(OS) Simple Picker

BIDViewController.h

//So the datasource and delegate will be this view controller.
//So drag and drop the picker outlets in the .xib to the File's Owner (this class)

//Necessary Protocols
//OR drag and drop the datasource and delegate from the connection inspector to the File's Owner
@interface BIDViewController : UIViewController <UIPickerViewDelegate, UIPickerViewDataSource>

@property (strong, nonatomic) IBOutlet UIPickerView *picker;
@property (strong, nonatomic) IBOutlet UILabel *resultLabel;
@property (strong, nonatomic) IBOutlet UITextField *dollarText;

@property (strong, nonatomic) NSArray *countryNames;
@property (strong, nonatomic) NSArray *exchangeRates;

//Action to dismiss the keyboard when hitting return (didEndOnExit signal)
- (IBAction)editingEnded:(id)sender;
@end

BIDViewController.mm

@implementation BIDViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
    
    //Initialize the arrays
    
    self.countryNames = [[NSArray alloc] initWithObjects:
                         @"Australia (AUD)", @"China (CNY)", @"France (EUR)",
                         @"Great Britain (GBP)", @"Japan (JPY)", nil];
    
    self.exchangeRates = [[NSArray alloc]
                          initWithObjects: [NSNumber numberWithFloat:0.9922],
                          [NSNumber numberWithFloat:6.5938],
                          [NSNumber numberWithFloat:0.7270],
                          [NSNumber numberWithFloat:0.6206],
                          [NSNumber numberWithFloat:81.57], nil];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
    self.resultLabel = nil;
    self.dollarText = nil;
    self.picker = nil;
}

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



- (IBAction)editingEnded:(id)sender
{
    [sender resignFirstResponder];
}



#pragma mark -
#pragma mark PickerView DataSource - UIPickerViewDataSource protocol

//Called by the PickerView to identify the number of components
//How many wheels
- (NSInteger)numberOfComponentsInPickerView:
(UIPickerView *)pickerView
{
    return 1;
}


//Number of rows in a wheel
- (NSInteger)pickerView:(UIPickerView *)pickerView
numberOfRowsInComponent:(NSInteger)component
{
    return [self.countryNames count];
}


//string to be displayed for a specified row in a specific component.
- (NSString *)pickerView:(UIPickerView *)pickerView
             titleForRow:(NSInteger)row
            forComponent:(NSInteger)component
{
    return [self.countryNames objectAtIndex:row];
}


#pragma mark -
#pragma mark PickerView Delegate - UIPickerViewDelegate protocol

-(void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row
      inComponent:(NSInteger)component
{
    //Get the exchange rate based on the row selected by the user
    float rate = [[self.exchangeRates objectAtIndex:row] floatValue];
    
    //Get the amount of dollars the user typed inthe text field
    float dollars = [self.dollarText.text floatValue];
    
    //Calculate the result
    float result = dollars * rate;
    
    NSString *resultString = [[NSString alloc] initWithFormat:@"%.2f USD = %.2f %@", dollars, result,
                              [self.countryNames objectAtIndex:row]];
    //show the result to the label
    self.resultLabel.text = resultString;
}

Read me

Before starting it is worth taking some time to talk about the delegate and datasource of the UIPickerView class.

In order to obtain the options to be displayed to the user, the PickerView needs a data source.

This data source takes the form of a protocol that defines the methods that must be implemented in order to provide
the Picker with data information. At a minimum the class designated as the data source must implement the following methods:

numberOfComponentsInPickerView: - Called by the PickerView to identify the number of components
                                  (i.e. selection wheels) that are to be displayed to the user.

numberOfRowsInComponent: - Informs the PickerView of the number of rows (in other words the selection options)
                          that are present in a specified component.

titleForRow: -  Called by the PickerView to identify the string that is to be displayed for a specified row
                in a specific component.

The PickerView also needs a mechanism for notifying the application code when a selection has been made by the user.
It achieves this by calling the

didSelectRow method 

of the class declared as the PickerView’s delegate. 

In order to fully implement a PickerView, therefore, it is necessary for the object to be assigned a data source and delegate.
Typically the view controller responsible for the PickerView is the best place to implement these two protocols.