What is the difference between using this
[self performSelectorOnMainThread:#selector(doStuff:)
withObject:myObject
waitUntilDone:YES];
instead of simply
[self doStuff:myObject];
in terms of CPU load? Or there are any other advantages?
thanks.
[self performSelectorOnMainThread:#selector(doStuff:)
withObject:myObject
waitUntilDone:YES];
This makes sure that the selector is performed on the main thread by adding it to the main run loop. When you set waitUntilDone:YES, you will let your current thread idle as long as the main run loop needs to perform the selector.
In a single threaded environment this will let the runloop run right after you called this, in a multithreaded environment, the main runloop will perform the selector once it does the next step.
You have in any of the both cases a small overhead as the selector isn't performed right after you called the function but at a later point while [self doStuff:myObject]; performs the selector immediately
The first one just ensure that your method will be invoked in main thread. If you are not doing multithreading, they should yield same result.
In your example, the second message will be sent straight away, while the first is added to the run loop of the main thread so it won't run immediately.
Related
I have a low-priority PDF-drawing NSOperation. I am using the function CGContextDrawPDFPage to draw the page.
As best I can tell, the app will not switch to another thread while the CGContextDrawPDFPage function is executing. I suppose I need to break up my graphics context into several smaller contexts to get around that. But I still have one problem -- how can I persuade the operation to allow other threads to perform their tasks before starting that draw? The way things are set up right now, the app draws the page right away, even though the thread priority is set to zero.
Here is the code that adds the NSOperation to the queue:
MyNSOperationSubclass* op = [[MyNSOperationSubclass alloc]initWithStuff: stuff];
[op setThreadPriority:0.];
[self.drawingQueue addOperation:op];
Here is some code within MyNSOperationSubclass:
-(void) drawStuff {
NSLog(#"drawStuff");
#autoreleasepool {
// some code to setup the graphics context and PDF page here
CGContextDrawPDFPage(context, page);
[self finish]; // finishes the NSOperation
}
}
-(void) main {
NSLog(#"main");
#autoreleasepool {
[self drawStuff];
}
}
-(void) start {
self.isExecuting = YES;
[NSThread detachNewThreadSelector:#selector(main) toTarget:self withObject:nil];
}
what happens is that even though the thread priority is set to zero, the setup code is so quick that the NSOperation gets into the CGContextDrawPDFPage call right as the operation is started. But once that call has started, the thread won't yield to other threads until it has finished. So the drawing operation happens right away, which is the opposite of what I want to do.
EDIT: After taking a closer look, the first answerer is correct that the CGContextDrawPDFPage call does not always prevent a thread from yielding. But even with a non-concurrent operation, I still have the problem that it finishes quickly, rather than waiting around for other, higher-priority stuff to happen.
EDIT 2: Apparently I don't understand how thread priorities work. After changing it to a nonconcurrent operation, the thread priority of the operation thread is 0.5, even though I called [op setThreadPriority: 0] before adding it to the queue.
So, the first thought is that your thread can't avoid "yielding" to other threads. Context switching is pre-emptive here. So you might want to talk about why you think this thread is not yielding. You may have some other bigger issue here with the other work that you are expecting to be done.
Second, when you set the thread priority for this operation, you're not setting the priority of this new thread that you are creating yourself here:
[NSThread detachNewThreadSelector:#selector(main) toTarget:self withObject:nil];
That's a new thread with normal priority.
But stepping back from that a second, why not just make this a non-concurrent operation and let your NSOperationQueue manage the thread for you? The queue will create a new thread for you as needed (and apply your thread priority that you asked for).
Alternatively, unless you still need to support iOS3, you can just use the global low priority GCD queue for this work (or target your own GCD queue to that global queue if you need finer control over order and still want all of this done with low priority).
So, consider letting either NSOperationQueue or GCD manage some of this for you. And think about why you think this thread is interfering with other work. That's probably some bigger issue.
Hope that helps.
I need ideas on the following -
In the main thread at some point of execution say Point A(sequential logic), I need to remember the state of execution and delegate the execution of some other logic onto another thread, and let the main thread handle the UI events etc. When the delegated logic completes on the other thread then the flow of execution should continue from the point A and should recollect the entire execution context and proceed as if it never paused there.
Regards,
Sunil Phani Manne
It's hard to implement this exactly the way you're saying (for example do(things)... yield(other_thread); ...do(more_things);.
