OC的运行时的总结：
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Objective-C 是一门面向对象的语言，通过一个运行时库<objc/runtime.h>给C语言赋予了类，对象，协议等面向对象的功能。在runtime.h中类和对象其实是对C的结构体的封装,OC的运行时库提供了一系列函数给我们来操作类（class），实例（Instance），实例变量（Ivars），实例方法（Method），这是开发者能够运用runtime机制的基础。
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Objective-C 是一门动态语言，所有的函数调用都是在运行时才确定相应的实现，而这一切都因为OC调用函数的机制是发消息(Message Dispatch),每一个类都维护一个方法列表(methodLists)，向类的实例发消息的时候，实例会通过isa指针去类的方法列表里通过SEL查找相应函数体的指针，从而调用相应的实现，而为了提高查询效率类还会维护函数调用的缓存。
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运行时系统赋予了OC高度的灵活性，但也是以牺牲一定的性能和安全性为代价的。
OC运行时实践：
本文的重点在于运行时特性在实际开发中的应用:
- 动态添加类或实例
- 关联对象的应用
- 动态方法处理：消息转发
- Method Swizzling
- 使用Block作为Method IMP
- KVC & KVO
1.动态添加类或实例
OBJC_EXPORT Class _Nullable
objc_allocateClassPair(Class _Nullable superclass, const char * _Nonnull name,
size_t extraBytes)
OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);//创建一个类或者源类
OBJC_EXPORT void
objc_registerClassPair(Class _Nonnull cls)
OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);//向运行时系统注册通过objc_allocateClassPair创建的类
OBJC_EXPORT void
objc_disposeClassPair(Class _Nonnull cls)
OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);//销毁一个类和它关联的源类
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
@interface Person : NSObject
- (id)initWithFirstName: (NSString *)firstName lastName: (NSString *)lastName age: (NSUInteger)age;
@property (readonly) NSString *firstName;
@property (readonly) NSString *lastName;
@property (readonly) NSUInteger age;
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
Class c = objc_allocateClassPair([NSObject class], "Person", 0);
class_addIvar(c, "firstName", sizeof(id), log2(sizeof(id)), @encode(id));
class_addIvar(c, "lastName", sizeof(id), log2(sizeof(id)), @encode(id));
class_addIvar(c, "age", sizeof(NSUInteger), log2(sizeof(NSUInteger)), @encode(NSUInteger));
Ivar firstNameIvar = class_getInstanceVariable(c, "firstName");
Ivar lastNameIvar = class_getInstanceVariable(c, "lastName");
ptrdiff_t ageOffset = ivar_getOffset(class_getInstanceVariable(c, "age"));
IMP initIMP = imp_implementationWithBlock(^(id self, NSString *firstName, NSString *lastName, NSUInteger age) {
object_setIvar(self, firstNameIvar, firstName);
object_setIvar(self, lastNameIvar, lastName);
char *agePtr = ((char *)(__bridge void *)self) + ageOffset;
memcpy(agePtr, &age, sizeof(age));
return self;
});
const char *initTypes = [[NSString stringWithFormat: @"%s%s%s%s%s%s", @encode(id), @encode(id), @encode(SEL), @encode(id), @encode(id), @encode(NSUInteger)] UTF8String];
class_addMethod(c, @selector(initWithFirstName:lastName:age:), initIMP, initTypes);
const char *objectGetterTypes = [[NSString stringWithFormat: @"%s%s%s", @encode(id), @encode(id), @encode(SEL)] UTF8String];
IMP descriptionIMP = imp_implementationWithBlock(^(id self) {
return [NSString stringWithFormat: @"<%@ %p: %@ %@ age %zd>", [self class], self, [self firstName], [self lastName], [self age]];
});
class_addMethod(c, @selector(description), descriptionIMP, objectGetterTypes);
IMP firstNameIMP = imp_implementationWithBlock(^(id self) {
