Swift 里的 dispatch_once 替代方案

ObjC 里的 dispatch_once 使用起来很方便，但是 Swift 里废弃使用这个方法了，原因是鼓励大家使用 static let 。

Objective-C

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    YogaSwizzleInstanceMethod(self, @selector(initWithFrame:), @selector(_yoga_initWithFrame:));
    ogaSwizzleInstanceMethod(self, @selector(setFrame:), @selector(_yoga_setFrame:));
    YogaSwizzleInstanceMethod(self, @selector(setBounds:), @selector(_yoga_setBounds:));
#if TARGET_OS_OSX
    YogaSwizzleInstanceMethod(self, @selector(setFrameSize:), @selector(_yoga_setFrameSize:));
    YogaSwizzleInstanceMethod(self, @selector(setBoundsSize:), @selector(_yoga_setBoundsSize:));
#endif
});

Swift

struct once {
    static let token = { () -> Bool in
        YogaSwizzleInstanceMethod(UIView.self, #selector(UIView.init(frame:)), #selector(_swift_yoga_init(frame:)))
        YogaSwizzleInstanceMethod(UIView.self, #selector(setter: UIView.frame), #selector(_swift_yoga_set(frame:)))
        YogaSwizzleInstanceMethod(UIView.self, #selector(setter: UIView.bounds), #selector(_swift_yoga_set(bounds:)))
        #if os(macOS)
        YogaSwizzleInstanceMethod(NSView.self, #selector(NSView.setFrameSize(_:)), #selector(_swift_yoga_set(frameSize:)))
        YogaSwizzleInstanceMethod(NSView.self, #selector(NSView.setBoundsSize(_:)), #selector(_swift_yoga_set(boundsSize:)))
        #endif

        return true
    }()
}

_ = once.token

token 是什么类型都可以，只是需要有个返回值能让编译通过，返回值也是没有用的。