由于公司项目，需要一个透明的tabbar,结果发现sb 中无法设置。于是在代码中实现
用到几个关键的属性
<pre><code>
//设置uiimage

1. image.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
   //设置背景

2. tabBar.backgroundImage
   //去掉横线

3. tabBar.shadowImage
   // 获取每个item
   4.var items:[UITabBarItem] = self.tabBar.items as! [UITabBarItem]
   //设置item 的图片，以及选中效果图片
   items[0].image
   items[0].selectedImage
   </code></pre>
   在viewDidLoad()中加入以下代码
   <pre><code>
   var image :UIImage=UIImage(named: "tabbarbg.png")! self.tabBar.backgroundImage=image.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
   self.tabBar.shadowImage=image.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
   self.tabBar.frame=CGRectMake(0, UIScreen.mainScreen().bounds.height-70,
   UIScreen.mainScreen().bounds.width, 84);

    var items:[UITabBarItem] = self.tabBar.items as! [UITabBarItem]
    var image1 :UIImage=UIImage(named: "commen_home_false")!
    items[0].image=image1.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    var select_image1 :UIImage=UIImage(named: "commen_home")!
    items[0].selectedImage=select_image1.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    var image2 :UIImage=UIImage(named: "commen_nearby_false")!
    items[1].image=image2.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    var select_image2 :UIImage=UIImage(named: "commen_nearby")!
    items[1].selectedImage=select_image2.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    
    var image3 :UIImage=UIImage(named: "commen_calendar_false")!
    items[2].image=image3.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    var select_image3 :UIImage=UIImage(named: "commen_calendar")!
    items[2].selectedImage=select_image3.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
    var image4 :UIImage=UIImage(named: "commen_calendar")!
    items[3].image=image4.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)
   

</code></pre>

实现效果

tab.png