Here are a couple other options I can think of (you'd have to implement these yourself, using delegates or notifications for example; I'm just giving a basic outline of how it would work):
do(things)
[object doStuffOnOtherThreadWithCallback:^{ // block-based
do(more_things)...
}];
or...
do(things)
[object doStuffOnOtherThreadWithCallbackTarget:self // target/selector-based
selector:#selector(callbackSelector)];
}
- (void)callbackSelector {
do(more_things)...
}
One option you have is encapsulating the whole sequential logic that comes after Point A in your delegate and then execute it on the main thread when the secondary thread ends.
In other words, when you start the thread by calling, e.g.
[NSThread detachNewThreadSelector:sel toTarget:target withObject:delegate]
you can implement your target target so that it has a specific selector completion that is called at the end of sel on the main thread, like this (this is the your delegate class):
#implementation YOURDelegateClass {
.....
-(void)completion {
}
-(void)sel {
...
...
[self performSelectorOnMainThread:#selector(#"completion") withObject:self];
}
}
Of course you have many sub-options available here, like using a different call to start the background execution, etc.
The important point is that: you have to encapsulate in a selector all the logic that comes after Point A, and that you have to schedule the execution of this selector on the main thread, in order to get back to your context (although your context will have changed in the meantime because you will also have updated the UI).
EDIT:
Having to schedule the execution on the main thread defeats blocks from being suitable for this kind of callback. On the other side, block have the advantage that they in some limited sense give you access to the same lexical context in which the block was defined (which is roughly what you call context).
A workaround for this could be the following. Before detaching the new thread, store in a delegate the block you would like to execute at completion:
typedef void(^CustomBlock)(void);
#property (nonatomic, copy) CustomBlock customBlock;
....
int a = ...
delegate.customBlock = ^{
NSLog(#"hello %d.....", a);
}
[NSThread detachNewThreadSelector:sel...
....
-(void)completion {
[self customBlock];
}
Of course, you only get the context preservation that is guaranteed to you by block. But here you hit against a limit of the language.
If you need more context preservation, then the only possibility is encapsulating that context in your delegate class ivars.
One thing is for sure. There, most probably, isn't any direct feature in Cocoa that does that. Since you're saying that you can't duplicate the resources onto the new thread (for a very good reason), I am going to suggest that you make use of NSUndoManager. For every change you make in the thread, push an undo operation for that change onto the undo manager. At the end of the thread, execute all the undo operations in the undo manager object. This should, if done correctly, restore your state. Now, since the idea is untested, there could be a chance that not all actions can be undone. You will have to check that out first.
I've used both GCD and performSelectorOnMainThread:waitUntilDone in my apps, and tend to think of them as interchangeable--that is, performSelectorOnMainThread:waitUntilDone is an Obj-C wrapper to the GCD C syntax. I've been thinking of these two commands as equivalent:
dispatch_sync(dispatch_get_main_queue(), ^{ [self doit:YES]; });
[self performSelectorOnMainThread:#selector(doit:) withObject:YES waitUntilDone:YES];
Am I incorrect? That is, is there a difference of the performSelector* commands versus the GCD ones? I've read a lot of documentation on them, but have yet to see a definitive answer.
As Jacob points out, while they may appear the same, they are different things. In fact, there's a significant difference in the way that they handle sending actions to the main thread if you're already running on the main thread.
I ran into this recently, where I had a common method that sometimes was run from something on the main thread, sometimes not. In order to protect certain UI updates, I had been using -performSelectorOnMainThread: for them with no problems.
When I switched over to using dispatch_sync on the main queue, the application would deadlock whenever this method was run on the main queue. Reading the documentation on dispatch_sync, we see:
Calling this function and targeting
the current queue results in deadlock.
where for -performSelectorOnMainThread: we see
wait
A Boolean that specifies whether the
current thread blocks until after the
specified selector is performed on the
receiver on the main thread. Specify
YES to block this thread; otherwise,
specify NO to have this method return
immediately.
If the current thread is also the main
thread, and you specify YES for this
parameter, the message is delivered
and processed immediately.
I still prefer the elegance of GCD, the better compile-time checking it provides, and its greater flexibility regarding arguments, etc., so I made this little helper function to prevent deadlocks:
void runOnMainQueueWithoutDeadlocking(void (^block)(void))
{
if ([NSThread isMainThread])
{
block();
}
else
{
dispatch_sync(dispatch_get_main_queue(), block);
}
}
Update: In response to Dave Dribin pointing out the caveats section ondispatch_get_current_queue(), I've changed to using [NSThread isMainThread] in the above code.