return object_getIvar(self, firstNameIvar);
});
class_addMethod(c, @selector(firstName), firstNameIMP, objectGetterTypes);
IMP lastNameIMP = imp_implementationWithBlock(^(id self) {
return object_getIvar(self, lastNameIvar);
});
class_addMethod(c, @selector(lastName), lastNameIMP, objectGetterTypes);
IMP ageIMP = imp_implementationWithBlock(^(id self) {
char *agePtr = ((char *)(__bridge void *)self) + ageOffset;
NSUInteger age;
memcpy(&age, agePtr, sizeof(age));
return age;
});
const char *ageTypes = [[NSString stringWithFormat: @"%s%s%s", @encode(NSUInteger), @encode(id), @encode(SEL)] UTF8String];
class_addMethod(c, @selector(age), ageIMP, ageTypes);
objc_registerClassPair(c);
Class PersonC = NSClassFromString(@"Person");
Person *alex = [[PersonC alloc] initWithFirstName: @"Alex" lastName: @"Trebek" age: 29];
Person *sean = [[PersonC alloc] initWithFirstName: @"Sean" lastName: @"Connery" age: 42];
NSArray *people = @[ alex, sean ];
NSLog(@"%@", people);
}
return 0;
}
输出结果：
2016-06-12 10:44:22.778865+0800 AllocateClass[26079:26214721] (
"<Person 0x100638aa0: Alex Trebek age 29>",
"<Person 0x100638c20: Sean Connery age 42>"
)
Program ended with exit code: 0
2.关联对象的应用
我们可以再OC的分类里面添加方法，但是无法添加属性，这个时候Runtime就要发挥作用了。
runtime提供了三个函数来设置关联对象:
//设置关联对象
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
//获取关联对象
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
//移除关联对象
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);
//参数解释
id object 被关联的对象
const void *key 关联的key 必须唯一
id value 关联的对象
objc_AssociationPolicy policy 关联策略
//其中的关联策略就相当于我们的property中的copy assign之类的
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.
添加公共属性
#import "Dog.h"
@interface Dog (Husky)
@property (copy, nonatomic) NSString *liveArea;
@property (copy, nonatomic) NSString *weight;
@end
#import "Dog+Husky.h"
#import <objc/runtime.h>
char * const identifier = "dogTypeIdentifier";
@implementation Dog (Husky)
@dynamic weight;
- (NSString *)liveArea {
id liveArea = objc_getAssociatedObject(self, identifier);
return liveArea;
}
- (void)setLiveArea:(NSString *)liveArea {
objc_setAssociatedObject(self, identifier, liveArea, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
@end
这样，我们可以直接用点语法，对属性直接操作。
@synthesize 和 @dynamic的区别：
这两者都是clang指定的编译器指令，@synthesize用来告诉编译器给我自动生成属性的getter\setter方法，@dynamic则告诉编译器属性的getter\setter方法没有在类中实现，而是在程序的其他地方实现（例如父类中或者是在运行时提供）。
添加私有成员变量
这里有一个好用的案例是给按钮添加事件回调，而不走target-action机制。
#import <UIKit/UIKit.h>
@interface UIButton (EventHandler)
//传入点击事件的回调
- (instancetype)initWithFrame:(CGRect)frame callback:(void (^)(UIButton *button))callbackBlock;
@end
#import "UIButton+EventHandler.h"
#import <objc/runtime.h>
const char *callback_identifier = "BUTTON_CALLBACK";
@interface UIButton()
@property (nonatomic, copy) void (^callbackBlock)(UIButton * button);
@end
@implementation UIButton (EventHandler)
- (void (^)(UIButton *))callbackBlock {
return objc_getAssociatedObject(self, callback_identifier);
}
- (void)setCallbackBlock:(void (^)(UIButton *))callbackBlock {
