图片拉伸

[backImage resizableImageWithCapInsets:UIEdgeInsetsMake(5, 5, 5, 5) resizingMode:UIImageResizingModeStretch]

主要是想用图层设置，想知道图层怎样控制拉伸
UIImage *backImage =[UIImage imageNamed:@"iamge"];
View.layer.contents=(id)backImage.CGImage;
View.layer.backgroundColor = [UIColor clearColor].CGColor;