I have a Quartz context declared in the main loop as
UIGraphicsBeginImageContext(mySize);
ctx = UIGraphicsGetCurrentContext();
// bla bla bla
This context is changing, as the user is doing stuff on the screen.
At some point in the app I need a new thread to be fired and grab what the context has, saving to a UIImageView.
on the thread I have something like
myImageView.image = UIGraphicsGetImageFromCurrentImageContext();
but this is giving me nil, as the thread is not able to know that the current context is.
How do I solve that?
thanks.
OK, it may be a kludge but you could try this
- (void) myUIGraphicsGetImageFromCurrentImageContext
{
myImageView.image= UIGraphicsGetImageFromCurrentImageContext();
}
Then in your thread replace the myImageView.Image=…line with this:
[self performSelectorOnMainThread:#selector(myUIGraphicsGetImageFromCurrentImageContext) withObject:nil waitUntilDone:YES];
Related
I've spent days trying every possible solution I can think of to this problem, but nothing seems to be working.
I run a background thread like this:
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext) {
Media *localMedia = [media inContext:localContext];
UIImage *image = localMedia.thumbnail;
dispatch_async(dispatch_get_main_queue(), ^{
[thumbnails setObject:image forKey:[NSNumber numberWithInt:i]];
[contentDict setObject:thumbnails forKey:#"MediaItems"];
[cell.entryView setNeedsDisplay];
});
}];
Or like this:
dispatch_queue_t cellSetupQueue = dispatch_queue_create("com.Journalized.SetupTimelineThumbnails", NULL);
dispatch_async(cellSetupQueue, ^{
NSManagedObjectContext *newMoc = [[NSManagedObjectContext alloc] init];
NSPersistentStoreCoordinator *coordinator = [NSManagedObjectContext contextForCurrentThread].persistentStoreCoordinator;
[newMoc setPersistentStoreCoordinator:coordinator];
NSNotificationCenter *notify = [NSNotificationCenter defaultCenter];
[notify addObserver:self
selector:#selector(mergeChanges:)
name:NSManagedObjectContextDidSaveNotification
object:newMoc];
Media *localMedia = [media inContext:[NSManagedObjectContext contextForCurrentThread];
UIImage *image = localMedia.thumbnail;
dispatch_async(dispatch_get_main_queue(), ^{
[thumbnails setObject:image forKey:[NSNumber numberWithInt:i]];
[contentDict setObject:thumbnails forKey:#"MediaItems"];
[cell.entryView setNeedsDisplay];
});
}];
Both of these give me a crash with UIImage returning as nil object, and a Cocoa Error 133000.
I've removed every other piece of background threading code, and have saved on the main thread directly before this just to make sure. Running the code above on the main thread also works, but freezes up my UI. Despite all of these efforts, the above code crashes every time.
I'm not sure what to do to make this work.
Edit:
My question, specifically, is how do I make this work without crashing? It seems to be a problem with objects from 1 context not existing in another, but how do i make them exist in another context?
Remember, the MR_inContext: method is using [NSManagedObjectContext objectWithID: ] method under the covers. You should look in there to make sure your object has:
1) Been saved prior to entering into the background context/block in your first code block
2) This method is returning something useful
I'm also not sure how you set up your thumbnail attribute. Ideally it shouldn't matter as long as you have the NSTransformable code write (there are samples on the internets that show you how to save a UIImage in core data using the transformable attribute)
Also, your code should look like this:
UIImage *image = nil;
[MagicalRecord saveInBackgroundWithBlock:^(NSManagedObjectContext *localContext) {
Media *localMedia = [media inContext:localContext]; //remember, this looks up an existing object. If your context is a child of a parent context that has contains this, the localContext should refresh the object from the parent.
//image processing/etc
image = localMedia.thumbnail;
} completion:^{
[thumbnails setObject:image forKey:#(i)]; //new literals are teh awesome
contentInfo[#"MediaItems"] = thumbnails; //as is the new indexer syntax (on the latest compiler)
[cell.entryView setNeedsDisplay];
}];
Fast answer:
NSManagedObjectReferentialIntegrityError = 133000,
// attempt to fire a fault pointing to an object that does not exist (we can see the store, we can't see the object)
EDIT:
It's pretty difficult to see something from the code. What is a managed object there?
I suppose the problem is that you are using temporary objects from one context in another context.
