Below is a block of code that runs in a separate thread from my app's main thread. How do I get the UI to update after each button gets its thumbnail? Right now it doesn't update until the whole method finishes. The buttons are already added to a UIScrollView.
(LotsGridButton is just a UIButton with some extra properties.)
- (void)fetchThumbnails {
CCServer* server = [[CCServer alloc] init];
for (int i=0; i<[buttons count]; i++) {
LotsGridButton* button = [buttons objectAtIndex:i];
if (button.lot.thumbnail) continue;
// load the thumbnail image from the server
button.lot.thumbnail = [server imageWithPath:button.lot.thumbnailURL];
[button setImage:button.lot.thumbnail forState:UIControlStateNormal];
}
[server release];
}
In place of setImage:forState:, take a look at the performSelectorOnMainThread: method, e.g.:
[myButton performSelectorOnMainThread:#selector(setThumbnail:) withObject:[server imageWithPath:myButton.lot.thumbnailURL] waitUntilDone:NO];
I've no experience with the iPhone but in Cocoa in general you're supposed to update the UI only from the main thread.
From a different thread you can execute code in the main thread by using NSObject's:
performSelectorOnMainThread:withObject:waitUntilDone:
Related
I have the following code that attempts to load a row of thumbnails in a tableview asynchronously:
for (int i = 0; i < totalThumbnails; i++)
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
__block GraphicView *graphicView;
__block Graphic *graphic;
dispatch_async(dispatch_get_main_queue(),
^{
graphicView = [[tableViewCell.contentView.subviews objectAtIndex:i] retain];
graphic = [[self.thumbnailCache objectForKey: [NSNumber numberWithInt:startingThumbnailIndex + i]] retain];
if (!graphic)
{
graphic = [[self graphicWithType:startingThumbnailIndex + i] retain];
[self.thumbnailCache setObject: graphic forKey:[NSNumber numberWithInt:startingThumbnailIndex + i]];
}
[graphicView setGraphic:graphic maximumDimension:self.cellDimension];
});
[graphicView setNeedsDisplay];
dispatch_async(dispatch_get_main_queue(),
^{
CGRect graphicViewFrame = graphicView.frame;
graphicViewFrame.origin.x = ((self.cellDimension - graphicViewFrame.size.width) / 2) + (i * self.cellDimension);
graphicViewFrame.origin.y = (self.cellDimension - graphicViewFrame.size.height) / 2;
graphicView.frame = graphicViewFrame;
});
[graphicView release];
[graphic release];
});
}
However when I run the code I get a bad access at this line: [graphicView setNeedsDisplay]; It's worth mentioning that the code works fine when I have it set up like this:
for (int i = 0; i < totalThumbnails; i++)
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
dispatch_async(dispatch_get_main_queue(),
^{
//put all the code here
});
}
It works fine and the UITableView loads asynchronously when it's first called, however the scrolling is still really choppy.
So I'd like to get the to get the first bit of code to work so I can get the drawing done in the global thread instead of the main thread (which I assume will fix the choppy scrolling?).
Since iOS4 drawing is able to be done asynchronously so I don't believe that is the problem. Possibly I'm misusing the __Block type?
Anyone know how I can get this to work?
You completely misunderstand how to use GCD. Looking at your code:
__block GraphicView *graphicView;
Your variable here is not initialised to nil. It is unsafe to send messages to.
__block Graphic *graphic;
dispatch_async(dispatch_get_main_queue(),
^{
//statements
});
Your dispatch statement here returns immediately. The system works for you to spin this task off on a different thread. Before, or perhaps at the same time as, the above statements are executed we move on to the next line of execution here...
[graphicView setNeedsDisplay];
At this point graphic view may or may not have been initialised by your dispatch statement above. Most likely not as there wont have been time. As it still hasn't been initialised it points to random memory and hence trying to send messages to it causes EXC_BAD_ACCESS.
If you want to draw cell contents asynchronously (or pre-render images or whatever.) I thouroughly reccommend watching WWDC 2012 session 211 "Building Concurrent User Interfaces on iOS". They do almost exactly what you seem to be attempting to do and explain all the pitfalls you can run into.
I think the issue is because you are trying to re-draw the UIView on a working thread. You should move this:
[graphicView setNeedsDisplay];
To the main queue.
I'm having a hard time figuring out how to put this all together.
I have a puzzle solving app on the mac.
You enter the puzzle, press a button, and while it's trying to find the number of solutions,
min moves and such I would like to keep the UI updated.
Then once it's finished calculating, re-enable the button and change the title.
