UITextView (editing) - detecting that next line event occured

The following delegate is called whenever a new text is entered in textView.

Set the delegate for UITextView, then code as follows

- (BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text;
{
    if ( [text isEqualToString:@"\n"] ) {
        //Do whatever you want
    }
    return YES;
}

For Swift use this

previousRect = CGRectZero

 func textViewDidChange(textView: UITextView) {

        var pos = textView.endOfDocument
        var currentRect = textView.caretRectForPosition(pos)
        if(currentRect.origin.y > previousRect?.origin.y){
            //new line reached, write your code
        }
        previousRect = currentRect

    }

For Objective C

CGRect previousRect = CGRectZero;
- (void)textViewDidChange:(UITextView *)textView{

    UITextPosition* pos = textView.endOfDocument;
    CGRect currentRect = [textView caretRectForPosition:pos];

    if (currentRect.origin.y > previousRect.origin.y){
            //new line reached, write your code
        }
    previousRect = currentRect;

}

Will detect line changes from anything to hitting "return", backspacing to reduce line-count, typing till the end of the line and a word-wrap occurs, etc. (*Note: MUST adjust variables for font size, I recommend not using hard coded numbers like in my example below).

previousNumberOfLines = ((hiddenText.contentSize.height-37+21)/(21));//numbers will change according to font size
NSLog(@"%i", previousNumberOfLines);