Suppose, I am presenting an UIAlertController, say myAlert, from main tread. myAlert has a action, defaultAction. I wonder if defaultActions handler by default runs on main queue or not. In other words, I wonder if there are some UI related operation inside doStuff in below code, am I required to wrap these UI tasks with main queue or it is guaranteed to run in main queue by OS?
UIAlertController* myAlert = [UIAlertController alertControllerWithTitle:#"My Alert"
message:#"This is an alert."
preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction* defaultAction = [UIAlertAction actionWithTitle:#"OK" style:UIAlertActionStyleDefault
handler:^(UIAlertAction * action) {
doStuff() // should I wrap doStuff in main queue, if doStuff has ui operations?
}
];
[alert addAction:defaultAction];
[self presentViewController:alert animated:YES completion:nil];
You are actually asking if it runs on the main queue, not if it is thread safe.
An object is "thread safe" if it can be accessed/modified from multiple threads without issue.
The answer to your question is that the action closure will run on the main queue as it results from a user interaction, so you do not need to explicitly dispatch UI updates onto the main queue.
Related
In the docs it says that coroutines are lighter than threads and so I wanted to use a kotlin coroutine instead of the BukkitRunnable.
//Defined as class field
private val scope = coroutineScope(Dispatchers.Default)
//In class method
scope.launch {/* wait some seconds and then change blockdata */}
Calling setBlockData from Dispatchers.Default thread throws an error because the spigot API is not thread safe and you can't call API stuff from a thread other than the main.
java.lang.IllegalStateException: Asynchronous block remove!
I was thinking that changing block data is the equivalent of android UI changes in Minecraft which means that the coroutine needs to be run/injected into the main thread. So it would make sense to run my coroutine in Dispatchers.Main. However, I can't find a way use Dispatchers.Main and set it to the main thread without getting an illegalStateException
I hope my logic is correct here
If you want a simple method that is able to bridge the suspending code with the main thread (with the possibility of fetching some information from the main thread and use that on your coroutine), you can use this method:
suspend fun <T> suspendSync(plugin: Plugin, task: () -> T): T = withTimeout(10000L) {
// Context: The current coroutine context
suspendCancellableCoroutine { cont ->
// Context: The current coroutine context
Bukkit.getScheduler().runTask(plugin) {
// Context: Bukkit MAIN thread
// runCatching is used to forward any exception that may occur here back to
// our coroutine, keeping the exception transparency of Kotlin coroutines
runCatching(task).fold({ cont.resume(it) }, cont::resumeWithException)
}
}
}
I've commented on what context each part of the code is executed so you can visualize the context switch. suspendCancellableCoroutine is a way of getting hold of the continuation object all coroutines use under the hood, so we can manually resume it once the main thread execute our task.
The outer block withTimeout is used so that if the main thread does not complete our task within 10 seconds, our coroutine gives up instead of hanging forever.
And the use is very simple too:
val plugin = // comes from somewhere
// example coroutine scope
CoroutineScope(Dispatchers.Default).launch {
// doing stuff async
// oh no, I need some data from the main thread!
val block = suspendSync(plugin) {
// this code runs on the MAIN thread
Bukkit.getWorld("blah").getBlockAt(0, 0, 0)
}
// back to async here, do stuff with block (just don't MODIFY it async, use more suspendSync if needed)
}
If you have any questions or think I can improve this answer, don't be afraid of letting me know.
This question is about GTK and threads.
You may find it useful if your application crashes, freezes or you want to have a multithreaded GTK application.
Main Loop
In order to understand GTK you must understand 2 concepts.
All contemporary GUIs are single-threaded. They have a thread which processes events from window system (like button, mouse events).
Such a thread is called main event loop or main loop.
GTK is also single threaded and not MT-safe. This means, that you must not call any GTK functions from other threads, as it will lead to undefined behaviour.
As Gtk documentation states,
Like all GUI toolkits, GTK+ uses an event-driven programming model. When the user is doing nothing, GTK+ sits in the “main loop” and waits for input. If the user performs some action - say, a mouse click - then the main loop “wakes up” and delivers an event to GTK+. GTK+ forwards the event to one or more widgets.
Gtk is event-based and asynchronous. It reacts to button clicks not in the exact moment of clicking, but a bit later.
It can be very roughly written like this (don't try this at home):
static list *pollable;
int main_loop (void)
{
while (run)
{
lock_mutex()
event_list = poll (pollable); // check whether there are some events to react to
unlock_mutex()
dispatch (event_list); // react to events.
}
}
void schedule (gpointer function)
{
lock_mutex()
add_to_list (pollable, something);
unlock_mutex()
}
I want a delayed action in my app
For example, hide a tooltip in several seconds or change button text.
