OC擁有很多的動態(tài)特性,這些動態(tài)特性在運行程序時發(fā)揮作用,而不是在編譯或者鏈接代碼時發(fā)揮作用.這一特性,也為OC提供了很多強大的功能和靈活性. 比如可以以實時的方式進行程序的開發(fā)和更新,而不需要重新編譯或者重新部署.
運行時系統(tǒng)的基本應(yīng)用
對象消息
在面向?qū)ο笾心?消息傳遞是指在對象之間發(fā)送和接收消息. 在OC中的對象消息傳遞用于類和對象方法的調(diào)用.
[接收器 方法選擇器:(參數(shù))];
接收器就是class或者obj. 消息就是由 selecter 和 parameter構(gòu)成. 對象消息的傳遞是以動態(tài)的方式實現(xiàn)的. 接收器的相應(yīng)類型和相應(yīng)的調(diào)用方法,是在運行時決定的(所以會存在一些與動態(tài)綁定有關(guān)的額外開銷,當(dāng)然oc有緩存機制來減少這種開銷).
所以O(shè)C對象消息傳遞的關(guān)鍵要素有:
.接收器:類,對象
.消息:選擇器+參數(shù)
. 方法:聲明方法的實現(xiàn),參數(shù),返回值等等....
. 方法綁定. 向接收器發(fā)送消息的時候,尋找并且執(zhí)行方法的過程
選擇器
在消息傳遞的過程中,選擇器是一個文本字符串.
` -(void)addValue1:(NSInteger)value1 value2:(NSInteger)value2{...}`
`addValue1:value2:`
選擇器是一個特殊的類型.
SEL類型 是用于在編寫源代碼時替換選擇器值的標(biāo)識符.
- (void)viewDidLoad {
[super viewDidLoad];
// SEL addValue1 = NSSelectorFromString(@"addvalue1:value2:");
SEL addValue1 = @selector(addvalue1:value2:);
//[self addvalue1:@"a" value2:@"b"];
[self performSelector:addValue1 withObject:@"a" withObject:@"b"];
}
-(void)addvalue1:(NSString *)value1 value2:(NSString *)value2
{
NSLog(@"%@",NSStringFromSelector(_cmd));
//addvalue1:value2:
}
動態(tài)類型 和 對象內(nèi)省
運行時系統(tǒng)通過動態(tài)類型(dynamic typing)功能可以在運行程序時,決定對象的類型.
id類型是一個OC所特有的類型,它的變量可以存儲任意類型的對象.
id list = [NSArray new];
list = @"I am a sting";
if([list isKindOfClass:[NSArray class]]){}
if([list isKindOfClass:[NSString class]]){}
好處就是可以簡化接口,提供非常大的靈活性.
動態(tài)綁定(dynamic binding)
是指在運行程序時(而不是編譯)將消息與方法對應(yīng)起來的處理過程.在運行程序和發(fā)送消息前,消息和接收消息的對象不會對應(yīng). 因為許多的接收器可能實現(xiàn)了相同的方法,調(diào)用方式會動態(tài)變化.動態(tài)綁定實現(xiàn)了 OOP的多態(tài)性.
id person = [[People alloc]init];
[person run];
因為person的類型為 People,運行時系統(tǒng)會搜尋 People的run方法. 如果沒有找到,就去父類中尋找對相應(yīng)方法.直到找到為止.
OC運行時系統(tǒng) 確定消息接收器的類型(動態(tài)類型)-> 然后確定實現(xiàn)的方法(動態(tài)綁定)->調(diào)用方法.
動態(tài)方法決議
動態(tài)方法決議能夠以動態(tài)的方法實現(xiàn)方法. 通過
+(BOOL)resolveClassMethod:(SEL)sel
+(BOOL)resolveInstanceMethod:(SEL)sel
Description: Dynamically provides an implementation for a given selector for an instance method.
This method and resolveClassMethod: allow you to dynamically provide an implementation for a given selector.
An Objective-C method is simply a C function that take at least two arguments—self and _cmd. Using the class_addMethod function, you can add a function to a class as a method. Given the following function:
parameters: name The name of a selector to resolve.
return: YES if the method was found and added to the receiver, otherwise NO.
以動態(tài)的方式,添加類方法,或者實例方法.
void doLogin(){
NSLog(@"dologin");
}
@implementation ViewController
+(BOOL)resolveInstanceMethod:(SEL)sel
{
NSString *string = NSStringFromSelector(sel);
if ([string isEqualToString:@"doLogin"]) {
class_addMethod([self class], sel, (IMP)doLogin,"@@:@");
return YES;
}
return [super resolveInstanceMethod:sel];
}
- (void)viewDidLoad {
[super viewDidLoad];
[self performSelector:NSSelectorFromString(@"doLogin")];
}
/// A pointer to the function of a method implementation.
