1、單例模式簡述

• 一個類有且僅有一個實例，并且自行實例化向整個系統(tǒng)提供。

2、單例模式UML類圖

• 單例模式

3、單例模式示意代碼

• SingletonA

/*********  SingletonA.h   **********/
@interface YSingletonA : NSObject
+ (instancetype)sharedInstance;
@end

/*********  SingletonA.m   **********/
@implementation YSingletonA

static YSingletonA *_instance = nil;
+ (instancetype)sharedInstance {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [[[self class] alloc] init];
    });
    return _instance;
}
@end

/*********  使用   **********/
YSingletonA *singleton1 = [YSingletonA sharedInstance];
NSLog(@"%@", singleton1);
YSingletonA *singleton2 = [YSingletonA sharedInstance];
NSLog(@"%@", singleton2);

YSingletonA *singleton3 = [[YSingletonA alloc] init];
NSLog(@"%@", singleton3);
YSingletonA *singleton4 = [[YSingletonA alloc] init];
NSLog(@"%@", singleton4);

YSingletonA *singleton5 = [YSingletonA new];
NSLog(@"%@", singleton5);
YSingletonA *singleton6 = [YSingletonA new];
NSLog(@"%@", singleton6);

/*
 運行結(jié)果：
 <YSingletonA: 0x60000000b550>
 <YSingletonA: 0x60000000b550>
 
 <YSingletonA: 0x60c00000bc30>
 <YSingletonA: 0x60400000b440>
 
 <YSingletonA: 0x60400000b450>
 <YSingletonA: 0x60c00000a9f0>

YSingletonA調(diào)用sharedInstance方法可以做到多次創(chuàng)建的實例是同一個
但是<無法做到>調(diào)用原有的init方法或者new方法創(chuàng)建的實例也是同一個
YSingletonA違背了單例類有且僅有一個實例的定義,或者說做的不夠完善
 */

• SingletonB

/*********  SingletonB.h   **********/
@interface YSingletonB : NSObject <NSCopying, NSMutableCopying>
+ (instancetype)sharedInstance;
@end

/*********  SingletonB.m   **********/
@implementation YSingletonB

static YSingletonB *_instance = nil;
+ (instancetype)sharedInstance {
    return [[self alloc] init];
}

+ (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;
}

@end

/*********  使用   **********/
YSingletonB *singleton1 = [YSingletonB sharedInstance];
NSLog(@"%@", singleton1);
YSingletonB *singleton2 = [YSingletonB sharedInstance];
NSLog(@"%@", singleton2);

YSingletonB *singleton3 = [[YSingletonB alloc] init];
NSLog(@"%@", singleton3);
YSingletonB *singleton4 = [[YSingletonB alloc] init];
NSLog(@"%@", singleton4);

YSingletonB *singleton5 = [YSingletonB new];
NSLog(@"%@", singleton5);
YSingletonB *singleton6 = [YSingletonB new];
NSLog(@"%@", singleton6);


YSingletonB *singleton7 = [singleton5 copy];
NSLog(@"%@", singleton7);
YSingletonB *singleton8 = [singleton5 mutableCopy];
NSLog(@"%@", singleton8);

/*
 <YSingletonB: 0x6080000069a0>
 <YSingletonB: 0x6080000069a0>
 
 <YSingletonB: 0x6080000069a0>
 <YSingletonB: 0x6080000069a0>
 
 <YSingletonB: 0x6080000069a0>
 <YSingletonB: 0x6080000069a0>
 
 <YSingletonB: 0x6080000069a0>
 <YSingletonB: 0x6080000069a0>

YSingletonB調(diào)用sharedInstance方法，alloc，init方法，new方法，甚至包括copy、mutableCopy方法我們得到的都是用一個實例
 */

• 完整Demo