Assuming your application is single-threaded, if you call sleep() it will be executed in main loop.
sleep() means, that this particular thread will be suspended for specified amount of seconds. No work will be done.
And if this thread is main thread, GTK will not be able to redraw or react to user interactions. The application freezes.
What you should do is schedule function call. It can be done with g_timeout_add or g_idle_add
In the first case our poll() from snippet above will return this event in several seconds. In the latter case it will be returned when there are no events of higher priority.
static int count;
gboolean change_label (gpointer data)
{
GtkButton *button = data;
gchar *text = g_strdup_printf ("%i seconds left", --count);
if (count == 0)
return G_SOURCE_REMOVE;
return G_SOURCE_CONTINUE;
}
void button_clicked (GtkButton *button)
{
gtk_button_set_label (button, "clicked");
count = 5;
g_timeout_add (1 * G_TIME_SPAN_SECOND, change_label, button);
}
Returning a value from function is very important. If you don't do it, the behaviour is undefined, your task may be called again or removed.
I have a long-running task
Long-running tasks aren't different from calling sleep. While one thread is busy with that task, it can't perform any other tasks, obviously. If that is a GUI thread, it can't redraw interface. That's why you should move all long-running tasks to other threads. There is an exception, though: non-blocking IO, but it's out of topic of my answer.
I have additional threads and my app crashes
As already mentioned, GTK is not MT-safe. You must not call Gtk functions from other threads.
You must schedule execution. g_timeout_add and g_idle_add are MT-safe, unlike other GTK functions.
That callbacks will be executed in main loop. If you have some shared resources between callback and thread you must read/write them atomically or use a mutex.
static int data;
static GMutex mutex;
gboolean change_label (gpointer data)
{
GtkButton *button = data;
int value;
gchar *text;
// retrieve data
g_mutex_lock (&mutex);
value = data;
g_mutex_unlock (&mutex);
// update widget
text = g_strdup_printf ("Current data value: %i", value);
return G_SOURCE_REMOVE;
}
gpointer thread_func (gpointer data)
{
GtkButton *button = data;
while (TRUE)
{
sleep (rand_time);
g_mutex_lock (&mutex);
++data;
g_mutex_unlock (&mutex);
g_idle_add (change_label, button);
}
}
Make sure mutexes are held as little as possible. Imagine you lock a mutex in another thread and do some IO. The main loop will be stuck until the mutex is released. There is g_mutex_try_lock() that returns immidiately, but it can bring additional syncronization problems because you can't guarantee that the mutex will be unlocked when mainloop tries to lock it.
Follow up: but python is single-threaded and GIL et cetera?
You can imagine that python is multi-threaded application run on a single-core machine.
You never know when the threads will be switched. You call a GTK function but you don't know in which state the main loop is. Maybe it free'd resources just a moment before. Always schedule.
What is not discussed and further reading
Detailed documentation on glib main loop can be found here
GSource as a more low-level primitive.
GTask
I have a worker thread doing calculation on the background and I want to send a event/message to call a update function to update the graphics on screen once the worker thread finish calculation.
How do I do that in cocos2d ?
Some demo code:
-(void) updateGraphic
{
//this one update all the graphics/sprite
}
//note workerThreadFunc is being used to start a new thread
-(void) workerThreadFunc
{
//...
//...
//finish calculation here
//since it's in a different thread, I cannot call updateGraphic directly here
//So I need a event to notify update Graphic here somehow
}
Cocos2D calls the -(void) draw {} method on all nodes automatically on the main thread. You do not need to call that method from another thread, and you can not perform custom OpenGL drawing outside the draw method.
In order to call a method that should be performed on the main thread, use the performSelectorOnMainThread method.
I've achieve it via pthreads, it needs to do some changes in CCDirector.cpp & CCDirector.h
the details is in this thread.
to use it, we can register handleMessageInUI in UI thread, then worker thread sends a message to UI thread, which will call handleMessageInUI to do UI drawing. some sample code is below:
In UI thread, we can register a handler to process message in UI thread.
bool HelloWorldScene::handleMessageInUIThread(const EXTCCMessage &msg) {
// implementations
// return true if this handler has processed this msg,
// otherwise, false is returned
switch (msg.msgId) {
case 2:
break;
default:
return false;
}
return true;
}
// register this Handler to UI Threader
CCDirector::mainLoopHandler()->registerHandler(this, (EXTCCHandleMessage)&HelloWorldScene::handleMessageInUIThread);
send a message to UI thread in a worker thread
EXTCCMessage msg;
msg.msgId = 2;
msg.msgData1 = NULL;
// "msg" will be processed by "handleMessageInUIThread" in UI thread
CCDirector::mainLoopHandler()->postMessage(msg);
I am writing an app that can revert the firmware of a particular device. While executing this revert code, I wish to display a progress indicator.
