I am developing an iphone application which has some data stored in a sqllite database. When my view loads i would like to load the data from the database on a background thread. The problem is the application keeps crashing and i dont know why.
The code:
-(id) init
{
if((self=[super init]))
{
[self performSelectorInBackground:#selector(loadList) withObject:nil];
}
}
-(void) loadList
{
#autoreleasepool
{
Loader * loader = [[Loader alloc] init];
NSMutableArray * array = [loader getItemList];
[array retain];
NSLog(#"Got %d items",[array count]);
[self performSelectorOnMainThread:#selector(createList:) withObject:array waitUntilDone:false];
[loader release];
}
}
-(void) createList: (NSMutableArray*) array
{
items = array;
int i;
Item * it;
for(i = 0; i < [items count]; i++)
{
it = [items objectAtIndex: i];
[it getName]; // crashes
// populate the list
}
}
Loader returns a NSMutableArray with Item objects. The application crashes when i call the item getName (which returns a NSString*). From what i understand it crashes because the item name properties is being released. What am i doing wrong?
Thanks!
It's likely to be a problem with whatever type of object you're using to populate array.
I'm unable to find finger-on-paper proof but I'm confident that performSelectorOnMainThread:withObject:waitUntilDone: retains its object. However if each of the items in array keeps a reference to loader then they need to take responsibility for retaining that object. It looks like you're attempting to keep it alive manually but — as Chuck alludes to — your call to performSelector... will return instantly and not wait for the call you've made to complete.
This particular bug appears to be that you're passing waitUntilDone:NO, so the array is being released immediately and consequently so are its items.
But in general, UIKit is not thread-safe, so this is just a touchy design. I would probably put the loading of this stuff in another class that handles the task for you instead of right in the view.
I'd put a breakpoint on the line:
it = [items objectAtIndex: i];
Then type
po it
in the debugger, and see what's in the name field. As a guess, I'd say one of two things: 1) the field that getName returns isn't initialized with an object (i.e. isn't a real NSString *) or that you're getting a C string from SQLite (which is what it usually returns) and you're trying to treat it as an NSString *. If it's the latter you can use [myCString stringWithUTF8String] to convert the C string into an NSString *
Related
I'm working on an iPhone app and facing some troubles with my shared singleton class.
I'm using a shared singleton to store two variables
int gameRuns and int totalScore
'gamRuns' just increments every time the user loads the app, and 'totalScore' is obvious :D
the issue is as follows, I load the singleton and init using my own method when the app loads using this code:
+ (SingletonLevelState*)sharedLevelStateInstance {
static SingletonLevelState *sharedLevelStateInstance;
#synchronized(self) {
if(!sharedLevelStateInstance) {
//Init a singleton
sharedLevelStateInstance = [[SingletonLevelState alloc] init];
sharedLevelStateInstance->gameRuns = 1;
sharedLevelStateInstance->totalScore = 0;
}
}
return sharedLevelStateInstance;
}
This is working great as I can reference this class from any other class and always get a pointer to the same object, so this works fine from other objects:
sharedLevelState = [SingletonLevelState sharedLevelStateInstance];
sharedLevelStateInstance.gameRuns++;
Now I added the NSCoder protocol, and added the two methods initWithCoder and encodeWithCoder as follows :
- (void) encodeWithCoder: (NSCoder *)coder
{
//encode level data
[coder encodeInt:self->gameRuns forKey:#"gameRuns"];
[coder encodeInt:self->totalScore forKey:#"totalScore"];
}
- (id) initWithCoder: (NSCoder *) coder
{
if(self = [super init]){
self->gameRuns = [coder decodeIntForKey:#"gameRuns"];
self->totalScore = [coder decodeIntForKey:#"totalScore"];
}
return self;
}
Now when the app loads, I check to see if we already have a saved sate, if it exists, I just unarchive the class with that file, if not, I init that class using my custom method above, then set its defaults, encode it to file so we have a saved state, here's the code:
//Load Level state
sharedLevelStateInstance = [SingletonLevelState sharedLevelStateInstance];
//Check if file is saved
NSFileManager *fm = [[NSFileManager alloc] init];
NSString *gameStatePath = [NSString stringWithString:[self getSavePath]];
if([fm fileExistsAtPath:gameStatePath]){
[self loadState];
sharedLevelStateInstance.gameRuns = sharedLevelStateInstance.gameRuns+1;
NSLog(#"Loaded %d times", [sharedLevelStateInstance gameRuns]);
}
[fm release];
Now the last line in the if statement works perfectly, it increments every time I load the app as expected and I feel really happy lol.
