There is some action that is executed in a loop, a certain number of iterations. In every iteration is formed string to be displayed in the UITextView.
- (IBAction)ButtonClick:(id)sender {
sTest *sT = new sTest();
int res;
for (int i=0; i<300; i++)
{
res = sT->test_fun(N1[i], M1[i], 10000);
[_textView setText:[NSString stringWithFormat:#"%#res=%d\n", _textView.text, res]];
}
With this approach, UI waits until cycle ends, and displays all the lines at once in textview. And I would like that each row displays in each iteration.
Using [NSThread detachNewThreadSelector:#selector(StringLoop) toTarget:self withObject:nil]; has no effect.
Tell me how to do it. Example is welcomed. Thank you.
I'm not sure that what you want is what you should be doing, but the most common implementation for concepts like you describe would be to run your loop on a background thread, and then make calls to the main thread to update the UI when needed. I've included one of the most "readable" and understandable implementations below, but as mentioned in comments, the Grand Central Dispatch is well-suited to your needs.
-(void)startItAll
{
[self performSelectorInBackground:#selector(doStuffInBackground) withObject:nil];
}
-(void)doStuffInBackground
{
//do math
for (int i = 0; i < 100; i++) {
[self performSelectorOnMainThread:#selector(doStuffInMain) withObject:nil waitUntilDone:NO];
}
}
-(void)doStuffInMain
{
//update necessary UI
}
Related
I have a UIViewController that does the following in viewDidLoad
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
items = [[DataFetcher sharedInstance] getItems handler:^(NSArray *currentItems){
if (currentItems.count % 30 == 0) { //don't wait for all the items to be ready show by chunks of 30
items = currentItems;
[tableView reloadData];
}
items = currentItems;
}];//Pretty complex call that takes some time to finish there is WebService calls, data parsing, storing some results ...
dispatch_async(dispatch_get_main_queue(), ^{
[tableView reloadData];
});
});
What I need to do is to stop getItems when I pop this viewController. It's pointless and it takes CPU Time and energy (This call may take up to a minute on some cases).
I am assuming I should be doing this in viewWillDisappear but how exactly?
You can use NSBlockOperation. Periodically check if it's been cancelled, and stop doing work if it has been:
- (void)getItemsWithHandler:(void (^)(NSArray *currentItems))handler {
self.operation = [NSBlockOperation blockOperationWithBlock:^{
if (self.operation.isCancelled) {
return;
}
// Do something expensive
if (self.operation.isCancelled) {
return;
}
// Do something else expensive
for (int i = 0; i < 10000; i++) {
if (self.operation.isCancelled) {
return;
}
// Do expensive things in a loop
}
}];
}
- (void) cancelGetItemsRequest {
[self.operation cancel];
self.operation = nil;
}
Alternatively, you can put a bunch of NSBlockOperations in an NSOperationQueue. You can set dependencies for the work, and cancel the entire queue at once if you want.
Cancelling asynchronous operations is nicely supported in NSOperation and NSOperationQueue, but it's quite a bit more complicated. In any case, your asynchronous code has to check from time to time whether it is cancelled or should still continue.
Best thing is to have a property "cancelled" somewhere, that you set when you don't want any more work to be done, and whenever you try to do more work, you check that property.
I'm having this wierd problem with the app freezing at a certain point. I'm guessing its got to do with how I'm using NSConditionLock.
Theres a library I have been given to use, which consists of a series of survey questions, but it works in such a way that it races directly to the last question without accepting answers, hence the need to pause the thread and accept input from the user.
I haven't used it before so maybe someone could help if I'm implementing it wrongly?
Please let me know if the code provided is insufficient.