This problem is of course best tackled with the use of multiple threads (http://stackoverflow.com/questions/1225700/can-i-start-a-thread-by-pressing-a-button-in-a-cocoa-interface-and-keep-using-in).
I have implemented the performSelectorInBackground method, which (according to the documentation) launches the specified selector in a separate thread. Meanwhile, my GUI is updated from the main thread by querying the 'reverter' object.
However, the GUI does not seem to be updating until the code in the secondary thread has finished executing. I obviously need the two to run in parallel. Here is what I've got so far - I'd be really grateful for any help as this is my first time with threading.
-(IBAction)pushButton:(id)sender{
//instatiate reverter object, which does all the firmware processing
Reverter *reverter = [[Reverter alloc] init];
//update the GUI to show a tab with a progress indicator
[tabView selectTabViewItemWithIdentifier:#"RevertProgressTab"];
//process revert code in a separate thread
[reverter performSelectorInBackground:#selector(revertFirmware) withObject:nil];
//process is complete when reverter progress reaches 100
while (!([reverter progress] == 100)) {
//check for failure
if ([reverter hasFailed]) {
[self showRevertFailureTab:nil];
return;
}
//update the progress indicator in the interface
[revertProgressBar setDoubleValue:(double)[reverter progress]];
[NSThread sleepForTimeInterval:0.05];
}
[self showRevertSuccessTab:nil];
}
Have I done anything obvious that would stop the GUI from being updated while the revertFirmware method runs?
Your while loop
while (/*condition*/) {
[NSThread sleepForTimeInterval:x];
}
will prevent your UI from updating. Your UI will only update as soon as your pushButton: method returns.
Instead of polling I would advice you start using an asynchronous event model:
Add a delegate to your reverter object
#protocol ReverterDelegate <NSObject>
- (void) reverterProgressDidUpdate:(float)progress;
#end
#interface Reverter : NSObject {
id<ReverterDelegate> delegate;
}
#property(assign) id<ReverterDelegate> delegate;
#end
Register your controller class as a delegate to your reverter
reverter.delegate = self;
and handle that event
- (void) reverterProgressDidUpdate:(float)progress {
// update ui
}
In your background thread send out events to the main thread
- (void) revertFirmware {
// once in a while send notifications of progress updates
if ([self.delegate respondsToSelector:#selector(reverterProgressDidUpdate:)]) {
[self.delegate performSelectorOnMainThread:#selector(reverterProgressDidUpdate:) withObject:[NSNumber numerWithFloat:progress] waitUntilDone:NO];
}
}
Make sure you retain your reverter somewhere, and release it when it's done working. You are now leaking in your pushButton: method. Also this is just a suggestion towards a better model. Instead of using performSelectorInBackground you could take a look at NSOperation and NSOperationQueue for example.
I am learning about threading and multithreading..so i just created a small application in which i will update
the progressbar and a static text using threading.I vl get two inputs from the user, start and end values
for how long the loop should rotate.I have 2threads in my application.
Thread1- to update the progressbar(according to the loop) the static text which will show the count(loop count).
Thread2 - to update the another static text which will just diplay a name
Basically if the user clicks start, the progressbar steps up and at the same time filecount and the name are displayed parallely.
There's is another operation where if the user clicks pause it(thread) has to suspend until the user clicks resume.
The problem is,the above will not work(will not suspend and resume) for both thread..but works for a singlw thread.
Please check the code to get an idea and reply me what can done!
on button click start
void CThreadingEx3Dlg::OnBnClickedStart()
{
m_ProgressBar.SetRange(start,end);
myThread1 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction1,this);
myThread2 = AfxBeginThread((AFX_THREADPROC)MyThreadFunction2,this);
}
thread1
UINT MyThreadFunction1(LPARAM lparam)
{
CThreadingEx3Dlg* pthis = (CThreadingEx3Dlg*)lparam;
for(int intvalue =pthis->start;intvalue<=pthis->end; ++intvalue)
{
pthis->SendMessage(WM_MY_THREAD_MESSAGE1,intvalue);
}
return 0;
}
thread1 function
LRESULT CThreadingEx3Dlg::OnThreadMessage1(WPARAM wparam,LPARAM lparam)
{
int nProgress= (int)wparam;
m_ProgressBar.SetPos(nProgress);
CString strStatus;
strStatus.Format(L"Thread1:Processing item: %d", nProgress);
m_Static.SetWindowText(strStatus);
Sleep(100);
return 0;
}
thread2
UINT MyThreadFunction2(LPARAM lparam)
{
CThreadingEx3Dlg* pthis = (CThreadingEx3Dlg*)lparam;
for(int i =pthis->start;i<=pthis->end;i++)
{
pthis->SendMessage(WM_MY_THREAD_MESSAGE2,i);
}
return 0;
}
thread2 function
LRESULT CThreadingEx3Dlg::OnThreadMessage2(WPARAM wparam,LPARAM lparam)
{
m_Static1.GetDlgItem(IDC_STATIC6);
m_Static1.SetWindowTextW(L"Thread2 Running");
Sleep(100);
m_Static1.SetWindowTextW(L"");
Sleep(100);
return TRUE;
}
void CThreadingEx3Dlg::OnBnClickedPause()
{
// TODO: Add your control notification handler code here
if(!m_Track)
{
m_Track = TRUE;
GetDlgItem(IDCANCEL)->SetWindowTextW(L"Resume");
myThread1->SuspendThread();
WaitForSingleObject(myThread1->m_hThread,INFINITE);
myThread2->SuspendThread();
m_Static.SetWindowTextW(L"Paused..");
}
else
{
m_Track = FALSE;
GetDlgItem(IDCANCEL)->SetWindowTextW(L"Pause");
myThread1->ResumeThread();
myThread2->ResumeThread();
/*myEventHandler.SetEvent();
WaitForSingleObject(myThread1->m_hThread,INFINITE);*/
}
}
I thought I should summarize some of the discussion in the comments into an answer.
