iOS设置导航栏透明度

As I support Colin's answer, I want to give you an additional hint to customize the appearance of an UINavigationBar including the alpha.

The trick is to use UIAppearance for your NavigationBar. This enables you to assign an UIImage to your NavigationBar's backgroundImage. You can generate these UIImages programmatically and use for that UIColors and set the colors' alpha properties as you want. I've done this in one of my own applications and it works as expected.

Here I give you some code snippets:

1. E.g. in your ..AppDelegate.m add these lines in didFinishLaunchingWithOptions

2. //create background images for the navigation bar
   UIImage *gradientImage44 = nil; //replace "nil" with your method to programmatically create a UIImage object with transparent colors for portrait orientation
   UIImage *gradientImage32 = nil; //replace "nil" with your method to programmatically create a UIImage object with transparent colors for landscape orientation
   
   //customize the appearance of UINavigationBar
   [[UINavigationBar appearance] setBackgroundImage:gradientImage44 forBarMetrics:UIBarMetricsDefault];
   [[UINavigationBar appearance] setBackgroundImage:gradientImage32 forBarMetrics:UIBarMetricsLandscapePhone];
   [[UINavigationBar appearance] setBarStyle:UIBarStyleDefault];

   2.Implement convenience methods to programmatically creates UIImage objects, e.g. create a new category for UIImage:

3. //UIImage+initWithColor.h
   //
   #import <UIKit/UIKit.h>
   
   @interface UIImage (initWithColor)
   
   //programmatically create an UIImage with 1 pixel of a given color
   + (UIImage *)imageWithColor:(UIColor *)color;
   
   //implement additional methods here to create images with gradients etc.
   //[..]
   
   @end
   
   //UIImage+initWithColor.m
   //
   #import "UIImage+initWithColor.h"
   #import <QuartzCore/QuartzCore.h>
   
   @implementation UIImage (initWithColor)
   
   + (UIImage *)imageWithColor:(UIColor *)color
   {
       CGRect rect = CGRectMake(0, 0, 1, 1);
   
       // create a 1 by 1 pixel context 
       UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0);
       [color setFill];
       UIRectFill(rect);
   
       UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
       UIGraphicsEndImageContext();
   
       return image;
   }

   3.Re-work your image creation in 1. (#import "UIImage+initWithColor.h" in AppDelegate.m and replace the "nil"s):

4. UIImage *gradientImage44 = [UIImage imageWithColor:[UIColor colorWithRed:1.0 green:0.0 blue:1.0 alpha:0.2]];
               UIImage *gradientImage32 = [UIImage imageWithColor:[UIColor colorWithRed:1.0 green:0.0 blue:1.0 alpha:0.2]];

   // 用图片当作背景 设置透明度