问题
I need some help. I am new to Events and handlers and I would like to begin using events for decoupling purposes but I am confused.
I have a class where I have a list view that is populated on OnAppearing. However to prevent onAppearing to happen each time the page is clicked I load the list once and then I would like to have items to get added or deleted to the list upon being added or removed from the server through the use of events.
The ListView page is a list of my favorite newspaper article Links. When clicking on any one of these links I get redirected to a LinksDetailsPage where I pass in the selected link and then display any details associated with the link.
Anyways...
I would like to add or remove an item on the my Favorites list seamlessly. So when I click on the AddItem or RemoveItem in the LinksDetailsPage I would like the item to either remove or add to theFavoritesPage List. Before I was only relying on the OnAppearing to work its magic and update the favorites list but it would lag to remove the item from the list, so this way I hope it would remove or add as soon as the even is invoked and not when the page loads on OnAppearing. However I think I am either not invoking the event properly or my subscriber is not subscribed properly. Events and delegates have been confusing for me from the get go. On top of that my favorites list is grouped so and it's my first time working with grouped lists as well. Check out my code:
Subscriber My Favorites Page:
public FavoritesPage()
{
InitializeComponent();
}
protected override async void OnAppearing()
{
if (_isDataLoaded)
return;
_isDataLoaded = true;
base.OnAppearing();
await LoadFavorites();
}
private async Task LoadFavorites()
{
groups = new ObservableCollection<LinksTypeGroup<string, NewspaperLink>>();
var links = await _db.GetAllFavorites();
var linkType = await _manager.GetLinkCategories();
foreach(var type in linkType)
{
var typegroup = links.FindAll(
delegate(NewspaperLink link)
{
return link.iLinkTypeID == type.iLinkTypeID;
});
groups.Add(new LinksTypeGroup<string, NewspaperLink>(type.Title, typegroup));
MyList.GroupDisplayBinding = new Binding("GroupKey");
MyList.ItemsSource = groups;
}
}
public void Listener(FavoritesPage P)
{
P.LinkAdded += Item_Added;
P.LinkDeleted += Item_Deleted;
}
void Item_Deleted(object sender, int e)
{
Console.WriteLine("Item_Deleted");
// remove item from groups ..see code above
}
void Item_Added(object sender, int e)
{
Console.WriteLine("Item_Added");
// add to groups ..see code above
}
I am not accessing anything so far.
Publisher LinksDetailsPage:
private NewspaperLink _link;
public event EventHandler< NewspaperLink> ItemAdded;
public event EventHandler< NewspaperLink> ItemDeleted;
public LinksDetailsPage(NewspaperLink link)
{
_link = link;
BindingContext = _link;
InitializeComponent();
}
protected override async void OnAppearing()
{
base.OnAppearing();
await LoadLink();
}
private async Task LoadLink()
{
var url = await db.ReturnRecipeLink(_link.iLinkID);
linkWebView.Source = url;
CheckifExists(_link);
}
}
void AddLink(object sender, System.EventArgs e)
{
var link = BindingContext as NewspaperLink;
db.InsertIntoMyList(_link);
ItemAdded?.Invoke(this, link);
}
void DeleteLink(object sender, System.EventArgs e)
{
var link = BindingContext as NewspaperLink;
db.DeleteFromMyList(_link);
ItemDeleted?.Invoke(this, link);
}
Can someone guide me on how to make this even process work?
