IOS开发（64）之GCD任务最多只执行一次

1 前言

使用 dispatch_once 函数 在 app 的生命周期内来保证你想确保每段代码只执行一次,即使它在代码的不同地方多次调用(比如单例的初始化)。

 

2 代码实例
zyappdelegate.m

 

[plain]
/一个用于调度一次函数的标识 
static dispatch_once_t oncetoken; 
//block object 
void (^executedonlyonce)(void) = ^{ 
    static nsuinteger numberofentries = 0; 
    numberofentries++; 
    nslog(@"executed %lu time(s)", (unsigned long)numberofentries); 
}; 
- (bool)application:(uiapplication *)application didfinishlaunchingwithoptions:(nsdictionary *)launchoptions 
{ 
    //声明一个队列 
    dispatch_queue_t concurrentqueue = dispatch_get_global_queue(dispatch_queue_priority_default, 0); 
    //执行一次的队列 
    dispatch_once(&oncetoken, ^{ dispatch_async(concurrentqueue, 
                                                executedonlyonce); 
    }); 
    dispatch_once(&oncetoken, ^{ dispatch_async(concurrentqueue, 
                                                executedonlyonce); 
    }); 
    self.window = [[[uiwindow alloc] initwithframe:[[uiscreen mainscreen] bounds]] autorelease]; 
    // override point for customization after application launch. 
    self.viewcontroller = [[[zyviewcontroller alloc] initwithnibname:@"zyviewcontroller" bundle:nil] autorelease]; 
    self.window.rootviewcontroller = self.viewcontroller; 
    [self.window makekeyandvisible]; 
    return yes; 
} 

//一个用于调度一次函数的标识
static dispatch_once_t oncetoken;
//block object
void (^executedonlyonce)(void) = ^{
    static nsuinteger numberofentries = 0;
    numberofentries++;
    nslog(@"executed %lu time(s)", (unsigned long)numberofentries);
};
- (bool)application:(uiapplication *)application didfinishlaunchingwithoptions:(nsdictionary *)launchoptions
{
    //声明一个队列
    dispatch_queue_t concurrentqueue = dispatch_get_global_queue(dispatch_queue_priority_default, 0);
    //执行一次的队列
    dispatch_once(&oncetoken, ^{ dispatch_async(concurrentqueue,
                                                executedonlyonce);
    });
    dispatch_once(&oncetoken, ^{ dispatch_async(concurrentqueue,
                                                executedonlyonce);
    });
    self.window = [[[uiwindow alloc] initwithframe:[[uiscreen mainscreen] bounds]] autorelease];
    // override point for customization after application launch.
    self.viewcontroller = [[[zyviewcontroller alloc] initwithnibname:@"zyviewcontroller" bundle:nil] autorelease];
    self.window.rootviewcontroller = self.viewcontroller;
    [self.window makekeyandvisible];
    return yes;
}
zyviewcontroller.m

 

[plain]
- (void)viewdidload 
{ 
    [super viewdidload]; 
    zymysingleton *test = [[zymysingleton alloc] init]; 
    //循环单例方法 
    for (int i=0; i<5; i++) { 
        [test sharedinstance]; 
    } 
    [test release]; 
} 

- (void)viewdidload
{
    [super viewdidload];
    zymysingleton *test = [[zymysingleton alloc] init];
    //循环单例方法
    for (int i=0; i<5; i++) {
        [test sharedinstance];
    }
    [test release];
}
zymysingleton.m

 

[plain] view plaincopyprint?- (id) sharedinstance{ 
    static zymysingleton *sharedinstance = nil; 
    //一个用于调度一次函数的标识 
    static dispatch_once_t oncetoken; 
    dispatch_once(&oncetoken, ^{ 
        sharedinstance = [zymysingleton new]; 
        nslog(@"sharedinstance is ======>%@",sharedinstance); 
    }); 
    return sharedinstance; 
} 

- (id) sharedinstance{
    static zymysingleton *sharedinstance = nil;
    //一个用于调度一次函数的标识
    static dispatch_once_t oncetoken;
    dispatch_once(&oncetoken, ^{
        sharedinstance = [zymysingleton new];
        nslog(@"sharedinstance is ======>%@",sharedinstance);
    });
    return sharedinstance;
}
运行后控制台显示结果

2013-05-10 17:30:49.334 gcdsingletest[2717:1303] executed 1 time(s)

2013-05-10 17:30:49.336 gcdsingletest[2717:c07] sharedinstance is ======><zymysingleton: 0x7195990>