Determine last line's width

Question

I have a label with multiple lines, lineBreakMode is set to UILineBreakModeWordWrap. How can I determine width of last line?

Answer 1

Since iOS 7.0 you can do it using this function (Maybe you'll have to tweak text container a bit more for your case):

public func lastLineMaxX(message: NSAttributedString, labelWidth: CGFloat) -> CGFloat {
    // Create instances of NSLayoutManager, NSTextContainer and NSTextStorage
    let labelSize = CGSize(width: bubbleWidth, height: .infinity)
    let layoutManager = NSLayoutManager()
    let textContainer = NSTextContainer(size: labelSize)
    let textStorage = NSTextStorage(attributedString: message)

    // Configure layoutManager and textStorage
    layoutManager.addTextContainer(textContainer)
    textStorage.addLayoutManager(layoutManager)

    // Configure textContainer
    textContainer.lineFragmentPadding = 0.0
    textContainer.lineBreakMode = .byWordWrapping
    textContainer.maximumNumberOfLines = 0

    let lastGlyphIndex = layoutManager.glyphIndexForCharacter(at: message.length - 1)
    let lastLineFragmentRect = layoutManager.lineFragmentUsedRect(forGlyphAt: lastGlyphIndex,
                                                                  effectiveRange: nil)

    return lastLineFragmentRect.maxX
}

Objective-C:

- (CGFloat)lastLineMaxXWithMessage:(NSAttributedString *)message labelWidth:(CGFloat)labelWidth
{
    CGSize labelSize = CGSizeMake(labelWidth, CGFLOAT_MAX);
    NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
    NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:labelSize];
    NSTextStorage *textStorage = [[NSTextStorage alloc] initWithAttributedString:message];
    [layoutManager addTextContainer:textContainer];
    [textStorage addLayoutManager:layoutManager];

    textContainer.lineFragmentPadding = 0;
    textContainer.lineBreakMode = NSLineBreakByWordWrapping;
    textContainer.maximumNumberOfLines = 0;
    NSUInteger lastGlyphIndex = [layoutManager glyphIndexForCharacterAtIndex:[message length] - 1];
    CGRect lastLineFragmentRect = [layoutManager lineFragmentUsedRectForGlyphAtIndex:lastGlyphIndex effectiveRange:nil];
    return CGRectGetMaxX(lastLineFragmentRect);
}

Then you can decide if there is enough place for your date label in the last line or not

usage example:

// you definitely have to set at least the font to calculate the result
// maybe for your case you will also have to set other attributes
let attributedText = NSAttributedString(string: label.text, 
                                        attributes: [.font: label.font])
let lastLineMaxX = lastLineMaxX(message: attributedText, 
                                labelWidth: label.bounds.width)

Answer 2

Here's how I did it. First put your label's lines in NSArray, and then check width of last line. In viewDidLoad:

NSArray* lines = [self getSeparatedLinesFromLbl:srcLabel];
NSString *lastLine=[lines lastObject];
float lastLineWidth=[lastLine sizeWithFont:srcLabel.font constrainedToSize:boundingSize lineBreakMode:NSLineBreakByWordWrapping].width;

And getSeparatedLinesFromLbl:

-(NSArray*)getSeparatedLinesFromLbl:(UILabel*)lbl
{
if ( lbl.lineBreakMode != NSLineBreakByWordWrapping )
{
    return nil;
}

NSMutableArray* lines = [NSMutableArray arrayWithCapacity:10];

NSCharacterSet* wordSeparators = [NSCharacterSet whitespaceAndNewlineCharacterSet];

NSString* currentLine = lbl.text;
int textLength = [lbl.text length];

NSRange rCurrentLine = NSMakeRange(0, textLength);
NSRange rWhitespace = NSMakeRange(0,0);
NSRange rRemainingText = NSMakeRange(0, textLength);
BOOL done = NO;
while ( !done )
{
    // determine the next whitespace word separator position
    rWhitespace.location = rWhitespace.location + rWhitespace.length;
    rWhitespace.length = textLength - rWhitespace.location;
    rWhitespace = [lbl.text rangeOfCharacterFromSet: wordSeparators options: NSCaseInsensitiveSearch range: rWhitespace];
    if ( rWhitespace.location == NSNotFound )
    {
        rWhitespace.location = textLength;
        done = YES;
    }

    NSRange rTest = NSMakeRange(rRemainingText.location, rWhitespace.location-rRemainingText.location);

    NSString* textTest = [lbl.text substringWithRange: rTest];

    CGSize sizeTest = [textTest sizeWithFont: lbl.font forWidth: 1024.0 lineBreakMode: NSLineBreakByWordWrapping];
    if ( sizeTest.width > lbl.bounds.size.width )
    {
        [lines addObject: [currentLine stringByTrimmingCharactersInSet:wordSeparators]];
        rRemainingText.location = rCurrentLine.location + rCurrentLine.length;
        rRemainingText.length = textLength-rRemainingText.location;
        continue;
    }

    rCurrentLine = rTest;
    currentLine = textTest;
}

[lines addObject: [currentLine stringByTrimmingCharactersInSet:wordSeparators]];

return lines;
}