Xamarin形式的ActivityIndi​​cator无法正常工作

Question

I am working on Xamarin form. I have xaml page where I want to display ActivityIndicator when button is clicked.

It is working when I set its property IsRunning = True in xaml but not working when i set this property from code.

Here is xaml

<ContentPage.Content>
        <StackLayout VerticalOptions="StartAndExpand">
            <StackLayout>
                <Image Source="imgmain.png" Aspect="AspectFit"></Image>
                <Image Source="Companylogo.png" Aspect="AspectFit"></Image>
            </StackLayout>
            <!--<BoxView WidthRequest="1"></BoxView>-->
            <StackLayout HorizontalOptions="StartAndExpand" Padding="10">
                <Grid Column="2" Row="4">
                    <Grid.RowDefinitions>
                        <RowDefinition Height="*" />
                        <RowDefinition  Height="*" />
                        <RowDefinition  Height="*" />
                        <RowDefinition  Height="*" />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="*" />
                        <ColumnDefinition Width="2*" />
                    </Grid.ColumnDefinitions>
                    <Label  Text="Username :" FontSize="20"  TextColor="DarkBlue"></Label>
                    <Entry Placeholder="Username" Grid.Row="0" Grid.Column="1" x:Name="EntryUsername"></Entry>
                    <Label Text="Password :" FontSize="20" TextColor="DarkBlue"  Grid.Row="1" Grid.Column="0"></Label>
                    <Entry Placeholder="Password" IsPassword="True"  Grid.Row="1" Grid.Column="1"  x:Name="EntryPassword"></Entry>
                    <Button Text="Login"  Grid.Row="2" Grid.ColumnSpan="2" Clicked="Button_OnClicked"></Button>
                    <ActivityIndicator x:Name="ActivityIndicatorLoading" Grid.Row="3" Grid.ColumnSpan="2" IsRunning="{Binding IsLoading}"  IsVisible="{Binding IsLoading}" Color="DarkBlue"></ActivityIndicator>
                </Grid>
            </StackLayout>
        </StackLayout>

    </ContentPage.Content>

code is as below

public partial class Login : ContentPage, INotifyPropertyChanged
{
    private bool isLoading;
    public bool IsLoading
    {
        set
        {
            if (isLoading != value)
            {
                isLoading = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this,
                        new PropertyChangedEventArgs("IsLoading"));
                }
            }
        }
        get { return isLoading; }
    }
    public Login()
    {
        InitializeComponent();
        IsLoading = false;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public void RaisePropertyChanged(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }

        public void Button_OnClicked(object sender, EventArgs e)
        {
            SetLoader(true);
            IsLoading = true;
            if (string.IsNullOrEmpty(EntryUsername.Text))
            {
                DisplayAlert("Alert", "Please enter username", "Ok");
                IsLoading = false;
                SetLoader(false);
                return;
            }
            if (string.IsNullOrEmpty(EntryPassword.Text))
            {
                DisplayAlert("Alert", "Please enter password", "Ok");
                IsLoading = false;
                SetLoader(false);
                return;
            }

            LoginManager loginManager=new LoginManager();
            User obj = loginManager.ValidateUser(EntryUsername.Text.Trim(), EntryPassword.Text.Trim());
            if (obj == null)
            {
                DisplayAlert("Error", "username/password is incorrect", "Ok");
            }
            else
            {
                Application.Current.Properties["FullName"] = obj.FirstName + " " + obj.LastName;
                Navigation.PushAsync(new MainPage());
            }

            SetLoader(false);
            IsLoading = false;
        }
    }

Answer 1

There is no ' PropertyChanged(this, new PropertyChangedEventArgs("IsLoading")); ' in getter of IsLoading property.

Then, there is no way to know its value's changed.

Please look here. https://developer.xamarin.com/guides/xamarin-forms/xaml/xaml-basics/data_binding_basics/

Edit : Sorry for my first answer.

You need to create a ViewModel class which has a 'IsLoading' property. And assign it to BindingContext property on the ContentPage. And set the PropertyChanged event on the IsLoading property at your ViewModel class. Finally, put boolean value on it, it would work. That's MvvM.