However, the problem arises when I try to get a reference of the singleton in another class by doing the following:
sharedLevelStateInstance = [SingletonLevelState sharedLevelStateInstance];
NSLog(#"Played: %d times", sharedLevelStateInstance.gameRuns);
It always counts back to 1, I know what happens but I'm not sue what's the best way to solve it, when I initWithCoder the singleton, It's not returning a static object, it creates a new one, when I init my sharedLevelStateInstance, it calls my first custom method, initializing it to the defaults hardcoded.
So StackOverflow, can you please help me ?!
I just need to know what's the best way to get a reference to the same object without allocating a new one every time I initWithCoder !
Thanks :)
So, you code should probably look like this:
if(self = [[SingletonLevelState sharedLevelStateInstance] retain])
Which sets the variables of the singleton, and returns the singleton. Be sure to retain the singleton, so that when the NSCoder releases this instance, it doesn't fully deallocate your singleton.
I'm still relatively new to iPhone development but thought I understood the basic principals of memory management. I have a class method that returns an NSMutableArray, I'm calling alloc and init on the object therefore know I'm responsible for releasing it. However because I'm returning the array I assumed I was supposed to use autorelease when creating the object instead of releasing it.
+(NSMutableArray *)getStations:(int)stationType {
if(database == nil){
[self openDataBase];
}
// Create a temporary array to hold the returned objects
NSMutableArray *holder = [[[NSMutableArray alloc] init] autorelease];
// Check if the statement has been defined
if(select4 == nil) {
const char *sql = "SELECT station_id, station_name, AVG(test_percent) FROM stations LEFT JOIN tests USING (station_id) WHERE station_type = ? GROUP BY station_id ORDER BY station_name ASC";
if(sqlite3_prepare_v2(database, sql, -1, &select4, NULL) != SQLITE_OK){
NSLog(#"Error while creating detail view statement. '%s'", sqlite3_errmsg(database));
}
}
sqlite3_bind_int(select4, 1, stationType);
// Check if the statement executed correctly
while(sqlite3_step(select4) == SQLITE_ROW) {
NSInteger primaryKey = sqlite3_column_int(select4, 0);
Tests *station = [[Tests alloc] initWithPrimaryKey:primaryKey];
station.station_name = [NSString stringWithUTF8String:(char *)sqlite3_column_text(select4, 1)];
station.average_score = [NSNumber numberWithDouble:sqlite3_column_double(select4, 2)];
[holder addObject:station];
[station release];
}
// Reset the detail statement.
sqlite3_reset(select4);
// Return the holder array
return holder;
}
There's the basic code - XCode no longer indicates a potential memory leak but it crashes everytime that code executes saying message sent to deallocated instance. Any help would be appreciated I've spent ages googling and can't see what's wrong with the code. I did find this thread but it doesn't appear to be the answer to my question - crash happens when NSMutableArray is returned?
The code you've posted appears to be managing memory correctly – you've got a one-to-one relationship between retains and (auto)releases, and you're making a textbook use of autorelease – so the problem is probably that the code calling this method needs to retain the resulting array before the autorelease pool kicks in and yanks the rug out from under you.
If your code is assigning the NSMutableArray to an ivar you've declared with #property, that ivar needs to be declared as
#property (retain) NSMutableArray *myStations;
If you're doing something else to store the array, you may need to call [myStations retain]. Your table view code will also need to release the array, probably in its dealloc method.
