When responding to an event in a textbox using C++/winrt I need to use ScrollViewer.ChangeView(). Trouble is, nothing happens when the call executes and I expect that is because at that moment the code is in the wrong thread; I have read this is the cause for lack of visible results from ChangeView(). It appears that the proper course is to use CoreDispatcher.RunAsync to update the scroller on the UI thread. The example code for this is provided only in C# and managed C++, however, and it is a tricky matter to figure out how this would look in normal C++. At any rate, I am not getting it. Does anyone have an example of the proper way to call a method on the UI thread in C++/winrt? Thanks.
[UPDATE:] I have found another method that seems to work, which I will show here, though I am still interested in an answer to the above. The other method is to create an IAsyncOperation that boils down to this:
IAsyncOperation<bool> ScrollIt(h,v, zoom){
co_await m_scroll_viewer.ChangeView(h,v,zoom);
}
The documentation entry Concurrency and asynchronous operations with C++/WinRT: Programming with thread affinity in mind explains, how to control, which thread runs certain code. This is particularly helpful in context of asynchronous functions.
C++/WinRT provides helpers winrt::resume_background() and winrt::resume_foreground(). co_await-ing either one switches to the respective thread (either a background thread, or the thread associated with the dispatcher of a control).
The following code illustrates the usage:
IAsyncOperation<bool> ScrollIt(h, v, zoom){
co_await winrt::resume_background();
// Do compute-bound work here.
// Switch to the foreground thread associated with m_scroll_viewer.
co_await winrt::resume_foreground(m_scroll_viewer.Dispatcher());
// Execute GUI-related code
m_scroll_viewer.ChangeView(h, v, zoom);
// Optionally switch back to a background thread.
// Return an appropriate value.
co_return {};
}
Related
I'm writing a small custom player based on libvlc. I've used much of the code from https://github.com/hartror/python-libvlc-bindings/blob/master/examples/gtkvlc.py that plays a single track just like I need.
Now I want to swtich to another track after previous has finished. To do that I catch callback "EventType.MediaPlayerEndReached" and in callback handler I write code:
<------>def endCallback(self,event):
<------><------>fname = vlc_controller.GetNextTrack()['url']
<------><------>self.w.set_title(fname)
<------><------>self.vlc.player.set_mrl(fname)
<------><------>self.w.set_title('after set_mrl')
<------><------>self.vlc.player.play()
<------><------>self.w.set_title('after play')
Now when this code gets executed it stucks on self.vlc.player.set_mrl(fname) and does not go any further and as a result I see NO NEXT TRACK.
I've tried different variations of this code using (vlc.stop(), vlc.set_media instead of vlc.set_mrl) but nothing works
Finally....
I think the best choise is to make multi threaded application on python
2 Threads:
Main thread - gtk.loop and displaying video an some other thinks
Additional thread that makes media switching.
For some time I was afraid of multithreading but now I know that this was the best way to do this task
For those facing similar issue in C# (Xamarin), per VLC documentation, calling the VLC object from an event tied to the object may freeze the application. So they said to use the code below inside the trigger event.
ThreadPool.QueueUserWorkItem(_ => PlayNext());
An example would be calling the code above inside the EndReach event.
Where PlayNext() is your own custom method to handle the next action you want to execute.
I struggled with this for about a day before seeing that portion of the documentation.
I am doing meta-programming with objective-C and try to automate some of an application functions. Thus, I am not changing the source code files and the view controllers of the application but from another file I am managing to get the UI navigation stack and I am using Objective-C Runtime Reference to find the tappable UI elements and the actions. for example for a button I found the target and action and call objc_msgSend to programatically fire the event.
step = (NSObject *)objc_msgSend(element.target, NSSelectorFromString(element.action));
However I need to be notified when the action was done, or in other word, I need to wait until the action was done and then continue my automation. I was thinking of using NSNotificationCenter
//To raise an event
[[NSNotificationCenter defaultCenter] postNotificationName:FIRE_EVENT_NOTIFICATION object:self];
but doesn't look like working.
I am even thinking of using Categories or
So I am not sure if there is anyway to wait for objc_msgSend and where should I continue.
