Confounding Cocoa problem — program hangs unless there’s an unrecognised method call - objective-c

Bear with me, this one is hard to explain. I hope some hero out there knows what’s going on here. Some history needed;
One of my cocoa objects, “Ball” represents a small graphic. It only makes sense within a view. In some of the Ball’s methods, it asks the view to redraw. Most importantly, it asks the view to redraw whenever the Ball’s position parameter is set. This is achieved in the setter.
Here’s the mouthful, as suggested:
In View.m
- (void)mouseUp:(NSEvent *)theEvent {
if (![runnerPath isEmpty]) {
[walkPath removeAllPoints];
[walkPath appendBezierPath:runnerPath];
[runnerPath removeAllPoints];
[[self held] setStep:0];
[[self held] setPath:walkPath];
[NSTimer scheduledTimerWithTimeInterval:.01 target:[self held] selector:#selector(pace) userInfo:nil repeats:YES];
}
}
In Ball.m
- (void)pace {
CGFloat juice = 10;
BOOL loop = YES;
while (loop) {
if ([self step] == [[self path] elementCount]) {
if ([[self timer] isValid]) {
[[self timer] invalidate];
}
[[self path] removeAllPoints];
// #throw([NSException exceptionWithName:#"test" reason:#"reason" userInfo:nil]);
}
if (loop) {
CGFloat distance;
NSPoint stepPoint;
if ([[self path] elementCount] > 0) {
NSPoint returnPoints[2];
[[self path] elementAtIndex:[self step] associatedPoints:returnPoints];
stepPoint = returnPoints[0];
distance = pixelDistance([self position], stepPoint);
}
if (distance <= juice) {
[self setPosition:stepPoint];
if (distance < juice) {
juice -= distance;
loop = YES;
[self setStep:[self step]+1];
} else {
loop = NO;
}
} else {
NSPoint cutPoint = moveAlongBetween([self position], stepPoint, juice);
[self setPosition:cutPoint];
loop = NO;
}
}
}
}

could you also tell how you handle exceptions? since normally an unrecognized selector will end your program. Maybe you need an exception rather than an unrecognized selector. Try:
#throw([NSException exceptionWithName:#"test" reason:#"reason" userInfo:nil]);
If this would fix it as well, you're doing something after this code which freezes the app.
edit: thanks for the code update.
There's some weird stuff going on here! I'm not going to rewrite the whole thing, so here's some pointers:
first of all: you're looping inside some routine that is called from a timer loop. Is that intended? There is no way to pause execution within that while() loop, so it will happen in a blink anyway. You would need to keep some state information in the class. E.g. adding a loop counter every time pace is called.
second: if you start a timer, it will call your selector with the timer as an argument. So define the function as -(void)pace:(NSTimer*)timer, and use timer, not [self timer] (the latter will not be your timer anyway, if you don't assign it!)
third: you're firing 100 times a second. That is a lot, and presumably higher than the refresh rate of any device you're writing this for. I think 20/sec is enough.
fourth: to be sure, if you change it to -(void)pace:(NSTimer*)timer, don't forget to use #selector(pace:) (i.e. don't forget the :)
fix those things, and if it's still broken, update your question again and put in comment so we will know. Good luck!

Try calling
for (NSView *each in [self views]) {
...
}
I'm assuming that views is an array, so fast enumeration applies to it directly and there is no need to call allObjects.
A couple of other points.
Have you set a Global breakpoint of objc_exception_throw? This will apply to all Xcode projects and is so useful I'm surprised it isn't set by default.
You say you looked at the Console for errors. I take it, then, that you didn't set a breakpoint on the code and step into it to see exactly what is happening when your execution reaches that point? Have a look at the Xcode Debugging Guide

Related

iOS 10 while(YES) never break

I have sample code below.
- (void)method1
{
breakFlag = NO;
[self performSelectorInBackground:#selector(method2) withObject:nil];
while (YES) {
if (breakFlag) {
break;
}
}
NSLog(#"End method1");
}
- (void)method2
{
for (int i = 0; i < 10000; i++) {
NSLog(#"value of i; %d", i);
}
breakFlag = YES;
NSLog(#"End method2");
}
I called method1 from viewDidAppear with
[self performSelectorInBackground:#selector(method1) withObject:nil];
breakFlag is variable instance.
Why while in method1 never be broken?
C and Objective-C compilers are not really aware of threading. They are, according to the language standard, perfectly entitled to examine code like your -method1 and see that nothing in the code of the loop changes breakFlag and optimize it by hoisting the check out of the loop. That is, it optimizes your method into:
- (void)method1
{
breakFlag = NO;
[self performSelectorInBackground:#selector(method2) withObject:nil];
if (!breakFlag) {
while (YES) {
}
}
NSLog(#"End method1");
}
It does need to check breakFlag after the call to -performSelectorInBackground:... because it can't know that that won't change breakFlag. But it doesn't need to check it within the loop.
If you were to declare breakFlag with the volatile qualifier, that would prevent this optimization. That tells the compiler that something can "spontaneously" (outside of what it can analyze) modify breakFlag at any time.
However, that's almost never the correct way to write code. Since you've refused to say what you're trying to accomplish, it's hard to advise you on better ways to achieve it. A general recommendation is to suggest that you use proper thread synchronization primitives, like semaphores, mutexes, etc. to have threads interact.
Instead of:
[self performSelectorInBackground:#selector(method2) withObject:nil];
Try:
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self method2];
});

