iOS中的单例模式

ARC

懒汉模式

#import "Singleton.h"

@implementation Singleton
static id _instance;

/**
*  alloc方法内部会调用这个方法
*/
+ (instancetype)allocWithZone:(struct _NSZone *)zone{
if (_instance == nil) { // 防止频繁加锁
@synchronized(self) {
if (_instance == nil) { // 防止创建多次
_instance = [super allocWithZone:zone];
}
}
}
return _instance;
}

+ (instancetype)sharedSingleton{
if (_instance == nil) { // 防止频繁加锁
@synchronized(self) {
if (_instance == nil) { // 防止创建多次
_instance = [[self alloc] init];
}
}
}
return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
return _instance;
}
@end

饿汉模式(不常用)

#import "HMSingleton.h"

@implementation Singleton
static id _instance;

/**
*  当类加载到OC运行时环境中（内存），就会调用一次（一个类只会加载1次）
*/
+ (void)load{
_instance = [[self alloc] init];
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone{
if (_instance == nil) { // 防止创建多次
_instance = [super allocWithZone:zone];
}
return _instance;
}

+ (instancetype)sharedSingleton{
return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
return _instance;
}
@end

GCD实现单例模式

@implementation Singleton
static id _instance;

+ (instancetype)allocWithZone:(struct _NSZone *)zone{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [super allocWithZone:zone];
});
return _instance;
}

+ (instancetype)sharedSingleton{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_instance = [[self alloc] init];
});
return _instance;
}

- (id)copyWithZone:(NSZone *)zone{
return _instance;
}
@end