- (void)viewDidLoad
{
[super viewDidLoad];
//INITIALISE CONDITION LOCK WITH CONDITION 0
condition=[[NSConditionLock alloc]initWithCondition: 0];
}
- (IBAction)startPressed:(UIButton*)sender {
if (sender.tag == 1) {
//START BACKGROUND THREAD
surveyThread = [[NSThread alloc] initWithTarget:self selector:#selector(runProjecttest) object:nil];
[surveyThread start];
}
else
{
//DO SOME STUFF AND THEN UNLOCK
[condition unlockWithCondition:1];
}
}
- (void) runProjecttest:(AbstractTask *)rendertask
{
// DO STUFF AND SHOW UI ON MAIN THREAD, THEN LOCK
[self performSelectorOnMainThread:#selector(showUI:) withObject:task waitUntilDone:YES];
[condition lockWhenCondition: 1];
}
EDIT: In short, I want the Objc equivalent of this java snippet...
this.runOnUiThread(showUI);
try
{
//SLEEP
Thread.sleep(1000*60*60*24*365*10);
}
catch (InterruptedException e)
{
//WAKE
setResponse(at,showUI);
}
EDIT 2: ShowUI method on Paul's request.
[self removePreviousSubViews];
switch ([task getType]) {
case SingleChoiceType:
{
NSLog(#"SingleChoiceType");
isMultipleChoice = NO;
[self addSingleChoiceView:nil];
break;
}
case TextType:
{
NSLog(#"TextType");
self.txtTextType.keyboardType=UIKeyboardTypeDefault;
[self addTextTypeView:nil];
break;
}
...more cases
}
-(void)addTextTypeView:(NSSet *)objects
{
self.txtTextType.text = #"";
CGRect frame = self.txtQuestionType.frame;
// frame.size = [self.txtQuestionType sizeThatFits: CGSizeMake(self.txtQuestionType.frame.size.width, FLT_MAX)];
frame.size.height = [self textViewHeightForAttributedText:self.txtQuestionType.text andWidth:self.txtQuestionType.frame.size.width andTextView:self.txtQuestionType];
self.txtQuestionType.frame=frame;
self.textTypeView.frame = CGRectMake((self.view.frame.size.width - self.textTypeView.frame.size.width)/2, ( self.txtQuestionType.frame.origin.y+self.txtQuestionType.frame.size.height), self.textTypeView.frame.size.width, self.textTypeView.frame.size.height);
[self.view addSubview: self.textTypeView];
}
I agree with BryanChen, I think you may have another issue. Without details on the survey library, it is impossible to confirm, but assuming that it is a UIViewController than accepts touch inputs to progress through a series of questions, it is hard to see why it is a threading issue - it simply shouldn't advance without user interaction.
That aside, your use of NSCondtionLock doesn't look right either.
Essentially an NSConditionLock has an NSInteger that represents the current 'condition', but just think of it of a number. There are then two basic operations you can perform -
lockWhenCondition:x will block the current thread until the 'condition' is 'x' and the lock is available. It will then claim the lock.
unlockWithCondition:y releases the lock and sets the condition to 'y'
There are also methods to set timeouts (lockBeforeDate) and try to claim the lock without blocking (tryLock, tryLockWhenCondition).
To synchronise two threads, the general pattern is
Initialise Lock to condition 'x'
Thread 1 lockWhenCondition:x -This thread can claim the lock because it is x
Thread 2 lockWhenCondition:y - This thread will block because the lock is x
Thread 1 completes work, unlockWithCondition:y - This will enable Thread 2 to claim the lock and unblock that thread
Your code looks strange, because you are starting a thread in your if clause but unlocking in an else clause. I would have thought you would have something like -
-(IBAction)startPressed:(UIButton*)sender {
if (sender.tag == 1) {
//START BACKGROUND THREAD
surveyThread = [[NSThread alloc] initWithTarget:self selector:#selector(runProjecttest) object:nil];
[surveyThread start];
[condition:lockWithCondition:1]; // This will block until survey thread completes
[condition:unlockWithCondition:0]; // Unlock and ready for next time
}
}
- (void) runProjecttest:(AbstractTask *)rendertask
{
// DO STUFF AND SHOW UI ON MAIN THREAD, THEN LOCK
[condition lockWhenCondition: 0];
[self performSelectorOnMainThread:#selector(showUI:) withObject:task waitUntilDone:YES];
[condition unlockWithCondition:1];
}
BUT This looks like a recipe for deadlock to me, because you are performing the showUI selector on the main thread that is blocked waiting for the survey thread to complete.