#if !OBJC_OLD_DISPATCH_PROTOTYPES
typedef void (*IMP)(void /* id, SEL, ... */ );
#else
typedef id (*IMP)(id, SEL, ...);
#endif
"@@:@" OC的類型編碼
-(id)forwardingTargetForSelector:(SEL)aSelector
Returns the object to which unrecognized messages should first be directed.
If an object implements (or inherits) this method, and returns a non-nil (and non-self) result, that returned object is used as the new receiver object and the message dispatch resumes to that new object.
NSInteger a = [self performSelector:NSSelectorFromString(@"length")];
}
-(id)forwardingTargetForSelector:(SEL)aSelector
{
if ([NSStringFromSelector(aSelector) isEqualToString:@"length"]) {
return @"沒有找到方法";
}
return 0;
}
運行時系統(tǒng)的結(jié)構(gòu)
介紹運行時系統(tǒng)的結(jié)構(gòu)以及它實現(xiàn)這些動態(tài)特性的方式.
運行時系統(tǒng)的組成部分
OC的運行時系統(tǒng)由兩個部分組成:編譯器 和 運行時系統(tǒng)庫
編譯器
在編譯階段,編譯器對源文件進行一系列的處理(預(yù)處理,詞法分析,語法分析,鏈接...)等.生成構(gòu)成可執(zhí)行程序的二進制文件. 運行時系統(tǒng)會為OC的面向?qū)ο筇卣魈峁?biāo)準(zhǔn)的API和實現(xiàn)代碼. OC的面向?qū)ο笤睾蛣討B(tài)特性,都是由運行時系統(tǒng)提供的.
運行時系統(tǒng)由這幾部分組成:
類元素:接口,實現(xiàn)代碼,協(xié)議,分類,方法,屬性,變量等等
類實例: 對象
對象消息的傳遞: 動態(tài)類型,動態(tài)綁定
動態(tài)方法決議
動態(tài)加載
對象內(nèi)省
當(dāng)編譯器解析使用了這些元素和特性的OC源碼時,它就會使用適當(dāng)?shù)倪\行時系統(tǒng)庫數(shù)據(jù)結(jié)構(gòu)和函數(shù),生成可執(zhí)行代碼.例子:
- 生成對象消息傳遞代碼
[接收器 消息];
它會生成調(diào)用運行時系統(tǒng)庫中函數(shù) objc_msgSend()的代碼.將源碼轉(zhuǎn)化為objc_msgSend(..)的代碼,每條消息都是以動態(tài)的方式處理的,這就意味著接收器的類型和方法的實際實現(xiàn),都是在運行程序的時候決定的.
OBJC_EXPORT id objc_msgSend(id self, SEL op, ...)
OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0);
- 生成類和代碼
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class isa OBJC_ISA_AVAILABILITY;
};
Class的數(shù)據(jù)類型就是一個 objc_class 結(jié)構(gòu)體指針
struct objc_class {
Class isa OBJC_ISA_AVAILABILITY;
#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
} OBJC2_UNAVAILABLE;
運行時庫提供接口,訪問這些信息:
objc_getClass(const char *name); // 對象的類定義
objc_getMetaClass(const char *name); // 對象的元類定義
class_getSuperclass(__unsafe_unretained Class cls); // 類的父類
class_copyMethodList(__unsafe_unretained Class cls, unsigned int *outCount); // 類的方法列表
class_copyIvarList(__unsafe_unretained Class cls, unsigned int *outCount)//實例變量列表
class_copyPropertyList(__unsafe_unretained Class cls, unsigned int *outCount);// 屬性列表
id 類型的定義
typedef struct objc_object *id;
運行時系統(tǒng)庫
運行時系統(tǒng)庫提供的有公開API,這些API使用C語言編寫.例如:
- 使用運行時系統(tǒng)創(chuàng)建類.
NSString* hello(){
return @"hello world";
}
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// 創(chuàng)建一個類
Class dynaClass = objc_allocateClassPair([NSObject class], "DynaClass", 0);
// 給類添加一個方法
class_addMethod(dynaClass, @selector(firstMethod), (IMP)hello, "@@:@");
objc_registerClassPair(dynaClass);
id dynaObj = [[dynaClass alloc]init];
id result = [dynaObj performSelector:@selector(firstMethod)];
NSLog(@"%@",result);
}
2.實現(xiàn)運行時系統(tǒng)的對象消息傳遞
運行時系統(tǒng)定義了一種方法數(shù)據(jù)類型(objc_method)
struct objc_method {
SEL method_name // SEL類型的變量
char *method_types
IMP method_imp // IMP的變量 方法的函數(shù)地址
}
typedef struct objc_method *Method;
執(zhí)行方法的邏輯:
1.通過對象的isa指針,找到對象所屬的類
2.通過搜索類的方法緩存,查找方法的IMP指針.如果找到了,就去調(diào)用函數(shù).
