NSManagedObject 的自定义初始值设定项 [英] Custom initializer for an NSManagedObject

问题描述

根据文档:

你不应该覆盖init.不鼓励您覆盖 initWithEntity:insertIntoManagedObjectContext:

You should not override init. You are discouraged from overriding initWithEntity:insertIntoManagedObjectContext:

你应该改用awakeFromInsert 或awakeFromFetch.

and you should instead use awakeFromInsert or awakeFromFetch.

如果我只想将某个属性设置为当前日期或类似日期，这很好，但是如果我想发送另一个对象并根据其信息设置属性怎么办?

This is fine if all I want to do is set some attribute to the current date or similar, but what if I want to send in another object and set attributes based on its information?

例如，在名为Item"的 NSManagedObject 子类中，我想要一个 initFromOtherThing:(Thing *)thing，其中项的名称设置为事物的名称.我想避免只需要记住"每次创建项目后立即设置名称，并且在我决定希望 Item 还基于 Thing 设置另一个默认属性时必须更新十五个不同的控制器类.这些是与模型相关的操作.

For example, in an NSManagedObject subclass called 'Item', I want an initFromOtherThing:(Thing *)thing, in which the item's name is set to that of the thing. I would like to avoid 'just having to remember' to set the name each time immediately after creating the item, and having to update fifteen different controller classes when I decide that I want Item to also set another default attribute based on Thing. These are actions tied to the model.

我打算如何处理这个问题?

How am I meant to handle this?

推荐答案

我认为处理这个问题的最好方法是将 NSManagedObject 子类化，然后创建一个类别来保存您想要添加到对象中的内容.例如几个用于唯一和方便创建的类方法:

I think the best way to handle this is by subclassing the NSManagedObject and then creating a category to hold what you want to add to the object. For example a couple of class methods for uniquing and conveniently creating:

+ (item *) findItemRelatedToOtherThing: (Thing *) existingThing inManagedObjectContext *) context {
    item *foundItem = nil;
    // Do NSFetchRequest to see if this item already exists...
    return foundItem;
}

+ (item *) itemWithOtherThing: (Thing *) existingThing inContext: (NSManagedObjectContext *) context {
    item *theItem;
    if( !(theItem = [self findItemRelatedToOtherThing: existingThing inManagedObjectContext: context]) ) {
        NSLog( @"Creating a new item for Thing %@", existingThing );
        theItem = [NSEntityDescription insertNewObjectForEntityForName: @"item" inManagedObjectContext: context];
        theItem.whateverYouWant = existingThing.whateverItHas;
    }
    return theItem; 
}

现在不要直接调用 initWithEntity:insertIntoManagedObjectContext: 只需使用您的便利类方法，例如:

Now don't ever call initWithEntity:insertIntoManagedObjectContext: directly just use your convenience class method like:

item *newItem = [item itemWithOtherThing: oldThing inContext: currentContext];

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