It isn't entirely clear what you are trying to do and the exact problem that you are having but I'll have a go at answering your question.
If I understand correctly you are trying to fire the action associated with a UI element, presumably something like a button press. You have a reference to the element in element and you want to call the associated action on the elements target. The following assumes the action is an IBAction.
The simplest way to do this would presumably be:
[element.target performSelector:element.action];
Note: element.action almost certainly returns a SEL (a selector) not an NSString so there is no need to run it through NSSelectorFromString().
Normally, an IBAction event would receive the clicked on element as a parameter so I think you might want to do:
[element.target performSelector:element.action withObject:element];
IBAction's have no return value so there is nothing to store when the method returns.
performSelector: and performSelector:withObject: will only return once the called method has run to completion. You shouldn't need to organise some sort of notification of the action completing.
However, if the action you are calling is launching code on another thread then it is possible that the called action will return before the result of pressing the button has completed. This will be difficult to monitor without knowledge of the code that is being run.
If, for some reason, you have to use objc_msgSend then you would use the following:
objc_msgSend(element.target, element.action, element);
Like performSelector:, objc_msgSend will only return when the called method has run to completion.
Hopefully I have understood your question and my answer makes sense, it is entirely possible I'm barking up the wrong tree though.
My WinForm apps needs to execute complex query with significant execution time, I have no influence (about 10mins)
When query is executing user sees 'application not responding' in task manager, which is really confusing to user, also not very professional...
I believe that query shall be executed in different thread or so. Have tried some approaches but have difficulties to make it really working (execute query, force main application wait result, return back to main app, possibility to cancel execution etc)
I wonder if you have own / good working solution for that. Code samples would be also very welcome :)
Also I believe there might exist some ready to use utilities / frameworks allowing simple execution of that.
The simplest approach here would be to do that work from a BackgroundWorker. MSDN has examples for this. This then executes on a worker thread, with events for completion/error/etc. It can also support cancel, but your operation needs to be coded to be interruptable.
Another approach is the Task API in 4.0; but if you use this you'll need to get back to the UI thread (afterwards) yourself. With BackgroundWorker this is automatic (the events are raised on the UI thread).
If ExecuteQuery if the method you want to execute, you can do:
void SomeMethod() {
var thread = new Thread(ExecuteQuery);
thread.Start();
}
void ExecuteQuery() {
//Build your query here and execute it.
}
If ExecuteQuery receives some parameters, like:
void ExecuteQuery(string query) {
//...
}
You can do:
var threadStarter = () => { ExecuteQuery("SELECT * FROM [Table]"); };
var thread = new Thread(ThreadStarter);
thread.Start();
If you want to stop the execution of the background thread, avoid calling thread.Abort() method. That will kill the thread, and you do not want this, because some incosistency could appear in your database.
Instead, you can have a bool variable visible from ExecuteQuery and from outside you can set it to True when you want to stop it. Then all you have to do is check in some parts of the code inside ExecuteQuery if that variable is still True. Otherwise, do some rollback to maintain the database stable.
Be sure you set that bool variable volatile
Edit:
If you want the UI to wait from the background thread, I usually do:
Start the background thread
Start some progress bar in the UI
Disable some controls (like buttons, etc) to avoid the user to click them while the background thread is working (eg: If you're executing the thread when a user clicks a button, then you should disable that button, otherwise multiple queries will be ocurring at the same time).
After finished the thread, you can stop progress bar and enable controls again.
How to know when the thread finished? You can use Events for that. Just create an event and fire it when it finishes, and do whatever you want inside the event handler...
Be sure you're accessing correctly to UI controls from the background thread, otherwise it will give you an error.
If you are new to threads and task is simple (as it seems), you should try to use standard background worker component.
Is there a way to update UI manually?
For example, I have a function which updates UI and execute some logic.
After the UI update, it will execute some logic that will take a long time and update of UI has to be wait until the execution of logic is finished.
Is there a way to update UI manually befor even the logic is even executed?
It seems that thread can be used in here.
But Is there a way to solve this by not using thread?
Also, using if thread can be used, what is the best practice?
Thanks!
