UIButton无法正确显示高亮背景图标，显示效果偏灰色

最近遇到一个奇怪的问题。把一个的黑色图片设置成UIButton的高亮背景图像后，UIButton确显示成把它显示成灰色的。经过一番调教，仍不得其解。后来被同事指点，才找出问题根源。现在记下来，以防忘记。

1、问题

　　//生成一张黑色图片，设置成按钮背景
    UIImage *backgroundImage = [UIImage imageWithPureColor:[UIColor blackColor]];

    [self.xibButton setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];

神奇之处就在于，最后显示效果如下：

2、调试

尝试手工创建一个button，然后设置背景色

    //手工创建一个按钮
    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(200, 28, 80, 80)];
    [button setTitle:@"code按钮" forState:UIControlStateNormal];
    [button setTitleColor:[UIColor colorWithRed:61.0/255 green:90.0/255 blue:249.0/255 alpha:1] forState:UIControlStateNormal];
    [self.view addSubview:button];

效果如下：

 

3、原因

UIButton有个属性 @property(nonatomic,readonly) UIButtonType buttonType;用来指示按钮样式。xib创建的默认样式为UIButtonTypeSystem，代码创建的样式为UIButtonTypeCustom。正式这个区别使得两种按钮的效果不同

4、验证

创建四个按钮。代码、xib与system、custom两两组合，效果如下：

代码如下：

- (void)viewDidLoad {
    [super viewDidLoad];
    UIImage *backgroundImage = [UIImage imageWithPureColor:[UIColor blackColor]];
    
    //创建第二个按钮
    UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(200, 28, 80, 80)];
    [button setTitle:@"code按钮" forState:UIControlStateNormal];
    [button setTitleColor:[UIColor colorWithRed:61.0/255 green:90.0/255 blue:249.0/255 alpha:1] forState:UIControlStateNormal];
    button.highlighted = YES;   //方便演示
    [self.view addSubview:button];
    
    self.codeButton = button;
    
    //创建第四个按钮
    button = [UIButton buttonWithType:UIButtonTypeSystem];
    button.frame = CGRectMake(40, 240, 240, 80);
    [button setTitle:@"code按钮 type=system" forState:UIControlStateNormal];
    [button setTitleColor:[UIColor colorWithRed:61.0/255 green:90.0/255 blue:249.0/255 alpha:1] forState:UIControlStateNormal];
    button.highlighted = YES;   //方便演示
    [self.view addSubview:button];
    
    self.codeSystemButton = button;
    
    [self.xibButton setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];
    [self.codeButton setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];
    [self.xibCustomButton setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];
    [self.codeSystemButton setBackgroundImage:backgroundImage forState:UIControlStateHighlighted];
}