UITabBarController 等待加载 UIViewController 直到选择 Xamarin

Question

是否可以在 UITabBarController 中选择新的 UIViewController 时调用方法？或者是否可以等待加载整个 UIViewController 直到选择视图？

我有一个 UITabBarController 看起来像：

    public sealed class VisualizationViewController : UITabBarController
{

    private UIViewController tabVand, tabVarme, tabEl;
    private UIToolbar toolbar;
    public VisualizationViewController()
    {
        toolbar = new UIToolbar(new RectangleF(0, 0, (float)View.Frame.Width, 60));
        toolbar.AutoresizingMask = UIViewAutoresizing.FlexibleWidth;
        View.AddSubview(toolbar);

        tabVand = new WaterVisualizationViewController();
        tabVand.Title = "Vand";
        tabVand.TabBarItem = new UITabBarItem("Vand", UIImage.FromFile("Images/Water.png"), 0); // Tag == initial order
        tabVand.TabBarItem.SetFinishedImages( UIImage.FromFile("Images/Water.png"), UIImage.FromFile("Images/Water.png"));
        tabVand.View.BackgroundColor = UIColor.LightGray;

        tabVarme = new TemperatureViewController();
        tabVarme.Title = "Varme";
        tabVarme.TabBarItem = new UITabBarItem("Varme", UIImage.FromFile("Images/Heat.png"), 1); // Tag == initial order
        tabVarme.TabBarItem.SetFinishedImages(UIImage.FromFile("Images/Heat.png"), UIImage.FromFile("Images/Heat.png"));

        tabVarme.View.BackgroundColor = UIColor.LightGray;

        tabEl = new ElVisualizationViewController();
        tabEl.Title = "EL";
        tabEl.TabBarItem = new UITabBarItem("EL", UIImage.FromFile("Images/Power.png"), 2); // Tag == initial order
        tabEl.TabBarItem.SetFinishedImages(UIImage.FromFile("Images/Power.png"), UIImage.FromFile("Images/Power.png"));

        tabEl.View.BackgroundColor = UIColor.LightGray;


        var tabs = new[]
        {
            tabVand, tabVarme, tabEl
        };
        ViewControllers = tabs;
        SelectedViewController = tabVand;
    }

现在，它会加载所有视图，这将是一个昂贵的调用，当我从虚拟数据移动到每个视图应加载的真实数据时。所以我只想在 UITabBarController.

中选择视图时进行数据调用

Answer 1

所以发生的事情是您的 ViewDidLoad() 方法正在为您的构造函数中的每个选项卡视图控制器触发（假设您正在那里调用您的网络服务）。每个 View Did Load 都在触发，因为您正在调用 View.Background，这会将视图加载到内存中。你不想在这里这样做，但在每个单独的选项卡视图控制器的 ViewDidLoad() 方法中：

public sealed class VisualizationViewController : UITabBarController
{

    private UIViewController tabVand, tabVarme, tabEl;
    private UIToolbar toolbar;
    public VisualizationViewController()
    {
        base.ViewDidLoad();
        toolbar = new UIToolbar(new RectangleF(0, 0, (float)View.Frame.Width, 60));
        toolbar.AutoresizingMask = UIViewAutoresizing.FlexibleWidth;
        View.AddSubview(toolbar);

        tabVand = new WaterVisualizationViewController();
        tabVand.Title = "Vand";
        tabVand.TabBarItem = new UITabBarItem("Vand", UIImage.FromFile("Images/Water.png"), 0); // Tag == initial order
        tabVand.TabBarItem.SetFinishedImages( UIImage.FromFile("Images/Water.png"), UIImage.FromFile("Images/Water.png"));

        //Don't do this, it will make its ViewDidLoad fire
        //and you don't want that.
        //tabVand.View.BackgroundColor = UIColor.LightGray;

        tabVarme = new TemperatureViewController();
        tabVarme.Title = "Varme";
        tabVarme.TabBarItem = new UITabBarItem("Varme", UIImage.FromFile("Images/Heat.png"), 1); // Tag == initial order
        tabVarme.TabBarItem.SetFinishedImages(UIImage.FromFile("Images/Heat.png"), UIImage.FromFile("Images/Heat.png"));

        //tabVarme.View.BackgroundColor = UIColor.LightGray;

        tabEl = new ElVisualizationViewController();
        tabEl.Title = "EL";
        tabEl.TabBarItem = new UITabBarItem("EL", UIImage.FromFile("Images/Power.png"), 2); // Tag == initial order
        tabEl.TabBarItem.SetFinishedImages(UIImage.FromFile("Images/Power.png"), UIImage.FromFile("Images/Power.png"));

        //tabEl.View.BackgroundColor = UIColor.LightGray;


        var tabs = new[]
        {
            tabVand, tabVarme, tabEl
        };
        ViewControllers = tabs;
        SelectedViewController = tabVand;
    }

然后在每个单独的选项卡 (UIViewController) 实现中，在它的 ViewDidLoad() 方法中调用您的 Web 服务，而不是在构造函数中。示例：

public class WaterVisualizationViewController : UIViewController
{
  public WaterVisualizationViewController()
  {
     //Don't call your web service here
  }

  public override ViewDidLoad()
  {
     //Set your background color here
     //Call your web service
     //Populate your UI with data
  }
}

现在，如果您的 Web 服务调用费用很高，您可能会在点击选项卡时看到一点延迟。为此，您应该考虑使用 Xamarin Component Store 中的 UIActivityIndicatorView to display a "Loading Data" type of indicator so that the user has feedback your app is doing something and not frozen. Alternatively, you can use BTProgressHUD，它非常易于使用

UITabBarController 等待加载 UIViewController 直到选择 Xamarin

UITabBarController wait loading UIViewController until selected Xamarin

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