Because the UI thread is the main thread of your app, it's generally not a good idea to process big operations on it because you froze your UI in the meantime (which is not user friendly).
The way you use thread depends on what you want to do, you can for example just use - (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg or you can create your own thread and be more specific.
Don't forget to call - (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait if you want to do some changes on the UI from other thread.
You'll find all you need about thread programming in this guide from iOS Reference Library.
Hope this helps !
Are you thinking about a simple progress bar? If that is the case, then you can use something using NSApp (see the section under Managing the Event Loop) and use runModalForWindow: and runModalSession: These will allow you to open a progress panel, report on status and allow a method to cancel the operation.
Because the operation is modal, other UI elements will be deactivated until the panel is dismissed.
I'm porting an archaic C++/Carbon program to Obj-C and Cocoa. The current version uses asynchronous usb reads and GetNextEvent to read the data.
When I try to compile this in Objective C, GetNextEvent isn't found because it's in the Carbon framework.
Searching Apple support yields nothing of use.
EDIT TO ADD:
Ok, so what I'm trying to do is run a document scanner through USB. I have set up the USBDeviceInterface and the USBInterfaceInterface (who came up with THAT name???) and I call (*usbInterfaceInterface)->WritePipeTO() to ask the scanner to scan. I believe this works. AT least the flatbed light moves across the page...
Then I try to use *(usbInterfaceInterface))->ReadPipeAsyncTO() to read data. I give this function a callback function, USBDoneProc().
The general structure is:
StartScan()
WaitForScan()
StartScan() calls the WritePipeTO and the ReadPipeAsyncTO
WaitForScan() has this:
while (deviceActive) {
EventRecord event;
GetNextEvent(0,&event);
if (gDataPtr != saveDataPtr) { // more data up the timeout
timeoutTicks = TickCount() + 60 * 60;
saveDataPtr = gDataPtr;
}
if (TickCount() > timeoutTicks) {
deviceActive = false;
}
}
Meanwhile, USBDoneProc incrementing gDataPtr to be the end of the data that we've read so far. It gets called several times during the asynchronous read, called automatically by the callback, as far as I can tell.
If I cake out the GetNextEvent() call in the WORKING code the USBDoneProc doesn't get called until the asynchronous readpipe timesout.
So it looks to me that I need something to give control back to the event handler so that the USBRead interrupts can actually interrupt and make the USBDoneProcget called...
Does that make any sense?
thanks.
I suppose the nearest thing to a Cocoa equivalent would be -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:]. But bear in mind that GetNextEvent is archaic even for Carbon. The preferred way of handing events is the "don't call us, we'll call you" scheme, where the app calls NSApplicationMain or RunApplicationEventLoop and events are dispatched to you.
EDIT to add: Does you app have a normal event loop? If so, maybe WaitForScan could start a Carbon timer and return to the event loop. Each time the timer fires, do what you did in the WaitForScan loop.
there is a USB hidapi that works for mac on windows.
http://www.signal11.us/oss/hidapi/
may this could be of help to you?
It works fine (I can list the connected USB devices and connect/write/read to a device);
however, if i USB device is connected/disconnected during the runtime of the application, I don't see the new connected/disconnected devices.
See: https://github.com/signal11/hidapi/issues/14
If I add the following code to hidapi, then hidapi detects the new USB devices.
#include <Carbon/Carbon.h>
void check_apple_events() {
printf("check_apple_events\n");
RgnHandle cursorRgn = NULL;
Boolean gotEvent=TRUE;
EventRecord event;
while (gotEvent) {
gotEvent = WaitNextEvent(everyEvent, &event, 0L, cursorRgn);
}
}
I need to compile this on OSX10.5 because it uses Carbon instead of Cocoa.
I am currently looking how to transform this to Cocoa.
you are also trying to move your code to cocoa, right?
let me know if you find out; I'll post it here if I get it.
regards,
David
Have you considered throwing the whole thing out and using Image Kit's new-in-10.6 scanner support instead? Even if it's custom, writing a TWAIN driver for it might be easier (and is certainly better) than trying to twist Cocoa into a GetNextEvent shape.