Objective-C:

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    // show loading
});

Swift 3:(注意 :block里面不可省略self)

DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 3) { 
    // show loading
}