The main form of my application has an event that gets triggered upon completion of a certain task. The event completion launches a background thread and the called method in that thread runs a loop which takes a while to complete. This loop updates (or is supposed to update) the user on what's going on during its run by editing the label->Text property of the UserControl along with a percentage of completion in a different label in the same UserControl. There are some more labels in this UserControl that are updated, but are not of the concern for this question.
Here is how the methods and statements are called.
Event Completed -> Background Thread
Background Thread -> Process()
Process() -> for loop()
for loop() -> UserControl->SetMessage(System::String^ msg) and UserControl->SetPercentage(int Percentage)
(1) UserControl->SetMessage(System::String^ msg) -> stores msg in a private UserControl member and executes this->Invoke->SetMessage_sub()
(2) UserControl->SetPercentage(int prcnt) -> stores prcnt in a private UserControl member and executes this->Invoke->SetPercentage_sub()
(1) SetMessage_sub() -> label1->Text = _msg;
(2) SetPercentage() -> label1->Text = _prcnt;
However, I found that the labels were not getting updated for as long as the loop ran. The labels only updated once the loop finished its run inside Process().
I used the System::Console::WriteLine method to figure out where the problem occurs. I found out that all the methods were being called. But the execution would stop at where the program would reach label->Text property. Please help me out.
I have run into a similar problem recently and found that the GUI update calls must be made from either a thread running in the background or a background worker. Try calling the method that updates your GUI from a background thread or a background worker.
Edit 1 - it sounds like you've already tried a background thread. Try using a BackgroundWorker object to accomplish the task.
Edit 2 - upon re-reading your question, it occurred to me that the problem you are facing is the GUI not loading until the event method has returned. I encountered the same problem and asked my question here. To my surprise, all I got was downvotes and no answers. A comment tried to refer me to using BGWorker class to update GUI but none of them read my question carefully. I know background threads and background workers. I have tried both but the GUI does not update until return is called from the event method. In my case, after calling the BackgroundWorker to do the processing, I ran an empty while loop in the event method and called break when the BGWorker finished. But the GUI never updated until break was executed. So, my solution was to create a separate event for the processing and processing completion. The completion of the initial event calls the processing method and then processing method's completion calls the processionf completion event method.
Related
I have a function that runs an sql query for data that may or may not be there. Since I need to run this function continually until it returns the proper value how can I run a progress bar until the loop finishes.
status = Logic.ProcessResource(currentInstance)
While status.woID.Count <= 0
status = Logic.ProcessResource(currentInstance)
End While
How can I run this and show another form with a progress bar until the loop exits?
My comments as an answer...
Put that code into a different thread, then use a ProgressBar in "Marquee" mode to indicate an operation that is ongoing, but has no known ending time.
Yes...but you still need to put the query/loop in a different thread...otherwise the main UI thread will to be to busy to animate and remain responsive to the user.
Look at the BackgroundWorker control, or using a Task, with Async/Await.
You'd show the form, start the worker, wait for worker to finish, then close the form. The BackgroundWorker() has UI friendly events like RunWorkerCompleted that are already marshaled to the UI thread for you.
Ok I'm pretty new to using threads but so far I've managed to get the following:
Private Delegate Sub dlgUpdateText(text as string)
Private Sub UpdateStatus(text as string)
If rtxStatus.InvokeRequired then
Dim dlg as new dlgUpdateText(AddressOf UpdateStatus)
Me.Invoke(dlg, text)
Else
rtxStatus.text = text
End If
End Sub
and from my Async BackgroundWorker I call
UpdateStatus("Some text")
which seems to work ok however in my original code (which generates errors because I'm updating the control from the wrong thread) I used the following code to append to the (rich)textbox:
rtxStatus.Select(rtxStatus.TextLength, 0)
rtxStatus.SelectionColor = Color.Red
rtxStatus.AppendText("Some error occurred gathering data")
My question is how should I modify my new code to allow me to do this rather than just replace the text? I have read several guides on using delegates but I'm still lost on a few points so I don't really know what's going on with the code I have.
Bonus questions (which probably serve best to show what needs explaining to me!):
What does the Delegate Sub actually do? It doesn't seem to serve any purpose other than hold the property (text) that was already passed to the main Sub (UpdateStatus)?
What is happening when Me.Invoke is called? Me is the current form so when I pass the Delegate Sub and the text where is it specified that the text should be passed to the rtxSTatus.Text property?
UpdateStatus runs in the main thread (?) and is called from one of the background worker threads so why is the Invoke even necessary or is the UpdateStatus Sub run under the background thread that called it? When I altered the text that is applied in the Else statement to see which was run it seems that Invoke is never used to change the text in the box.
Any help would be really appreciated, I'm completely baffled by this - thanks!
Rather than creating a delegate I would suggest using the existing methods offered from a backgroundworker. The backgroundworker provides two methods to access the main thread:
The ProgressChanged event to update the main thread during backgroundworker processing and the RunWorkerCompleted event to update the main thread once the backgroundworker process is complete.
