Ios – String length with given font to fit UITextView

cocoa-touchiosiphone

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.

Best Solution

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;   
}