ReactiveCocoa: throttle never executes / not working

I try to subscribe to a signal with throttle, but it never executes.
I have a UISearchController (Attention: UISearchController from iOS8, not the older UISearchDisplayController, which works quiet better and has thousands of working tutorials and examples in the web) and want to make API-Requests while the user is typing.
To let the traffic being low, i don't want to start API-Requests with each key the user is pressing, but when the user stops for a while, say 500 ms after the last keypress.
Since we're unable to reference the TextField in the SearchBar of the UISearchController, we use the delegates from UISearchController:
To get the latest typed text of the Textfield in the Searchbar, I use this:
#pragma mark - UISearchResultsUpdating
- (void)updateSearchResultsForSearchController:(UISearchController *)searchController {
NSString *searchText = searchController.searchBar.text;
// strip out all the leading and trailing spaces
NSString *strippedString = [searchText stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
if([strippedString isEqualToString:self.currentFilter]) {
return;
}
self.currentFilter = strippedString;
}
The property currentFilter keeps the current search string.
Also, i have a RACObserve on the currentFilter-Property to react on every change which is made to this property:
[RACObserve(self, currentFilter) subscribeNext:^(NSString* x) {
NSLog(#"Current Filter: %#", x);
// do api calls and everything else
}];
Now i want to throttle this signal. But when i implement the call to throttle, nothing happens. SubscribeNext will never be called:
[[RACObserve(self, currentFilter) throttle:500] subscribeNext:^(NSString* x) {
NSLog(#"%#", x); // will never be called
}];
How to achieve to throttle inputs in a searchbar? And what is wrong here?
UPDATE
i found a workaround besides ReactiveCocoa thanks to #malcomhall. I moved the code within the updateSearchResultsForSearchController-delegate method into a separate method and schedule it with performSelector and cancel this scheduler with cancelPreviousPerformRequestsWithTarget.
- (void)updateSearchResultsForSearchController:(UISearchController *)searchController {
[NSObject cancelPreviousPerformRequestsWithTarget:self selector:#selector(useTextSearchQuery) object:nil];
[self performSelector:#selector(useTextSearchQuery) withObject:nil afterDelay:1.0];
}
Anyway, i want still understand how "throttle" from ReactiveCocoa is working and why not in this case :)
-throttle: accepts an NSTimeInterval, which is a floating-point specification of seconds, not milliseconds.
Given the code in the question, I expect you would see results after 500 seconds have elapsed.

Draw the contents of an array modified in the background

I have a background thread that runs and modifies the contents of a NSMutableArray within an object. This takes a long time to run (several hours) and I periodically want to draw the contents of an array within the drawRect of a NSView to check on progress and see the intermediate results.
My object has a protocol with a method called: didChange:
// How I start my background thread
[self performSelectorInBackground:#selector(startProcessing) withObject:nil];
- (void)startProcessing {
myObject.delegate = self;
[myObject start];
}
// My protocol implementation
- (void)myObjectDidChange:(myObjectClass *)sender {
[myView setNeedsDisplay:YES];
}
// My View's drawRect (pseudo code)
- (void)drawRect {
[myObject drawInContext:context];
}
All works, except that the NSMutableArray backing all this is being changed whilst the drawing takes place. How should I do this? Do I somehow pause the processing in the background thread whilst the update is taking place?
EDIT: This is the sort of display I am drawing (although much more complicated):
Any help appreciated.
If you are doing something in background thread and you want to update UI, its usually done on the main thread, so in your object did change you would do it, probably like this:
// My protocol implementation
- (void)myObjectDidChange:(myObjectClass *)sender {
dispatch_async(dispatch_get_main_queue(), ^{
[self drawRect]; //Or any drawing function you are trying to do
});
}
I have done it using NSLock to lock the outer loop of the start and the drawInContext methods. I am still not sure if this is the best approach and will not accept this answer for a few days in case there is a better answer out there.
- (void)start {
for(int i=0; i < MAX; i++) {
[self.updateLock lock];
....
[self.updateLock unlock];
}
}
- (void)drawInContext:(CGContextRef)context {
[self.updateLock lock];
...
[self.updateLock unlock];
}

Why does my retained CMAttitude instance become nil after I call stopDeviceMotionUpdates and startDeviceMotionUpdates?

