应用程序启动时iAd为白色（未加载）

Question

我的iAd是白色的。当iAd未完全加载时，看起来bannerViewDidLoadAd运行。它只在我第一次显示我的ViewController（应用程序启动）时才会发生。当我转到另一个控制器并返回时，iAd正确加载。你知道为什么吗？

- (void)bannerViewDidLoadAd:(ADBannerView *)banner {
    _bannerView.hidden = NO;
    [self.view setNeedsLayout]; 
}

- (void)bannerView:(ADBannerView *)banner didFailToReceiveAdWithError:(NSError *)error {
    _bannerView.hidden = YES;
    [self.view setNeedsLayout]; 
}

-(void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    if (!_bannerView) {
        _bannerView = [[ADBannerView alloc] initWithAdType:ADAdTypeBanner];
        _bannerView.delegate = self;
        _bannerView.hidden = YES;
            CGRect bannerFrame = _bannerView.frame;
        bannerFrame.origin.y = self.view.bounds.size.height - _bannerView.bounds.size.height;
           _bannerView.frame = bannerFrame;
        [self.view addSubview:_bannerView];
    } 
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    _bannerView.delegate = nil;
    [_bannerView removeFromSuperview];
    _bannerView = nil; 
}