How can you make a background web request and then update the UI, but have all the code that does the web requesting/parsing in a separate class so you can use it in multiple places? I thought I could use the classes methods as event handlers for a BackgroundWorker class, like
APIHelper mHelper = new APIHelper("http://example.com?foo=bar");
BackgroundWorker bw = new BackgroundWorker();
bw.DoWork +=new DoWorkEventHandler(mHelper.GetResponse);
bw.RunWorkerCompleted +=new RunWorkerCompletedEventHandler(mHelper.HandleResponse);
bw.RunWorkerAsync();
where APIHelper has the method
public void GetResponse(object sender, DoWorkEventArgs e)
{
BackgroundWorker worker = (BackgroundWorker) sender;
WebRequest request = HttpWebRequest.Create(this.URL);
IAsyncResult result = (IAsyncResult)
request.BeginGetResponse(ResponseCallback, request);
}
but then I don't know how to access the worker thread from ResponseCallback and, anyway, HandleResponse gets called first (obviously). (I tried putting in result.AsyncWaitHandle.WaitOne(); but I get a NotSupportedException error.) Yet I can't work out how to make the web request call synchronously. I'm clearly trying to go about this the wrong way, but I have no idea what the right way is.
ETA:
My aim is to be able to go:
user clicks (a) button(s) (on various pages)
a "working" message is displayed on the UI thread (and then input is blocked)
in a background thread my APIHelper class makes the relevant API call, gets the response, and passes it back to the UI thread; I only seem to be able to do this by starting another thread and waiting for that to return, because there's no synchronous web requests
the UI thread updates with the returned message (and input continues as before)
I can do the first two bits, and if I have the response, I can do the last bits, but I can't work out how to do the middle bit. Hopefully that made it clearer!
It took me several tried before I found there is a Dispatcher.
During the BackgroundWorker's dowork and complete methods you can call:
this.Dispatcher.BeginInvoke(() =>
{
// UPDATE UI BITS
});
I think the Dispatcher is only available in the view. So I'm not sure if the methods can exist outside of the xaml.cs
Put whatever you want to update in your UI; when updating an ObservableCollection you must do the update of you items in the Dispatcher.BeginInvoke too
This link might be a good read too:
http://www.windowsphonegeek.com/articles/All-about-Splash-Screens-in-WP7-ndash-Creating-animated-Splash-Screen
Update to assist notes
This is just a rough idea mind you...
bw.DoWork +=new DoWorkEventHandler(DoWork);
bw.RunWorkerCompleted +=new RunWorkerCompletedEventHandler(Complete)
// At least I think the EA is DoWork....
public void DoWork(object sender, DoWorkEventArgs e)
{
mHelper.GetResponse();
this.Dispatcher.BeginInvoke(() =>
{
UIObject.Visibility Collapse.
});
// Wait and do work with response.
});
}
public void Complete(object sender, RunWorkerCompleteEventArgs e)
{
this.Dispatcher.BeginInvoke(() =>
{
UIObject.Visible ....
});
}
I'd put all this logic in a viewmodel that the viewmodel of each page inherits from.
Have the pages bind to properties on the viewmodel (such as ShowLoading, etc.) which the model updates appropriately. i.e. before making the webrequest and in the callback.
As you won't be running the viewmodel code in the UI thread you also wouldn't need to run in a separate BackgroundWorker and you'll be able to access the properties of the viewmodel without issue.
It might be useful if you use a helper class that I have developed for WebDownload purposes during WP7 development.
I'm using it in 2-3 WP7 apps and no problem so far. Give it a go to see if it helps. You can get the class from the my blog linked bellow:
http://www.manorey.net/mohblog/?p=17#content
[NOTE] When working with this class you don't need to run anything in a background worker or new thread; it handles it all asynchronously.
