如何等待命令的任务？

Question

I am trying to wait for the end of a task. This task loads a bunch of things from the server and usually takes about 2 to 3 seconds. This task is started by the command LoadItemsCommand. Once the task is over, it should check if anything has been loaded, and if any of the loaded items already contain values. If yes, it should remove a button from the Toolbar.

But it doesn't wait. The command call and specifically ask to await the task, but on the main page, things procede without waiting for the end of the task. i tried to put a loop with 30 second wait time to wait for the command to work, but nothing.

Here is the command in the viewmodel:

LoadItemsCommand = new Command(async () => await ExecuteLoadItemsCommand());

Here is the LoadItems Function in the ViewModel

async Task ExecuteLoadItemsCommand()
{
    if (IsBusy)
        return;
    IsBusy = true;

    List<string> stockIds = new List<string>();
    foreach (var orderline in this._order)
    {
        stockIds.Add(orderline.StockId);
    }
    string[] array = new string[stockIds.Count];
    stockIds.CopyTo(array, 0);
    this._products = await Common.GetProducts(new ProductSearchFilter { StockId = array });

    _scanStart = Common.Now;
    Items.Clear();

    bool already_scanned = false; // Todo trouver un meilleur non car pas tres explicite

    foreach (PickingOrderLineToHandle orderLine in _order)
    {
        if (orderLine.ScannedQuantity < orderLine.Quantity)
        {
            if ((bool)orderLine.WasSentToEnd && !already_scanned)
            {
                Items.Add(new ListItem() { Name = " ----- ", Id = "-1" });
                already_scanned = true;
            }

            Product product = null;
            foreach (var prod in _products)
            {
                if (prod.StockId == orderLine.StockId)
                {
                    product = prod;
                    break;
                }
            }
            Items.Add(new ListItem() { Name = $"{(orderLine.Quantity - orderLine.ScannedQuantity).ToString()} / {orderLine.Quantity}\t + {orderLine.Table_bin.LabelAddress} {product.ProductName} {orderLine.stock.Platform}", Id = orderLine.StockId });
        }
    }

    IsBusy = false;
}

And here is the call in the page:

protected override void OnAppearing()
{
    base.OnAppearing();

    viewModel.LoadItemsCommand.Execute(null);

    int loop = 0;
    while (viewModel.IsBusy && loop < 60)
    {
        System.Threading.Thread.Sleep(500);
        loop++;
    };

    if (loop == 60)
    {
        DisplayAlert("Erreur", "Une erreur est survenue lors du chargment. Veuillez réessayer.", "Ok");
        Navigation.PopAsync();
    }

    var cantCancel = viewModel.Items.Any(i => i.BookedQuantity > 0);

    if (Common.IsTeamLeader)
        cantCancel = false;

    if (cantCancel)
    {
        var cancelButton = this.ToolbarItems.Where(b => b.Text == "Annuler").First();
        this.ToolbarItems.Remove(cancelButton);
    }
}

Answer 1

Problem

The following statement seems like a blocking call, but actually, since the method that is called by the Command is an asynchronous anonymous function, it will just execute in a fire and forget fashion and will not be awaited:

viewModel.LoadItemsCommand.Execute(null);

That's because of how the Command is defined:

LoadItemsCommand = new Command(async () => await ExecuteLoadItemsCommand());

A Command like this always executes synchronously. However, when calling an async void method from within a synchronous context, it will just begin execution, but won't await the method (hence fire and forget ).

Solution

Now, the way you're using the Command is unusual. Normally, you would bind to the Command from a UI control or you would just trigger it's execution. You are expecting it to finish before the rest of the code is executed, which isn't the case as I've explained above.

Instead of executing the Command like that, you could instead do the following:

1. Make sure ExecuteLoadItemsCommand() is returning a Task
2. Change the visibility of ExecuteLoadItemsCommand() to public
3. Change the name to ExecuteLoadItemsAsync() which is a common notation for methods that return asynchronous Tasks

Your new method signature should look something like this:

public async Task ExecuteLoadItemsAsync()
{
    //...
}

Then, in your page's code-behind, you could change the OnAppearing() method override to run asynchronously by adding the async keyword to the signature and then awaiting the ExecuteLoadItemsAsync() method:

protected override async void OnAppearing()
{
    base.OnAppearing();

    await viewModel.ExecuteLoadItemsAsync();

    //...
}

This way, you're executing and awaiting the method directly. The Command can still be used for any UI elements to bind to, such as buttons.

Optional improvement

In your ViewModel, you could use an AsyncRelayCommand instead of Command like this:

private AsyncRelayCommand _loadItemsCommand;
public AsyncRelayCommand LoadItemsCommand => _loadItemsCommand ??= new AsyncRelayCommand(ExecuteLoadItemsAsync);

However, this doesn't change the fact that you shouldn't execute the Command in your page's code behind the way you've been attempting.