动手实现 NSObject

#Original (English)

Source: https://www.mikeash.com/pyblog/friday-qa-2013-01-25-lets-build-nsobject.html

The NSObject class lies at the root of (almost) all classes we build and use as part of Cocoa programming. What does it actually do, though, and how does it do it? Today, I’m going to rebuild NSObject from scratch, as suggested by friend of the blog and occasional guest author Gwynne Raskind.

Components of a Root ClassWhat exactly does a root class do? In terms of Objective-C itself, there is precisely one requirement: the root class’s first instance variable must be isa, which is a pointer to the object’s class. The isa is used to figure out what class an object is when dispatching messages. That’s all there has to be, from a strict language standpoint.

A root class that only provides that wouldn’t be very useful, of course. NSObject provides a lot more. The functionality it provides can be broken down into three categories:

• Memory management: standard memory management methods like retain and release are implemented in NSObject. The alloc method is also implemented there.

• Introspection: NSObject provides a bunch of methods that are essentially wrappers around Objective-C runtime functionality, such as class, respondsToSelector:, and isKindOfClass:.

• Default implementations of miscellaneous methods: there are a bunch of methods that we count on every object implementing, such as isEqual: and description. In order to ensure that every object has an implementation, NSObject provides a default implementation that every subclass gets if it doesn’t bring its own.

CodeI’ll be reimplementing NSObject functionality as MAObject. I’ve posted the full code for this article on GitHub:

https://github.com/mikeash/MAObject

Note that this code is built without ARC. Although ARC is great and should be used whenever possible, it really gets in the way when implementing a root class, because a root class needs to implement memory management and ARC prefers that you leave memory management up to the compiler.

Instance VariablesMAObject has two instance variables. The first is the isa pointer. The second is the object’s reference count:

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    @implementation MAObject {
2
        Class isa;
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        volatile int32_t retainCount;
4
    }

The reference count will be managed using functions from OSAtomic.h to ensure thread safety, which is why it has a somewhat unusual definition rather than just using NSUInteger or similar.

NSObject actually holds reference counts externally. There’s a global table which maps an object’s address to its reference count. This saves memory, because the table represents the common reference count of 1 by not having an entry in the table at all. However, this technique is complex and a bit slow, so I opted not to follow it for my own version.

Memory ManagementThe first thing that MAObject needs to be able to do is to create instances. This is done by implementing the +alloc method. (I’m skipping the deprecated and rarely used +allocWithZone:, which these days does the same thing and ignores its parameter anyway.)

Subclasses rarely override +alloc, and rely on the root class to allocate memory for them. That means that MAObject needs to be able to allocate instances not only of MAObject, but of any subclass. This is done by taking advantage of the fact that the value of self in a class method is the class the message was actually sent to. If code does [SomeSubclass alloc], then self holds a pointer to SomeSubclass. That class can then be used to query the runtime to figure out how much memory to allocate, and to set the isa pointer correctly. The retain count is also initialized to 1, as suits a newly allocated object:

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    + (id)alloc
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    {
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        MAObject *obj = calloc(1, class_getInstanceSize(self));
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        obj->isa = self;
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        obj->retainCount = 1;
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        return obj;
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    }

The retain method simply uses OSAtomicIncrement32 to bump up the retain count, and returns self:

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    - (id)retain
2
    {
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        OSAtomicIncrement32(&retainCount);
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        return self;
5
    }

The release method does a bit more. It first decrements the retain count. If the retain count was decremented to 0, then the object needs to be destroyed, so the code calls dealloc:

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    - (oneway void)release
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    {
3
        uint32_t newCount = OSAtomicDecrement32(&retainCount);
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        if(newCount == 0)
5
            [self dealloc];
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    }

The implementation of autorelease calls NSAutoreleasePool to add self to the current autorelease pool. Autorelease pools are part of the runtime these days, so this is a somewhat indirect route, but the autorelease APIs in the runtime are private, so this is the best we can do for now:

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    - (id)autorelease
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    {
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        [NSAutoreleasePool addObject: self];
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        return self;
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    }

The retainCount method simply returns the value held in the ivar:

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    - (NSUInteger)retainCount
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    {
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        return retainCount;
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    }

Finally, there’s the dealloc method. In normal classes, dealloc needs to clean up any instance variables and then call super. The root class has to actually dispose of the memory occupied by the object itself. In this case, it’s just a simple call to free:

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    - (void)dealloc
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    {
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        free(self);
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    }

There are a couple of helper methods as well. NSObject provides a do-nothing init method for consistency, so that subclasses can always call [super init]:

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    - (id)init
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    {
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        return self;
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    }

There’s also a new method, which is just a wrapper around alloc and init:

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    + (id)new
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    {
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        return [[self alloc] init];
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    }

There’s also an empty finalize method. NSObject implements this as part of its garbage collection support. MAObject doesn’t support garbage collection in the first place, but I included this just because NSObject has it:

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    - (void)finalize
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    {
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    }

IntrospectionMany of the introspection methods are just wrappers around runtime functions. Since that’s not too interesting, I’ll give a brief discussion of what the runtime function is doing behind the scenes as well.