In Windows programming, you should never try to manipulate a GUI control from a background thread, as doing so can cause your program to deadlock . This means only the main thread should ever touch elements of the GUI. (Technically, what matters is which thread created the control, but it's not common to create controls in background threads).
This requirement is detailed in Joe Newcomer's article on worker threads (see "Worker Threads and the GUI II: Don't Touch the GUI").
You are using SendMessage in your thread procedures. This causes the appropriate message handler for the target control to be invoked, but in the thread that called SendMessage. In your case, that means the background threads run the message handlers and therefore update the progress bar and label.
The alternative is to use PostMessage. This causes the message to be added to a queue to be processed by the main thread's message loop. When the main thread gets to run, it processes the messages in the order they were added to the queue, calling the message handlers itself. Since the main thread owns the windows, it is safe for it to update the controls.
You should also beware that SuspendThread and ResumeThread are tricky to get right. You might want to read this section of Joe Newcomer's article, which describes some of the dangers.
Tasks like this are often better achieved by using a timer. This is a mechanism for having the operating system notify your program when a particular amount of time has passed. You could implement this with a timer as below:
BEGIN_MESSAGE_MAP(CThreadingEx3Dlg, CDialog)
ON_WM_DESTROY()
ON_WM_TIMER()
END_MESSAGE_MAP()
void CThreadingEx3Dlg::OnTimer(UINT_PTR nTimerID)
{
static int progress = 0;
if (nTimerID == 1)
{
m_ProgressBar.SetPos(progress);
CString strStatus;
strStatus.Format(_T("Processing item: %d"), progress);
m_Static.SetWindowText(strStatus);
progress++;
if (progress > end) // If we've reached the end of the updates.
KillTimer(1);
}
}
BOOL CThreadingEx3Dlg::OnInitDialog()
{
// ... initialize controls, etc, as necessary.
SetTimer(1, 100, 0);
}
void CThreadingEx3Dlg::OnDestroy()
{
KillTimer(1);
}
If you want both updates handled at the same time, they can use the same timer. If they need to happen at different times (such as one at a 100 ms interval and another at a 150 ms interval) then you can call SetTimer twice with different IDs. To pause the action, call KillTimer. To resume it, call SetTimer again.
Multi-threading and message queuing is quite a complex game. When you SendMessage from ThreadA to the same thread then it just calls the message handler. If you do it from ThreadA to another thread (ThreadB) then it gets more complicated. ThreadA then posts a message to the ThreadB's message queue and waits on a signal to say that ThreadB has finished processing the message and sent the return value. This raises an instant problem. If ThreadB is not pumping messages then you have a deadlock as the message in ThreadB's will never get "dispatched". This also raises an EVEN bigger problem. If ThreadB's message needs to send a message to a control created in ThreadA then you have a massive architectural problem. As ThreadA is currently suspended waiting for ThreadB to return and ThreadB is suspended waiting for ThreadA to return. Nothing will happen ... They will both sit suspended.
Thats about it really. Its pretty easy as long as you bear these issues in mind. ie It absoloutely IS possible despite what the others have said.
In general though your threading is pretty pointless because you straight away send a message to the main thread to do some processing. Why bother starting the threads in the first place. You may as well not bother because the threads will just sit there waiting for the main thread to return.
Why do you "WaitForSingleObject" anyway when you suspend the first thread? Why not just suspend them both.
All round, though, you aren't giving enough information about what you are doing to say exactly whats going on. What happens when you click pause, for example?
Windows will not operate properly when more than one thread interacts with the GUI. You'll need to reorganize your program so that does not happen.