Detect moment when newline starts in UITextView

This is how I would do it:

• Get the UITextPosition of the last character.
• Call caretRectForPosition on your UITextView.
• Create a CGRect variable and initially store CGRectZero in it.
• In your textViewDidChange: method, call caretRectForPosition: by passing the UITextPosition.
• Compare it with the current value stored in the CGRect variable. If the new y-origin of the caretRect is greater than the last one, it means a new line has been reached.

Sample code:

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

    UITextPosition* pos = yourTextView.endOfDocument;//explore others like beginningOfDocument if you want to customize the behaviour
    CGRect currentRect = [yourTextView caretRectForPosition:pos];

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

}

Also, you should read the documentation for UITextInput protocol reference here. It is magical, I'm telling you.

Let me know if you have any other issues with this.

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

    }

answer of @n00bProgrammer in Swift-4 with more precise line break detection.

@n00bProgrammer answer is perfect except one thing it reacts differently when the user starts typing in a first line, it presents that Started New Line too.
Overcoming issue, here is the refined code

    var previousRect = CGRect.zero
    func textViewDidChange(_ textView: UITextView) {
        let pos = textView.endOfDocument
        let currentRect = textView.caretRect(for: pos)
        self.previousRect = self.previousRect.origin.y == 0.0 ? currentRect : self.previousRect
        if currentRect.origin.y > self.previousRect.origin.y {
            //new line reached, write your code
            print("Started New Line")
        }
        self.previousRect = currentRect
    }