If you want to use the returned NSMutableArray as a data source to fill in rows in a table view, then you're going to need to retain it in your UITableView class (or your UITableViewDataSource delegate class). It's going to be called repeatedly whenever the view is scrolled or otherwise needs updating.
Easiest thing to do is make it a retained property in that class.
#property (nonatomic, retain) Tests * stationArray;
Then, say, if you want to get your data in your viewDidLoad method:
self.stationArray = [self getStations: self.stationID]; // property retains
Access it in numberOfRowsInSection:
return self.stationArray.count;
Access it in cellForRowAtIndexPath:
Tests *station = [self.stationArray objectAtIndex:indexPath.row];
And, of course, in dealloc...
[stationArray release];
The autorelease in the method is correct (not an init or copy method), but the class will need to retain it if it wants to use it later on, after the current event.
I'm new to cocoa / objective-c and i'm struggeling with the releases of my objects. I have the following code:
gastroCategoryList = [[NSMutableArray alloc] init];
for (NSDictionary *gastrocategory in gastrocategories) {
NSString *oid = [gastrocategory objectForKey:#"id"];
GastroCategory *gc = [[GastroCategory alloc] initWithId:[oid intValue] name:[gastrocategory objectForKey:#"name"]];
[gastroCategoryList addObject:gc];
}
The analyzer shows me that the "gastrocategory" defined in the for is a potential memory leak. But i'm not sure if i can release this at the end of the for loop?
Also at the following code:
- (NSArray *)eventsForStage:(int)stageId {
NSMutableArray *result = [[NSMutableArray alloc] init];
for (Event *e in eventList) {
if ([e stageId] == stageId) {
[result addObject:e];
}
}
return result;
}
The Analyzer tells me that my "result" is a potential leak. But where should I release this?
Is there also a simple rule to memorize when i should use assign, copy, retain etc. at the #property ?
Another problem:
- (IBAction)showHungryView:(id)sender {
GastroCategoriesView *gastroCategoriesView = [[GastroCategoriesView alloc] initWithNibName:#"GastroCategoriesView" bundle:nil];
[gastroCategoriesView setDataManager:dataManager];
UIView *currentView = [self view];
UIView *window = [currentView superview];
UIView *gastroView = [gastroCategoriesView view];
[window addSubview:gastroView];
CGRect pageFrame = currentView.frame;
CGFloat pageWidth = pageFrame.size.width;
gastroView.frame = CGRectOffset(pageFrame,pageWidth,0);
[UIView beginAnimations:nil context:NULL];
currentView.frame = CGRectOffset(pageFrame,-pageWidth,0);
gastroView.frame = pageFrame;
[UIView commitAnimations];
//[gastroCategoriesView release];
}
I don't get it, the "gastroCategoriesView" is a potential leak. I tried to release it at the end or with autorelease but neither works fine. Everytime I call the method my app is terminating. Thank you very much again!
In your loop, release each gc after adding it to the list since you won't need it in your loop scope anymore:
gastroCategoryList = [[NSMutableArray alloc] init];
for (NSDictionary *gastrocategory in gastrocategories) {
NSString *oid = [gastrocategory objectForKey:#"id"];
GastroCategory *gc = [[GastroCategory alloc] initWithId:[oid intValue] name:[gastrocategory objectForKey:#"name"]];
[gastroCategoryList addObject:gc];
[gc release];
}
In your method, declare result to be autoreleased to absolve ownership of it from your method:
NSMutableArray *result = [[[NSMutableArray alloc] init] autorelease];
// An alternative to the above, produces an empty autoreleased array
NSMutableArray *result = [NSMutableArray array];
EDIT: in your third issue, you can't release your view controller because its view is being used by the window. Setting it to autorelease also causes the same fate, only delayed.
You'll have to retain your GastroCategoriesView controller somewhere, e.g. in an instance variable of your app delegate.