Related
I've been having some issues with threading in monotouch. My app makes use of an external library which I've linked with and it works fine. Because of the nature of the app and the library I have to make all the calls to it on a single separate thread.These calls will generally be :
Random non deterministic caused by user
Every t miliseconds(around 20ms). Like an update function
After reading for a bit I decided to try out NSThread. I've managed to call the Update function by attaching an NSTimer to the thread's RunLoop and it's all working fine. The problem that I'm having now is calling other methods on the same thread. I read somewhere that using PerformSelector on the RunLoop adds the selector invocation to the RunLoop's queue and invokes it when available, which is basically exactly what I need. However the methods that I need to call :
Can have multiple paramteres
Have callbacks, which I need to invoke on the main thread, again with multiple parameters
For the multiple parameters problem I saw that NSInvocation can be a solution, but the life of me I can't figure out how to do it with monotouch and haven't found any relevant examples.
For the actuals calls that I need to make to the library, I tried doing a generic way in which I can call any function I choose via delegates on a particular thread, which sort of works until I'm hit with the multiple parameters and/or callbacks to the main thread again with multiple parameters. Should I maybe just register separate selectors for each (wrapped)function that I need to call from the library?
I'm not hellbent on using this approach, if there is a better way I'm open to it, it's just that after searching for other options I saw that they don't fit my case:
GCD(not even sure I have it in monotouch) spawns threads on it's own whenever necessary. I need a single specific thread to schedule my work on
NSInvocationQueue(which uses GCD internally from what I read) does the same thing.
pThreads, seem overkill and managing them will be a pain(not even sure I can use them in monotouch)
I'm not an iOS developer, the app works fine with monodroid where I had Runnables and Handlers which make life easier :) . Maybe I'm not looking at this the right way and there is a simple solution to this. Any input would be appreciated.
Thanks
UPDATE
I was thinking of doing something along these lines :
Have a simple wrapper :
class SelectorHandler : NSObject
{
public static Selector Selector = new Selector("apply");
private Action execute;
public SelectorHandler(Action ex)
{
this.execute = ex;
}
[Register("apply")]
private void Execute()
{
execute();
}
}
Extend NSThread
public class Daemon : NSThread
{
public void Schedule(Action action)
{
SelectorHandler handler = new SelectorHandler(action);
handler.PerformSelector(SelectorHandler.Selector, this, null, true);
}
}
Then, when I want to call something I can do it like this :
private Daemon daemon;
public void Call_Library_With_Callback(float param, Action<int> callback)
{
daemon.Schedule(() =>
{
int callbackResult = 0;
//Native library calls
//{
// Assign callback result
//}
daemon.InvokeOnMainThread(() =>
{
callback(callbackResult);
});
});
}
I'm trying to do a simple chat application on javafx
my actual problem is the thread to insert updates into a observablelist and set it on a listview
the code im using :
String message_reçu;
try {
out = new PrintWriter(socket.getOutputStream());
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
while (true) {
message_reçu = in.readLine();
if (message_reçu != null) {
messagesReçus.add(message_reçu);
}
if (message_reçu.equals("QUIT")) {
break;
}
}
in.close();
out.close();
socket.close();
I did this inside of a runnable class and once the server fire a msg the thread insert the msg on the list and shows on the listview but the thread dies instead of keep the work
I did a search on it and every one says to use a runlater but I’m completely lost there I did declare a runlater but I’m not sure how to execute it so any help is welcome
Thanks
Other Answers
This question is largely a duplicate of the following questions, so also refer to the answers to those:
Usage of JavaFX Platform.runLater and access to UI from a different thread
Platform.runLater and Task in JavaFX
Solution
Your question is little more specific than those though, so I'll provide some extra info.
For your particular code you want to wrap the add call in Platform.runLater:
Platform.runLater(new Runnable() {
public void run() {
messagesReçus.add(message_reçu);
}
});
Everything else in your example stays as it is.
Background Information
JavaFX UI updates must be made on the JavaFX application thread or there is a high likelihood that your program will malfunction in unexpected ways.
The ListView control listens for changes to the ObservableList backing the ListView cell values. When that list changes, a UI update is immediately triggered on the thread the originally updated the list.
