CODEWITHSUNDEEP
.nettask-parallel-library
Goran
Goran

Reputation: 6528

Asynchronous parent task with synchronous children tasks

I was wondering if this can be done without using coroutines:

private void GetAll()
{
    if (_refreshA)
    {
        collA = new ObservableCollection(GetA<A>());
        _refreshA = false;
    }

    if (_refreshB)
    {
        collB = new ObservableCollection(GetB<B>(param))
        _refreshB = false;
    }

    if (_refreshC)
    {
        collC = new ObservableCollection(GetC<C>())
        _refreshC = false;
    }
}

CollA, CollB and CollC are used on UI thread. I need GetAll do be executed on different thread than UI, and I need GetA(), GetB(param) and GetC() to be executed one after another (not in parallel).

Result shoul be (if all 3 _refreshX are true):

create new thread
execute GetA() on new thread
wait for data to arrive
update UI collection with new data
create new thread
execute GetB(param) on new thread
wait for data to arrive
update UI collection with new data
create new thread
execute GetC() on new thread
wait for data to arrive
update UI collection with new data

Can this be done only with TPL, or I need to use coroutines also?

Edit: since I have had wrong impression that async await cannot be used on .NET 4, and svick and Adam Robinson pointed that out to me, I will try to achieve this using async await:

System.Diagnostics.Debug.WriteLine(string.Format("Loading data started. Thread: {0}, {1}", System.Threading.Thread.CurrentThread.GetHashCode(), DateTime.Now));

IsBusy = true;
//Task.Factory.StartNew(() => GetDataBatch()); // UI is responsive
GetDataBatch(); // UI is not freezed (it is shown), but it is not responsive

System.Diagnostics.Debug.WriteLine(string.Format("Loading data completed. Thread: {0}, {1}", System.Threading.Thread.CurrentThread.GetHashCode(), DateTime.Now));

private async Task GetAll()
{
    if (_refreshA)
    {
        collA = new ObservableCollection(await Task.Run(() => GetA<A>()));
        _refreshA = false;

        System.Diagnostics.Debug.WriteLine(string.Format("GetA items loaded. Thread: {0}, {1}", System.Threading.Thread.CurrentThread.GetHashCode(), DateTime.Now));

    }

    if (_refreshB)
    {
        collB = new ObservableCollection(await Task.Run(() => GetB<B>(param)));
        _refreshB = false;

        System.Diagnostics.Debug.WriteLine(string.Format("GetB items loaded. Thread: {0}, {1}", System.Threading.Thread.CurrentThread.GetHashCode(), DateTime.Now));
    }

    System.Threading.Thread.Sleep(10000);

    if (_refreshC)
    {
        collC = new ObservableCollection(await Task.Run(() => GetC<C>()));
        _refreshC = false;

        System.Diagnostics.Debug.WriteLine(string.Format("GetC items loaded. Thread: {0}, {1}", System.Threading.Thread.CurrentThread.GetHashCode(), DateTime.Now));
    }
}

The result is:

Loading data started. Thread: 9, 15-Oct-12 02:35:00
Loading data completed. Thread: 9, 15-Oct-12 02:35:00
GetA items loaded. Thread: 9, 15-Oct-12 02:35:00
GetB items loaded.  Thread: 9, 15-Oct-12 02:35:01
GetC items loaded.  Thread: 9, 15-Oct-12 02:35:11

Problem: UI is not freezed (view is shown), but it is not responsive also. For example, if I hover with mouse over a menu item, nothing happens. I have a template (busy template) that is shown during data loading, which should indicate to user what is going on. This template is not being shown, looks like it doesn't have enough CPU time to draw itself. If I use this code:

Task.Factory.StartNew(() => GetDataBatch()); // UI is responsive
//GetDataBatch(); // UI is not freezed (it is shown), but it is not responsive

then UI is responsive, busy data template is shown on screen, but the problems is that now all collection data belong to other thread than UI, so I cannot do any operation on them from UI thread.

How does async await handle this problem?

Upvotes: 1

Views: 323

Answers (2)

svick
svick

Reputation: 245046

Using C# 5 async-await, this would be quite simple: run the code that needs to run on the background thread using Task.Run() and then await the Task, to asynchronously wait for it to complete and resume on the UI thread:

private async Task GetAll()
{
    if (_refreshA)
    {
        collA = new ObservableCollection(await Task.Run(() => GetA<A>()));
        _refreshA = false;
    }

    if (_refreshB)
    {
        collB = new ObservableCollection(await Task.Run(() => GetB<B>(param)));
        _refreshB = false;
    }

    if (_refreshC)
    {
        collC = new ObservableCollection(await Task.Run(() => GetC<C>()));
        _refreshC = false;
    }
}

Upvotes: 3

Dax Fohl
Dax Fohl

Reputation: 10781

void GetAll() {
    new Thread(() => {
        if (_refreshA) {
            var alist = GetA<A>();
            Dispatcher.Invoke(new Action(() => { collA = new ObservableCollection<A>(alist); }));
            _refreshA = false;
        }

        if (_refreshB) {
            var blist = GetB<B>(param);
            Dispatcher.Invoke(new Action(() => { collB = new ObservableCollection<B>(blist); }));
            _refreshB = false;
        }

        if (_refreshC) {
            var clist = GetC<C>();
            Dispatcher.Invoke(new Action(() => { collC = new ObservableCollection<C>(clist); }));
            _refreshC = false;
        }
    }).Start();
}

Dispatcher.Invoke guarantees that the actions each complete before the method continues. Just for comparison, if you had wanted to do them in parallel you'd have needed to use Dispatcher.BeginInvoke instead.

Upvotes: 1

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