BoltClock's answer is spot-on as to the first part of your question. I'll try to tackle the rest.
Assign is for simple, non-object types such as int, double, or struct. It generates a setter that does a plain old assignment, as in "foo = newFoo". Copy & retain will, as their names imply, either make a copy of the new value ("foo = [newFoo copy]") or retain it ("foo = [newFoo retain]"). In both cases, the setter will release the old value as appropriate.
So the question is, when to copy and when to retain. The answer is... it depends. How does your class use the new value? Will your class break if some other code modifies the incoming object? Say, for example, you have an NSString* property imaginatively named "theString." Other code can assign an NSMutableString instance to theString - that's legal, because it's an NSString subclass. But that other code might also keep its own reference to the mutable string object, and change its value - is your code prepared to deal with that possibility? If not, it should make its own copy, which the other code can't change.
On the other hand, if your own code makes no assumptions about whether theString might have been changed, and works just as well whether or not it was, then you'd save memory by retaining the incoming object instead of unnecessarily making a copy of it.
Basically, the rule, which is unfortunately not so simple sometimes, is to think carefully about whether your own code needs its own private copy, or can correctly deal with a shared object whose value might be changed by other code.
The reason you can release gc after it is added to the gastroCategoryList is that when an object is added to an array, the array retains that object. So, even though you release your gc, it will still be around; retained by the gastroCategoryList.
When you are returning a newly created object from a method, you need to call autorelease. This will cause the object to be released only after the runtime leaves the scope of the calling method, thereby giving the calling method a chance to do something with the returned value.
Note that if your method starts with the word copy or new, then you should not autorelease your object; you should leave it for the calling method to release.
As for copy vs retain vs assign... as a general rule, copy objects that have a mutable version, such as NSArray, NSSet, NSDictionary, and NSString. This will ensure that the object you have a pointer to is not mutable when you don't want it to be.
Otherwise, use retain whenever you want your class to be ensured that an object is still in memory. This will apply to almost every object except for objects that are considered parents of your object, in which case you would use assign. (See the section on retain cycles here).
Also note that you have to use assign for non-object types such as int.
Read through the Memory Management Programming Guide a bit; it's quite helpful.
So in my view controller, I run code to populate an NSArray of Customer (custom class) objects. This custom class has objects that are of ANOTHER custom class called Address (a customer has a billing address and a shipping address). In the view controller when a customer in the list is selected, it passes a new view controller a customer object, like so:
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
InfoViewController *customerinfoViewController = [[InfoViewController alloc] initWithStyle:UITableViewStyleGrouped andCustomer:[[[customers objectAtIndex:indexPath.section] objectAtIndex:indexPath.row] retain]];
[self.navigationController pushViewController:customerinfoViewController animated:YES];
[customerinfoViewController release];
}
The first time I visit this view controller while running the application, it works fine. However, when I revisit the view controller, something interesting happens. The application crashes, with unrecognized selector sent to instance 0x00whatever. Using the mouseover debugging feature in xCode, I am finding that the first object of the customer's shipAddress variable has its type changed from NSString to NSIndexPath. This does not happen to the customer's billAddress object. Anyone have any idea what is going on here? It seems like I may be having memory management issues but I would definitely like a confirmation on this before I tear my code apart tracking down all the retains and releases....
