for example I have a viewmodel IndexViewModel: public class IndexViewModel { public ObservableCollection<Order>Items{get;set;} } I want create a thread .how to change the List in the thread?

How to change the ViewModel data from a thread?

Accepted answer

Leon Lu (Shanghai Wicresoft Co,.Ltd.) 78,511 Microsoft Vendor

Hello,

Welcome to our Microsoft Q&A platform!

Do you want to achieve the thread-safe when we then try to update the collection from different threads, and want to get rid this error because ObservableCollection isn't thread safe:

System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. Parameter name: index

If so, we can change the viewModel like following code. We just need to call this method in our constructor after initializing our ObservableCollection and our collection becomes thread safe:

   public class IndexViewModel  
       {  
     
     
           public IndexViewModel()  
           {  
               Items = new ObservableCollection<Order>();  
     
               Items.Add(new Order() { Name= "2222" });  
               Items.Add(new Order() { Name = "3256" });  
     
                
               Xamarin.Forms.BindingBase.EnableCollectionSynchronization(Items, null, ObservableCollectionCallback);  
     
           }  
     
           public ObservableCollection<Order> Items { get; }  
     
           void ObservableCollectionCallback(IEnumerable collection, object context, Action accessMethod, bool writeAccess)  
           {  
               // `lock` ensures that only one thread access the collection at a time  
               lock (collection)  
               {  
                   accessMethod?.Invoke();  
               }  
           }  
       }

Best Regards,

Leon Lu

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mc 5,061 Reputation points

2021-09-06T06:58:11.403+00:00

Then how to change the data in the viewmodel in other thead?

the Items as a parameter to the thread?

Leon Lu (Shanghai Wicresoft Co,.Ltd.) 78,511 Microsoft Vendor

It depends on your needs, Normally, we can handle data in the viewModel. For example, I have a Commad to open a new thread, then execute the add data like following code format.

public ICommand ChangeCommand { protected set; get; }

        public IndexViewModel()
        {
            Items = new ObservableCollection<Order>();

            Items.Add(new Order() { Name= "2222" });
            Items.Add(new Order() { Name = "3256" });

            ChangeCommand= new Command(() =>
            {
                Thread thr = new Thread(() => { ChangeList(); });
                thr.Start();

            });
            Xamarin.Forms.BindingBase.EnableCollectionSynchronization(Items, null, ObservableCollectionCallback);

        }

        void ChangeList()
        {
            for (int i = 0; i < 100; i++)
            {
                Items.Add(new Order() { Name = "test" + i });
            }

        }

        public ObservableCollection<Order> Items { get; }

        void ObservableCollectionCallback(IEnumerable collection, object context, Action accessMethod, bool writeAccess)
        {
            // `lock` ensures that only one thread access the collection at a time
            lock (collection)
            {
                accessMethod?.Invoke();
            }
        }

mc 5,061 Reputation points

2021-09-06T08:50:05.493+00:00
I know what you said and thank you.

but it can not add a parameter to the function right ?

So I have to create one class with property to hold the parameter in the thread.

In other class I can not use Items.

public class ChangeListClass { public int Id{get;set;} public void ChangeList() { for (int i = 0; i < 100; i++) { Items.Add(new Order() { Name = "test" + i }); } } }

So I can use the parameter Id but can not use Items

Leon Lu (Shanghai Wicresoft Co,.Ltd.) 78,511 Microsoft Vendor

but it can not add a parameter to the function right ?

You can add paramters whatever you want, I just for testing, So I do not add it. For example, I have a command for button in the row of listview, I can get the current object by command, I can pass it to the thread.

`
 public IndexViewModel()
        {
            Items = new ObservableCollection<Order>();

            Items.Add(new Order() { Name= "2222" });
            Items.Add(new Order() { Name = "3256" });

            ChangeCommand= new Command((sender) =>
            {
                Order order= sender as Order;
                Thread thr = new Thread(() => { ChangeList(order); });
                thr.Start();

            });
            Xamarin.Forms.BindingBase.EnableCollectionSynchronization(Items, null, ObservableCollectionCallback);

        }

        void ChangeList(Order order)
        {       
                Items.Add(order);
        }

Leon Lu (Shanghai Wicresoft Co,.Ltd.) 78,511 Microsoft Vendor

If you have other class, then you want to change the data in the ObservableCollection of viewModel, you have init viewModel, use viewModel's object to add item.

public partial class MainPage : ContentPage
    {
        IndexViewModel indexViewModel;
        public MainPage()
        {
            InitializeComponent();
             indexViewModel=     new IndexViewModel();
            BindingContext = indexViewModel;
        }

        private void Button_Clicked(object sender, EventArgs e)
        {

            Thread thr = new Thread(()=> { ChangeList(indexViewModel); });
            thr.Start();
        }
        void ChangeList(IndexViewModel indexViewModel1)
        {
            for (int i = 0; i < 100; i++)
            {
                indexViewModel1.Items.Add(new Order() { Name = "test" + i });
            }

        }

mc 5,061 Reputation points

2021-09-06T09:36:22.377+00:00

I will try thank you.

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How to change the ViewModel data from a thread?

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