回答1:
- Event
If want to use Events, LinksDetailsPage should be declared something like following:
public partial class LinksDetailsPage : ContentPage
{
public event EventHandler<NewspaperLink> ItemAdded;
public event EventHandler<NewspaperLink> ItemDeleted;
public LinksDetailsPage()
{
InitializeComponent();
}
protected virtual void AddLink(NewspaperLink e)
{
EventHandler<NewspaperLink> handler = ItemAdded;
if (handler != null)
{
handler(this, e);
}
}
protected virtual void DeleteLink( NewspaperLink e)
{
EventHandler<NewspaperLink> handler = ItemDeleted;
if (handler != null)
{
handler(this, e);
}
}
// Add click event
private void Add_Clicked(object sender, EventArgs e)
{
AddLink(new NewspaperLink() {link="first link" });
}
// Delete click event
private void Delete_Clicked(object sender, EventArgs e)
{
DeleteLink(new NewspaperLink() { link = "first link" });
}
}
public class NewspaperLink : EventArgs
{
public string link { get; set; }
}
Then you need to subscribe it in the ListView page when navigating to the LinksDetailsPage page:
private async void Button_Clicked(object sender, EventArgs e)
{
LinksDetailsPage detailPage = new LinksDetailsPage();
detailPage.ItemAdded += DetailGridPage_ItemAdded;
detailPage.ItemDeleted += DetailGridPage_ItemDeleted;
await Navigation.PushModalAsync(detailGridPage);
}
private void DetailGridPage_ItemDeleted(object sender, NewspaperLink e)
{
Console.WriteLine("The tlink was deleted : " + e.link);
}
private void DetailGridPage_ItemAdded(object sender, NewspaperLink e)
{
Console.WriteLine("The link was added : "+e.link);
}
- Delegate
Similarly, if want to use Delegate, you only need to declare something in List Page as follows:
public partial class ListViewPage : ContentPage
{
public delegate void ItemAddedDelegate(NewspaperLink e);
public delegate void ItemDeletedDelegate(NewspaperLink e);
public ListViewPage()
{
InitializeComponent();
}
private async void Button_Clicked(object sender, EventArgs e)
{
ItemAddedDelegate itemAddedDelegate = AddMethod;
ItemDeletedDelegate itemDeletedDelegate = DeleteMethod;
DetailGridPage detailGridPage = new DetailGridPage(itemAddedDelegate, itemDeletedDelegate);
await Navigation.PushModalAsync(detailGridPage);
}
public static void AddMethod(NewspaperLink item)
{
Console.WriteLine("Add" + item.link);
}
public static void DeleteMethod(NewspaperLink link)
{
Console.WriteLine("Delete" + item.link);
}
}
Then in LinksDetailsPage, you can pass the add or delete delegate method to ListViewPage.
public partial class LinksDetailsPage : ContentPage
{
private ListViewPage.ItemAddedDelegate itemAddedDelegate;
private ListViewPage.ItemDeletedDelegate itemDeletedDelegate;
public DetailGridPage()
{
InitializeComponent();
}
public LinksDetailsPage(ListViewPage.ItemAddedDelegate itemAddedDelegate, ListViewPage.ItemDeletedDelegate itemDeletedDelegate)
{
InitializeComponent();
this.itemAddedDelegate = itemAddedDelegate;
this.itemDeletedDelegate = itemDeletedDelegate;
}
// Add click event
private void Add_Clicked(object sender, EventArgs e)
{
itemAddedDelegate(new NewspaperLink() { link = "first link" });
}
// Delete click event
private void Delete_Clicked(object sender, EventArgs e)
{
itemDeletedDelegate(new NewspaperLink() { link = "first link" });
}
}
public class NewspaperLink : EventArgs
{
public string link { get; set; }
}
- MessageCenter
If using MessageCenter, it should be the best convenient method to achieve that.
Only Subscribe it in ListView page:
public ListViewPage()
{
InitializeComponent();
MessagingCenter.Subscribe<object, NewspaperLink>(this, "Add", async (sender, arg) =>
{
await DisplayAlert("Message received", "arg=" + arg.link, "OK");
});
MessagingCenter.Subscribe<object, NewspaperLink>(this, "Delete", async (sender, arg) =>
{
await DisplayAlert("Message received", "arg=" + arg.link, "OK");
});
}
And send message in LinksDetailsPage as follows:
// Add click event
private void Add_Clicked(object sender, EventArgs e)
{
NewspaperLink newspaperLink= new NewspaperLink() { link = "first link" };
MessagingCenter.Send<object, NewspaperLink>(this, "Add", newspaperLink);
}
// Delete click event
private void Delete_Clicked(object sender, EventArgs e)
{
NewspaperLink newspaperLink = new NewspaperLink() { link = "first link" };
MessagingCenter.Send<object, NewspaperLink>(this, "Delete", newspaperLink);
}
来源：https://stackoverflow.com/questions/65174611/events-and-delegates-in-xamarin-forms-parent-and-child-pages