限制UITextField的输入长度

#pragma mark -UITextFieldDelegate-
#pragma mark 限制输入框的字数
/**
*  限制输入框的字数
*
*/
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
if (textField == noteTF) {
if (string.length == 0) return YES;
NSInteger existedLength = textField.text.length;
NSInteger selectedLength = range.length;
NSInteger replaceLength = string.length;
if (existedLength - selectedLength + replaceLength > 20) {
return NO;
}
}
return YES;
}