Below is some sample code from the button selector, and the solving function:
( Please keep in mind I copy/paste from Xcode so there might be some missing {} or
some other typos.. but it should give you an idea what I'm trying to do.
Basicly, user presses a button, that button is ENABLED=NO, Function called to calculate puzzle. While it's calculating, keep the UI Labels updated with moves/solution data.
Then once it's finished calculating the puzzle, Button is ENABLED=YES;
Called when button is pressed:
- (void) solvePuzzle:(id)sender{
solveButton.enabled = NO;
solveButton.title = #"Working . . . .";
// I've tried using this as a Background thread, but I can't get the code to waitTilDone before continuing and changing the button state.
[self performSelectorInBackground:#selector(createTreeFromNode:) withObject:rootNode];
// I've tried to use GCD but similar issue and can't get UI updated.
//dispatch_queue_t queue = dispatch_queue_create("com.gamesbychris.createTree", 0);
//dispatch_sync(queue, ^{[self createTreeFromNode:rootNode];});
}
// Need to wait here until createTreeFromNode is finished.
solveButton.enabled=YES;
if (numSolutions == 0) {
solveButton.title = #"Not Solvable";
} else {
solveButton.title = #"Solve Puzzle";
}
}
Needs to run in background so UI can be updated:
-(void)createTreeFromNode:(TreeNode *)node
{
// Tried using GCD
dispatch_queue_t main_queue = dispatch_get_main_queue();
...Create Tree Node and find Children Code...
if (!solutionFound){
// Solution not found yet so check other children by recursion.
[self createTreeFromNode:newChild];
} else {
// Solution found.
numSolutions ++;
if (maxMoves < newChild.numberOfMoves) {
maxMoves = newChild.numberOfMoves;
}
if (minMoves < 1 || minMoves > newChild.numberOfMoves) {
solutionNode = newChild;
minMoves = newChild.numberOfMoves;
// Update UI on main Thread
dispatch_async(main_queue, ^{
minMovesLabel.stringValue = [NSString stringWithFormat:#"%d",minMoves];
numSolutionsLabel.stringValue = [NSString stringWithFormat:#"%d",numSolutions];
maxMovesLabel.stringValue = [NSString stringWithFormat:#"%d",maxMoves];
});
}
GCD and performSelectorInBackground samples below. But first, let's look at your code.
You cannot wait where you want to in the code above.
Here's the code you had. Where you say wait in the comment is incorrect. See where I added NO.
- (void) solvePuzzle:(id)sender{
solveButton.enabled = NO;
solveButton.title = #"Working . . . .";
// I've tried using this as a Background thread, but I can't get the code to waitTilDone before continuing and changing the button state.
[self performSelectorInBackground:#selector(createTreeFromNode:) withObject:rootNode];
// NO - do not wait or enable here.
// Need to wait here until createTreeFromNode is finished.
solveButton.enabled=YES;
}
A UI message loop is running on the main thread which keeps the UI running. solvePuzzle is getting called on the main thread so you can't wait - it will block the UI. It also can't set the button back to enabled - the work hasn't been done yet.
It is the worker function's job on the background thread to do the work and then when it's done to then update the UI. But you cannot update the UI from a background thread. If you're not using blocks and using performSelectInBackground, then when you're done, call performSelectorOnMainThread which calls a selector to update your UI.
performSelectorInBackground Sample:
In this snippet, I have a button which invokes the long running work, a status label, and I added a slider to show I can move the slider while the bg work is done.
// on click of button
- (IBAction)doWork:(id)sender
{
[[self feedbackLabel] setText:#"Working ..."];
[[self doWorkButton] setEnabled:NO];
[self performSelectorInBackground:#selector(performLongRunningWork:) withObject:nil];
}
- (void)performLongRunningWork:(id)obj
{
// simulate 5 seconds of work
// I added a slider to the form - I can slide it back and forth during the 5 sec.
sleep(5);
[self performSelectorOnMainThread:#selector(workDone:) withObject:nil waitUntilDone:YES];
}
- (void)workDone:(id)obj
{
[[self feedbackLabel] setText:#"Done ..."];
[[self doWorkButton] setEnabled:YES];
}
GCD Sample:
// on click of button
- (IBAction)doWork:(id)sender
{
[[self feedbackLabel] setText:#"Working ..."];
[[self doWorkButton] setEnabled:NO];
// async queue for bg work
// main queue for updating ui on main thread
dispatch_queue_t queue = dispatch_queue_create("com.sample", 0);
dispatch_queue_t main = dispatch_get_main_queue();
// do the long running work in bg async queue
// within that, call to update UI on main thread.
