控制器view的背景图片的方法, 四种：
1.直接在控制器view上添加一个imageView大小设置的和view一样
UIImageView *beijingimage = [UIImageView alloc]initWithImage:[UIImage imageNamed:@"beijing"];
beijingimage.frame = self.view.frame;
[self.view addSubview:beijingimage];
2.重写控制器view, 直接drawRect画在上面
UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, NO, 0);
UIImage * image = [UIImage imageNamed:@"beijing"];
[image drawAsPatternInRect:self.view.bounds];
3.把image转换成UIColor对象, 再通过设置背景颜色设置
self.view.backgroundColor = [UIColor colorWithPatternImage:@"beijing"];
4.设置layer.contents, 直接设置图片内容
self.view.layer.contents =[UIImage imageNamed:@"beijing"];