Question

我有一个完整的菜鸟问题。 obj-c我显然很生气。我有一个简单的购物车类实现为单身，只是希望它存储一个NSMutableDictionary。我希望能够从应用程序的任何位置向此字典添加对象。但对于一些人（我很确定很简单）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

在我的VC中，我试图将其设置为：

- (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]); 
}

由于

Answer 1

您永远不会创建名为items的NSMutableDictionary对象。

您可以在ShoppingCart的初始化中创建它。

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

或在sharedInstance

中

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

Answer 2

我可能还会更好地（可以说）设置您的共享实例，如下所示：

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

return instance;

Singleton NSMutableDictionary属性不允许使用setObject：forKey

2 个答案: