如何通过静态（+）方法将对象添加到数组中？

Question

// Marketplace.h
#import <Foundation/Foundation.h>
#import "Item.h"

@interface Marketplace : NSObject

+ (void)addItemToMarketplace:(Item *)newItem; // METHOD IN QUESTION

@end

// Marketplace.m
#import "Marketplace.h"

@interface Marketplace()
@property (strong, nonatomic) NSMutableArray *listOfItems;
@end

@implementation Marketplace


+ (void)addItemToMarketplace:(Item *)newItem // METHOD IN QUESTION
{
    [self.listOfItems addObject:newItem]; // Raises 3 errors
}

@end

我已经声明了一个类方法 addItemToMarketplace，它接受一个 Item 类型的对象，并将这个 Item 添加到我在实现文件的接口中声明的 listOfItems 属性中（我不确定我是否希望其他类摆弄这个性质）。我在另一个类中使用过这种方法，例如 [Marketplace addItemToMarketplace:newItem]。我不确定如何处理在编写 [self.listOfItems addObject:newItem] 时引发的三个错误。

The 3 errors are as follows:
    1. Member reference type 'struct objc_class *' is a pointer; maybe you meant to use '->'?
    2. Definition of 'struct objc_class' must be imported from module 'ObjectiveC.runtime' before it is required
    3. No member named 'listOfItems' in 'struct objc_class'

Making the change proposed in #1, which changes self.listOfItems to self->listOfItems, raises the error "Member reference base type 'Class' is not a structure or union"

任何帮助将不胜感激。

// NEW CHANGES!

在进行了一些更改并遵循我在其他网站上找到的一些建议后，以下是我目前所拥有的：

// Marketplace.h

#import <Foundation/Foundation.h>
#import "Item.h"

@interface Marketplace : NSObject {}

+ (Marketplace *)sharedMarket;
- (void)addItemToMarketplace:(Item *)newItem;

@end

// Marketplace.m

#import "Marketplace.h"

@interface Marketplace()
@property (strong, nonatomic)NSMutableArray *listOfItems;
@end

static Marketplace *sharedMarketplace = nil;

@implementation Marketplace

+ (Marketplace *)sharedMarket
{
    if (sharedMarketplace == nil) {
        sharedMarketplace = [[super allocWithZone:NULL] init];
    }
return sharedMarketplace;
}


- (void)addItemToMarketplace:(Item *)newItem
{
    [self.listOfItems addObject:newItem];
}

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

+ (id)allocWithZone:(NSZone *)zone {
    return [self sharedMarket];
}

- (id)copyWithZone:(NSZone *)zone {
    return self;
}

@end

问题、cmets、疑虑？

Answer 1

只有实例方法可以对实例数据进行操作。因此，您需要一个实例或将数据也更改为静态：

@interface Marketplace()
@end

static NSMutableArray *listOfItems;  // static is implied, so not really necessary

@implementation Marketplace

+ (void)addItemToMarketplace:(Item *)newItem {

    if (!listOfItems) listOfItems = [NSMutableArray array];
    [listOfItems addObject:newItem];
}

@end

请注意，这里没有线程安全。

Answer 2

+ 前缀定义了一个类方法。在该方法中self 指的是类本身，这几乎肯定不是您想要的。该类没有listOfItems，因为这是该类的每个实例上可用的属性。

不清楚您在这里尝试做什么。如果您来自 C/C++ 背景，那么您可能已经为此方法假设了不同的行为。您是想将项目添加到由此类的特定实例管理的列表中，还是添加到所有实例共享的列表中？

Answer 3

如果您的应用中只有一个 marketplace 实例，请考虑使用单例..

然后您可以像这样将商品添加到单一市场：

[[Marketplace theMarketplace] addItem:<*item*>]

这是一种设置方法：

@interface Marketplace : NSObject
@property ( nonatomic ) NSArray * items ;
@end

@implementation Marketplace

static Marketplace * __marketplace ;

+(void)load
{
    __marketplace = [ Marketplace new ] ;
}

+(instancetype)theMarketplace
{
    return __marketplace ;
}

-(void)addItem:(Item*)item
{
    self.items = [ ( self.items ?: @[] ) arrayByAddingObject:item ] ;
}

@end

（或在 Swift 中）

class Marketplace
{
    struct Static
    {
        static let marketplace = Marketplace()
    }

    var items:Array<Item> = [] ;

    class func get() -> Marketplace { return Static.marketplace }

    func addItem( item: Item ) { self.items += item }
}