- 類的方法列表
- 一系列的動態(tài)方法決議方法.
- 父類的方法列表
與運行時系統(tǒng)的交互
- 運行時系統(tǒng)api. C風(fēng)格的.
- NSObject提供了一套api.其他類繼承NSObject.
isKindOfClass..
performerSelect...
運行時系統(tǒng)的應(yīng)用
AFN里的方法混淆
+ (void)swizzleResumeAndSuspendMethodForClass:(Class)theClass {
Method afResumeMethod = class_getInstanceMethod(self, @selector(af_resume));
Method afSuspendMethod = class_getInstanceMethod(self, @selector(af_suspend));
if (af_addMethod(theClass, @selector(af_resume), afResumeMethod)) {
af_swizzleSelector(theClass, @selector(resume), @selector(af_resume));
}
if (af_addMethod(theClass, @selector(af_suspend), afSuspendMethod)) {
af_swizzleSelector(theClass, @selector(suspend), @selector(af_suspend));
}
}
// AFN
static inline void af_swizzleSelector(Class theClass, SEL originalSelector, SEL swizzledSelector) {
Method originalMethod = class_getInstanceMethod(theClass, originalSelector);
Method swizzledMethod = class_getInstanceMethod(theClass, swizzledSelector);
method_exchangeImplementations(originalMethod, swizzledMethod);
}
- (void)af_suspend {
NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
NSURLSessionTaskState state = [self state];
[self af_suspend]; // 方法交換后 [self suspend]; 掛起,結(jié)束
if (state != NSURLSessionTaskStateSuspended) {
// dataTask suspend 后發(fā)送通知
[[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidSuspendNotification object:self];
}
}
//
* @note This is an atomic version of the following:
* \code
* IMP imp1 = method_getImplementation(m1);
* IMP imp2 = method_getImplementation(m2);
* method_setImplementation(m1, imp2);
* method_setImplementation(m2, imp1);
* \endcode
*/
OBJC_EXPORT void method_exchangeImplementations(Method m1, Method m2)
OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0);
YYModel
// 屬性列表
objc_property_t *properties = class_copyPropertyList(cls, &propertyCount);
if (properties) {
NSMutableDictionary *propertyInfos = [NSMutableDictionary new];
_propertyInfos = propertyInfos;
for (unsigned int i = 0; i < propertyCount; i++) {
YYClassPropertyInfo *info = [[YYClassPropertyInfo alloc] initWithProperty:properties[i]];
if (info.name) propertyInfos[info.name] = info;
}
free(properties);
}
- (instancetype)initWithProperty:(objc_property_t)property {
if (!property) return nil;
self = [super init];
_property = property;
const char *name = property_getName(property);
if (name) {
_name = [NSString stringWithUTF8String:name];
}
YYEncodingType type = 0;
unsigned int attrCount;
objc_property_attribute_t *attrs = property_copyAttributeList(property, &attrCount);
for (unsigned int i = 0; i < attrCount; i++) {
switch (attrs[i].name[0]) {
case 'T': { // Type encoding
if (attrs[i].value) {
...
case 'v': return YYEncodingTypeVoid | qualifier;
case 'B': return YYEncodingTypeBool | qualifier;
case 'c': return YYEncodingTypeInt8 | qualifier;
case 'C': return YYEncodingTypeUInt8 | qualifier;
case 's': return YYEncodingTypeInt16 | qualifier;
case 'S': return YYEncodingTypeUInt16 | qualifier;
case 'i': return YYEncodingTypeInt32 | qualifier;
case 'I': return YYEncodingTypeUInt32 | qualifier;
case 'l': return YYEncodingTypeInt32 | qualifier;
case 'L': return YYEncodingTypeUInt32 | qualifier;
case 'q': return YYEncodingTypeInt64 | qualifier;
case 'Q': return YYEncodingTypeUInt64 | qualifier;
case 'f': return YYEncodingTypeFloat | qualifier;
case 'd': return YYEncodingTypeDouble | qualifier;
case 'D': return YYEncodingTypeLongDouble | qualifier;
}
....
case 'R': {
type |= YYEncodingTypePropertyReadonly;
} break;
case 'C': {
type |= YYEncodingTypePropertyCopy;
} break;
case '&': {
type |= YYEncodingTypePropertyRetain;
} break;
case 'N': {
type |= YYEncodingTypePropertyNonatomic;
} break;
case 'D': {
type |= YYEncodingTypePropertyDynamic;
} break;
-----------------------完----------------------