The simplest introspection method is class, which just returns the value of isa:

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    - (Class)class
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    {
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        return isa;
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    }

Technically, this method will fail on tagged pointers. A proper implementation should call object_getClass, which behaves correctly for tagged pointers, and extracts the isa from a normal pointer.

The superclass instance method is equivalent to just invoking the superclass class method on the object’s class, so that’s exactly what the method does:

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    - (Class)superclass
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    {
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        return [[self class] superclass];
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    }

There are also class methods for these. The +class method just returns self, which is the class object. This is a little weird, but it’s how NSObject does things. [obj class] returns the object’s class, but [MyClass class] just returns a pointer to MyClass itself. It’s not consistent, as MyClass also has a class, which is the MyClass metaclass, but it’s how things are done:

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    + (Class)class
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    {
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        return self;
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    }

The +superclass method does what it says. This is implemented by calling class_getSuperclass, which just grovels around inside the class structure maintained by the runtime and pulls out the pointer to the superclass.

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    + (Class)superclass
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    {
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        return class_getSuperclass(self);
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    }

There are also methods for querying whether an object’s class matches a particular class. The simple one is isMemberOfClass:, which does a strict check, ignoring subclasses. Its implementation is simple:

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    - (BOOL)isMemberOfClass: (Class)aClass
2
    {
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        return isa == aClass;
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    }

The isKindOfClass: method checks subclasses too, so that [subclassInstance isKindOfClass: [Superclass class]] returns YES. The output of this method is essentially the same as that of the class method isSubclassOfClass:, so it just calls through:

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    - (BOOL)isKindOfClass: (Class)aClass
2
    {
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        return [isa isSubclassOfClass: aClass];
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    }

That method gets a bit more interesting. Starting from self, it walks up the class hierarchy, comparing with the target class at each level. If it finds a match, it returns YES. If it runs off the top of the class hierarchy without ever finding a match, it returns NO:

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    + (BOOL)isSubclassOfClass: (Class)aClass
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    {
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        for(Class candidate = self; candidate != nil; candidate = [candidate superclass])
4
            if (candidate == aClass)
5
                return YES;
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        return NO;
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    }

It’s interesting to note that this check is not particularly efficient. If you call this method on a class that’s deep in the class hierarchy, it can take a lot of loop iterations before it returns NO. Because of that, isKindOfClass: checks can be quite a lot slower than message sends, and can actually be substantial bottlenecks in certain cases. Just one more reason to avoid them when possible.

The respondsToSelector: method just calls through to the runtime function class_respondsToSelector. That, in turn, looks up the selector in the class’s method table to see if it has an entry:

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    - (BOOL)respondsToSelector: (SEL)aSelector
2
    {
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        return class_respondsToSelector(isa, aSelector);
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    }

There’s a class method, instancesRespondToSelector:, which is nearly identical. The only difference is passing self, which is the class in this context, rather than isa, which would be the metaclass here:

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    + (BOOL)instancesRespondToSelector: (SEL)aSelector
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    {
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        return class_respondsToSelector(self, aSelector);
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    }

There are also two conformsToProtocol: methods, one for instances and one for classes. These also just wrap a runtime function, which in this case just consults a table of every protocol that the class conforms to in order to see if the given protocol is present:

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    - (BOOL)conformsToProtocol: (Protocol *)aProtocol
2
    {
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        return class_conformsToProtocol(isa, aProtocol);
4
    }
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    + (BOOL)conformsToProtocol: (Protocol *)protocol
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    {
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        return class_conformsToProtocol(self, protocol);
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    }

Next is methodForSelector:, and its classy cousin instanceMethodForSelector:. These both call through to class_getMethodImplementation, which looks up the selector in the class’s method table and returns the corresponding IMP:

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    - (IMP)methodForSelector: (SEL)aSelector
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    {
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        return class_getMethodImplementation(isa, aSelector);
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    }
5

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    + (IMP)instanceMethodForSelector: (SEL)aSelector
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    {
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        return class_getMethodImplementation(self, aSelector);
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    }

An interesting aspect of these methods is that class_getMethodImplementation always returns an IMP, even for unknown selectors. When the class doesn’t actually implement a method, it returns a special forwarding IMP which wraps up the message arguments starts down the path to invoking forwardInvocation:.

