Objective C Converter应用

我正在为我的学校项目开发一个小型转换器应用程序，该应用程序不会在App Store中发布，仅适用于学校年级。我想知道是否有人知道这个应用程序中的开发人员如何

建立转换值的连接？我能想到的最简单的方法是使用开关盒，但这会产生过多不必要的代码：

if (first column is distance) then if(second column is metres) 
case0:third column is yards -> do something
case1:third column is kilometers ->do something
case2:third column is metros ->do nothing

等等....

谢谢回答。

编辑：

感谢您的快速回答，这就是我到目前为止所做的，iOS模拟器正确显示了三列选择器，其中第二个显示在第二个选择器中，具体取决于第一个选择器的行。你能告诉我这有什么好处吗？也许有些事情是不必要的复杂？

#import "konverterViewController.h"

@interface konverterViewController  ()

@end

@implementation konverterViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.fizVelicine  = [[NSArray alloc] initWithObjects:@"distance",@"mass" , nil];
    self.udaljenostJedinice  = [[NSArray alloc]         initWithObjects:@"meter", @"kilometer", @"yard", @"inch",  nil];
    self.masaJedinice  = [[NSArray alloc]         initWithObjects:@"kilogram",@"dekagram",@"gram",@"tone" , nil];
   }

- (UIView *)pickerView:(UIPickerView *)pickerView viewForRow:(NSInteger)row forComponent:(NSInteger)component reusingView:(UIView *)view
{
    UILabel *retval = (id)view;
    if (!retval) {
        retval = [[UILabel alloc] initWithFrame:CGRectMake(0.0f, 0.0f,
                                                           [pickerView rowSizeForComponent:component].width,
                                                           [pickerView rowSizeForComponent:component].height)];

    }
    retval.opaque = NO;
    retval.backgroundColor = [UIColor orangeColor];
    retval.adjustsFontSizeToFitWidth = YES;
    retval.font = [UIFont systemFontOfSize:12];
    if (component == 0) {
        retval.textAlignment = NSTextAlignmentCenter;
        retval.text = [self.fizVelicine objectAtIndex:row];
    }
    else if ([self.odabir isEqualToString:@"distance"]){
            retval.textAlignment = NSTextAlignmentCenter;
            retval.text = [self.udaljenostJedinice objectAtIndex:row];
            }
    else  {
            retval.textAlignment = NSTextAlignmentCenter;
            retval.text = [self.masaJedinice objectAtIndex:row];
             }

    return retval;
}

// returns the number of 'columns' to display.
- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView
{
    return 3;


}

// returns the # of rows in each component..
- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent: (NSInteger)component
{
    if(component== 0)
    {
        return [self.fizVelicine count];
    }
    else
    {
        if([self.odabir isEqualToString:@"distance"]){  return [self.udaljenostJedinice count];}
        else {return [self.masaJedinice count];}
    }


}

-(NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row   forComponent:(NSInteger)component
{
    if(component == 0)
    {
        return [self.fizVelicine objectAtIndex:row];
    }

    else {
        if([self.odabir isEqualToString:@"distance]){

            return [self.udaljenostJedinice objectAtIndex:row];}
        else {
            return [self.masaJedinice objectAtIndex:row];
        }
    }


}


- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row   inComponent:(NSInteger)component
{
    if (component == 0) {
        self.odabir=[[NSString alloc] initWithFormat:@"%@" , [self.fizVelicine objectAtIndex:row]];


        [pickerView reloadAllComponents];}


}



- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end