EDIT: More information here. with the following code, I have an NSMutableArray at the class level. At each iteration of the loop, I am looping through nodes in XML (which works fine). Every time a new letter is detected as the first letter of the name, I create a new subarray and add the customer to it, thus filling my class-level NSMutableArray (customers) with subArrays of customers for each letter of the alphabet detected. My question is about the retains and releases of the cycling customer object. Clang Static says there is an over-retaining error on the customer, but when I fix it according to Clang, the loop crashes. what gives? Related code below:
DDXMLDocument *rootDoc = [[[DDXMLDocument alloc] initWithData:xmlData options:0 error:nil] autorelease];
NSArray *elems = [rootDoc nodesForXPath:#"QBXML/QBXMLMsgsRs/CustomerQueryRs/CustomerRet" error:nil];
DDXMLNode *node;
sectionTitles = [[[NSMutableArray alloc] initWithCapacity:1] retain]; // Letters for UITableView section titles
NSMutableArray *subArray;
NSString *lastchar = #"A";
NSString *testchar;
int indexCount = -1;
customers = [[[NSMutableArray alloc] initWithCapacity:[elems count]] retain];
Customer *newCust;
for (int i = 0; i < [elems count]; i++) {
node = [elems objectAtIndex:i];
newCust = [[Customer alloc] initWithCustomerRetNode:node];
testchar = [[newCust fullName] substringToIndex:1];
if (i == 0 || ![[testchar uppercaseString] isEqualToString:lastchar]) {
[sectionTitles addObject:testchar];
lastchar = testchar;
indexCount++;
subArray = [[NSMutableArray alloc] initWithCapacity:1];
[customers addObject:subArray];
[subArray release];
[[customers lastObject] addObject:[newCust retain]];
}
else {
[[customers lastObject] addObject:[newCust retain]];
}
[newCust release];
}
NOTE: this code works for the most part, but clang doesn't like it.
EDIT: Addresses in the Customer class are assigned like so (which now does not work after Clang fixes)
...
else if ([tempname isEqualToString:#"BillAddress"])
billAddress = [billAddress initWithAddressNode:tempnode];
else if ([tempname isEqualToString:#"ShipAddress"])
shipAddress = [shipAddress initWithAddressNode:tempnode];
...
It sounds like you are having a over release issue, so yes memory management, you might be overreleasing that array you are storing your objects in.Cant really tell from the snippet of code though. Youll have to go and look through the code and find the source. Also using Clang Static Analyzer might be of help to you.
I have an iPhone app where one view, View A, updates another view in a tab bar, View B, using:
// This works.
- (void) reloadData
{
MyDB * db = _GET_DB_CLASS;
if(data != nil) // data is a property of type NSMutableArray
[data release];
NSMutableArray * d = [db getDataQuery];
data = s; // Don't release since we are not using the accessor. And retain count should be 1.
}
If I do this, it doesn't work (e.g. I update B, then switch to B, crash. I can't see anything useful in the logs either ... ).
NSMutableArray * d = [db getDataQuery];
self.data = s; // Doesn't work
[data release];
I have not used a custom setter. What is going on?
I totally forgot about this. Too much work. So I am not sure what the exact nature of the problem was. But I took a closer look at the retainCounts in the debugger (before trying out what Lou suggested).
It is a retain/release issue. I guess the rule is to be consistent with your usage. Anyway this works:
- (void) reloadFridgeData
{
MyDB * db = _GET_DB_CLASS;
if(self.data != nil)
{
self.data = nil;
}
NSMutableArray * newData = [db getData];
self.data = newData;
[newData release];
}
If data is definitely a property with no underlying value of the same name, then it must be accessed from self as in [[self data] release] or [self.data release]. However as mentioned if the property is set to auto-retain, you can accomplish the same thing by setting the property to a new value, such as nil. (As you've done in the code you noted works.)
Have you declared your data property to be "retain", as opposed to "assign"? You should have something like:
#property(retain) NSMutableArray *data;
If not, then when you assign to the data property, it won't increment the reference count. Your subsequent release will dealloc the object and the next reference to the data property will crash.
Take a look at the Apple docs for setter semantics for more information about retain.
Generally, if you are just using a retain property, you should always use self.data and call the generated setter.
It:
Calls release on the old value automatically (checks for nil)
Calls retain on the new value
Then your code would be simply:
self.data = s;
[s release];
Two things to do to debug:
Use static analysis -- I have personally found this tool to be 100% accurate with retain/release issues: http://clang.llvm.org/StaticAnalysisUsage.html
Follow the instructions on my blog post on memory debugging on the iPhone:
http://loufranco.com/blog/files/debugging-memory-iphone.html