I am having trouble in calling a method that I defined screenCap by a UIWebView. Basically, I have two UIWebView declared: firstWebView and secondWebView.
I ran a javascript to extract some HTML loaded by the firstWebView and display it to the secondWebView by calling [secondWebView loadHTMLString: extractString baseURL:nil];
What I want to do is after secondWebView is loaded, I want to call
[secondWebView screenCap: self];
The screenCap method is defined by:
-(void) screenCap: (UIWebView *)webView; {
UIGraphicsBeginImageContext(webView.frame.size);
[webView.layer renderInContext: UIGraphicsGetCurrentContext()];
viewImage; = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
theImageView.image = viewImage;
NSLog(#"write Image done!");
I, however got a warning "UIWebView may not respond to '-screenCap' and of course the screenCap never got called. can anyone direct me how to fix this problem?
There's nothing wrong in the code of the ScreenCap method.
But instead of doing
[secondWebView screenCap: self];
write
[self screenCap:secondWebView];
I have a view that generates an image based on a series of layers. I have images for the background, for the thumbnail, and finally for an overlay. Together, it makes one cohesive display.
It seems to work a dream, except for when it doesn't. For seemingly no reason, I get an EXC_BAD_ACCESS on the specified line below after it's generated somewhere between 8 and 20 images. I've run it through the memory leak tool and allocation tool, and it's not eating up tons of memory and it's not leaking. I'm totally stumped.
Here's the relevant code:
- (UIImage *)addLayer:(UIImage *)layer toImage:(UIImage *)background atPoint:(CGPoint)point {
CGSize size = CGSizeMake(240, 240);
UIGraphicsBeginImageContext(size);
[background drawAtPoint:CGPointMake(0, 0)]; // <--- error here
[layer drawAtPoint:point];
UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return result;
}
// Build the layered image -- thingPage onto thingBackground,
// then the screenshot on that, then the thingTop on top of it all.
// thingBackground, thingPage and thingTop are all preloaded UIImages.
-(UIImage *)getImageForThing:(Thing *)t {
[self loadImageCacheIfNecessary];
if (!t.screenshot) {
return [UIImage imageNamed:#"NoPreview.png"];
} else {
UIImage *screenshot = t.screenshot;
UIImage *currentImage = [self addLayer:thingPage toImage:thingBackground atPoint:CGPointMake(0, 0)];
currentImage = [self addLayer:screenshot toImage:currentImage atPoint:CGPointMake(39, 59)];
currentImage = [self addLayer:thingTop toImage:currentImage atPoint:CGPointMake(0, 1)];
return currentImage;
}
}
I can't find anywhere that this is going wrong, and I've been tearing my hair out for a couple of hours on this. It's the final known bug in the system, so you can imagine how antsy I am to fix it! :-)
Thanks in advance for any help.
As to me, I always use -(void)drawInRect: instead of -(void)drawAtPoint:
CGRect rtDraw;
rtDraw.origin = CGPointZero;
rtDraw.size = size;
[background drawInRect:rtDraw];
[layer drawInRect:rtDraw];
And ....
The paint method with UIGraphicsBeginImageContext(size) and UIGraphicsEndImageContext() is not thread-safe.
Those functions will push or pop a context with stack struct, which is managed by system.
EXC_BAD_ACCESS is almost always due to accessing an object that has already been released. In your code this seems to be t.screenshot. Check creation (and retaining if it is an instance variable) of the object returned by Thing's screenshot property.
As it turns out, the error wasn't in the code I posted, it was in my caching of the thingBackground, thingPage and thingTop images. I wasn't retaining them. Here's the missing code, fixed:
-(void)loadImageCacheIfNecessary {
if (!folderBackground) {
thingBackground = [[UIImage imageNamed:#"ThingBack.png"] retain];
}
if (!folderPage) {
thingPage = [[UIImage imageNamed:#"ThingPage.png"] retain];
}
if (!folderTop) {
thingTop = [[UIImage imageNamed:#"ThingTop.png"] retain];
}
}
I will admit I'm still not comfortable with the whole retain/release/autorelease stuff in Objective C. Hopefully it'll sink in one day soon. :-)
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.
This appears to be the the classic method for scanning images from the iPhone. I have a thread that is dispatched from the main thread to go and scan for Codes. It essentially creates a new UIImage each time then removes it.
