直接在代理方法中寫代碼

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    
    if (textField == yourTextField) {
        
        NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //得到輸入框的內容
        
        if ([toBeString containsString:@"."]) {
            
            NSRange subRange = [toBeString rangeOfString:@"."];
            
            if (toBeString.length - subRange.location > 3) { //如果輸入框小數點后大于2位小數
                
                textField.text = [toBeString substringToIndex:toBeString.length - 1];
                
                UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message:@"最多保留2位小數哦!" delegate:nil cancelButtonTitle:@"好的" otherButtonTitles:nil, nil];
                
                [alert show];
                
                return NO;
            }
        }
    }
    
    return YES;
}