为什么绑定没有更新选择器选定项目
问题描述:
我的Xamarin表单中有一个Picker
,绑定到模型(下面的代码)。为什么绑定没有更新选择器选定项目
Load
方法设置SelectedVehicle
但该选取器未显示选定的车辆。首次加载页面时,选取器在列表中显示正确的项目。但在App.VehicleId
更改后的页面重新加载时,选取器显示空白。
即使我明确地在OnAppearing
在选择器设置SelectedIndex
,捡拾工具显示空白,且SelectedIndex
已设置回-1
当我后来的样子。
如何正确更新页面重新加载时的选择器选择?
的XAML:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:DefectReport"
x:Class="DefectReport.MainPage">
<ContentPage.Content>
<ScrollView>
<StackLayout>
<Label x:Name="Message" TextColor="Red" />
<Label Text="Welcome to Vehicle Check" />
<Label Text="Choose vehicle" />
<Picker ItemsSource="{Binding Vehicles}" ItemDisplayBinding="{Binding RegistrationNumber}" SelectedItem="{Binding SelectedVehicle}" SelectedIndexChanged="Picker_SelectedIndexChanged" />
<Label Text="{Binding SelectedVehicle.Description}" />
<Button Text="Add vehicle not in list" Clicked="SelectVehicle_Clicked" />
<Button Text="Check vehicle" Clicked="CheckVehicle_Clicked" />
<Button Text="Log out" Clicked="LogOut_Clicked" />
</StackLayout>
</ScrollView>
</ContentPage.Content>
</ContentPage>
后面的代码是这样的:
public partial class MainPage : ContentPage {
public class VehicleInfo : Model {
public async void Load() {
Vehicles = new ObservableCollection<Vehicle>(await App.Database.ReadAsync<Vehicle>().ToListAsync());
SelectedVehicle = Vehicles.FirstOrDefault(v => v.DeviceRecordId == App.VehicleId) ?? Vehicles.FirstOrDefault();
System.Diagnostics.Debug.WriteLine("App.VehicleId = " + App.VehicleId);
System.Diagnostics.Debug.WriteLine("SelectedVehicle.Id = " + selectedVehicle.DeviceRecordId);
}
private Vehicle selectedVehicle;
public Vehicle SelectedVehicle {
get { return selectedVehicle; }
set {
if (selectedVehicle != value) {
selectedVehicle = value;
OnPropertyChanged("SelectedVehicle");
}
}
}
private ObservableCollection<Vehicle> vehicles;
public ObservableCollection<Vehicle> Vehicles {
get { return vehicles; }
set {
if (vehicles != value) {
vehicles = value;
OnPropertyChanged("Vehicles");
}
}
}
}
VehicleInfo data;
public MainPage() {
data = new VehicleInfo();
BindingContext = data;
InitializeComponent();
}
protected override void OnAppearing() {
base.OnAppearing();
data.Load();
}
Model
是一个简单的类实现INotifyPropertyChanged
:
public class Model : System.ComponentModel.INotifyPropertyChanged {
public void OnPropertyChanged(string name) {
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(name));
}
public event PropertyChangedEventHandler PropertyChanged;
}
答
原来,这个问题是创建一个新的ObservableCollection
。我改变了代码
Vehicles = new ObservableCollection<Vehicle>(await App.Database.ReadAsync<Vehicle>().ToListAsync());
,而是清除现有的集合,并添加了新的记录它。