在iOS中，UIWebView使用stringByEvaluatingJavaScriptFromString:方法无法跳转网页

Question

-(void)webViewDidFinishLoad:(UIWebView *)webView{
    NSString *title = [webView stringByEvaluatingJavaScriptFromString:@"document.title"];
    [self setTitle:title];

    // 禁用用户选择
    [webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.style.webkitUserSelect='none';"];
    // 禁用长按弹出框
    [webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.style.webkitTouchCallout='none';"];
    // 隐藏titleBar
    [webView stringByEvaluatingJavaScriptFromString:@"document.getElementById('titlebar').style.display = 'none';"];

     NSString *backUrl = [webView stringByEvaluatingJavaScriptFromString:@"document.getElementById('back').href"];
    _backUrl = backUrl;
    if ([backUrl isEqualToString:@""]) {
        _backBtnItem.customView.hidden = YES;
    }else{
        _backBtnItem.customView.hidden = NO;
    }
}

我在webViewDidFinishLoad中获取到了返回的url，想把返回写到应用中，但是压根用（确保绝对获取到到了），不知道为什么，求解答，求指导

- (void)back:(UIButton *)sender{
    [_webView stringByEvaluatingJavaScriptFromString:[NSString stringWithFormat:@"document.location.replace(%@)",_backUrl]];
//    NSURL *url = [[NSURL alloc]initWithString:_backUrl];
//    [_webView loadRequest:[NSURLRequest requestWithURL:url]];
}