组合根

//entity
@interface Order : NSObject
@property (nonatomic, readonly) NSInteger number;
@property (nonatomic, readonly) NSInteger total;
@property (nonatomic, readonly) NSInteger payable;

- (instancetype)initWithNumber:(NSInteger)number
                         total:(NSInteger)total;
@end

@implementation Order

- (instancetype)initWithNumber:(NSInteger)number
                         total:(NSInteger)total {
    self = [super init];
    if (self) {
        _number = number;
        _total = total;
    }
    return self;
}

- (NSInteger)payable {
    return _total >= 200 ? _total - _total / 10 : _total;
}
@end

//gateway
@protocol Catalog <NSObject>
- (NSNumber *)priceOf:(NSString *)sku;
@end

//repository
@protocol Orders <NSObject>
- (void)save:(Order *)order;
- (NSUInteger)count;
@end

//output port: one job, so a block type
typedef void (^Report)(NSString *text);

//use case: asks for its parts, never builds them; they
//come in as properties, the way Cocoa wires outlets and
//delegates, and one forgotten would stay nil, silently:
//another reason to set them in exactly one place
@interface PlaceOrder : NSObject
@property (nonatomic, strong) id<Catalog> catalog;
@property (nonatomic, strong) id<Orders> orders;
@property (nonatomic, copy) Report report;

//a dictionary is fine here: only the sum is seen, never
//the order in which the lines were walked
- (void)place:(NSDictionary *)items;
@end

@implementation PlaceOrder

- (void)place:(NSDictionary *)items {
    NSInteger total = 0;
    for (NSString *sku in items) {
        NSNumber *price = [self.catalog priceOf:sku];
        if (price == nil) {
            self.report([@"no "
                         stringByAppendingString:sku]);
            return;
        }
        total += price.integerValue
                 * [items[sku] integerValue];
    }
    Order *order = [[Order alloc]
        initWithNumber:1001 + self.orders.count
                 total:total];
    [self.orders save:order];
    self.report([NSString stringWithFormat:
        @"order %ld for %ld", (long)order.number,
        (long)order.payable]);
}
@end

//adapters
@interface MemoryCatalog : NSObject <Catalog>
@end

@implementation MemoryCatalog
- (NSNumber *)priceOf:(NSString *)sku {
    return @{@"pen": @30, @"lamp": @190}[sku];
}
@end

@interface MemoryOrders : NSObject <Orders>
@end

@implementation MemoryOrders {
    NSArray<Order *> *_rows;
}

- (void)save:(Order *)order {
    NSArray *before = _rows ? _rows : @[];
    _rows = [before arrayByAddingObject:order];
}

- (NSUInteger)count {
    return _rows.count;
}
@end

//composition root: the only place that sees every layer
//at once, and the only place where an adapter is built;
//no NSClassFromString and no list of class names in a
//plist: the runtime could wire the app by strings, and
//the compiler would check none of it
@interface ShopApp : NSObject
@property (nonatomic, strong, readonly) PlaceOrder *useCase;
@property (nonatomic, strong, readonly) id<Orders> orders;

+ (instancetype)loudApp;
+ (instancetype)quietApp;
@end

@implementation ShopApp

- (instancetype)initWithReport:(Report)report {
    self = [super init];
    if (self) {
        _orders = [[MemoryOrders alloc] init];
        _useCase = [[PlaceOrder alloc] init];
        _useCase.catalog = [[MemoryCatalog alloc] init];
        _useCase.orders = _orders;
        _useCase.report = report;
    }
    return self;
}

+ (instancetype)loudApp {
    return [[ShopApp alloc] initWithReport:
        ^(NSString *text) { NSLog(@"%@", text); }];
}

//a report that says nothing ignores the text, and says
//so with (void): Apple's __unused is missing on GNUstep
+ (instancetype)quietApp {
    return [[ShopApp alloc] initWithReport:
        ^(NSString *text) { (void)text; }];
}
@end

//client code
ShopApp *app = [ShopApp loudApp];
[app.useCase place:@{@"pen": @2, @"lamp": @1}];
[app.useCase place:@{@"chair": @1}];

//a second root builds the same use case differently
ShopApp *night = [ShopApp quietApp];
[night.useCase place:@{@"lamp": @1}];
NSLog(@"saved %lu", (unsigned long)night.orders.count);
//printed: order 1001 for 225
//         no chair
//         saved 1