There is a need to run some code when one object get dealloc. E.g, I set up one observer which updates the label A's text when object B's name changed. When label A's retain count reach 0, I want to remove the observer from B.
Possible solutions:
1 Subclass and call the clean code in dealloc.
2 Create a wrapper class which able to run arbitrary code in dealloc and associate this object to label A. When A get dealloc, the associated object get dealloc too (suppose only A holds the strong ref to it), then the code get called.
I don't like the 1st one since it is so intrusive that makes it barely useless, need to subclass just for some easy stuff. So I am using No.2.
Do you have any comments? How do you do it?
I put my solution here in case someone needed.
#interface ExecuteWrapper : NSObject
#property (nonatomic, copy) void(^block)();
-(void)dealloc;
#end
#implementation ExecuteWrapper
-(void)dealloc{
if(self.block){
self.block();
}
}
#end
#implementation NSObject (SLUtil)
+(void)executeWhenDealloc:(NSObject *)object block:(void(^)())block{
static char key;
NSMutableArray *executeWrapperArray = [object associatedValueForKey:&key];
if ( executeWrapperArray == nil){
executeWrapperArray = [NSMutableArray array];
[object associateValue:executeWrapperArray withKey:&key];
}
ExecuteWrapper *executeWrapper = [[ExecuteWrapper alloc] init];
executeWrapper.block = block;
[executeWrapperArray addObject:executeWrapper];
}
#end
In client code
[NSObject executeWhenDealloc:labelA block:^{
// clean up code
}];
Note: Keep in mind that don't hold a strong ref to label A in the label
Related
I'm trying to learn about delegation in Objective-C, and am having a minor issue with a variable losing it's data in the transfer process. I have Class1 that contains an NSMutableArray. The array gets populated, then I would like to transfer the array's values to Class2, and display it. Here is the relevant code in Class1:
//Class1.h
#class Class1;
// define the protocol for the delegate
#protocol Class1Delegate
#required
-(void)sayHello:(Class1 *)customClass withAntArray:(NSMutableArray *)antArray;
#end
#interface Class1 : MySuperClassName
#property (nonatomic, assign) id delegate;
-(void)helloDelegate;
#end
//Class1.m:
#interface Class1 ()
#property (nonatomic, strong) NSMutableArray *antOccurenceTimes;
#end
#implementation Class1
#synthesize antOccurenceTimes;
-(void)helloDelegate
{
// send the message to the delegate
[_delegate sayHello:self withAntArray:self.antOccurenceTimes];
}
Now, this is what I have in Class2:
#import "Class1.h"
#interface Class2 : UIView <Class1Delegate>
#end
// Class2.m:
- (void)appropriateTimeToCallMethod {
Class1 *initAntMarks = [[Class1 alloc] init];
initAntMarks.delegate = self;
[initAntMarks helloDelegate];
}
-(void)sayHello:(Class1 *)customClass withAntArray:(NSMutableArray *)antArray {
NSLog(#"Hello! %#", antArray.description);
}
The antArray.description reads as "NULL". Now, I figured that obviously it will be null, because I just created an entirely new instance of the class right before calling upon the needed method. I feel like I may have something mixed up, and being so new to delegation, I'm not sure exactly what. Does anyone know what I need to tweak to utilize Delegation?
I forgot to add that I did initialize it in Class1, and it gets populated just fine. It's only in class2 that this is occurring.
I initalize antOccuranceTimes in a separate method in ClassA in the snippet below, and the NSLog fires twice...
NSLog(#"Array initalized in class A");
antOccurenceTimes = [NSMutableArray new];
Change this line:
#property (nonatomic, assign) id delegate;
to:
#property (nonatomic, weak) id <Class1Delegate> delegate;
assign should only be used for C primitives, not Objective-c object references. You should also be checking if your object actually conforms to the delegate before messaging the delegate.
Edit:
I think you may be confused about the purpose of delegation.
Class1 *initAntMarks = [[Class1 alloc] init];
initAntMarks.delegate = self;
[initAntMarks helloDelegate];
Why are you calling a method on an object which in turn calls a delegate method when you could simply create a method that returns the NSMutableArray? The way you have your code currently set up requires that before the call to -helloDelegate you have to have filled the array with the appropriate objects. The purpose of delegation in MVC is to inform an object about an event that took place inside of another object. You are "delegating" the task off to another object, or you could say, that another object if responsible for the fulfillment of the task. Read the Apple Docs on Delegation. Delegation in your code is not the correct pattern to implement, as I stated you can simply return that array with a method call.
Edit 2:
There are two ways you can achieve this, through property methods or through an explicit method that returns your array. If you choose to use property methods, the property declaration must be in the public interface i.e. the .h file so that your class can all the accessors when the object is being implemented.
//Inside the .h
#property (nonatomic, strong) NSMutableArray *antOccurenceTimes;
This will automatically provide you with two accessor methods for the antOccurenceTimes property. These are the getter -antOccurenceTimes and setter -setAntOccurenceTimes: methods. Now after you initialize the class and fill your array you can call -antOccurenceTimes to return the array.
