target_locations[ 0] = [[CLLocation alloc] initWithLatitude : 51.50373056
longitude : 0.129986111];
[target_locations[ 0] release];
Consider the above code, is it the proper way to keep the assigned object to have a retain count of 1 ?
*Assuming ARC is not activated.
Given that target_locations is an NSMutableArray, and that ARC is not enabled, the correct procedure here is as follows:
CLLocation * newLocation = [[CLLocation alloc] initWithLatitude : 51.50373056
longitude : 0.129986111];
target_locations[0] = newLocation;
[newLocation release];
You shouldn't send release to the result of an array access because you don't own that object through that pointer. While it does work in this case, it's incorrect semantically and too likely to cause problems if you get into the habit.
Also, consider renaming target_locations to targetLocations, which is consistent with Cocoa style. Using the underscore makes it look like a plain-C array rather than an object.
Related
Currently i am working on a Location based application for iPhone/iPad . I have several annotations in my MapKit , what i want to do is to track the location of the user and shows the annotations that are within the 3km . Can somebody give me a start ?
Sorry for the delayed response... the question just fell off my radar.
I'm going to suppose that you have a method that returns a set of NSValue-wrapped CLLocationCoordinate2D structs (the basic approach is the same regardless of what your internal data representations are). You can then filter the list using a method something akin to the following (warning: typed in browser):
NSSet *locations = ...;
CLLocation centerLocation = ...; // Reference location for comparison, maybe from CLLocationManager
CLLocationDistance radius = 3000.; // Radius in meters
NSSet *nearbyLocations = [locations objectsPassingTest:^(id obj, BOOL *stop) {
CLLocationCoordinate2D testCoordinate;
[obj getValue:&testCoordinate];
CLLocation *testLocation = [[CLLocation alloc] initWithLatitude:testCoordinate.latitude
longitude:testCoordinate.longitude];
BOOL returnValue = ([centerLocation distanceFromLocation:testLocation] <= radius);
[testLocation release];
return returnValue;
}
];
With the filtered set of coordinates in hand, you can create MKAnnotation instances and add them to the map in the usual manner, as described in Apple's documentation.
If you have many thousands of test locations then I suppose this approach could start to incur performance issues. You would then want to switch your point storage approach to use, e.g., quadtrees, to reduce the number of points that need to be precision-filtered. But don't optimize prematurely!
Hope that helps!
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.
I'm trying to create then retrieve an array of CLLocationCoordinate2D objects, but for some reason the array is always empty.
I have:
NSMutableArray *currentlyDisplayedTowers;
CLLocationCoordinate2D new_coordinate = { currentTowerLocation.latitude, currentTowerLocation.longitude };
[currentlyDisplayedTowers addObject:[NSData dataWithBytes:&new_coordinate length:sizeof(new_coordinate)] ];
I've also tried this for adding the data:
[currentlyDisplayedTowers addObject:[NSValue value:&new_coordinate withObjCType:#encode(struct CLLocationCoordinate2D)] ];
And either way, the [currentlyDisplayedTowers count] always returns zero. Any ideas what might be going wrong?
Thanks!
To stay in object land, you could create instances of CLLocation and add those to the mutable array.
CLLocation *towerLocation = [[CLLocation alloc] initWithLatitude:lat longitude:lon];
[currentDisplayedTowers addObject:towerLocation];
To get the CLLocationCoordinate struct back from CLLocation, call coordinate on the object.
CLLocationCoordinate2D coord = [[currentDisplayedTowers lastObject] coordinate];
As SB said, make sure your array is allocated and initialized.
You’ll also probably want to use NSValue wrapping as in your second code snippet. Then decoding is as simple as:
NSValue *wrappedCoordinates = [currentlyDisplayedTowers lastObject]; // or whatever object you wish to grab
CLLocationCoordinate2D coordinates;
[wrappedCoordinates getValue:&coordinates];
You need to allocate your array.
NSMutableArray* currentlyDisplayedTowers = [[NSMutableArray alloc] init];
Then you can use it. Be sure to call release when you are done with it or use another factory method.
