使用KVO监听WKWebView的“estimatedProgress”属性，通过监听该属性的变化才是进度条的长度。
@property (nonatomic, retain) CALayer *progresslayer;
并监听estimatedProgress

[_webView addObserver:self forKeyPath:@"estimatedProgress" options:NSKeyValueObservingOptionNew context:nil];

UIView *progress = [[UIView alloc]initWithFrame:CGRectMake(0, 0, CGRectGetWidth(self.view.frame), 2)];
    progress.backgroundColor = [UIColor clearColor];
    [self.view addSubview:progress];
CALayer *layer = [CALayer layer];
    layer.frame = CGRectMake(0, 0, 0, 2);
    layer.backgroundColor = JHMaincolor.CGColor;
    [progress.layer addSublayer:layer];
    self.progresslayer = layer;

监听estimatedProgress属性变化，并修改进度条长度

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{
    if ([keyPath isEqualToString:@"estimatedProgress"]) {
        self.progresslayer.opacity = 1;
        //不要让进度条倒着走...有时候goback会出现这种情况
        if ([change[@"new"] floatValue] < [change[@"old"] floatValue]) {
            return;
        }
        self.progresslayer.frame = CGRectMake(0, 0, self.view.bounds.size.width * [change[@"new"] floatValue], 2);
        if ([change[@"new"] floatValue] == 1) {
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(.4 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                self.progresslayer.opacity = 0;
            });
            dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
                self.progresslayer.frame = CGRectMake(0, 0, 0, 3);
            });
        }
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

移除监听

- (void)dealloc {
    [self.webView removeObserver:self forKeyPath:@"estimatedProgress"];
}