問題描述

一個簡單的問題.

如果我有一個屬性和一個用相同名稱聲明的 ivar:

if I have a property and an ivar declared with the same name:

在 .h 文件中:

(Reminder*)reminder;
@property(nonatomic,strong)(Reminder*)reminder;

在 .m 文件中，如果我使用 ARC，我應該使用 ivar 還是 init 方法中的屬性?

in the .m file, should I use the ivar or the property in the init method if I'm using ARC?

- (id)initWithReminder:(Reminder*)reminder_ {
    self = [super init];
    if (self) {
        reminder = reminder_;
    }
    return self;
}

或者我應該像這樣使用該屬性來獲得自動引用計數的好處:

Or should I use the property to get the benefit of the automatic reference counting like this:

- (id)initWithReminder:(Reminder*)reminder_ {
    self = [super init];
    if (self) {
        self.reminder = reminder_;
    }
    return self;
}

我不確定在對象初始化的哪個時間點可以使用點符號訪問屬性.

I'm not sure at which point in the object's initialization the properties become accessible with the dot notation.

推薦答案

在部分構造的狀態下使用直接訪問，不管 ARC:

Use direct access in partially constructed states, regardless of ARC:

- (id)initWithReminder:(Reminder*)reminder_ {
    self = [super init];
    if (self) {
        reminder = reminder_;
        // OR
        reminder = [reminder_ retain];
    }
    return self;
}

這是因為 self.whatever 會觸發其他副作用，例如 Key-Value Observing (KVO) 通知，或者您的類實現(顯式)或子類覆蓋 setWhatever:——這可能會將你部分初始化的實例暴露給其他 API(包括它自己的)，這些 API 正確地假設它們正在處理一個完全構造的對象.

This is because self.whatever will trigger other side effects, such as Key-Value Observing (KVO) notifications, or maybe your class implements (explicitly) or a subclass overrides setWhatever: -- and that could expose your partially initialized instance to other APIs (including its own), which rightly assume they are dealing with a fully constructed object.

您可以手動驗證一個類是否能夠在部分初始化的狀態下運行，但這需要大量維護并且(坦率地說)當其他人想要繼承您的類時是不切實際或不可能的.它需要大量的時間和維護，這樣做并沒有實質性的好處，尤其是如果您嘗試將這種方法用作慣例.

You could manually verify that a class is capable of operating in a partially initialized state, but that requires a lot maintenance and is (frankly) impractical or impossible when other people want to subclass your class. It requires a lot of time and maintenance, and there isn't substantiative benefit doing so, especially if you try to use the approach as a convention.

所以保證正確性的統一方式是在部分構造狀態下使用直接訪問，避免使用訪問器.

So the uniform manner which guarantees correctness is to use direct access in partially constructed states, and avoid using the accessors.

注意:我使用的是部分構造"，因為初始化只是圖片的一半；-dealloc 有類似的注意事項.

Note: I am using "partially constructed" because initialization is only half of the picture; -dealloc has similar caveats.

關于為什么應該在部分構造狀態 (ARC || MRC) 中使用直接訪問的更多詳細信息可以在此處找到:初始化屬性，點表示法

Some more detail as to why you should use direct access in partially constructed states (ARC || MRC) can be found here: Initializing a property, dot notation

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