according to the size returned here.

Option (b), being the simplest approach, should work. After all, it works perfectly with UILabels. But when I applied the same frame calculation to CATextLayer, the frame was always turning out to be a bit bigger than expected or needed.

As it turns out, the line-spacing in CATextLayers and UILabels (for the same font and size) is different. As a result, sizeWithFont (whose line-spacing calculations would match with that of UILabels) does not return the expected size for CATextLayers.

This is further proven by printing the same text using a UILabel, as against a CATextLayer and comparing the results. The text in the first line overlaps perfectly (it being the same font), but the line-spacing in CATextLayer is just a little shorter than in UILabel. (Sorry I can't upload a screenshot right now as the ones I already have contain confidential data, and I presently don't have the time to make a sample project to get clean screenshots. I'll upload them later for posterity, when I have the time)

This is a weird difference, but I thought it would be possible to adjust the spacing in the CATextLayer by specifying the appropriate attribute for the NSAttributedString I use there, but that does not seem to be the case. Looking into CFStringAttributes.h I can't find a single attribute that could be related to line-spacing.

Bottomline:

So it seems like it's not possible to use CATextLayer on iOS in a scenario where the layer is required to fit to its text. Am I right on this or am I missing something?

P.S:

1. The reason I wanted to use CATextLayer and NSAttributedString's is because the string to be displayed is to be colored differently at different points. I guess I'd just have to go back to drawing the strings by hand as always....of course there's always the option of hacking the results from sizeWithFont to get the proper line-height.

2. Abusing the 'code' tags a little to make the post more readable.

3. I'm not able to tag the post with 'CATextLayer' - surprisingly no such tags exist at the moment. If someone with enough reputation bumps into this post, please tag it accordingly.

回答1:

Try this:

- (CGFloat)boundingHeightForWidth:(CGFloat)inWidth withAttributedString:(NSAttributedString *)attributedString {
    CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString( (CFMutableAttributedStringRef) attributedString); 
    CGSize suggestedSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, CFRangeMake(0, 0), NULL, CGSizeMake(inWidth, CGFLOAT_MAX), NULL);
    CFRelease(framesetter);
    return suggestedSize.height;
}

You'll have to convert your NSString to NSAttributedString. In-case of CATextLayer, you can use following CATextLayer subclass method:

- (NSAttributedString *)attributedString {

    // If string is an attributed string
    if ([self.string isKindOfClass:[NSAttributedString class]]) {
        return self.string;
    }

    // Collect required parameters, and construct an attributed string
    NSString *string = self.string;
    CGColorRef color = self.foregroundColor;
    CTFontRef theFont = self.font;
    CTTextAlignment alignment;

    if ([self.alignmentMode isEqualToString:kCAAlignmentLeft]) {
        alignment = kCTLeftTextAlignment;

    } else if ([self.alignmentMode isEqualToString:kCAAlignmentRight]) {
        alignment = kCTRightTextAlignment;

    } else if ([self.alignmentMode isEqualToString:kCAAlignmentCenter]) {
        alignment = kCTCenterTextAlignment;

    } else if ([self.alignmentMode isEqualToString:kCAAlignmentJustified]) {
        alignment = kCTJustifiedTextAlignment;

    } else if ([self.alignmentMode isEqualToString:kCAAlignmentNatural]) {
        alignment = kCTNaturalTextAlignment;
    }

    // Process the information to get an attributed string
    CFMutableAttributedStringRef attrString = CFAttributedStringCreateMutable(kCFAllocatorDefault, 0);

    if (string != nil)
        CFAttributedStringReplaceString (attrString, CFRangeMake(0, 0), (CFStringRef)string);

    CFAttributedStringSetAttribute(attrString, CFRangeMake(0, CFAttributedStringGetLength(attrString)), kCTForegroundColorAttributeName, color);
    CFAttributedStringSetAttribute(attrString, CFRangeMake(0, CFAttributedStringGetLength(attrString)), kCTFontAttributeName, theFont);

    CTParagraphStyleSetting settings[] = {kCTParagraphStyleSpecifierAlignment, sizeof(alignment), &alignment};
    CTParagraphStyleRef paragraphStyle = CTParagraphStyleCreate(settings, sizeof(settings) / sizeof(settings[0]));
    CFAttributedStringSetAttribute(attrString, CFRangeMake(0, CFAttributedStringGetLength(attrString)), kCTParagraphStyleAttributeName, paragraphStyle);    
    CFRelease(paragraphStyle);

    NSMutableAttributedString *ret = (NSMutableAttributedString *)attrString;

    return [ret autorelease];
}

HTH.

回答2:

I have a much easier solution, that may or may not work for you.

If you aren't doing anything special with the CATextLayer that you can't do a UILabel, instead make a CALayer and add the layer of the UILabel to the CALayer

UILabel*label = [[UILabel alloc]init];

//Do Stuff to label

CALayer *layer = [CALayer layer];

//Set Size/Position

[layer addSublayer:label.layer];

//Do more stuff to layer

回答3:

This page gave me enough to create a simple centered horizontally CATextLayer : http://lists.apple.com/archives/quartz-dev/2008/Aug/msg00016.html

- (void)drawInContext:(CGContextRef)ctx {
  CGFloat height, fontSize;

  height = self.bounds.size.height;
  fontSize = self.fontSize;

  CGContextSaveGState(ctx);
  CGContextTranslateCTM(ctx, 0.0, (fontSize-height)/2.0 * -1.0);
  [super drawInContext:ctx];
  CGContextRestoreGState(ctx);
}