dispatch_async(queue,
^{
[self performLongRunningWork];
dispatch_async(main, ^{ [self workDone]; });
});
}
- (void)performLongRunningWork
{
// simulate 5 seconds of work
// I added a slider to the form - I can slide it back and forth during the 5 sec.
sleep(5);
}
- (void)workDone
{
[[self feedbackLabel] setText:#"Done ..."];
[[self doWorkButton] setEnabled:YES];
}
dispatch_queue_t backgroundQueue;
backgroundQueue = dispatch_queue_create("com.images.bgqueue", NULL);
- (void)process {
dispatch_async(backgroundQueue, ^(void){
//background task
[self processHtml];
dispatch_async(main, ^{
// UI updates in main queue
[self workDone];
});
});
});
}
By and large, any work to be submitted to a background queue needs to follow this pattern of code:
dispatch_queue_t queue = dispatch_queue_create("com.myappname", 0);
__weak MyClass *weakSelf = self; //must be weak to avoid retain cycle
//Assign async work
dispatch_async(queue,
^{
[weakSelf doWork];
dispatch_async(dispatch_get_main_queue(),
^{
[weakSelf workDone];
});
});
queue = nil; //Using ARC, we nil out. Block always retains the queue.
Never Forget:
1 - queue variable above is a reference counted object, because it is a private queue, not a global one. So it is retained by the block which is executing inside that queue. Until this task is complete, it is not released.
2 - Every queue got its own stack which will be allocated / deallocated as part of recursive operation. You only need to worry about class member variables which are reference counted (strong, retain etc.) which are accessed as part of doWork above.
3 - While accessing those reference counted vars inside background queue operation, you need to make them thread-safe, depending on use cases in your app. Examples include writes to objects such as strings, arrays etc. Those writes should be encapsulated inside #synchronized keyword to ensure thread-safe access.
#synchronized ensures no another thread can get access to the resource it protects, during the time the block it encapsulates gets executed.
#synchronized(myMutableArray)
{
//operation
}
In the above code block, no alterations are allowed to myMutableArray inside the #synchronized block by any other thread.
I have an app that loads multiple thumbnail images into a UIScrollVIew. This is a lengthy operation, and so as not to block up the display of the rest of the UI, I am running it in a separate thread. This works fine the first time at application launch, but later a new set of images needs to be loaded into the UIScrollView. When I detach a thread a second time the app crashes (sometimes). Code follows:
// this call is in a separate method
[NSThread detachNewThreadSelector:#selector(addThumbnailsToScrollView) toTarget:self withObject:nil];
// this is the main entry point for the thread
- (void) addThumbnailsToScrollView {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // Top-level pool
// now place all the thumb views as subviews of the scroll view
float xPosition = THUMB_H_PADDING;
int pageIndex = 0;
for (Page *page in self.pages) {
// get the page's bitmap image and scale it to thumbnail size
NSString *name = [page valueForKey:#"pageBackground"];
NSString *basePath = [[NSBundle mainBundle] pathForResource:page.pageBackground ofType:#"jpg" inDirectory:nil];
UIImage *thumbImage = [UIImage imageWithContentsOfFile:basePath];
thumbImage = [thumbImage imageScaledToSize:CGSizeMake(80, 100)];
// create a ThumbImageView for each page and add it to the thumbnailScrollView
if (thumbImage) {
ThumbImageView *thumbView = [[ThumbImageView alloc] initWithImage:thumbImage];
[thumbView setDelegate:self];
[thumbView setImageName:name];
[thumbView setImageSize:CGSizeMake(80, 100)];
[thumbView setPageIndex:pageIndex];
pageIndex ++;
CGRect frame = [thumbView frame];
frame.origin.y = 0;
frame.origin.x = xPosition;
[thumbView setFrame:frame];
[thumbnailPagesScrollView addSubview:thumbView];
[thumbView release];
xPosition += (frame.size.width + THUMB_H_PADDING);
}
}
[self hightlightThumbnailPageAtIndex:0];
[(UIActivityIndicatorView *)[thumbnailPagesScrollView.superview viewWithTag:100] stopAnimating];
[pool release]; // Release the objects in the pool.
}
I thought that a detached thread exits as soon as the main entry routine was completed. Wouldn't the second call to detach a thread be a new thread? Why is the app crashing, but sometimes not?
Thanks
Jk
You cannot touch UIKit (meaning UIScrollVIew) in a secondary thread - you need to reorganize so that the fetch takes place in a secondary thread but you make a NSData object (containing the image binary) available to your primary thread for each thumbnail so that it can do everything related to actually displaying them.
Apple repeatedly warn in documentation that UIKit is not thread-safe.
