Need to call retain for a property with retain flag specified - iphone

Question

I'm newbie in Objective-C, and as most of the newbies I have a questions about references management.
I've written a class which downloads data using NSURLConnection. The code is similar to Apple's example in http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html. The only difference is that receivedData variable is declared as "#property (nonatomic,retain) NSMutableData *receivedData;" In .m file I have "#synthesize receivedData = _receivedData;".
I have connectionStart function which starts downloading data. In this function I have this code:
if (theConnection) {
// Create the NSMutableData to hold the received data.
// receivedData is an instance variable declared elsewhere.
self.receivedData = [NSMutableData data];
} else {
// Inform the user that the connection failed.
}
The program crashes with this message:
2011-06-12 12:47:22.298 WebGallery[1212:207] *** -[NSConcreteMutableData release]: message sent to deallocated instance 0x118a6fe0
If I change receivedData assignment to this code:
self.receivedData = [[NSMutableData data] retain];
Then the program works correctly and no memory leaks are detected.
As you see I need to call retain on NSMutableData and I'm using property, which is declared as "retain".
Why does this happen?
EDIT: Full contents of .m file:
#import "GalleryData.h"
#import "XmlParser.h"
#implementation GalleryData
#synthesize receivedData = _receivedData;
#synthesize imagesData = _imagesData;
#synthesize delegate = _delegate;
#synthesize currentObjectType = _currentObjectType;
#synthesize currentObjectIndex = _currentObjectIndex;
- (id) init
{
[super init];
_imagesData = [[NSMutableArray alloc] init];
return self;
}
- (void) dealloc
{
[_imagesData release];
_imagesData = nil;
[super dealloc];
}
- (void) connectionStart:(NSURL *)theURL
{
NSURLRequest *theRequest = [NSURLRequest requestWithURL:theURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:60.0];
NSURLConnection *theConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
if (theConnection) {
// Create the NSMutableData to hold the received data.
// receivedData is an instance variable declared elsewhere.
//ASK: Kodėl čia reikia daryti retain jei #property jau nustatyta retain?
self.receivedData = [[NSMutableData data] retain];
} else {
// Inform the user that the connection failed.
}
}
- (void) startLoading
{
NSLog(#"Loading started");
self.currentObjectIndex = 0;
self.currentObjectType = ObjectTypeXML;
[self connectionStart:[NSURL URLWithString:#"http://www.aleksandr.lt/gallery/data.xml"]];
}
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
[self.receivedData setLength:0];
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
[self.receivedData appendData:data];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
[connection release];
[self.receivedData release];
NSLog(#"Connection failed! Error - %# %#",
[error localizedDescription],
[[error userInfo] objectForKey:NSURLErrorFailingURLStringErrorKey]);
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
[connection release];
if (self.currentObjectType == ObjectTypeXML) {
NSXMLParser *nsXmlParser = [[NSXMLParser alloc] initWithData:self.receivedData];
XmlParser *parser = [[XmlParser alloc] initXmlParser:self.imagesData];
[nsXmlParser setDelegate:parser];
[nsXmlParser parse];
[nsXmlParser release];
[parser release];
[self.receivedData release];
self.receivedData = nil;
if ([self.imagesData count]) {
self.currentObjectIndex = 0;
self.currentObjectType = ObjectTypeThumbImage;
ImageData *firstImage = [self.imagesData objectAtIndex:0];
NSURL *theURL = [NSURL URLWithString:firstImage.thumbImageURL];
[self connectionStart:theURL];
} else {
[self.delegate loadingFinished];
return;
}
} else if (self.currentObjectType == ObjectTypeThumbImage) {
ImageData *currentImage;
currentImage = [self.imagesData objectAtIndex:self.currentObjectIndex];
UIImage *thumbImage = [[UIImage alloc] initWithData:self.receivedData];
if (thumbImage == nil) {
NSLog(#"image was not created");
}
[currentImage setThumbImageScaled:thumbImage];
[thumbImage release];
[self.receivedData release];
self.receivedData = nil;
if (self.currentObjectIndex == ([self.imagesData count] - 1)) {
[self.delegate loadingFinished];
return;
}
self.currentObjectIndex++;
currentImage = [self.imagesData objectAtIndex:self.currentObjectIndex];
NSLog(#"'%#'", currentImage.thumbImageURL);
NSURL *theURL = [NSURL URLWithString:currentImage.thumbImageURL];
[self connectionStart:theURL];
}
}
#end

Answer 1

Do not call [self.receivedData release] - this leaves the internal pointer dangling. The whole point of a retained property is that it releases itself. Just do self.receivedData = nil.

Answer 2

Here is your problem:
[self.receivedData release];
self.receivedData = nil;
You're releasing the attribute twice (first time explicitly and second time implicitly by assigning nil).