I have a class with an ivar:
#interface myCoolClass:NSObject
{
CMAttitude *referenceAttitude;
}
In my implementation have these selectors:
- (void) startTrackingMotion
{
if (motionManager == nil) {
motionManager = [[CMMotionManager alloc] init];
motionManager.accelerometerUpdateInterval = 0.01;
motionManager.deviceMotionUpdateInterval = 0.01;
referenceAttitude = [motionManager.deviceMotion.attitude retain];
}
[motionManager startDeviceMotionUpdates];
if (referenceAttitude == nil) {
CMDeviceMotion *dm = motionManager.deviceMotion;
referenceAttitude = [dm.attitude retain];
}
}
- (void) stopTrackingMotion
{
[motionManager stopDeviceMotionUpdates];
}
I want to take the referenceAttitude when I init the motion manager and use it over the life of the application. Sometimes, I need to track motion and other times I do not.
Here's the app flow:
call startTrackingMotion since I'm ready for motion
referenceAttitude stays retained and I use it to track motion
I call stopTrackingMotion since I'm going to do non motion stuff
the app do some other stuff
I call startTrackingMotion again since I'm ready for motion again
At this point, as I step through the code, I step over the "if (motionManager == nil)" loop since it's still there. However, everytime it comes to the "if (referenceAttitude == nil)" loop, the if statement resolves to true.
Am I retaining it incorrectly? Does calling stopDeviceMotionUpdates nil out my instance?
Thanks.
I dont think you are giving enough time for the motionManager to update itself.
You should get the referenceAttitude after the first tick of deviceMotionUpdateInterval (0.1).
Calling stopDeviceMotionUpdates doesn't nil out your referenceAttitude variable. What version of XCode are you using, and what kind of device are you running it on?
Have you verified that it is set to a valid CMAttitude instance at this point in code?
referenceAttitude = [motionManager.deviceMotion.attitude retain];
If you execute po [dm attitude] in the debugger within the referenceAttitude == nil block, what do you get?

How can I make a method stall for a fixed amount of time?

I have an app that calls a sometimes-fast, sometimes-slow method. I know an upper bound for how long it will take (2 seconds). I'd like to set a timer to start when the method is called, run the code, but then not produce the output until 2 seconds has passed, no matter how long it actually takes. That way the user perceives the action as always taking the same amount of time. How can I implement this?
What I would like is something along the lines of this:
-(IBAction)doStuff {
// START A TIMER, LOOK BUSY
[activityIndicator startAnimating];
... real work happens here ...
... NSString *coolString gets assigned ...
// WHEN TIMER == 2 SECONDS, REVEAL COOLNESS
[activityIndicator stopAnimating];
[textField setText:coolString];
}
There are a couple of ways to delay an action in Cocoa. The easiest may be to use performSelector:withObject:afterDelay:. This method sets up a timer for you and calls the specified method when the time comes. It's an NSObject method, so your objects all get it for free.
The tricky part here is that the first method will block the main thread, so you need get it onto a background thread, and then get back to the main thread in order to update the UI. Here's a stab at it:
// Put the method which will take a while onto another thread
[self performSelectorInBackground:#selector(doWorkForUnknownTime)
withObject:nil];
// Delay the display for exactly two seconds, on the main thread
[self performSelector:#selector(displayResults)
withObject:nil
afterDelay:2.0];
- (void)doWorkForUnknownTime {
// results is an ivar
results = ...; // Perform calculations
}
- (void)displayResults {
if( !results ){
// Make sure that we really got results
[self performSelector:#selector(displayResults:)
withObject:nil
afterDelay:0.5];
return;
}
// Do the display!
}
The only other thing I can think of is to store the time that the "work" method is called in an NSDate, and check how long it took when you get the results. If it isn't two seconds yet, sleep the background thread, then call back to the main thread when you're done.
[self performSelectorInBackground:#selector(doWorkForUnknownTime:)
withObject:[NSDate date]];
- (void)doWorkForUnknownTime:(NSDate *)startTime {
// All threads must have an autorelease pool in place for Cocoa.
#autoreleasepool{
// This will take some time
NSString * results = ...; // Perform calculations
NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startTime];
if( elapsedTime < 2.0 ){
// Okay to do this to wait since we're on a background thread,
// although not ideal; sleeping threads are kind of wasteful.
// Try not to do this a lot.
sleep(2.0 - elapsedTime);
}
// Don't forget to retain results on the main thread!
[self performSelectorOnMainThread:#selector(displayResults:)
withObject:results
waitUntilDone:YES];
// [results release]; // if necessary
}
}
[self performSelector:#selector(myfunc) withObject: afterDelay:];
should help.
-(IBAction)doStuff {
// START A TIMER, LOOK BUSY
[activityIndicator startAnimating];
... real work happens here ...
... NSString *coolString gets assigned ...
// WHEN TIMER == 2 SECONDS, REVEAL COOLNESS
[self performSelector:#selector(revealCoolnessWithString:) withObject:coolString afterDelay:2];
}
- (void)revealCoolnessWithString:(NSString *)coolString
{
[activityIndicator stopAnimating];
[textField setText:coolString];
}
Hope this helps