objc_setAssociatedObject(self, callback_identifier, callbackBlock, OBJC_ASSOCIATION_COPY);
}
- (instancetype)initWithFrame:(CGRect)frame callback:(void (^)(UIButton *))callbackBlock {
if (self = [super initWithFrame:frame]) {
self.callbackBlock = callbackBlock;
[self addTarget:self action:@selector(didClickAction:) forControlEvents:UIControlEventTouchUpInside];
}
return self;
}
- (void)didClickAction:(UIButton *)button {
__weak typeof(self) weakSelf = self;//注意这里应该用weak指针，避免循环引用
weakSelf.callbackBlock(button);
}
@end
这样我们就可以在初始化button的时候就可以直接调用事件处理了。
self.btn = [[UIButton alloc] initWithFrame:CGRectMake(100, 100, 100, 50) callback:^(UIButton *button) {
NSLog(@"点击事件");
}];
ORM（Object Relational Mapping）对象模型关系映射
关联对象的另一个重要应用是解析json数据直接给模型赋值，一些第三方的json解析库（MJExtension等）就是运用了这个特性，下面是这种方式的一个简单实现：
- (instancetype)initWithDict:(NSDictionary *)dict {
if (self = [self init]) {
//(1)获取类的属性及属性对应的类型
NSMutableArray *keys = [NSMutableArray array];
NSMutableArray *attributes = [NSMutableArray array];
/*
* 例子
* name = value3 attribute = T@"NSString",C,N,V_value3
* name = value4 attribute = T^i,N,V_value4
*/
unsigned int outCount;
objc_property_t *properties = class_copyPropertyList([self class], &outCount);
for (int i = 0; i < outCount; i++) {
objc_property_t property = properties[i];
//通过property_getName函数获得属性的名字
NSString *propertyName = [NSString stringWithCString:property_getName(property) encoding:NSUTF8StringEncoding];
[keys addObject:propertyName];
//通过property_getAttributes函数可以获得属性的名字和@encode编码
NSString *propertyAttribute = [NSString stringWithCString:property_getAttributes(property) encoding:NSUTF8StringEncoding];
[attributes addObject:propertyAttribute];
}
//立即释放properties指向的内存
free(properties);
//(2)根据类型给属性赋值
for (NSString *key in keys) {
if ([dict valueForKey:key] == nil) continue;
[self setValue:[dict valueForKey:key] forKey:key];
}
}
return self;
}
同样的利用这个特性也能实现模型的本地化存储，用SQLite做本地化的时候，可以利用这个特性避免复杂的SQL的编写
var propertyNames: [String]? {
get {
var count: UInt32 = 0
guard let properties = class_copyPropertyList(self.classForCoder, &count) else { return nil }
var propertyNames: [String] = []
for i in 0..<count {
if let property: objc_property_t = properties[Int(i)] {
let name = String.init(cString: property_getName(property))
propertyNames += [name]
}
}
return propertyNames
}
}
var propertyAttributes: [String]? {
get {
var count: UInt32 = 0
guard let properties = class_copyPropertyList(self.classForCoder, &count) else { return nil }
var propertyAttributes: [String] = []
for i in 0..<count {
if let property: objc_property_t = properties[Int(i)] {
let attribute = String.init(cString: property_getAttributes(property))
propertyAttributes.append(attribute)
}
}
return propertyAttributes
}
}
这是两段Swift3的代码，用于从属性中获取相应的类型和属性标识，虽然Swift 3.x版本对于运行时的支持甚少，但是只要我们的类继承自NSObject就可以用到Faundation框架中的runtime的特性了。
访问私有变量
我们知道，如果成员变量放在了.m文件中，就成了私有变量，但是我们依然可以通过Runtime获取，这时候，我们就需要知道成员变量的名称了,OC没有绝对的私有变量和方法，方法当然也可以这样获取出来.
Ivar ivar = class_getInstanceVariable([Model class], "_str1");
NSString * str1 = object_getIvar(model, ivar);
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