IOS透明NavigationBar

    // 全透明

    [self setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];

    self.shadowImage = [UIImage new];

    // 透明度

    UIView * barBackgroundView = [[UIView alloc] init];

    barBackgroundView.frame = CGRectMake(0, -20, [ZFUtils screenW], 64);

    barBackgroundView.backgroundColor = [[UIColor whiteColor]colorWithAlphaComponent:alpha];

    [self insertSubview:barBackgroundView atIndex:0];

    barBackgroundView.userInteractionEnabled = NO;  // 關鍵之處