从Images.xcassets获取默认启动图LaunchImage

有很多童鞋在开发中，需要获取默认启动图，但是又不想多添加一次，使程序包增大，而以前的获取办法又不管用，到底怎样才能获取到呢？

self.startImageView = [[UIImageView alloc] initWithFrame:self.window.bounds];
f (iPhone5) {
    self.startImageView.image = [UIImage imageNamed:@"LaunchImage-700-568h"];
}else if (iPhone6) {
    self.startImageView.image = [UIImage imageNamed:@"LaunchImage-800-667h"];
}else if (iPhone6Plus) {
    self.startImageView.image = [UIImage imageNamed:@"LaunchImage-800-Portrait-736h"];
}else {
    self.startImageView.image = [UIImage imageNamed:@"LaunchImage-700"];
}
[self.window addSubview:self.startImageView];

如果你程序兼容7.0以下，那么你就需要判断版本，因为在ios6上获取启动图是@“LaunchImage”