Shrink UITextField placeholder to fit

gistfile1.m

// call from a view controller's -viewDidLayoutSubviews or -viewWillAppear:
// pass it a text field, etc, assign result to that text field's attributedPlaceholder property
// note that the maxKerning parameter must be a negative value (a "maximally negative" kerning value)

// pro tip: i've sometimes found that a text field still has incorrect bounds in -viewDidLayoutSubviews
// and needs to forcibly have its layout done:
//- (void)viewDidLayoutSubviews {
//   [self.field layoutIfNeeded];
//   self.field.attributedPlaceholder = [self resizedPlaceholderForField:self.field withPadding:4.0 maxKerningReduction:0.3];
//}

- (NSAttributedString *)resizedPlaceholderForField:(UITextField *)field withPadding:(CGFloat)extraPadding maxKerningReduction:(double)maxKerning
{
    NSAttributedString *placeholder = field.attributedPlaceholder;
    UIFont *font = [placeholder attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL];
    if (!font) {
        return placeholder;
    }
    
    CGFloat fontSize = font.pointSize;
    NSMutableAttributedString *mutablePlaceholder = placeholder.mutableCopy;
    
    CGSize boundsSize = [field placeholderRectForBounds:field.bounds].size;
    CGSize unlimitedSize = (CGSize){ FLT_MAX, boundsSize.height };
    CGFloat maxWidth = boundsSize.width - extraPadding - extraPadding;
    
    CGFloat fontDecrement = 0.5;
    CGFloat kernDecrement = 0.05;
    
    // if doesn't fit, first tighten the letter spacing by increments of .05
    BOOL fits = NO;
    double kern = 0; // goes down from 0, since loop condition being 'kern > maxKerning', will skip this loop is maxKerning >= 0
    while (!fits && kern > maxKerning) {
        CGRect measuredRect = [mutablePlaceholder boundingRectWithSize:unlimitedSize options:0 context:nil];
        if (!(fits = measuredRect.size.width <= maxWidth)) {
            kern -= kernDecrement;
            [mutablePlaceholder addAttribute:NSKernAttributeName value:@(kern) range:(NSRange){0, mutablePlaceholder.string.length}];
        }
    }
    
    // as long as it still doesn't fit, reduce the text size by .5 and try again, down to its minimum
    while (!fits && fontSize >= field.minimumFontSize) {
        CGRect measuredRect = [mutablePlaceholder boundingRectWithSize:unlimitedSize options:0 context:nil];
        if (!(fits = measuredRect.size.width <= maxWidth)) {
            fontSize -= fontDecrement;
            font = [font fontWithSize:fontSize];
            [mutablePlaceholder addAttribute:NSFontAttributeName value:font range:(NSRange){0, mutablePlaceholder.string.length}];
        }
    }
    
    return mutablePlaceholder;
}