常规写法
#import <Foundation/Foundation.h>
@interface JeySingletonTest : NSObject
+ (instancetype) shareInstance;
@end
#import "JeySingletonTest.h"
@implementation JeySingletonTest
static JeySingletonTest *_instance = nil;
+ (instancetype) shareInstance
{
static dispatch_once_t onceToken ;
dispatch_once(&onceToken, ^{
_instance = [[self alloc] init] ;
}) ;
return _instance ;
}
@end
JeySingletonTest* test_obj1 = [JeySingletonTest shareInstance] ;
NSLog(@"test_obj1 = %@", test_obj1) ;
JeySingletonTest* test_obj2 = [JeySingletonTest shareInstance] ;
NSLog(@"test_obj2 = %@", test_obj2) ;
JeySingletonTest* test_obj3 = [[JeySingletonTest alloc] init] ;
NSLog(@"test_obj3 = %@", test_obj3) ;
输出：
2018-12-02 16:02:51.996393+0800 001--Block[1351:40278] test_obj1 = <JeySingletonTest: 0x60000001a400>
2018-12-02 16:02:51.996561+0800 001--Block[1351:40278] test_obj2 = <JeySingletonTest: 0x60000001a400>
2018-12-02 16:02:51.996698+0800 001--Block[1351:40278] test_obj3 = <JeySingletonTest: 0x60400001a800>
看到init出来的对象内存地址不一样了
安全写法
创建对象的步骤分为申请内存(alloc)、初始化(init)这两个步骤，我们要确保对象的唯一性，因此在第一步这个阶段我们就要拦截它。当我们调用alloc方法时，oc内部会调用allocWithZone这个方法来申请内存，我们覆写这个方法，然后在这个方法中调用shareInstance方法返回单例对象，这样就可以达到我们的目的。拷贝对象也是同样的原理，覆写copyWithZone方法，然后在这个方法中调用shareInstance方法返回单例对象。
#import <Foundation/Foundation.h>
@interface JeySingleton : NSObject
+ (instancetype) shareInstance;
@end
#import "JeySingleton.h"
@implementation JeySingleton
static JeySingleton* _instance = nil;
+ (instancetype) shareInstance
{
static dispatch_once_t onceToken ;
dispatch_once(&onceToken, ^{
_instance = [[super allocWithZone:NULL] init] ;
}) ;
return _instance ;
}
+ (id) allocWithZone:(struct _NSZone *)zone
{
return [JeySingleton shareInstance] ;
}
- (id) copyWithZone:(struct _NSZone *)zone
{
return [JeySingleton shareInstance] ;
}
@end
JeySingleton* obj1 = [JeySingleton shareInstance] ;
NSLog(@"obj1 = %@", obj1) ;
JeySingleton* obj2 = [JeySingleton shareInstance] ;
NSLog(@"obj2 = %@", obj2) ;
JeySingleton* obj3 = [[JeySingleton alloc] init] ;
NSLog(@"obj3 = %@", obj3) ;
输出：
2018-12-02 16:02:51.995880+0800 001--Block[1351:40278] obj1 = <JeySingleton: 0x60400001a820>
2018-12-02 16:02:51.996067+0800 001--Block[1351:40278] obj2 = <JeySingleton: 0x60400001a820>
2018-12-02 16:02:51.996211+0800 001--Block[1351:40278] obj3 = <JeySingleton: 0x60400001a820>
init出来的对象还是一样的内存地址
