iOS 单例线程安全问题

1. OC 中不允许编译时初始化变量, 静态变量和全局变量, 只能赋值常量, 所以只能通过 dispatch_once_t 来避免多线程多次初始化问题.

static NSString *a = @"asdf";
static int b = 10;
static NSString *c = [[NSString alloc] init]; /// Initializer element is not a compile-time constant

@implementation AAA
+ (id)shared{
    static AAA * staticInstance = nil;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        staticInstance = [[self alloc] init];
    });
    return staticInstance;
}
@end

2. ObjectiveC++ 允许编译时初始化变量, 所以不需要处理线程问题

@interface AA : NSObject

@end

@implementation AA

- (instancetype)init
{
    self = [super init];
    if (self) {
        NSLog(@"init");
    }
    return self;
}

@end

static AA *aa = [[AA alloc] init];

3. Swift 允许编译时初始化变量, 所以不需要处理线程问题

class MyClass {
    static let shared = MyClass()
    private init() {
        print("init")
        sleep(10)
    }
}

DispatchQueue.global().async {
    for _ in 0 ..< 100 {
        DispatchQueue.global().async {
            _ = MyClass.shared
            print(Thread.current)
        }
    }
}