iOS空心圆下载进度指示器控件

self.layer = [CAShapeLayer layer];
    self.layer.frame = CGRectMake(0, 0, 100, 100);
    self.layer.position = self.view.center;
    self.layer.lineWidth = 10.0;
    self.layer.fillColor = [UIColor clearColor].CGColor;
    self.layer.strokeColor = [UIColor lightGrayColor].CGColor;
    self.layer.lineCap = kCALineCapSquare;
    /*
     CA_EXTERN NSString *const kCALineCapButt
     __OSX_AVAILABLE_STARTING (__MAC_10_6, __IPHONE_3_0);
     CA_EXTERN NSString *const kCALineCapRound
     __OSX_AVAILABLE_STARTING (__MAC_10_6, __IPHONE_3_0);
     CA_EXTERN NSString *const kCALineCapSquare
     */
    UIBezierPath *path = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(0, 0, 100, 100)];
    self.layer.path = path.CGPath;
    [self.view.layer addSublayer:self.layer];

    self.layer.strokeEnd = 0.0;
    _progress = 0;
    self.timer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(updateProgress) userInfo:nil repeats:YES];


//更新下载进度
_progress += 0.01;

    if (self.layer.strokeEnd >= 1) {
        self.timer.fireDate = [NSDate distantFuture];
    }
    else
    {
        self.layer.strokeEnd = _progress;
    }