單例模式在iOS開發(fā)中是最為常用的模式之一，在應(yīng)用這個(gè)模式時(shí)，單例對象的類必須保證只有一個(gè)實(shí)例存在。許多時(shí)候整個(gè)系統(tǒng)只需要擁有一個(gè)的全局對象，這樣有利于我們協(xié)調(diào)系統(tǒng)整體的行為。一般情況下，許多人都是按下面的方式寫單例模式：

#import "Singleton.h"

@implementation Singleton

static Singleton *shareSingleton = nil;

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

@end

測試一下這種方法可行不：

Singleton *one = [Singleton shareSingleton];
    NSLog(@"one = %@", one);

    Singleton *two = [Singleton shareSingleton];
    NSLog(@"two = %@", two);

    Singleton *three = [[Singleton alloc] init];
    NSLog(@"three = %@", three);

控制臺輸出內(nèi)容：
2015-12-22 15:31:38.230 Singleton[2264:1996979] one = <Singleton: 0x7fe598f4d3c0>
2015-12-22 15:31:38.230 Singleton[2264:1996979] two = <Singleton: 0x7fe598f4d3c0>
2015-12-22 15:31:38.231 Singleton[2264:1996979] three = <Singleton: 0x7fe598edc570>

由此可見，只有調(diào)用shareSingleton方法獲得的對象是相通的，用alloc init來構(gòu)造對象的時(shí)候，得到的確實(shí)不一樣的結(jié)果。究其原因，當(dāng)使用alloc init創(chuàng)建對象時(shí)，在調(diào)用alloc方法時(shí)，oc內(nèi)部會調(diào)用allocWithZone這個(gè)方法來申請內(nèi)存，為了避免allocWithZone申請新的內(nèi)存，可以重寫allocWithZone方法，在該方法中調(diào)用shareSingleton方法返回單例對象?？截悓ο笠彩峭瑯拥牡览?。

完整代碼如下：

#import "Singleton.h"

@implementation Singleton

static Singleton *shareSingleton = nil;

  (instancetype)shareSingleton {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        shareSingleton = [[super allocWithZone:NULL] init];
    });
    return shareSingleton;
}

  (instancetype)allocWithZone:(struct _NSZone *)zone {
    return [Singleton shareSingleton];
}

- (id)copyWithZone:(struct _NSZone *)zone {
    return [Singleton shareSingleton];
}

@end

測試：

Singleton *one = [Singleton shareSingleton];
    NSLog(@"one = %@", one);

    Singleton *two = [Singleton shareSingleton];
    NSLog(@"two = %@", two);

    Singleton *three = [[Singleton alloc] init];
    NSLog(@"three = %@", three);

    Singleton *four = [[Singleton alloc] init];
    NSLog(@"four = %@", [four copy]);

    Singleton *five = [Singleton new];
    NSLog(@"five = %@", five);

結(jié)果：
2015-12-22 17:01:52.350 Singleton[2399:2262945] one = <Singleton: 0x7f9e6be4ae30>
2015-12-22 17:01:52.350 Singleton[2399:2262945] two = <Singleton: 0x7f9e6be4ae30>
2015-12-22 17:01:52.351 Singleton[2399:2262945] three = <Singleton: 0x7f9e6be4ae30>
2015-12-22 17:01:52.351 Singleton[2399:2262945] four = <Singleton: 0x7f9e6be4ae30>
2015-12-22 17:01:52.351 Singleton[2399:2262945] five = <Singleton: 0x7f9e6be4ae30>

轉(zhuǎn)載于:https://my.oschina.net/zyboy/blog/617435