Selecting a word in a UITextView

Update

This got a lot easier for iOS 7, thanks to the addition of NSLayoutManager in CoreText. If you're dealing with a UITextView you can access the layout manager as a property of the view. In my case I wanted to stick with a UILabel, so you have to create a layout manager with the same size, i.e:

NSTextStorage *textStorage = [[NSTextStorage alloc] initWithAttributedString:labelText];
NSLayoutManager *layoutManager = [[NSLayoutManager alloc] init];
[textStorage addLayoutManager:layoutManager];
CGRect bounds = label.bounds;
NSTextContainer *textContainer = [[NSTextContainer alloc] initWithSize:bounds.size];
[layoutManager addTextContainer:textContainer];

Now you just need to find the index of the character that was clicked, which is simple!

NSUInteger characterIndex = [layoutManager characterIndexForPoint:location
                                                  inTextContainer:textContainer
                         fractionOfDistanceBetweenInsertionPoints:NULL];

Which makes it trivial to find the word itself:

if (characterIndex < textStorage.length) {
  [labelText.string enumerateSubstringsInRange:NSMakeRange(0, textStorage.length)
                                       options:NSStringEnumerationByWords
                                    usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
                                      if (NSLocationInRange(characterIndex, enclosingRange)) {
                                        // Do your thing with the word, at range 'enclosingRange'
                                        *stop = YES;
                                      }
                                    }];
}

Original Answer, which works for iOS < 7

Thanks to @JP Hribovsek for some tips getting this working, I managed to solve this well enough for my purposes. It feels a little hacky, and likely wouldn't work too well for large bodies of text, but for paragraphs at a time (which is what I need) it's fine.

I created a simple UILabel subclass that allows me to set the inset value:

#import "WWLabel.h"

#define WWLabelDefaultInset 5

@implementation WWLabel

@synthesize topInset, leftInset, bottomInset, rightInset;

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        self.topInset = WWLabelDefaultInset;
        self.bottomInset = WWLabelDefaultInset;
        self.rightInset = WWLabelDefaultInset;
        self.leftInset = WWLabelDefaultInset;
    }
    return self;
}

- (void)drawTextInRect:(CGRect)rect
{
    UIEdgeInsets insets = {self.topInset, self.leftInset,
        self.bottomInset, self.rightInset};

    return [super drawTextInRect:UIEdgeInsetsInsetRect(rect, insets)];
}

Then I created a UIView subclass that contained my custom label, and on tap constructed the size of the text for each word in the label, until the size exceeded that of the tap location - this is the word that was tapped. It's not prefect, but works well enough for now.

I then used a simple NSAttributedString to highlight the text:

#import "WWPhoneticTextView.h"
#import "WWLabel.h"

#define WWPhoneticTextViewInset 5
#define WWPhoneticTextViewDefaultColor [UIColor blackColor]
#define WWPhoneticTextViewHighlightColor [UIColor yellowColor]

#define UILabelMagicTopMargin 5
#define UILabelMagicLeftMargin -5

@implementation WWPhoneticTextView {
    WWLabel *label;
    NSMutableAttributedString *labelText;
    NSRange tappedRange;
}

// ... skipped init methods, very simple, just call through to configureView

- (void)configureView
{
    if(!label) {
        tappedRange.location = NSNotFound;
        tappedRange.length = 0;

        label = [[WWLabel alloc] initWithFrame:[self bounds]];
        [label setLineBreakMode:NSLineBreakByWordWrapping];
        [label setNumberOfLines:0];
        [label setBackgroundColor:[UIColor clearColor]];
        [label setTopInset:WWPhoneticTextViewInset];
        [label setLeftInset:WWPhoneticTextViewInset];
        [label setBottomInset:WWPhoneticTextViewInset];
        [label setRightInset:WWPhoneticTextViewInset];

        [self addSubview:label];
    }


    // Setup tap handling
    UITapGestureRecognizer *singleFingerTap = [[UITapGestureRecognizer alloc]
                                               initWithTarget:self action:@selector(handleSingleTap:)];
    singleFingerTap.numberOfTapsRequired = 1;
    [self addGestureRecognizer:singleFingerTap];
}

- (void)setText:(NSString *)text
{
    labelText = [[NSMutableAttributedString alloc] initWithString:text];
    [label setAttributedText:labelText];
}

- (void)handleSingleTap:(UITapGestureRecognizer *)sender
{
    if (sender.state == UIGestureRecognizerStateEnded)
    {
        // Get the location of the tap, and normalise for the text view (no margins)
        CGPoint tapPoint = [sender locationInView:sender.view];
        tapPoint.x = tapPoint.x - WWPhoneticTextViewInset - UILabelMagicLeftMargin;
        tapPoint.y = tapPoint.y - WWPhoneticTextViewInset - UILabelMagicTopMargin;

        // Iterate over each word, and check if the word contains the tap point in the correct line
        __block NSString *partialString = @"";
        __block NSString *lineString = @"";
        __block int currentLineHeight = label.font.pointSize;
        [label.text enumerateSubstringsInRange:NSMakeRange(0, [label.text length]) options:NSStringEnumerationByWords usingBlock:^(NSString* word, NSRange wordRange, NSRange enclosingRange, BOOL* stop){

            CGSize sizeForText = CGSizeMake(label.frame.size.width-2*WWPhoneticTextViewInset, label.frame.size.height-2*WWPhoneticTextViewInset);
            partialString = [NSString stringWithFormat:@"%@ %@", partialString, word];

            // Find the size of the partial string, and stop if we've hit the word
            CGSize partialStringSize  = [partialString sizeWithFont:label.font constrainedToSize:sizeForText lineBreakMode:label.lineBreakMode];

            if (partialStringSize.height > currentLineHeight) {
                // Text wrapped to new line
                currentLineHeight = partialStringSize.height;
                lineString = @"";
            }
            lineString = [NSString stringWithFormat:@"%@ %@", lineString, word];

            CGSize lineStringSize  = [lineString sizeWithFont:label.font constrainedToSize:label.frame.size lineBreakMode:label.lineBreakMode];
            lineStringSize.width = lineStringSize.width + WWPhoneticTextViewInset;

            if (tapPoint.x < lineStringSize.width && tapPoint.y > (partialStringSize.height-label.font.pointSize) && tapPoint.y < partialStringSize.height) {
                NSLog(@"Tapped word %@", word);
                if (tappedRange.location != NSNotFound) {
                    [labelText addAttribute:NSForegroundColorAttributeName value:[UIColor blackColor] range:tappedRange];
                }

                tappedRange = wordRange;
                [labelText addAttribute:NSForegroundColorAttributeName value:[UIColor redColor] range:tappedRange];
                [label setAttributedText:labelText];
                *stop = YES;
            }
        }];        
    }
}