问题描述
StackOverflow小组敬上
我正在尝试在我的应用程序中运行后台进程。此后台进程应每15秒更新一次应用程序页面中的“背景图片”。到目前为止,我尝试在App OnStart()方法中创建一个计时器,并在BeginInvokeOnMainThread()方法中更新页面的背景图像,但没有成功。有人可以帮我吗?
我的代码-
{
private static Stopwatch stopWatch = new Stopwatch();
private const int defaultTimespan = 20;
private readonly HomePage homePage;
public App()
{
InitializeComponent();
try
{
MainPage = new MainPage();
homePage = new HomePage();
}
catch(Exception ex)
{
string str = ex.Message;
}
}
protected override void OnStart()
{
if (!stopWatch.IsRunning)
{
stopWatch.Start();
}
Device.StartTimer(new TimeSpan(0,10),() =>
{
// Logic for logging out if the device is inactive for a period of time.
if (stopWatch.IsRunning && stopWatch.Elapsed.Seconds >= defaultTimespan)
{
//prepare to perform your data pull here as we have hit the 1 minute mark
// Perform your long running operations here.
Device.BeginInvokeOnMainThread(() =>
{
// If you need to do anything with your UI,you need to wrap it in this.
// homePage.BackgroundImageSource = "goldengate.jpg";
homePage.ChangeBackgroundImage();
});
stopWatch.Restart();
}
// Always return true as to keep our device timer running.
return true;
});
}
protected override void OnSleep()
{
//stopWatch.Reset();
}
protected override void OnResume()
{
//stopWatch.Start();
}
//void ChangeHomePageImage()
//{
// Navigation.PushAsync(new HomePage(appBackground));
// Navigation.RemovePage(this);
//}
}
MainPage -
<MasterDetailPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:d="http://xamarin.com/schemas/2014/forms/design"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
xmlns:local="clr-namespace:Excercise.Views"
x:Class="Excercise.MainPage" IsPresented="False">
<MasterDetailPage.Master>
<local:MenuPage x:Name="menuPage"></local:MenuPage>
</MasterDetailPage.Master>
<MasterDetailPage.Detail>
<NavigationPage>
<x:Arguments>
<local:HomePage x:Name="homePage"></local:HomePage>
</x:Arguments>
</NavigationPage>
</MasterDetailPage.Detail>
</MasterDetailPage>
HomePage -
public partial class HomePage : ContentPage
{
private sqliteAsyncConnection _connection;
public HomePage()
{
InitializeComponent();
// BindingContext = new HomePageviewmodel();
_connection = DependencyService.Get<IsqliteDb>().GetConnection();
loadData("");
}
public HomePage(string BackgroundimgPath)
{
InitializeComponent();
// BindingContext = new HomePageviewmodel();
_connection = DependencyService.Get<IsqliteDb>().GetConnection();
loadData(BackgroundimgPath);
}
public HomePage(string City,string LocationKey,string StateID)
{
InitializeComponent();
_connection = DependencyService.Get<IsqliteDb>().GetConnection();
// BindingContext = new HomePageviewmodel();
try
{
// Method Calls
}
catch (Exception)
{
displayAlert("Error","There was an error loading this page.","OK");
}
}
protected override void OnAppearing()
{
this.Title = App.AppTitle;
this.firstStacklayout.Margin = new Thickness(0,(Application.Current.MainPage.Height * 0.25),0);
base.OnAppearing();
}
解决方法
您正在创建HomePage
的实例并尝试对其进行更新,但是它与MasterDetail
中显示的实例不同
尝试这样的事情
var md = (MasterDetailPage)MainPage;
var nav = (NavigationPage)md.DetailPage;
var home = (HomePage)nav.CurrentPage;
home.ChangeBackgroundImage();
或者,您可以使用MessagingCenter
向HomePage
发送一条消息,要求其更新