Which brings us back to the question, what does showUI do and why is it skipping directly to the end?
Here is what I need to do:
Something like:
for int i = 0; i < 3; i++)
call method #selector(spin) with delay 0.1
//once this is done (e.g., in 0.3 seconds)
call method #selector(move) with delay 1 second.
What I need is to pile on events, and have other events start relative to when the previous one finishes.
So something like:
wait 100ms //total time = 100ms
spin
wait 100ms //total time = 200ms
spin
wait 100ms //total time = 300ms
spin
wait 1000ms //total time = 1300ms
move
Is something like this possible with dispatch_after? If so could someone give me an example? I cannot seem to find one for this case.
Note, none of this should cause the UI thread to wait / block.
Thanks
You can use NSThread's sleepForTimeInterval method. The following code will block the thread sleepForTimeInterval is running on.
dispatch_queue_t yourThread = dispatch_queue_create("com.xxx.queue", nil);
dispatch_async(yourThread, ^
{
[NSThread sleepForTimeInterval:10.0];
dispatch_async(dispatch_get_main_queue(), ^
{
});
});
This should point you to the right direction.
I personally think that you're trying to achive the goal in wrong way. When you deal with asynchronous process you should use blocks
Based on what your request :
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
[self performSelector:#selector(move:) withObject:[NSNumber numberWithInt:0] afterDelay:2];
}
-(void) move:(NSNumber *)i {
int val = [i intValue] + 1;
//do what you need here
if (val < 3) [self performSelector:#selector(move:) withObject:[NSNumber numberWithInt:val] afterDelay:2];
}
But Of course if you need to process a lot of data, use other thread instead...
dispatch_group_t group = dispatch_group_create();
dispatch_time_t timeNow = DISPATCH_TIME_NOW;
for (int i = 0; i < 3; ++i)
{
dispatch_group_enter(group);
dispatch_after(dispatch_time(timeNow, i*100), queue, ^(){
[self spin];
dispatch_group_leave(group);
});
}
dispatch_group_notify(group, queue, ^(){
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 1000), queue, ^(){
[self move];
});
});
I want to illustrate the progress on MBProgressHUD item, but when i triger this method :
- (IBAction)signInBttn:(id)sender {
MBProgressHUD *hudd = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
hudd.mode = MBProgressHUDModeAnnularDeterminate;
hudd.labelText = #"Loading";
__block float value = 0;
for (int j = 0; j<2000; j++) {
dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for (int i = 0; i<20000 ; i++) {
}
value += 0.001;
dispatch_async( dispatch_get_main_queue(), ^{
hudd.progress = value;
});
});
}
}
hud appears fully to 100%. This is only for my information, I dont have idea how to create background task which calculate something and when he done with e.g. 40% the HUD is refreshing to 40% of his progress. I hope I made myself clear, and if anyone has time to help improve my code, thanks a lot for any answers
In this case, you can solve the problem by decoupling the updating of the counter from the updating of your HUD in your UI. Apple refers to this as "updating the state asynchronously" in WWDC 2012 video Asynchronous Design Patterns with Blocks, GCD, and XPC.