I would suggest adding the thumbView to the thumbnailPagesScrollView on the main thread rather than a separate thread. There might be issues on the retain count of the object across threads. There is a convenience method performSelectorOnMainThread I think it is to do that. You could pass thumbView to that and then add it to the subview.
Alternatively you could do the whole if statement on the main thread as thats not the thing that will interrupt the user.
Also with your activity indicator this should be stopped on the main thread. Everything UI related should be done on the main thread.
A frequently answered question, I'm afraid but I am pretty much in the dark on this.
Within my view controller I have the following method to switch back and forward between two images a total of 5 times
- (IBAction)cycle{
BOOL select1;
select1=YES;
UIImage *image1 = [UIImage imageNamed:#"image1.png"];
UIImage *image2 = [UIImage imageNamed:#"image2.png"];
for (int i=0; i<5; i++) {
if (select1){
[imageview setImage:image1];
} else {
[imageview setImage:image2];
}
[NSThread sleepForTimeInterval:1.0];
[imageview setNeedsDisplay];
}
}
The problem is that the setNeedsDisplay message does not work within the loop and the view is only updated when the method quits.
Is there any thing I can do here? Is the approach feasible or am I completely down the wrong track. This is very much a test program (I am new to this language) but it would be useful to control something like this programmatically. The next step app will implement randomly changing times between picture changes.
Can anyone help me on this?
Wrong track. Calling
[NSThread sleepForTimeInterval:1.0];
Is only trying to put the main thread asleep. The problem is that this is the thread that's actually does the drawing. You need to end what you're doing and give control bakc to the NS runtime so that it can update gui elements.
Try using NSTimer to create a scheduled timer. Have it repeat with the time interval that you want. All that you then have to do is set the image:
if (select1){
[imageview setImage:image1];
} else {
[imageview setImage:image2];
}
there's no need for
[imageview setNeedsDisplay];
As the imageview will handle that for itself.
Kinda new to iPhone programming and was experimenting with threads
- (void)viewDidLoad {
[super viewDidLoad];
[NSThread detachNewThreadSelector:#selector(changeMain) toTarget:self withObject:nil];
[NSThread detachNewThreadSelector:#selector(changeThread) toTarget:self withObject:nil];
}
- (void)changeMain{
NSAutoreleasePool* arp = [[NSAutoreleasePool alloc] init];
for (int i = 0; i < 1000000; i++) {
[mainValue setText:[NSString stringWithFormat:#"%d",i]];
[self.view setNeedsDisplay];
}
[arp release];
}
- (void)changeThread{
NSAutoreleasePool* arp = [[NSAutoreleasePool alloc] init];
for (int i = 0; i < 1000000; i++) {
[threadValue setText:[NSString stringWithFormat:#"%d",i]];
[self.view setNeedsDisplay];
}
[arp release];
}
mainValue and threadValue are both just UILabels. I expected this to run and see both labels run up to 999999 but instead it starts at some low number (what it is when the screen initally refreshing i assume), pauses for a bit, then updates to 999999. I'm thinking the screen just isn't refreshing.
Is this correct? Am I doing it wrong?
You have to perform any Cocoa Touch operations in main thread, in other case results are unpredictable.
You don't have to call setNeedsDisplay manually.
So I'd recommend to use the following construction:
[threadValue performSelectorOnMainThread:#selector(setText:) withObject:[NSString stringWithFormat:#"%d",i] waitUntilDone:YES];
Additional notes:
1. 100000 runs can overflow the main thread queue so some values will disappear
2. You can use waitUntilDone:NO too
The setNeedsDisplay message triggers a redraw, but it only happens during the next time the main thread becomes active. So your side threads trigger a million redraws but they are queued. As soon as the main thread continues, it "collapses" all requests into one redraw.
Most likely setNeedsDisplay just sets a flag that is checked once during each run of the main loop, so setting it 1000000 to true doesn't influence anything. Try to let the "worker threads" sleep after each iteration to give the main thread some time to redraw.
Don't use for() for animations. The for will process in the same "frame". Maybe just have an ivar i and in changeMain you can have if (i<10000) { mainValue.text = [NSString stringWithFormat:#"%d",i]; i++;} or something like that. This way the setText only happens once per "frame".
I'm not sure if this will work, but you could try to force the setNeedsDisplay method to be executed on the main thread, using e.g. [self performSelectorOnMainThread:#selector(setNeedsDisplay) withObject:nil waitUntilDone:YES]. This should (hopefully, i didn't test it!) update the view after every increment. You could also try to set waitUntiDone:NO, but I'm unsure what will happen then.
See here