WKWebView有一個屬性estimatedProgress，就是當前網頁載入的進度，所以首先監聽這個屬性。

WKWebView *webView = [[WKWebView alloc]initWithFrame:[UIScreen mainScreen].bounds];
[webView addObserver:self forKeyPath:@"estimatedProgress" options:NSKeyValueObservingOptionNew context:nil];

接下來，就是弄一個進度條啦，在viewDidLoad方法裡面寫：

    UIView *progress = [[UIView
 
 alloc]initWithFrame:CGRectMake(0, 64, CGRectGetWidth(self.view.frame), 3)];
    progress.backgroundColor = [UIColor clearColor];
    [self.view addSubview:progress];

    CALayer *layer = [CALayer layer];
    layer.frame = CGRectMake(0, 0, 0, 3);
    layer.backgroundColor = COLOR_BAR_TIN.CGColor;
    [progress.layer
 
 addSublayer:layer];
    self.progresslayer = layer;

為什麼要用CALayer？隱式動畫啊。然後就是監聽方法啦

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context{
    if ([keyPath isEqualToString:@"estimatedProgress"]) {
        self.progresslayer.opacity = 1
 
;
        //不要讓進度條倒著走...有時候goback會出現這種情況
        if ([change[@"new"] floatValue] < [change[@"old"] floatValue]) {
            return;
        }
        self.progresslayer.frame = CGRectMake(0, 0, self.view.bounds.size.width * [change[@"new"] floatValue], 3);
        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];
    }
}

拷貝到自己的專案裡面就OK啦，是不是很爽呢？最後別忘了取消監聽...

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