Once you wrap the list modifications in Platform.runLater, you ensure that the subsequently triggered UI update is performed on the JavaFX application thread rather than your user thread.
I'm not very experienced with this topic so forgive me if this isn't very clear.
I've created a Portable Class Library that has an ObservableCollection of Sections, and each secion has an ObservableCollection of Items.
Both of these collections are bound to the UI of separate Win8 and WP8 apps.
I'm trying to figure out the correct way to populate these collections correctly so that the UI gets updated from the PCL class.
If the class was inside the win8 project I know I could do something like Dispatcher.BeginInvoke, but this doesn't translate to the PCL, nor would I be able to reuse that in the WP8 project.
In this thread (Portable class library equivalent of Dispatcher.Invoke or Dispatcher.RunAsync) I discovered the SynchroniationContext class.
I passed in a reference to the main app's SynchroniationContext, and when I populate the sections I can do so because it's only the one object being updated:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
UpdateSections(sections);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post(UpdateSections, sections);
}
However, when I try to do the same thing with articles, I have to have a reference to both the section AND the article, but the Post method only allows me to pass in a single object.
I attempted to use a lambda expression:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
section.Items.Add(item);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post((e) =>
{
section.Items.Add(item);
}, null);
}
but I'm guessing this is not correct as I'm getting an error about being "marshalled for a different thread".
So where am I going wrong here? how can I update both collections correctly from the PCL so that both apps can also update their UI?
many thanks!
Hard to say without seeing the rest of the code but I doubt is has anything to do with Portable Class Libraries. It would be good to see the details about the exception (type, message and stack trace).
The way you call Post() with more than argument looks correct. What happens if you remove the if check and simply always go through SynchronizationContext.Post()?
BTW: I don't explicitly pass in the SynchronizationContext. I assume that the ViewModel is created on the UI Thread. This allows me to capture it like this:
public class MyViewModel
{
private SynchronizationContext _context = SynchronizationContext.Current;
}
I would recommend that at least in your ViewModels, all publicly observable state changes (ie property change notifications and modifications to ObservableCollections) happen on the UI thread. I’d recommend doing the same thing with your model state changes, but it might make sense to let them make changes on different threads and marshal those changes to the UI thread in your ViewModels.
To do this, of course, you need to be able to switch to the UI thread in portable code. If SynchronizationContext isn’t working for you, then just create your own abstraction for the dispatcher (ie IRunOnUIThread).
The reason you were getting the "marshalled on a different thread" error is that you weren't passing the item to add to the list as the "state" object on the Post(action, state) method.
Your code should look like this:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
section.Items.Add(item);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post((e) =>
{
var item = (YourItemnType) e;
section.Items.Add(item);
}, item);
}
If you make that change, your code will work fine from a PCL.
Im working on a Windows Service in which I would like to have two threads. One thread should look for updates (in a RSS feed) and insert rows into a DB when updates is found.
When updates are found I would like to send notification via another thread, that accesses the DB, gets the messages and the recipients and then sends notifications.
Perhaps the best practice isn't to use two threads. Should I have db-connections in both threads?
Could anyone provide me with tips how to solve this?
The major reason to make an application or service multithreaded is to perform database or other background operations without blocking (i.e. hanging) a presentation element like a Windows form. If your service depends on very rapid polling or expects db inserts to take a very long time, it might make sense to use two threads. But I can't imagine that either would be the case in your scenario.
If you do decide to make your service multithreaded, the two major classes in C# that you want to look into are BackgroundWorker and ThreadPool. If you want to do multiple concurrent db inserts (for example, if you want to execute an insert for each of multiple RSS feeds polled at the same time), you should use a ThreadPool. Otherwise, use a BackgroundWorker.
