防止先出现第一页再跳：
截屏，将截屏的image放置在window的最前边

UIGraphicsBeginImageContextWithOptions(view.bounds.size,YES,[UIScreen mainScreen].scale);
[view.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

UIWindow *window = [MainTabBarController shared].view.window;
if (window && [MainTabBarController shared].screenShotForLaunchImg) {
[window insertSubview:[MainTabBarController shared].screenShotForLaunchImg atIndex:window.subviews.count - 1];
}