This method will dispatch the `block` to self. If `self` is the main queue, and current thread is main thread, the block. will be invoked immediately instead of being dispatched.

safeAsync.swift

extension DispatchQueue {
    func safeAsync(_ block: @escaping ()->()) {
        if self === DispatchQueue.main && Thread.isMainThread {
            block()
        } else {
            async { block() }
        }
    }
}