viewwillappear 之后访问 以autosizeing为 基准的 view 的frame,
UIImageView *mImageView = [[UIImageView alloc]init];
[mImageView setImage:[[UIImageimageNamed:@"activities_input_bg.png"] stretchableImageWithLeftCapWidth:10topCapHeight:10]];
mImageView.frame = CGRectMake(0, 0, self.mScrollView.frame.size.width, self.mScrollView.contentSize.height);
[self.mScrollViewinsertSubview:mImageView belowSubview:picView];