ios – 为什么Apple在自己的类构造函数中使用instancetype不一致？

看看NSArray.h中NSArray创建方法的块.

有没有正当的原因,方法返回id不返回instancetype？

苹果甚至经历了添加内联注释的努力,让我们知道,在这种情况下,id返回一个NSArray.

@interface NSArray (NSArrayCreation)

+ (instancetype)array;
+ (instancetype)arrayWithObject:(id)anObject;
+ (instancetype)arrayWithObjects:(const id [])objects count:(NSUInteger)cnt;
+ (instancetype)arrayWithObjects:(id)firstObj,... NS_REQUIRES_NIL_TERMINATION;
+ (instancetype)arrayWithArray:(NSArray *)array;

- (instancetype)init;   /* designated initializer */
- (instancetype)initWithObjects:(const id [])objects count:(NSUInteger)cnt; /* designated   initializer */

- (instancetype)initWithObjects:(id)firstObj,... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithArray:(NSArray *)array;
- (instancetype)initWithArray:(NSArray *)array copyItems:(BOOL)flag;

+ (id /* NSArray * */)arrayWithContentsOfFile:(Nsstring *)path;
+ (id /* NSArray * */)arrayWithContentsOfURL:(NSURL *)url;
- (id /* NSArray * */)initWithContentsOfFile:(Nsstring *)path;
- (id /* NSArray * */)initWithContentsOfURL:(NSURL *)url;

@end

我唯一可以想出这些特定的方法是苹果的指导

“The array representation at the location identified by aURL must contain only property list >objects (Nsstring,NSData,NSArray,or NSDictionary objects). The objects contained by this >array are immutable,even if the array is mutable.”

然而,这还是给我没有解释使用id超过实例类型,因为他们仍然允许NSArray子类返回自己的实例类型

NSDictionary遵循完全相同的模式,其中使用文件或URL的内容创建字典使用id,所有其他创建方法使用instancetype

- (instancetype)initWithObjectsAndKeys:(id)firstObject,... NS_REQUIRES_NIL_TERMINATION;
- (instancetype)initWithDictionary:(NSDictionary *)otherDictionary;
- (instancetype)initWithDictionary:(NSDictionary *)otherDictionary copyItems:(BOOL)flag;
- (instancetype)initWithObjects:(NSArray *)objects forKeys:(NSArray *)keys;

+ (id /* NSDictionary * */)dictionaryWithContentsOfFile:(Nsstring *)path;
+ (id /* NSDictionary * */)dictionaryWithContentsOfURL:(NSURL *)url;
- (id /* NSDictionary * */)initWithContentsOfFile:(Nsstring *)path;
- (id /* NSDictionary * */)initWithContentsOfURL:(NSURL *)url;

我知道苹果公司正在逐渐将基础类中的id替换为实例类型,但是在单个类中使用的图形不一致性可以作为我们自己使用的指导,或者他们没有去完成他们开始工作的类上？

要扩展一点我想探索的返回类型的dictionaryWithContentsOfFile当调用NSMutableDictionary

Nsstring * plistPath = [[NSBundle mainBundle] pathForResource:@"myFile" ofType:@"plist"];
NSMutableDictionary *myDictionary = [NSMutableDictionary dictionaryWithContentsOfFile:plistPath]; 
    if ([ myDictionary isKindOfClass:[NSMutableDictionary class]])
    {
        NSLog(@"This is a mutable dictionary why id and not instancetype?");
        [myDictionary setobject:@"I can mutate the dictionary" forKey:@"newKey"];
    }
NSLog (@"%@",myDictionary[@"newKey"]); 
    return YES;
}

以下输出到我的控制台：

This is a mutable dictionary why id and not instancetype?

I can mutate the dictionary

因此,我可以在字典中添加新的键和对象.

解决方法

类集群可能会返回除您创建它们的类之外的类.
对于基础类,这通常是正确的,因为它们将在许多情况下创建某种优化类.该类仍然会从isKindOfClass返回YES：

还有一些免费的桥接类返回基础和核心基金会共享的类.一个例子是NSCFString.