目录
一，基本概念
二，优缺点
三，如何实现
四，封装成宏
五，实现可继承
一，基本概念
保证一个类全局只有一个实例，并且提供一个统一的入口访问该实例
二，优缺点
1，优点：全局只创建一个实例，节省内存资源
2，缺点：不能继承（下面会介绍如何实现）
三，如何实现
-
第一步：实现shared方法来创建实例
方法一：线程不安全，多线程情况下可能会创建多个实例
static id _instance = nil;
+ (instancetype)sharedInstance {
if(!_instance) {
_instance = [[self alloc] init];
}
return _instance;
}
方法二：加锁可以保证线程安全，但是会消耗性能
static id _instance = nil;
+ (instancetype)sharedInstance {
@synchronized(self) { // 加锁
if(!_instance) {
_instance = [[self alloc] init];
}
}
return _instance;
}
方法三：GCD是线程安全的，也能保证性能(官方推荐)
static id _instance = nil;
+ (instancetype)sharedInstance {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [[self alloc] init];
});
return _instance;
}
-
第二步：避免外部直接调用alloc/new/copy/mutableCopy来创建实例
方法一：重写alloc/copy/mutableCopy方法
+ (instancetype)allocWithZone:(struct _NSZone *)zone {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [super allocWithZone:zone];
});
return _instance;
}
- (id)copyWithZone:(NSZone *)zone {
return _instance;
}
- (id)mutableCopyWithZone:(NSZone *)zone {
return _instance;
}
方法二：禁用init/new/copy/mutableCopy方法
- (instancetype)init OBJC_UNAVAILABLE("use sharedInstance instead");
+ (instancetype)new OBJC_UNAVAILABLE("use sharedInstance instead");
- (instancetype)copy OBJC_UNAVAILABLE("use sharedInstance instead");
- (instancetype)mutableCopy OBJC_UNAVAILABLE("use sharedInstance instead");
四，封装成宏
作用：避免每个类都写一遍实现代码
1，重写方法的宏
// .h文件
#define singletonH \
+ (instancetype)sharedInstance;
// .m文件
#define singletonM \
static id _instance = nil; \
+ (instancetype)sharedInstance { \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^{ \
_instance = [[self alloc] init]; \
}); \
return _instance; \
} \
+ (instancetype)allocWithZone:(struct _NSZone *)zone { \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^{ \
_instance = [super allocWithZone:zone]; \
}); \
return _instance; \
} \
- (id)copyWithZone:(NSZone *)zone { \
return _instance; \
} \
- (id)mutableCopyWithZone:(NSZone *)zone { \
return _instance; \
}
2，禁用方法的宏
// .h文件
#define singletonH \
+ (instancetype)sharedInstance; \
- (instancetype)init OBJC_UNAVAILABLE("use sharedInstance instead"); \
+ (instancetype)new OBJC_UNAVAILABLE("use sharedInstance instead"); \
- (instancetype)copy OBJC_UNAVAILABLE("use sharedInstance instead"); \
- (instancetype)mutableCopy OBJC_UNAVAILABLE("use sharedInstance instead");
// .m文件
#define singletonM \
static id _instance = nil; \
+ (instancetype)sharedInstance { \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^{ \
_instance = [[self alloc] init]; \
}); \
return _instance; \
}
3，如何使用
// .h文件
@interface HttpManager : NSObject
singletonH
@end
// .m文件
@implementation HttpManager
singletonM
@end
五，实现可继承
1，为什么不能继承：子类和父类共用一个实例，导致子类对象无法访问子类的属性和方法
// Person实现了单例，Man继承自Person，在Man中定义run方法
Person *p1 = [Person sharedInstance];
Person *p2 = [[Person alloc] init];
NSLog(@"p1: %@", p1);
NSLog(@"p2: %@", p2);
Man *m1 = [Man sharedInstance];
Man *m2 = [[Man alloc] init];
NSLog(@"m1: %@", m1);
NSLog(@"m2: %@", m2);
[m1 run];
// 打印结果
p1: <Person: 0x600000b95650>
p2: <Person: 0x600000b95650>
m1: <Person: 0x600000b95650>
m2: <Person: 0x600000b95650>
-[Person run]: unrecognized selector sent to instance 0x600000b95650
2，如何实现继承：使用runtime的关联函数
// Person.m的实现代码
+ (instancetype)sharedInstance {
id _instance = objc_getAssociatedObject(self, @"instance");
// 每个类都需要创建一次，所以不能用GCD
@synchronized(self) {
if(!_instance) {
_instance = [[self alloc] init];
objc_setAssociatedObject(self, @"instance", _instance, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
}
return _instance;
}
+ (instancetype)allocWithZone:(struct _NSZone *)zone {
id _instance = objc_getAssociatedObject(self, @"instance");
@synchronized(self) {
if(!_instance) {
_instance = [super allocWithZone:zone];
objc_setAssociatedObject(self, @"instance", _instance, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
}
return _instance;
}
- (id)copyWithZone:(NSZone *)zone {
return objc_getAssociatedObject(self, @"instance");
}
- (id)mutableCopyWithZone:(NSZone *)zone {
return objc_getAssociatedObject(self, @"instance");
}
// 打印结果
p1: <Person: 0x600000eeb4a0>
p2: <Person: 0x600000eeb4a0>
m1: <Man: 0x600000ef0ce0>
m2: <Man: 0x600000ef0ce0>
man is running
