C# Xamarin形成相对论，但不出现

我正在遵循一些相当标准，我真的在努力让相对的年轻人工作。最终，我希望在主要内容上覆盖一个ActivityIndicator：

        BindingContext = new LoginViewModel(this);
        Padding = new Thickness(20);

        Title = "Login";

        var image = new Image
        {
            Source = ImageSource.FromFile("logo.png"),
            HeightRequest = 50
        };

        var label = new Label
        {
            Text = "...",
            FontSize = 20,
            HorizontalTextAlignment = TextAlignment.Center
        };
        var errorLabel = new Label
        {
            Text = "",
            TextColor = Color.Red,
            FontSize = 20,
            HorizontalTextAlignment = TextAlignment.Center
        };

        var loginButton = new Button
        {
            Text = "Log In",
            BackgroundColor = Color.Black,
            TextColor = Color.White,
            FontSize = 20,
            HeightRequest = 50
        };

        var loginEntry = new Entry
        {
            Placeholder = "Username"
        };

        var passwordEntry = new Entry
        {
            Placeholder = "Password"
        };

        var copywrite = new Label
        {
            Text = "© 2016",
            FontSize = 15,
            HorizontalTextAlignment = TextAlignment.Center
        };

        var loadingIndicator = new ActivityIndicator
        {
            BackgroundColor = Color.Blue,
            IsVisible = true
        };

        ...

        var topLayer = new StackLayout
        {
            Spacing = 10,
            Children = { image, label, loginEntry, passwordEntry, loginButton, errorLabel },
            VerticalOptions = LayoutOptions.Start
        };

        var bottomLayer = new StackLayout
        {               
            Spacing = 10,
            Children = { copywrite },
            VerticalOptions = LayoutOptions.End
        };

        var mainContent = new StackLayout
        {
            Children =
            {
                topLayer,
                new StackLayout 
                {
                    VerticalOptions = LayoutOptions.CenterAndExpand,
                },
                bottomLayer
            },
            VerticalOptions = LayoutOptions.FillAndExpand,
            BackgroundColor = Color.Green
        };

        var r = new RelativeLayout()
        {
            BackgroundColor = Color.Pink
        };
        r.Children.Add(mainContent, 
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Width;
            }),
            Constraint.RelativeToParent((parent) =>
            {
                return parent.Height;
            })
        );

        Content = r;

当我设置Content=mainContent时，我看到的一切都很好，但是通过上面的代码，我得到了一个白色屏幕。我一直在找

当我尝试这个：

        var overlay = new AbsoluteLayout()
        {
            BackgroundColor = Color.Pink
        };
        AbsoluteLayout.SetLayoutFlags(mainContent, AbsoluteLayoutFlags.PositionProportional);
        AbsoluteLayout.SetLayoutBounds(mainContent, new Rectangle(0f, 0f, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
        AbsoluteLayout.SetLayoutFlags(loadingIndicator, AbsoluteLayoutFlags.PositionProportional);
        AbsoluteLayout.SetLayoutBounds(loadingIndicator, new Rectangle(0.5, 0.5, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
        overlay.Children.Add(mainContent);
        overlay.Children.Add(loadingIndicator);

        Content = overlay;

---

我可以看到绿色和粉色视图，但它们也可以堆叠（而不是重叠），但我也看不到粉色绝对布局中的活动指示器。

对于RelativeLayout，您调用的Add方法是在X和Y上设置约束，而不是在宽度和高度上设置约束。该Add变量的参数顺序为：

子视图

X约束

Y约束

宽度限制

高度限制

所有约束都是可选的

要明确地将其放置在整个屏幕上，请执行以下操作：

r.Children.Add(mainContent, 
    Constraint.Constant(0),
    Constraint.Constant(0),
    Constraint.RelativeToParent((parent) =>
    {
        return parent.Width;
    }),
    Constraint.RelativeToParent((parent) =>
    {
        return parent.Height;
    })
);

对于AbsoluteLayout，请尝试一组稍微不同的约束：

    AbsoluteLayout.SetLayoutFlags(mainContent, AbsoluteLayoutFlags.All);
    AbsoluteLayout.SetLayoutBounds(mainContent, new Rectangle(0f, 0f, 1f, 1f));

---

这明确规定mainContent将占据整个AbsoluteLayout，而不是依赖于mainContent的实际布局大小。

谢谢-我需要做的是说：AbsoluteLayout.SetLayoutBounds（加载指示器，新矩形（0.5,0.5，-1，-1））；