Singleton NSMutableDictionary属性将不允许setObject：forKey

Question

我有一个完整的noob问题给你。我明显与obj-c生锈。我有一个简单的购物车类实现为单例，只是希望它存储单个NSMutableDictionary。我希望能够从应用程序中的任何位置将对象添加到此字典中。但对于一些（我敢肯定简单）的原因，它只是返回null。没有错误消息。

ShoppingCart.h：

#import <Foundation/Foundation.h> 

@interface ShoppingCart : NSObject 

// This is the only thing I'm storing here. 
@property (nonatomic, strong) NSMutableDictionary *items; 

+ (ShoppingCart *)sharedInstance; 

@end 

ShoppingCart.m：

// Typical singelton. 
#import "ShoppingCart.h" 

@implementation ShoppingCart 

static ShoppingCart *sharedInstance = nil; 

+ (ShoppingCart *)sharedInstance 
{ 
    @synchronized(self) 
    { 
     if (sharedInstance == nil) 
      sharedInstance = [[self alloc] init]; 
    } 
    return(sharedInstance); 
} 

@end 

而在我的VC我试图用设置：

- (IBAction)addToCartButton:(id)sender 
{ 
    NSDictionary *thisItem = [[NSDictionary alloc] initWithObjects:@[@"test", @"100101", @"This is a test products description"] forKeys:@[@"name", @"sku", @"desc"]]; 

    // This is what's failing. 
    [[ShoppingCart sharedInstance].items setObject:thisItem forKey:@"test"]; 

    // But this works. 
    [ShoppingCart sharedInstance].items = (NSMutableDictionary *)thisItem; 

    // This logs null. Specifically "(null) has been added to the cart" 
    DDLogCInfo(@"%@ has been added to the cart", [[ShoppingCart sharedInstance] items]); 
} 

谢谢

Answer 1

你永远不会创建一个名为items的NSMutableDictionary对象。

您可以在ShoppingCart的init中创建它。

-(id)init 
{ 
    if(self = [super init]) { 
     _items = [NSMutableDictionary dictionary]; 
    } 
    return self; 
} 

或sharedInstance

+ (ShoppingCart *)sharedInstance 
{ 
    @synchronized(self) 
    { 
     if (sharedInstance == nil) 
      sharedInstance = [[self alloc] init]; 
      sharedInstance.items = [NSMutableDictionary dictionary]; 
    } 
    return(sharedInstance); 
} 

Answer 2

我也想补充它的更好（可以说）设置你的共享实例，像这样：

static ShoppingCart *instance = nil; 
static dispatch_once_t onceToken; 
dispatch_once(&onceToken, ^{ 
    instance = [[self alloc] init]; 
    instance.items = [NSMutableDictionary dictionary]; 
}); 

return instance;