The methodSignatureForSelector: method just wraps the equivalent class method:

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    - (NSMethodSignature *)methodSignatureForSelector: (SEL)aSelector
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    {
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        return [isa instanceMethodSignatureForSelector: aSelector];
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    }

The class method in turn wraps some runtime calls. It first fetches the Method for the given selector. If it can’t be found, then the class doesn’t implement that method, and this code returns nil. Otherwise, it extracts the C string representing the method’s types, and wraps the in an NSMethodSignature object:

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    + (NSMethodSignature *)instanceMethodSignatureForSelector: (SEL)aSelector
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    {
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        Method method = class_getInstanceMethod(self, aSelector);
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        if(!method)
5
            return nil;
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        const char *types = method_getTypeEncoding(method);
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        return [NSMethodSignature signatureWithObjCTypes: types];
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    }

Finally, there’s performSelector:, and the two withObject: variants that take arguments. These aren’t strictly introspection, but they fall in the same general category of wrapping lower-level runtime functionality. They simply retrieve the IMP for the given selector, cast it to the appropriate function pointer type, and call it:

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    - (id)performSelector: (SEL)aSelector
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    {
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        IMP imp = [self methodForSelector: aSelector];
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        return ((id (*)(id, SEL))imp)(self, aSelector);
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    }
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    - (id)performSelector: (SEL)aSelector withObject: (id)object
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    {
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        IMP imp = [self methodForSelector: aSelector];
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        return ((id (*)(id, SEL, id))imp)(self, aSelector, object);
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    }
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    - (id)performSelector: (SEL)aSelector withObject: (id)object1 withObject: (id)object2
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    {
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        IMP imp = [self methodForSelector: aSelector];
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        return ((id (*)(id, SEL, id, id))imp)(self, aSelector, object1, object2);
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    }

Default ImplementationsMAObject provides default implementations of a bunch of methods. We’ll start off with default implementations of isEqual: and hash, which just use the object’s pointer for identity purposes:

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    - (BOOL)isEqual: (id)object
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    {
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        return self == object;
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    }
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    - (NSUInteger)hash
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    {
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        return (NSUInteger)self;
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    }

Any subclasses with a more expansive notion of equality will have to override these methods, but any subclass where an object is only ever equal to itself can just use these implementations.

The description method is another handy one to have a default implementation. This implementation just generates a string of the form <MAObject: 0xdeadbeef>, containing the object’s class and pointer value.

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    - (NSString *)description
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    {
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        return [NSString stringWithFormat: @"<%@: %p>", [self class], self];
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    }

The standard for classes is to just return the class name from their own description, so there’s a class method as well that fetches that name from the runtime and returns it:

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    + (NSString *)description
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    {
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        return [NSString stringWithUTF8String: class_getName(self)];
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    }

doesNotRecognizeSelector: is a lesser-known utility method. It throws an exception to make it look like the object doesn’t actually respond to the given selector. This is useful for things like creating override points where subclasses have to implement a particular method:

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    - (void)subclassesMustOverride
2
    {
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        // pretend we don't actually implement this here
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        [self doesNotRecognizeSelector: _cmd];
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    }

The code is fairly simple. The only really tricky bit is formatting the method name. We want to display something like -[Class method], but class methods need a + at the front, as in +[Class classMethod]. To figure out which context it’s in, the code checks to see whether isa is a metaclass. If it is, then self is a class, and the + variant should be used. Otherwise, self is an instance, and the - variant is used. The rest of the code just raises the appropriate NSException:

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    - (void)doesNotRecognizeSelector: (SEL)aSelector
2
    {
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        char *methodTypeString = class_isMetaClass(isa) ? "+" : "-";
4
        [NSException raise: NSInvalidArgumentException format: @"%s[%@ %@]: unrecognized selector sent to instance %p", methodTypeString, [[self class] description], NSStringFromSelector(aSelector), self];
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    }

Finally, there are a bunch of little methods that either provide obvious answers to obvious questions (e.g. the self method), exist to let subclasses always safely call super (e.g. the empty +initialize method), or exist as override points (e.g. the copy implementation that throws an exception). None of these are particularly interesting, but I include them for completeness:

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    - (id)self
2
    {
3
        return self;
4
    }
5

6
    - (BOOL)isProxy
7
    {
8
        return NO;
9
    }
10

11
    + (void)load
12
    {
13
    }
14

15
    + (void)initialize
16
    {
17
    }
18

19
    - (id)copy
20
    {
21
        [self doesNotRecognizeSelector: _cmd];
22
        return nil;
23
    }
24

25
    - (id)mutableCopy
26
    {
27
        [self doesNotRecognizeSelector: _cmd];
28
        return nil;
29
    }
30

31
    - (id)forwardingTargetForSelector: (SEL)aSelector
32
    {
33
        return nil;
34
    }
35

36
    - (void)forwardInvocation: (NSInvocation *)anInvocation
37
    {
38
        [self doesNotRecognizeSelector: [anInvocation selector]];
39
    }
40

41
    + (BOOL)resolveClassMethod:(SEL)sel
42
    {
43
        return NO;
44
    }
45

46
    + (BOOL)resolveInstanceMethod:(SEL)sel
47
    {
48
        return NO;
49
    }

ConclusionNSObject is a big bundle of different functionality, but nothing too strange. Its main function is to handle memory allocation and management so that you can actually create objects. It also provides a bunch of handy override points for methods that every object is expected to support, and wraps a bunch of runtime functions in a nicer API.

I’ve skipped over a big piece of functionality provided by NSObject: key-value coding. This is complex enough that it deserves its own article, so I will come back to that another time.

That’s it for today. Friday Q&A is driven by reader ideas, in case you somehow didn’t already know, so please send in your topic suggestions. Until next time, don’t code anything I wouldn’t code.