String length with given font to fit UITextView

Question

I need to move text that the user has entered into a large multi-line UITextView into a smaller (but still multi-line) UITextView*. If the user has entered more text than will display in the smaller view, I want to truncate the text so that it fits with all the (truncated) text visible. (Neither the large UITextView nor the smaller one should scroll.)

What's the best way to do this?

I can use a loop, shortening the string by a character each time, and then use NSString's sizeWithFont: constrainedToSize: lineBreakMode: to find out the height this shorter string would need, and then compare that against the height I have available in my smaller UITextView, ending the loop when the string will fit - but that seems slow and awkward. There must be a better way.

I'd like to just tell the destination UITextView to truncate its displayText member as it displays it on screen, but I've not been able to find a way to do that.

*More context on this, from a comment I made below:

I've got a landscape app. I change the layout of the view depending on the photo the user chooses. If it's a landscape photo, the caption is smaller - just a line at the bottom of the photo. If she chooses a portrait photo, then there's plenty of space I can use for the caption at the side of the photo, so the caption is bigger.

If the user changes her photo orientation from portrait to landscape, then I want to truncate the text and then allow her to edit it so that it makes sense. I could just zap it, but I'd prefer to preserve it to minimize her typing.

Answer 1

I wrote the following recursive method and public API to do this properly. The ugly fudge factor is the subject of this question.

#define kFudgeFactor 15.0
#define kMaxFieldHeight 9999.0

// recursive method called by the main API
-(NSString*) sizeStringToFit:(NSString*)aString min:(int)aMin max:(int)aMax
{
if ((aMax-aMin) <= 1)
    {
    NSString* subString = [aString substringToIndex:aMin];
    return subString;
    }

int mean = (aMin + aMax)/2; 
NSString* subString = [aString substringToIndex:mean];

CGSize tallerSize = CGSizeMake(self.frame.size.width-kFudgeFactor,kMaxFieldHeight);
CGSize stringSize = [subString sizeWithFont:self.font constrainedToSize:tallerSize lineBreakMode:UILineBreakModeWordWrap];

if (stringSize.height <= self.frame.size.height)
    return [self sizeStringToFit:aString min:mean max:aMax]; // too small
else    
        return [self sizeStringToFit:aString min:aMin max:mean];// too big
}

-(NSString*)sizeStringToFit:(NSString*)aString
{

CGSize tallerSize = CGSizeMake(self.frame.size.width-kFudgeFactor,kMaxFieldHeight);
CGSize stringSize = [aString sizeWithFont:self.font constrainedToSize:tallerSize lineBreakMode:UILineBreakModeWordWrap];

// if it fits, just return
if (stringSize.height < self.frame.size.height)
    return aString; 

// too big - call the recursive method to size it       
NSString* smallerString = [self sizeStringToFit:aString min:0 max:[aString length]];
return smallerString;   
}

Answer 2

This isn't actually a fix but it does provide a good starting poing for the calculation.

If you use NSString's sizeWithFont: constrainedToSize: lineBreakMode: you get a vertical height for your text. If you divide that by your font's leading height, you get the number of lines in the whole string. Dividing [NSString count] by that number gives you an approximation to number of characters per line. This assumes the string is homogeneuous and will be inaccurate if someone types (e.g.) 'iiiiiiiiiii..." as oposed to "MMMMMMMMM...".

You can also divide you bounding box by the relevent font's leading height to get the number of lines that fit within your bounding box.

Multiplying characters per line by number of lines gives you a starting point for finding text that fits.

You could calculate the margin for error in this figure by doing the same calculation for those 'iiiiii...' and "MMMMMM...'" strings.

Answer 3

I would suggest taking a slightly different approach and seeing if you can use a UILabel instead of the smaller UITextView.

UILabels can be setup to be multi-line like a UITextView through their numberOfLines property.

UILabels also have a lineBreakMode property and I believe that the default value of that property will do the exact truncation effect that you are looking for.