Generally this isn't necessary (most of the time the stuff we're doing asynchronously is slow enough that we don't have problems), but if doing something that is running faster than the UI can hope to keep up with, you create a "dispatch source" for this. I'm going to illustrate it with a UIProgressView, but the same applies to pretty much any UI:
// create source for which we'll be incrementing a counter,
// and tell it to run the event handler in the main loop
// (because we're going to be updating the UI)
dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_DATA_ADD, 0, 0, dispatch_get_main_queue());
// specify what you want the even handler to do (i.e. update the HUD or progress bar)
dispatch_source_set_event_handler(source, ^{
self.iterations += dispatch_source_get_data(source);
[self.progressView setProgress: (float) self.iterations / kMaxIterations];
});
// start the dispatch source
dispatch_resume(source);
// now, initiate the process that will update the source
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for (long i = 0; i < kMaxIterations; i++)
{
// presumably, do something meaningful here
// now increment counter (and the event handler will take care of the UI)
dispatch_source_merge_data(source, 1);
}
// when all done, cancel the dispatch source
dispatch_source_cancel(source);
});
In my example, iterations is just a long property:
#property (nonatomic) long iterations;
And I defined my kMaxIterations constant as follows:
static long const kMaxIterations = 10000000l;
First off, if you want to delay execution use dispatch_after: Apple Doc since it could be that Clang is optimizing your loop (i.e. by making it not exist).
Within that block call dispatch_sync on the main thread to update the UI, since dispatch_async is not guaranteed to execute 'evenly'. Something like this ought to work...
for (...) {
dispatch_after(<some formula of i>, DEFAULT_PRIORITY, ^{
dispatch_sync(MAIN_QUEUE, ^{ hudd.progress = value });
}
}
I want to synchronize some data with a web service. For each item I have to make a asynchronous call.
I want to have a completion block witch is called, when each item was synchronized. For each item I am able to perform a completion block. Now, I don't know a good way how to do it.
This is the interface:
-(void) synchronizeItemsOnComplete:(CompleteBlock) block {
NSArray* items = // get items
for (int i = 0, n = [items count]; i < n; i++) {
[self synchronizeItem:[items objectAtIndex:i] onComplete:^{
// What do do here?
}];
}
// And/or here?
}
-(void) synchronizeItemOnComplete:(CompleteBlock) block {
// do something
block();
}
How can I wait for the synchronization and then perform the block?
I tried something like this:
NSArray* items = // get items
__block int countOfItemsUntilDone = [items count];
for (int i = 0, n = countOfItemsUntilDone; i < n; i++) {
[self synchronizeItem:[items objectAtIndex:i] onComplete:^{
countOfItemsUntilDone--;
}];
}
dispatch_queue_t queue = dispatch_queue_create("wait for syncing", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
while (countOfItemsUntilDone > 0) {
usleep(1000); // wait a little bit
}
block();
});
dispatch_release(queue);
But I think this is a quite bad way. Any ideas?
Instead of spinning in a loop waiting for the counter to equal zero, check the counter value each time you decrement it, then fire an event when it reaches zero.
-(void) synchronizeItemsOnComplete:(CompleteBlock) block {
NSArray* items = // get items
__block NSUInteger remaining = [items count];
for (ItemClass* item in items) {
[self synchronizeItemImage:item onComplete:^{
--remaining;
if (remaining == 0) {
block();
}
}];
}
}
To explain why it feels wrong, there are two things you're doing here that you should do either never or rarely:
Using background queues. This is difficult and bug-prone. Don't do it without reading up a lot about writing concurrent code. You also only really need to do this if an operation blocks for a substantial amount of time (eg., to read a file from disk, or perform an intensive calculation). Don't assume you need to do it unless you have a good reason (eg., a measurable performance problem).
Spinning in a loop, checking a variable for changes and calling sleep. You should never do this.
Also, if you're looping over the elements in an array, the for ... in syntax is much nicer (and potentially more efficient) calling objectAtIndex: on each index.
Never check or decrement shared memory in different threads like this, it can cause races. Use a dispatch group to do what you're doing.
dispatch_queue_t myBGQueue;
dispatch_group_t itemsGroup = dispatch_group_create();
for (ItemClass *item in items) {
dispatch_group_async(itemsGroup, myBGQueue, ^{
[self synchronizeItemImage:item];
});
}
/* execution will sleep here until all the blocks added in the `for` complete */
dispatch_group_wait(itemsGroup, DISPATCH_TIME_FOREVER);
dispatch_release(itemsGroup);
You can use these to use synchronously.
GCD and this
performSelector:waitUntilDone:YES