I then use
runOnMainQueueWithoutDeadlocking(^{
//Do stuff
});
to perform the actions I need to secure on the main thread, without worrying about what thread the original method was executed on.
performSelectorOnMainThread: does not use GCD to send messages to objects on the main thread.
Here's how the documentation says the method is implemented:
- (void) performSelectorOnMainThread:(SEL) selector withObject:(id) obj waitUntilDone:(BOOL) wait {
[[NSRunLoop mainRunLoop] performSelector:selector target:self withObject:obj order:1 modes: NSRunLoopCommonModes];
}
And on performSelector:target:withObject:order:modes:, the documentation states:
This method sets up a timer to perform the aSelector message on the current thread’s run loop at the start of the next run loop iteration. The timer is configured to run in the modes specified by the modes parameter. When the timer fires, the thread attempts to dequeue the message from the run loop and perform the selector. It succeeds if the run loop is running and in one of the specified modes; otherwise, the timer waits until the run loop is in one of those modes.
GCD's way is suppose to be more efficient and easier to handle and is only available in iOS4 onwards whereas performSelector is supported in the older and newer iOS.
I am trying to perform a selector which returns an NSString in the background thread, and the NSString returned will depend on the input object albumlink.
I am performing it in the background, as it takes a while to shorten the URL.
I would really appreciate if you could tell me how I could get the return string.
My code to perform that selector is:
[self performSelectorInBackground:#selector(shortenURL:) withObject:albumlink];
You can write another method in your class (let's call it -handleResponse:(NSString *)response), and then from the backgrounded process you can call:
[self performSelectorOnMainThread:#selector(handleResponse:) withObject:#"My response string" waitUntilDone:NO];
You can't get a function's return value outside of the thread it runs in. The whole point of doing something in a background thread is that it's taken out of the normal flow for the main thread, so there's no place for it to return to.. The most sensible approach is to create a block that's performed in the background (either through NSOperation or GCD directly) which updates either updates the value on the main thread — if you need to store the value afterward — or which just does whatever you were going to do with the value if it was only going to be used in one branch of code.
I have an NSOperationQueue on my main thread running a set of NSOperations (max concurrent set to 1) that I want to be able to cancel at any time. When I press a button I tell the queue to cancel all operations and wait until finished. This should hang the main thread until the operation queue is empty, however it is hanging my main thread indefinitely.
Here's the code I use to stop it:
...
[myQueue cancelAllOperations];
[myQueue waitUntilAllOperationsAreFinished];
return YES; // This line never gets called
Note: I need to use waitUntilAllOperationsAreFinished as further processes require that the queue be empty.
The strange thing is this is only occurring on the device. When running in the simulator it works as expected.
I have watched breakpoints and I can follow the currently running operation until it finishes. It detects [self isCancelled], stops what it's doing and zips through to the end of the main method. I can see that nothing in the operation is causing it to hang, and by cancelling all operations, none of the other operations should start, and the queue should finish. I have checked by adding breakpoints and none of the other operations start.
Why is this happening?
In any of your operations (or in any other thread), are you using -performSelectorOnMainThread:withObject:waitUntilDone:? -waitUntilAllOperationsAreFinished will block whatever thread it's called on until all operations are complete. Odds are, if you're calling this as the application is terminating, that thread would be the main thread. If the main thread is blocked, and one of the operations uses -performSelectorOnMainThread:withObject:waitUntilDone:, your application will freeze because the operation will never complete.
I've had this exact thing happen to me before. Unfortunately, it's pretty difficult to work around.
You should never, ever block the main thread. It handles all your UI updates for one thing, for another as you have noted you managed to create a deadlock.
Instead, try creating a method like:
- (void) notifyOnFinish
{
[myQueue waitUntilAllOperationsAreFinished];
[self performSelectorOnMainThread:(queueEmpty) withObject:nil waitUntilDone:NO];
}
Then where you have your code now, call:
[myQueue cancelAllOperations];
[self performSelectorInBackground:#selector(notifyOnFinish) withObject:nil];
And in a queueEmpty method you just do whatever you want to do when the queue is emptied.
Basically, just create a background thread to block instead of the main thread.
Perhaps the waitUntilAllOperationsArefFinished is causing the block... Maybe the operations all cancel and finish before the call to waitUntilAllOperationsArefFinished and then the queue is sat hanging, waiting for operations that are already finished to finish...?
I don't know this for a fact, but maybe try not calling waitUntilAllOperationsArefFinished.