实现背景渐变效果

UIView *bgView = [[UIView alloc] initWithFrame:CGRectMake(0, 60, SCREEN_WIDTH, 175-60)];
[_headerView addSubview:bgView];

    //渐变效果
    _gradientLayer = [CAGradientLayer layer];  // 设置渐变效果
    _gradientLayer.bounds = self.view.bounds;
    _gradientLayer.borderWidth = 0;
    _gradientLayer.frame = self.view.bounds;
    _gradientLayer.colors = [NSArray arrayWithObjects:
                                 (id)[[UIColor clearColor] CGColor],
                                 (id)[[UIColor colorWithRed:0 green:0 blue:0 alpha:0.5] CGColor], nil,nil];
    _gradientLayer.startPoint = CGPointMake(0.5, 0.5);
    _gradientLayer.endPoint = CGPointMake(0.5, 1.0);
    [bgView.layer insertSublayer:_gradientLayer atIndex:0];