ActivityIndi​​cator未在XAML中显示绑定到ViewModel

Question

你有一个看似相当简单的登录表单，我已经绑定了一个viewmodel。

除活动指标外，一切都在游泳。我已经尝试过各种方式让它出现。

据我了解，当设置为true时，IsVisible和IsRunning属性应该使指示符出现。我将这些绑定到bool属性，该属性在整个登录命令中设置为true / false。

要让它显示，我需要做什么？

Login.xaml

ns2

Login.xaml.cs

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="TechsportiseApp.Views.Login">
    <ContentPage.ToolbarItems>
        <ToolbarItem Name="Register" Order="Primary" Icon="addperson.png" Text="Register" Priority="0" Command="{Binding RegisterCommand}" />
        <ToolbarItem Name="Help" Order="Primary" Icon="help.png" Text="Help" Priority="1" Command="{Binding HelpCommand}" />
    </ContentPage.ToolbarItems>
    <ContentPage.Content>
        <RelativeLayout VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand">
            <StackLayout HorizontalOptions="FillAndExpand">
                <StackLayout HorizontalOptions="FillAndExpand" BackgroundColor="Red" Padding="2" IsVisible="{Binding IsOffline}">
                    <Label Text="OFFLINE" BackgroundColor="Red" TextColor="White" FontAttributes="Bold" FontSize="Small" HorizontalOptions="FillAndExpand" HorizontalTextAlignment="Center" />
                </StackLayout>
                <StackLayout VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand" Orientation="Vertical">
                    <ScrollView Orientation="Vertical" VerticalOptions="StartAndExpand">
                        <StackLayout VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand" Orientation="Vertical" Margin="10">
                            <Image Source="splash.png" HorizontalOptions="Center" />
                            <Label Text="Race Director" FontAttributes="Bold" FontSize="Large" HorizontalTextAlignment="Center" HorizontalOptions="CenterAndExpand" />
                            <Label Text="by Techsportise" HorizontalTextAlignment="Center" HorizontalOptions="CenterAndExpand" />
                            <BoxView HeightRequest="20" HorizontalOptions="FillAndExpand" />
                            <Entry x:Name="email" Text="{Binding Email}" Placeholder="Email address" />
                            <Entry x:Name="password" Text="{Binding Password}" IsPassword="true" Placeholder="Password" />
                            <StackLayout Padding="3" Orientation="Horizontal">
                                <Label Style="{StaticResource EntryFormLabels}" Text="REMEMBER ME" FontSize="Small" HorizontalOptions="StartAndExpand" VerticalTextAlignment="Center" />
                                <Switch IsToggled="{Binding RememberMe}" HorizontalOptions="End" />
                            </StackLayout>
                            <Button x:Name="loginButton" Text="Login" Command="{Binding LoginCommand}" Style="{StaticResource Buttons}" />
                            <ActivityIndicator IsVisible="{Binding IsBusy}" IsRunning="{Binding IsBusy}" Color="#80000000" />
                            <Label Text="{Binding ValidationError}" HorizontalTextAlignment="Center" HorizontalOptions="CenterAndExpand" TextColor="Red" FontSize="Small" IsVisible="{Binding Invalid}" />
                        </StackLayout>
                    </ScrollView>
                </StackLayout>
            </StackLayout>
            <ActivityIndicator IsVisible="{Binding IsBusy}"
                               IsRunning="{Binding IsBusy}"
                               Color="Black"
                               VerticalOptions="CenterAndExpand"
                               HorizontalOptions="CenterAndExpand"
                               RelativeLayout.XConstraint="{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=1}"
                               RelativeLayout.YConstraint="{ConstraintExpression Type=RelativeToParent,
                                        Property=Height,
                                        Factor=1}" />
        </RelativeLayout>
    </ContentPage.Content>
</ContentPage>

Loginviewmodel

using RestSharp;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using TechsportiseApp.Views;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
using TechsportiseApp.API;
using TechsportiseApp.ViewModels;
using TechsportiseApp.Models;
using Newtonsoft.Json;

namespace TechsportiseApp.Views
{
    public partial class Login : ContentPage
    {
        public Login ()
        {
            InitializeComponent ();
            var viewModel = new LoginViewModel();
            BindingContext = viewModel;
        }

        public Login(string email)
        {
            InitializeComponent();
            var viewModel = new LoginViewModel(email);
            BindingContext = viewModel;
        }
    }
}

Answer 1

尝试将ViewModel方法Login更改为：

private void Login()
{
    Task.Run(() => 
    {
        try
        {
            Device.BeginInvokeOnMainThread(() => IsBusy = true);
            var isvalidemail = GlobalFunctions.IsValidEmail(Email);
            var IsThereConnection = GlobalFunctions.CheckForInternetConnection();

            if (IsThereConnection == false)
                throw new Exception("You cannot login whilst you are offline");
            else if (Email == "")
                throw new Exception("You must enter an email address");
            else if (Password == "")
                throw new Exception("You must enter a password");
            else if (isvalidemail == false)
                throw new Exception("You must enter a valid email address");
            //We are good to go
            else
            {
                var LoginStatus = AccountsAPI.Login(Email, Password);
                if (LoginStatus.Code == "OK")
                {
                    var tokenobject = JsonConvert.DeserializeObject<TokenModel>(LoginStatus.Content);
                    Application.Current.Properties["Token"] = tokenobject.Access_token;
                    Application.Current.Properties["IsRemembered"] = RememberMe;
                    string token = Application.Current.Properties["Token"].ToString();
                    if ((bool)Application.Current.Properties["ShowHelpOnStartup"] == true)
                        Device.BeginInvokeOnMainThread(() => App.Current.MainPage = new StartupHelp(true));
                    else
                        Device.BeginInvokeOnMainThread(() => App.Current.MainPage = new MainMenuMasterDetail());
                }
                //Error response
                else
                    throw new Exception("Your login has failed. Please check your details and try again.");         
            }
        }
        catch(Exception ex)
        {

            Device.BeginInvokeOnMainThread(() => 
            {
                ValidationError = ex.Message;
                Invalid = true;
            });
        }
        finally
        {
            Device.BeginInvokeOnMainThread(() => IsBusy = true);
        }
    });
}