一款UIWebView的加载进度条，基于CAShapeLayer，现在支持两种显示风格：纯颜色进度条和渐变色进度条。
DKProgressLayer *layer1 = [DKProgressLayer alloc] initWithFrame:CGRectMake(0, 40, DEVICE_WIDTH, 2)];
//设置进度条纯颜色显示
layer.progressColor = [UIColor greenColor];
//设置进度条的显示风格，默认纯色显示
layer.progressStyle = DKProgressStyle_Noraml;
DKProgressLayer *layer2 = [DKProgressLayer alloc] initWithFrame:CGRectMake(0, 40, DEVICE_WIDTH, 2) color:[UIColor greenColor]];
//设置进度条为渐变色显示
layer.progressStyle = DKProgressStyle_Gradual;
//根据需求，可以添加到NavigationController或者WebView
[self.navigationController.navigationBar.layer addSublayer:layer];