Runtime浅谈

oc/swift是一种类面向对象语言
c是一种结构化的语言

1:为什么我们使用oc和c很容易融合一起编写代码？下面可以直观理解。
2:oc经过LLVM编译器编译会转换为c++代码，转换过程是怎样的？
可以通过下面思考下。
3:一切皆为对象，类也是对象（可理解为类是元类的对象）
4:那我们可以通过下面理解下objc_msgSend是如何实现的？
下章会为大家谈谈我的理解

Class 定义

typedef struct objc_class *Class;

Object定义

struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

id定义

typedef struct objc_object *id;

SEL定义

typedef struct objc_selector *SEL;

IMP定义

typedef void (*IMP)(void /* id, SEL, ... */ ); 
or
typedef id _Nullable (*IMP)(id _Nonnull, SEL _Nonnull, ...); 

objc_class结构体里内容相关

/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method;

/// An opaque type that represents an instance variable.
typedef struct objc_ivar *Ivar;

/// An opaque type that represents a category.
typedef struct objc_category *Category;

/// An opaque type that represents an Objective-C declared property.
typedef struct objc_property *objc_property_t;

objc_class (objc2)

struct objc_class {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;

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

} OBJC2_UNAVAILABLE;

Protocol

typedef struct objc_object Protocol;

method

/// Defines a method
struct objc_method_description {
    SEL _Nullable name;               /**< The name of the method */
    char * _Nullable types;           /**< The types of the method arguments */
};

property

/// Defines a property attribute
typedef struct {
    const char * _Nonnull name;           /**< The name of the attribute */
    const char * _Nonnull value;          /**< The value of the attribute (usually empty) */
} objc_property_attribute_t;

protocol_list

struct objc_protocol_list {
    struct objc_protocol_list * _Nullable next;
    long count;
    __unsafe_unretained Protocol * _Nullable list[1];
};

category

struct objc_category {
    char * _Nonnull category_name                            OBJC2_UNAVAILABLE;
    char * _Nonnull class_name                               OBJC2_UNAVAILABLE;
    struct objc_method_list * _Nullable instance_methods     OBJC2_UNAVAILABLE;
    struct objc_method_list * _Nullable class_methods        OBJC2_UNAVAILABLE;
    struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

method_description_list

struct objc_method_description_list {
    int count;
    struct objc_method_description list[1];
};

ivar

struct objc_ivar {
    char * _Nullable ivar_name                               OBJC2_UNAVAILABLE;
    char * _Nullable ivar_type                               OBJC2_UNAVAILABLE;
    int ivar_offset                                          OBJC2_UNAVAILABLE;
#ifdef __LP64__
    int space                                                OBJC2_UNAVAILABLE;
#endif
}                                                            OBJC2_UNAVAILABLE;

ivar_list

struct objc_ivar_list {
    int ivar_count                                           OBJC2_UNAVAILABLE;
#ifdef __LP64__
    int space                                                OBJC2_UNAVAILABLE;
#endif
    /* variable length structure */
    struct objc_ivar ivar_list[1]                            OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

method

struct objc_method {
    SEL _Nonnull method_name                                 OBJC2_UNAVAILABLE;
    char * _Nullable method_types                            OBJC2_UNAVAILABLE;
    IMP _Nonnull method_imp                                  OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

method_list

struct objc_method_list {
    struct objc_method_list * _Nullable obsolete             OBJC2_UNAVAILABLE;

    int method_count                                         OBJC2_UNAVAILABLE;
#ifdef __LP64__
    int space                                                OBJC2_UNAVAILABLE;
#endif
    /* variable length structure */
    struct objc_method method_list[1]                        OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

symtab 符号表

typedef struct objc_symtab *Symtab                           OBJC2_UNAVAILABLE;

struct objc_symtab {
    unsigned long sel_ref_cnt                                OBJC2_UNAVAILABLE;
    SEL _Nonnull * _Nullable refs                            OBJC2_UNAVAILABLE;
    unsigned short cls_def_cnt                               OBJC2_UNAVAILABLE;
    unsigned short cat_def_cnt                               OBJC2_UNAVAILABLE;
    void * _Nullable defs[1] /* variable size */             OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

cache

struct objc_cache {
    unsigned int mask /* total = mask + 1 */                 OBJC2_UNAVAILABLE;
    unsigned int occupied                                    OBJC2_UNAVAILABLE;
    Method _Nullable buckets[1]                              OBJC2_UNAVAILABLE;
};

module

typedef struct objc_module *Module                           OBJC2_UNAVAILABLE;

struct objc_module {
    unsigned long version                                    OBJC2_UNAVAILABLE;
    unsigned long size                                       OBJC2_UNAVAILABLE;
    const char * _Nullable name                              OBJC2_UNAVAILABLE;
    Symtab _Nullable symtab                                  OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;