You can find this information and how to implement it from the following link:
http://msdn.microsoft.com/en-us/library/ywkkz4s1.aspx
I have a background worker I'm using to run through a large function in hopes of quickening my window load and responsiveness. When I put this function in the backgrounderworker's doWork event I'm just wondering if there's anyway I can prioritize that text to update as soon as the BGW is complete rather than waiting for the UI thread to finish and then update the textbox. Is this possible? I only ask because it seems like it's taking quite some to update the textbox after running through the function(which takes about 1.5-2s) but even if I start the BGW to begin even before the window loads(takes a good 5 seconds to load), it's still the last item on my window to update so I would like to(if possible), halt the UI thread from updating the UI until I update with what this function returns, then continue updating the rest of the labels.
Also, is there anyway to update two separate items at once or is it restricted to the one UI thread?
If you want to update UI elements, you have to do it on the UI thread. When the BackgroundWorker is finished, the RunWorkerCompleted event is raised on the UI thread. If the UI thread is busy doing something, then the completed event has to wait. In general, there's no safe way to interrupt the UI thread, make it process the RunWorkerCompleted event, and then go back to what it was doing.
UI elements must be updated from the UI thread. So you can't update two separate items at once.
I don't know how your initialization is structured, but if you have one group of items that you can initialize before the BGW is finished, and another group that can't update before the BGW is done, then do the first group and stop. Then have the RunWorkerCompleted handler do its update and all the rest of the updates. So it would look something like:
FormLoad()
start background worker
do first group of updates
RunWorkerCompletedHandler()
update from BGW calculation
do rest of updates
I am running a lengthly task in an Action and I would like to have a display of where I am at. For that I created a Text Field and I tried it with setStringValue:
[textField setStingValue: [NSSting stringWithFormat:#"%ld",currentValue]]
The code works but unfortunately it is not updating the NSTextField after every iteration but rather when the whole Action is done.
What am I doing wrong?
This is because applications with the Cocoa framework use an event loop to perform operations, and events occur in a completely serial fashion.
An event is basically any kind of action that the framework designer could not predict or found convenient to have run in a delayed manner. Since you can't predict when clicks will be performed, they need to be considered events; and for efficiency reasons (since you don't want to repaint a component multiple times if you don't need to), the repaint actions are events too.
Your action runs in response to a user event (for instance, a click on a button is an event) and therefore blocks all other events waiting in the queue until it's complete. However, components are repainted in response to a different, framework-triggered event, and as such the text field must wait until your action completes to repaint itself. This is why you cannot visually change the value of a text field from inside an action.
In order to notify your user of the progress of your task, you'll need to run it on a different thread. There's a lot to say about threads, so you should probably read some about them. I'm also sure that there are plenty of examples of how to run a long action in a background thread and update the UI accordingly for Cocoa all over the Internet.
When you click on a UI component, and it enters the Action block, the code is running on the main thread, the same thread that is painting the UI. If you run a long running operation in that block, it isn't going to paint until you are done because it is busy doing whatever you have it doing - you have hijacked the paint thread.
As said elsewhere, you need to spawn another thread, and then have the new thread perform the long running operation, and occasionally send messages to have the UI be updated by the main thread.
As a next step, go read the Apple documentation on NSThread, specifically:
+ (void)detachNewThreadSelector:(SEL)aSelector toTarget:(id)aTarget withObject:(id)anArgument
Be aware that threading is a non-trivial domain area, and be ready for some wierd behavior if you aren't careful.
I'm having some issues using dispatch_async. On my applications main/UI thread, I call dispatch_async on the global queue, and tell it to go do some function call which has a completion handler. I'm expecting the completion handler to get called but it does not appear to sometimes.
dispatch_queue_t hiq = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_async(hiq, ^(void){
[object doSomethingAndThenCompletionHandler:^(){
//Do some stuff because I'm done
}];
});
What is interesting is, I'm doing this in response to a controlTextDidChange whenever I type something into a textfield. When I click out of the textfield, all of the completion handlers fire at once.
My guess is that all of my completion handlers are getting queued up behind something on the run loop and then when my UI element loses focus, that frees up the queue. I'm not enough of an expert on run loops to know what exactly is happening as I'm typing in a textfield or combobox but perhaps it's tying up the runloop?
EDIT: I think it has to do with the run loop mode because when I print out the run loop mode it prints out as NULL except for in the completion handler when it prints out as the default mode. I'm using a combo box and the issue is only present when the combobox is open and expanded. UIScrollView blocks run loop? I think it might have to do with this issue. What do you guys think?
When you do [[NSRunLoop currentRunLoop] runUntilDate:fiveSecondsFromNow], it enters the run loop recursively, and the run loop processes events normally. So it will call your method again if the text field receives another event. The stack trace would look something like this (with lots more frames related to NSRunLoop):
main
NSApplicationMain
-[NSRunLoop runUntilDate:]
your method
-[NSRunLoop runUntilDate:]
your method
I have no idea why you're calling runUntilDate:, but it's probably not for a good reason.
If you want help understanding why your completion handler isn't getting called, then you need to show us the definition of your doSomethingAndThenCompletionHandler: method.
Figured out the issue I think. Some of the libraries I'm using are most likely sending NSURLConnections out without scheduling to run in common run loop modes. When the combo box is open the run loop mode changes to event tracking mode and then can't get call backs for them.