GCD全解-06-dispatch_once

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单次执行一个任务，此方法中的任务只会执行一次，重复调用也没办法重复执行（单例模式中常用此方法）

static AutoPayManager *manager;
@implementation AutoPayManager
+ (instancetype)defaultPayManager {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        manager = [[self alloc] init];
    });
    return manager;
}
+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    if (!manager) {
        manager = [super allocWithZone:zone];
    }
    return manager;
}

dispatch_once(&onceToken, ^{
    doSomething;
});