理解Objective-C运行时

#include < stdio.h >
 
int main(int argc, const char **argv[])
{
        printf("Hello World!");
        return 0;
}

.text  .align 4,0x90  .globl _main _main: Leh_func_begin1:  pushq %rbp Llabel1:  movq %rsp, %rbp Llabel2:  subq $16, %rsp Llabel3:  movq %rsi, %rax  movl %edi, %ecx  movl %ecx, -8(%rbp)  movq %rax, -16(%rbp)  xorb %al, %al  leaq LC(%rip), %rcx  movq %rcx, %rdi  call _printf  movl $0, -4(%rbp)  movl -4(%rbp), %eax  addq $16, %rsp  popq %rbp  ret Leh_func_end1:  .cstring LC:  .asciz "Hello World!"

[self doSomethingWithVar:var1];

objc_msgSend(self,@selector(doSomethingWithVar:),var1);

-(NSString *)movieTitle
{
    return @"Futurama: Into the Wild Green Yonder";
}

typedef struct objc_selector  *SEL;

SEL aSel = @selector(movieTitle);

[target getMovieTitleForObject:obj];

typedef struct objc_class *Class;
typedef struct objc_object {
    Class isa;
} *id;

struct Block_literal_1 {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved; 
    void (*invoke)(void *, ...);
    struct Block_descriptor_1 {
         unsigned long int reserved; // NULL
         unsigned long int size;  // sizeof(struct Block_literal_1)
         // optional helper functions
         void (*copy_helper)(void *dst, void *src);
         void (*dispose_helper)(void *src); 
    } *descriptor;
    // imported variables
};

typedef id (*IMP)(id self,SEL _cmd,...);

@interface MyClass : NSObject {
    // vars
    NSInteger counter;
}
// methods
-(void)doFoo;
@end

#if !__OBJC2__
    Class super_class                                        OBJC2_UNAVAILABLE;
    const char *name                                         OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
    struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
    struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
#endif

译者注：Objective-C类体系结构图如下所示：

MyObject *object = [[MyObject alloc] init];

MyObject *obj = [[MyObject alloc] init];
 
@implementation MyObject
-(id)init 
{
    if(self = [super init])
    {
        [self setVarA:@”blah”];
    }
    return self;
}
@end

#import < Foundation/Foundation.h>
 
@interface MyObject : NSObject
{
    NSString *aString;
}
 
@property(retain) NSString *aString;
 
@end
 
@implementation MyObject
 
-(id)init
{
    if (self = [super init])
    {
      [self setAString:nil];
    }
    return self;
}
 
@synthesize aString;
 
@end

 
int main (int argc, const char * argv[]) 
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
 
    id obj1 = [NSMutableArray alloc];
    id obj2 = [[NSMutableArray alloc] init];
 
    id obj3 = [NSArray alloc];
    id obj4 = [[NSArray alloc] initWithObjects:@"Hello",nil];
      
    NSLog(@"obj1 class is %@",NSStringFromClass([obj1 class]));
    NSLog(@"obj2 class is %@",NSStringFromClass([obj2 class]));
  
    NSLog(@"obj3 class is %@",NSStringFromClass([obj3 class]));
    NSLog(@"obj4 class is %@",NSStringFromClass([obj4 class]));
  
    id obj5 = [MyObject alloc];
    id obj6 = [[MyObject alloc] init];
  
    NSLog(@"obj5 class is %@",NSStringFromClass([obj5 class]));
    NSLog(@"obj6 class is %@",NSStringFromClass([obj6 class]));
          
    [pool drain];
    return 0;
}

NSMutableArray
NSMutableArray 
NSArray
NSArray
MyObject
MyObject

obj1 class is __NSPlaceholderArray
obj2 class is NSCFArray
obj3 class is __NSPlaceholderArray
obj4 class is NSCFArray
obj5 class is MyObject
obj6 class is MyObject

2015-09-07 13:43:06.922 ObjMessage[5185:1441448] obj1 class is __NSPlaceholderArray
2015-09-07 13:43:11.201 ObjMessage[5185:1441448] obj2 class is __NSArrayM
2015-09-07 13:43:17.987 ObjMessage[5185:1441448] obj3 class is __NSPlaceholderArray
2015-09-07 13:43:18.503 ObjMessage[5185:1441448] obj4 class is __NSArrayI
2015-09-07 13:43:32.228 ObjMessage[5185:1441448] obj5 class is MyObject
2015-09-07 13:43:33.478 ObjMessage[5185:1441448] obj6 class is MyObject

[self printMessageWithString:@"Hello World!"];

objc_msgSend(self, @selector(printMessageWithString:), @"Hello World!");

-(int)doComputeWithNum:(int)aNum

int aClass_doComputeWithNum(aClass *self,SEL _cmd,int aNum)

//declare C function pointer
int (computeNum *)(id,SEL,int);
 