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
{
while (![thread isCancelled]) {
#ifdef DEBUG
NSLog(#"Decoding Loop");
#endif
// [self performSelectorOnMainThread:#selector(updateImageBuffer) withObject:nil waitUntilDone:YES];
CGImageRef cgScreen = UIGetScreenImage();
UIImage *uiimage = [UIImage imageWithCGImage:cgScreen];
if (uiimage){
CGSize size = [uiimage size];
CGRect cropRect = CGRectMake(0.0, 80.0, size.width, 360); // Crop to centre of the screen - makes it more robust
#ifdef DEBUG
NSLog(#"picked image size = (%f, %f)", size.width, size.height);
#endif
[decoder decodeImage:uiimage cropRect:cropRect];
}
[uiimage release];
CGImageRelease(cgScreen);
}
}
[pool release];
the problem is that the [pool release] causes an ERROR_BAD_EXC (that old classic) and the program bombs. I'm told that there is no need to call [uiimage release] as I havent explicitly allocated a UIImage but this doesn't seem to be the case. If I take that line out, Memory usage goes through the roof and the program quits dues to lack of memory. It appears I can't have this work the way I'd like.
Is there a way to create a UIImage "in-place"? I.e, have a buffer that is written to again and again as a UIImage? I suspect that would work?
Update!
Tried executing the UIKit related calls on the main thread as follows:
-(void)performDecode:(id)arg{
// Perform the decoding in a seperate thread. This should, in theory, bounce back with a
// decoded or not decoded message. We can quit at the end of this thread.
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
{
while (![thread isCancelled]) {
#ifdef DEBUG
NSLog(#"Decoding Loop");
#endif
[self performSelectorOnMainThread:#selector(updateImageBuffer) withObject:nil waitUntilDone:YES];
if (uiimage){
CGSize size = [uiimage size];
CGRect cropRect = CGRectMake(0.0, 80.0, 320, 360); // Crop to centre of the screen - makes it more robust
#ifdef DEBUG
NSLog(#"picked image size = (%f, %f)", size.width, size.height);
#endif
[decoder decodeImage:uiimage cropRect:cropRect];
}
}
}
[pool drain];
#ifdef DEBUG
NSLog(#"finished decoding.");
#endif
}
-(void) updateImageBuffer {
CGImageRef cgScreen = UIGetScreenImage();
uiimage = [UIImage imageWithCGImage:cgScreen];
//[uiimage release];
CGImageRelease(cgScreen);
}
No joy however as EXC_BAD_ACCESS rears its ugly head when one wishes to grab the "Size" of the UIImage
As has been stated by others, you should not release the UIImage returned from imageWithCGImage: . It is autoreleased. When your pool drains, it tries sending a release message to your already-released image objects, leading to your crash.
The reason why your memory usage keeps climbing is that you only drain the autorelease pool outside of the loop. Your autoreleased objects keep accumulating inside of the loop. (By the way, you need to release your autorelease pool at the end of that method, because it is currently being leaked.) To prevent this accumulation, you could drain the pool at regular intervals within the loop.
However, I'd suggest switching to doing [[UIImage alloc] initWithCGImage:cgScreen] and then releasing the image when done. I try to avoid using autoreleased objects whereever I can within iPhone applications in order to have tighter control over memory usage and overall better performance.
UIGetScreenImage() is private and undocumented so you flat-out cannot use it. Saying that nothing about it suggests that you now own CGImageRef cgScreen so why do you release it? You also have no way of knowing if it is thread safe and so should assume it isn't. You then go on to release the IImage *uiimage which you did not init, retain or copy, so again - you don't own it. Review the docs.
[uiimage release] is definitely wrong in this context. Also, Apple stresses that all UIKit methods must be executed on the main thread. That includes UIGetScreenImage() and +[UIImage imageWithCGImage:].
Edit: So you get an exception when calling -[UIImage size] on the wrong thread. This probably shouldn't surprise you because it is not permitted.
UIImage *uiimage = [[UIImage alloc] initWithCGImage: cgScreen];
Explicitly stating that I know best when to release the object seemed to work. Virtual Memory still increases but physical now stays constant. Thanks for pointing out the UIKit Thread Safe issues though. That is a point I'd missed but seems not affect the running at this point.
Also, I should point out, Red Laser and Quickmark both use this method of scanning camera information ;)