Typically, you'd have a db access class that would have a method to insert a row. That method would create a background worker, add DoWork handler to some static method in that db access class to the background worker, then call DoWorkAsync. You should only have db connection settings in that one class in order to make maintaining the code easier. For example:
public static class DbAccess
{
public void InsertRow(SomeObject entity)
{
BackgroundWorker bg = new BackgroundWorker();
bg.DoWork += InsertRow_DoWork;
bg.RunWorkerCompleted += InsertRow_RunWorkerCompleted;
bg.RunWorkerAsync(entity);
}
private void InsertRow_DoWork(object sender, DoWorkEventArgs e)
{
BackgroundWorker bg = sender as BackgroundWorker;
SomeObject entity = e.Argument as SomeObject;
// insert db access here
}
private void InsertRow_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
// send notifications
// alternatively, pass the InsertRow method a
// delegate to a method in the calling class that will notify
}
}
I have a Silverlight application that uses WCF services and also uses the Wintellect Power Threading library to ensure logic executes fully before the application continues. This is achieved by calling back to the application using delegates so it can continue after the service call has completely finished.
I wish to achieve the same thing in another part of my application but without the use of callbacks e.g. call method that uses WCF service to say load an object from the database, wait for this to return and then return the Id of the object from the original method called.
The only way I could see to do this was to carry out the call to the WCF service in a helper library which loads the object on a different thread and the original method would keep checking the helper library (using static variables) to wait for it to complete and then return it.
Is this the best way to achieve this functionality? If so here are details of my implementation which is not working correctly.
public class MyHelper
{
private static Thread _thread;
private static User _loadedObject;
public static GetUser()
{
return _loadedObject;
}
public static void LoadObject(int userId)
{
_loadedObject = null;
ParameterizedThreadStart ts = new ParameterizedThreadStart(DoWork);
_thread = new Thread(ts);
_thread.Start(userId);
}
private static void DoWork(object parameter)
{
var ae = new AsyncEnumerator();
ae.BeginExecute(DoWorkWorker(ae, Convert.ToInt32(parameter)), ae.EndExecute);
}
private static IEnumerator<Int32> DoWorkWorker(AsyncEnumerator ae, int userId)
{
// Create a service using a helper method
var service = ServiceHelper.GetService<IUserServiceAsync>();
service.BeginGetUserById(userId, ae.End(), null);
yield return 1;
_loadedObject = service.EndGetUserById(ae.DequeueAsyncResult());
_thread.Abort();
}
}
My method then is:
public int GetUser(int userId)
{
MyHelper.LoadObject(userId);
User user = MyHelper.GetUser();
while (user == null)
{
Thread.Sleep(1000);
user = MyHelper.GetUser();
}
return user.Id;
}
The call to the get the user is executed on a different thread in the helper method but never returns. Perhaps this is due to the yield and the calling method sleeping. I have checked the call to get the user is on a different thread so I think everything should be kept separate,
The whole construct you are using does not match current best practices of Silverlight. In Silverlight your data access methods (via WebServices of course) are executed asynchronously. You should not design around that, but adapt your design accordingly.
However calling services sequentially (which is different than synchonously) can be valid in some scenarios. In this blog post I have shown how to achieve this by subscribing the Completed event of the remote call and block the UI in the meantime, with which the workflow looks and feels like normal async calls.
I believe calls to the server from Silverlight apps use events that fire on the UI thread; I think that's part of the Silverlight host environment in the browser and can't be worked around. So trying to call back to the server from another thread is never going to end well. If you are waiting in program code in the UI thread, your never going to get the call result events from your WCF calls.
You can simulate a synchronous call from a non-UI thread with a callback on the UI thread, but that is probably not what you want. It's better to bite the bullet and make your program logic work with the async calls Silverlight gives you.
If you code against the Interface created for your service reference you can call the Begin and End methods 'synchronously' for each one of your service calls, we then pass in an Action<T> to execute after the End methods has completed. Take note that you have to do this from a dispatcher. This is very close to making a synchronous call as the code to run after the call is still written where the call is made, and it executes after the service call is completed. It does however involve creating wrapper methods but we also worked around that by hiding our wrappers and generating them automatically. Seems like a lot of work but isn't, and ends up being more elegant than all the event handlers etc. Let me know if you need more info on this pattern