目前UIWebView的进度条实现可能有2种

自己模拟进度条

Web View Progress

第二种是调用私有API

Chrome Progress Bar

在UIWebView体系架构一文中详细介绍UIWebView的结构

利用webView的私有方法

-(id)webView:(id)view identifierForInitialRequest:(id)initialRequest fromDataSource:(id)dataSource
{
    [super webView:view identifierForInitialRequest:initialRequest fromDataSource:dataSource];
    return [NSNumber numberWithInt:resourceCount++];
}

- (void)webView:(id)view resource:(id)resource didFailLoadingWithError:(id)error fromDataSource:(id)dataSource {
    [super webView:view resource:resource didFailLoadingWithError:error fromDataSource:dataSource];
    resourceCompletedCount++;
    if ([self.progressDelegate respondsToSelector:@selector(webView:didReceiveResourceNumber:totalResources:)]) {
        [self.progressDelegate webView:self didReceiveResourceNumber:resourceCompletedCount totalResources:resourceCount];
    }
}

-(void)webView:(id)view resource:(id)resource didFinishLoadingFromDataSource:(id)dataSource
{
    [super webView:view resource:resource didFinishLoadingFromDataSource:dataSource];
    resourceCompletedCount++;
    if ([self.progressDelegate respondsToSelector:@selector(webView:didReceiveResourceNumber:totalResources:)]) {
        [self.progressDelegate webView:self didReceiveResourceNumber:resourceCompletedCount totalResources:resourceCount];
    }
}

- (void)webView:(IMTWebView *)_webView didReceiveResourceNumber:(int)resourceNumber totalResources:(int)totalResources {
    //Set progress value
    [chromeBar setProgress:((float)resourceNumber) / ((float)totalResources) animated:NO];

    //Reset resource count after finished
    if (resourceNumber == totalResources) {
        _webView.resourceCount = 0;
        _webView.resourceCompletedCount = 0;
    }
}