//methodForSelector is COCOA & not ObjC Runtime
//gets the same function pointer objc_msgSend gets
computeNum = (int (*)(id,SEL,int))[target methodForSelector:@selector(doComputeWithNum:)];
 
//execute the C function pointer returned by the runtime
computeNum(obj,@selector(doComputeWithNum:),aNum);

void fooMethod(id obj, SEL _cmd)
{
    NSLog(@"Doing Foo");
}

+(BOOL)resolveInstanceMethod:(SEL)aSEL
{
    if(aSEL == @selector(doFoo:))
    {
        class_addMethod([self class],aSEL,(IMP)fooMethod,"v@:");
        return YES;
    }
    return [super resolveInstanceMethod];
}

- (id)forwardingTargetForSelector:(SEL)aSelector
{
    if(aSelector == @selector(mysteriousMethod:))
    {
        return alternateObject;
    }
    return [super forwardingTargetForSelector:aSelector];
}

-(void)forwardInvocation:(NSInvocation *)invocation
{
    SEL invSEL = invocation.selector;
 
    if([altObject respondsToSelector:invSEL]) 
    {
        [invocation invokeWithTarget:altObject];
    } 
    else 
    {
        [self doesNotRecognizeSelector:invSEL];
    }
}

#import < Cocoa/Cocoa.h> //Cocoa
#include < objc/runtime.h> //objc runtime api’s
 
@interface NSView (CustomAdditions)
@property(retain) NSImage *customImage;
@end
 
@implementation NSView (CustomAdditions)
 
static char img_key; //has a unique address (identifier)
 
-(NSImage *)customImage
{
    return objc_getAssociatedObject(self,&img_key);
}
 
-(void)setCustomImage:(NSImage *)image
{
    objc_setAssociatedObject(self,&img_key,image,
                             OBJC_ASSOCIATION_RETAIN);
}
 
@end

/* Associative References */

/**
 * Policies related to associative references.
 * These are options to objc_setAssociatedObject()
 */
enum {
    OBJC_ASSOCIATION_ASSIGN = 0,           /**< Specifies a weak reference to the associated object. */
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_COPY_NONATOMIC = 3,   /**< Specifies that the associated object is copied. 
                                            *   The association is not made atomically. */
    OBJC_ASSOCIATION_RETAIN = 01401,       /**< Specifies a strong reference to the associated object.
                                            *   The association is made atomically. */
    OBJC_ASSOCIATION_COPY = 01403          /**< Specifies that the associated object is copied.
                                            *   The association is made atomically. */
};

/***********************************************************************
* vtable dispatch
* 
* Every class gets a vtable pointer. The vtable is an array of IMPs.
* The selectors represented in the vtable are the same for all classes
*   (i.e. no class has a bigger or smaller vtable).
* Each vtable index has an associated trampoline which dispatches to 
*   the IMP at that index for the receiver class's vtable (after 
*   checking for NULL). Dispatch fixup uses these trampolines instead 
*   of objc_msgSend.
* Fragility: The vtable size and list of selectors is chosen at launch 
*   time. No compiler-generated code depends on any particular vtable 
*   configuration, or even the use of vtable dispatch at all.
* Memory size: If a class's vtable is identical to its superclass's 
*   (i.e. the class overrides none of the vtable selectors), then 
*   the class points directly to its superclass's vtable. This means 
*   selectors to be included in the vtable should be chosen so they are 
*   (1) frequently called, but (2) not too frequently overridden. In 
*   particular, -dealloc is a bad choice.
* Forwarding: If a class doesn't implement some vtable selector, that 
*   selector's IMP is set to objc_msgSend in that class's vtable.
* +initialize: Each class keeps the default vtable (which always 
*   redirects to objc_msgSend) until its +initialize is completed.
*   Otherwise, the first message to a class could be a vtable dispatch, 
*   and the vtable trampoline doesn't include +initialize checking.
* Changes: Categories, addMethod, and setImplementation all force vtable 
*   reconstruction for the class and all of its subclasses, if the 
*   vtable selectors are affected.
**********************************************************************/

static const char * const defaultVtable[] = {
    "allocWithZone:", 
    "alloc", 
    "class", 
    "self", 
    "isKindOfClass:", 
    "respondsToSelector:", 
    "isFlipped", 
    "length", 
    "objectForKey:", 
    "count", 
    "objectAtIndex:", 
    "isEqualToString:", 
    "isEqual:", 
    "retain", 
    "release", 
    "autorelease", 
};
static const char * const defaultVtableGC[] = {
    "allocWithZone:", 
    "alloc", 
    "class", 
    "self", 
    "isKindOfClass:", 
    "respondsToSelector:", 
    "isFlipped", 
    "length", 
    "objectForKey:", 
    "count", 
    "objectAtIndex:", 
    "isEqualToString:", 
    "isEqual:", 
    "hash", 
    "addObject:", 
    "countByEnumeratingWithState:objects:count:", 
};