I had currentlyDisplayedTowers = nil which was causing all the problems. Also, the previous advice to init and alloc were necessary. Thanks everyone for the help!
For anyone else with this issue, there's another solution if you are planning on using MapKit.
(The reason I say IF, of course, is because importing a module such as MapKit purely for a convenient wrapper method is probably not the best move.. but nonetheless here you go.)
#import MapKit;
Then just use MapKit's coordinate value wrapper whenever you need to:
[coordinateArray addObject:[NSValue valueWithMKCoordinate:coordinateToAdd]];
In your example..
[currentlyDisplayedTowers addObject:[NSValue valueWithMKCoordinate:new_coordinate]];
I have defined a class called FGLocation that conforms to the MKAnnotation protocol, what I am trying to do is measure the distance between two of these objects. I noticed that I could use the method distanceFromLocation: that is defined as a method for the CLLocation class. As you can see below I am creating two temp CLLocation objects to do the calculation, but I can't help thinking that I am maybe missing a better / easier way. Does anyone have any comments on what I am doing or how I might do it better?
// IS THERE A BETTER WAY TO DO THIS?
FGLocation *root = [annotations objectAtIndex:counter-1];
FGLocation *leaf = [annotations objectAtIndex:counter];
CLLocation *rootLocation = [[CLLocation alloc] initWithLatitude:[root coordinate].latitude longitude:[root coordinate].longitude];
CLLocation *leafLocation = [[CLLocation alloc] initWithLatitude:[leaf coordinate].latitude longitude:[leaf coordinate].longitude];
CLLocationDistance thisDistance = [rootLocation distanceFromLocation:leafLocation];
[rootLocation release];
[leafLocation release];
NB: my FGLocation object is defined as (see below) I noticed in the docs that I should not be subclassing CLLocation.
#interface FGLocation : NSObject <MKAnnotation> {
I think that what you are doing is a fair approach, with the possible exception that you might want to implement some sort of caching mechanism/threshold for recalculation if you are doing this often or on a lot of points. Calculating distances accurately involves non-Euclidean geometry and is computationally expensive.
http://en.wikipedia.org/wiki/Haversine_formula
Depending on where the points are located on the globe and whether or not precision is highly important, you can also fudge things by pretending to be working in a Euclidean space and leveraging the Pythagorean theorem. I have used this kind of scheme to create a gross filter over a large dataset and then resorted to more precise calculations on points that pass the filter.
I will try to make myself as clear as possible. Let start from the beginning. I have an application with a tableview that contains a list of places with distances from myLocation. Now everytime I get an update in the gps location I run the following code
- (void)locationUpdate:(CLLocation *)location {
myLocation = location;
for (Trek * trek in list) {
CLLocation *loc = [[CLLocation alloc] initWithLatitude:[trek latitude_start] longitude:[trek longitude_start]];
double dis = [locationManager getDistance: loc];
[trek setDistance:dis];
[trek setDistanceUnit];
[loc release];
}
[self.tableView reloadData];
}
Now this piece of code [trek setDistanceUnit]; calls
-(void) setDistanceUnit {
if (self.distance < 1000.0)
self.distanceString = [NSString stringWithFormat:#"%.0lf m", self.distance];
}
Now if I use only distanceString the application crash. Now I think it may have something to do with the fact that those updates may run concurrently (in parallel) to the access required by the view to draw the cells. Anyone has any idea? I can post more code if helpful, I just didn't want to post too much to make this post too long.
I tried to search everywhere but I could not found anything so far.
Thanks in advance,
Umberto
PS Now the application is working but I would like to understand what is going on.
If your distanceString is a retain property, assigning it without self sets it up for a crash because you bypass the setter, and assign the string without retaining it. So when the string gets deallocated on being sent to the autorelease pool, your app crashes.
By synthesizing the accessors using #synthesize and using the dot notation (or setDistanceString:), the object will retain the string for you so that it always has a pointer to it for itself (until it's released).