Xcode - Is this MVC?

Question

I'm a part-time notepad website coder taking steps into IOS apps. I got my first mac last week and have cobbled together a half working app. Now, convinced by this SO answer, I am restarting trying to learn MVC. I am on a very steep learning curve so please bear with me.

I have read up on MVC, separation of layers, three boxes and some arrows, I get it. However, translating the theory into the real world is frustrating. Most example apps I have looked at do not seem to use MVC. Even this LazyTableImages example from Apple seems to use app delegate as the model which confuses me. I have built on this excellently simple MVC example.

My app retrieves location markers from a web service. I have a tabbed application using ARC. One tab has a Mapkit map to display the markers.

A simple class to hold a marker record:

@interface MarkerRecord : NSObject

@property (strong, nonatomic) NSDecimalNumber *lat;
@property (strong, nonatomic) NSDecimalNumber *lon;
@property (strong, nonatomic) NSString *des;

A Model class, holding a collection of Marker classes, to serve both views. This is my Model class .h:

@interface MarkersModel : NSObject

- (void)getMarkers; // Send HTTP GET to web service to retrieve JSON markers
- (void)postMarker; // Send HTTP POST to web service to INSERT new marker record

And the implementation:

@interface MarkersModel ()
@property (strong, nonatomic) NSArray *data;
@end

@implementation MarkersModel
@synthesize data;

- (void)getMarkers
{
    // Send HTTP GET to web service to retrieve JSON markers
    ...
}

- (void)objectLoader:(RKObjectLoader *)objectLoader didLoadObjects:(NSArray *)objects
{
    // Thanks to Restkit data is an array of Marker classes
    data = objects;

    // Pass data to controller

}

A mapViewController class .h:

#import <MapKit/MapKit.h>
#import "MarkersModel.h"

@interface MapViewController : UIViewController<MKMapViewDelegate> {
    MarkersModel *markersModel;
}

@property (nonatomic, strong) IBOutlet MKMapView *mapView;

@end

And the implementation:

@interface MapViewController ()
@end

@implementation MapViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Create instance of markers model
    markersModel = [[MarkersModel alloc] init];
}

-(void)viewWillAppear:(BOOL)animated
{
    [markersModel getMarkers];
}

- (void)putMarkersOnMap
{
    // To be called by Markers Model 
    ...
}

Here's my main question:

• Is my code outline above (the beginnings of) a decent MVC implementation?

If so:

• how do I get the Model to update the Controller ( i.e. how do I get MarkersModel.objectLoader function to call mapViewController.putMarkersOnMap)?

I strongly suspect the answer is a custom delegate on the controller but I cannot for the life of me see how to implement it. I cannot relate any examples to my situation. I think I need something like this on the controller:

@class MapViewController;
@protocol MapViewControllerDelegate;

While I am at it some additional questions:

• Will I create any dangling pointers - I suspect something should be unsafe_unretained (I am compiling to 4.3)?
• Is my MarkersModel declaration in the controller class correct? I am concerned that it will not be managed by ARC. I also suspect it is public but should be private.
• Should the MarkersModel.getMarkers method be static?

I hope this is clear and an acceptable question. Thanks for your patience, Polly.

Answer 1

Is my code outline above (the beginnings of) a decent MVC implementation?

By my standards, yes

how do I get the Model to update the Controller

You are correct, you want to use a delegate. You declare the delegate protocol along with the class it is a delegate for. So expanding on your MarkersModel header:

@protocol MarkersModelDelegate<NSObject>
-(void) markersDidUpdate:(MarkersModel*)model;
@end

@interface MarkersModel : NSObject
{
    __weak id<MarkersModelDelegate> delegate;
}

@property(weak,nonatomic) id<MarkersModelDelegate> delegate;

Then in your view controller:

@interface MapViewController : UIViewController<MKMapViewDelegate, MarkersModelDelegate> {

Then in the model implementation:

@implementation MarkersModel

@synthesize delegate;

//...

-(void)objectLoader:(RKObjectLoader *)objectLoader didLoadObjects:(NSArray *)objects
{
    // Thanks to Restkit data is an array of Marker classes
    data = objects;

    // Pass data to controller
    if( delegate != nil )
    {
        [delegate markersDidUpdate:self];
    }
}

Will I create any dangling pointers - I suspect something should be unsafe_unretained (I am compiling to 4.3)?

Not that I am aware of

Is my MarkersModel declaration in the controller class correct? I am concerned that it will not be managed by ARC. I also suspect it is public but should be private.

You can declare it as private by adding the @private header beforehand:

@interface MapViewController...
{
@private
MarkersModel *markersModel;
...

Should the MarkersModel.getMarkers method be static?

If by static, you mean as a class-level instead of instance-level method, that entirely depends on your implementation. If all MarkersModel objects are supposed to retain the same data, then yes it makes sense to make it class-level. But if each MarkersModel object is supposed to retain its own set of Markers then no, it should remain an instance-level method