xamarin - 如何在屏幕中间显示 ActivityIndi​​cator？

Question

我创建了一个事件指示器并将其添加到 StackLayout，当我让它运行时，它在模拟器中显示在右上角 Android 4.4，在 iOS 中不显示，在 Android 6 手机中不显示.

var indicator = new ActivityIndicator()
            {
                Color = Color.Blue,
            };
            indicator.SetBinding(ActivityIndicator.IsVisibleProperty, "IsBusy", BindingMode.OneWay);
            indicator.SetBinding(ActivityIndicator.IsRunningProperty, "IsBusy", BindingMode.OneWay);
AbsoluteLayout.SetLayoutFlags(indicator, AbsoluteLayoutFlags.PositionProportional);
            AbsoluteLayout.SetLayoutBounds(indicator, new Rectangle(0.5, 0.5, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
mainLayout.Children.Add(indicator);

我想将事件指示器显示在屏幕中央，因为该操作需要时间才能完成。

Answer 1

您在状态栏中看到的指示器是基页类的 IsBusy 属性的默认行为。您的代码不起作用的原因是因为您试图将 ActivityIndi​​cator 的可见性绑定(bind)到该属性 - 但您没有指定绑定(bind)源。如果您查看调试器的应用程序输出日志，那么您可能会看到类似“在类型‘Object’上找不到属性‘IsBusy’”的消息。

要修复它，您只需将每个绑定(bind)的绑定(bind)上下文指向表单。试一试:

public partial class App : Application
{
    public App ()
    {
        var mainLayout = new AbsoluteLayout ();
        MainPage = new ContentPage {
            Content = mainLayout
        };

        var containerPage = Application.Current.MainPage;

        var indicator = new ActivityIndicator() {
            Color = Color.Blue,
        };
        indicator.SetBinding(VisualElement.IsVisibleProperty, new Binding("IsBusy", BindingMode.OneWay, source: containerPage));
        indicator.SetBinding(ActivityIndicator.IsRunningProperty, new Binding("IsBusy", BindingMode.OneWay, source: containerPage));
        AbsoluteLayout.SetLayoutFlags(indicator, AbsoluteLayoutFlags.PositionProportional);
        AbsoluteLayout.SetLayoutBounds(indicator, new Rectangle(0.5, 0.5, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
        mainLayout.Children.Add(indicator);

        containerPage.IsBusy = true;
    }
}