You can also create an explicit method that return the array:
- (NSMutableArray *)hello{
//Do something here
return _antOccurenceTimes;
}
You have not yet initialized the antOccurenceTimes. Of cause it is nil. There are many options depending on what you need. You can, for example, initialize it in a init function:
- (instancetype)init {
self = [super init];
if( self ) {
antOccurenceTimes = [NSMutableArray array];
[antOccurenceTimes addObject:#"Hello World"];
}
}
Or maybe initialize it before you call the delegate the function.
-(void)helloDelegate
{
// send the message to the delegate
self.antOccurenceTimes = [NSMutableArray array];
[self.antOccurenceTimes addObject:#"Hello World"];
[_delegate sayHello:self withAntArray:self.antOccurenceTimes];
}
I think you get my point.
I create a NSMutableArray that I need as long as my app lives, lets call it suseranArray, just after the #implementation of my main class. This Array will hold several objects of a class called Vassal. A Vassal is simply:
1) A NSMutableString
2) Another NSMutableString
3) A NSMutableArray
4) Another NSMutable Array
Each Vassal created is also needed for the life of the app, and they never change.
These objects are made as (retain) properties in an .h file, synthesized in the .m file, and each given an alloc+init whenever the object Vassal is created during the init function. Each vassal has data filled in and stored in the suzerain Array. the 3rd item always has several elements, and after a bug appeared, I put a line to check if it is ever empty, but it never is, and life is good.
Now, later on when a certain Vassal object is needed, we try to access its 3rd property to fetch the data in there, and sometimes that array empty... I checked to see if it disappeared somehow, but it is always there on the debug, carrying a nice address like 0x2319f8a0 which makes sense since the NSMutableString just above it is at address 0x2319fb40 - (I was expecting 00000000 after a lot of headache). What is happening? I my head, I am creating an RETAINed objects, which retains data put in by default, and that object is put inside another, but somehow the data inside the array vanishes. What possible scenario could lead to this? Thank you for your time :)
Note: the last array currently just holds one item at this stage of development, and curiously enough, that one item is never missing, despite the two arrays being 'brothers'
Vassal.h
#interface Vassal : NSObject
#property (retain) NSMutableString *wordBody;
#property (retain) NSMutableString *wordCode;
#property (retain) NSMutableArray *wordRelations;
#property (retain) NSMutableArray *wordLinks;
#end
Vassal.m
#implementation Vassal:NSObject
#synthesize wordBody;
#synthesize wordCode;
#synthesize wordRelations;
#synthesize wordLinks;
-(NSObject*) init
{
if(self=[super init])
{
wordBody=[[NSMutableString alloc] init];
wordCode=[[NSMutableString alloc] init];
wordRelations=[[NSMutableArray alloc] init];
wordLinks=[[NSMutableArray alloc] init];
}
return self;
}
//Somewhere in Suseran:
-(void)fillStuff
{
...
Vassal *vassal=[Vassal new];
for (int i=0;i<[originalDataString length];i++)
{
...
[vassal.wordRelations addObject:anItem];
...
}
int errorTest=[vassal.wordRelations count];
if (errorTest==0)
{
//breakpoint here. Program NEVER comes here
}
[bigArrayOfVassals addObject:vassal];
}
//these arrays are never touched again but here:
-(void) getVassalstuff:(NSMutableString*)codeOfDesiredVassal
{
Vassal *aVassal;
for (int i=0;i<[bigArrayOfVassals count];i++)
{
aVassal=bigArrayOfVassals[i];
if ([codeOfDesiredVassal isEqualToString:aVassal.wordCode)
{
int errorTest=[aVassal.wordRelations count];
if (errorTest==0)
{
//yay! this breakpoint sometimes is hit, sometimes not,
//depending on code's mood. Why is this happening to me? :,(
}
}
}
}
I see that that you have properties that are mutable (which is itself a bad idea except for specific cases) and that you are retaining them.
Mutability means that if you have set the array as a property based on some other array, and if that original array is changed, the array in your property is also changed. It may be, and I don't know because you haven't shown any code, that you are emptying the original array, and thus changing the array you have as a property
Solutions:
My preferred solution is to use the immutable versions of these classes for your properties; NSString, NSArray and instead of retain use copy
A second solution is to leave the properties as mutable, but write a custom setter for each of them that stores a mutableCopy of the object that you pass in.
In both of these cases, your property will be a copy of the object used to set the property, so that if the object is changed outside of your class it will not affect your class's properties.
edited to add, after a comment
If you declare your property as
#property (copy) NSArray wordRelations;
Then simply writing
vassal wordArray = tempArray;
will do the same thing and is cleaner and more readable..
I have a singleton that I'd like to use to manage the onscreen animation of my views. Here's my.
#import <Foundation/Foundation.h>
#interface OAI_AnimationManager : NSObject {
NSMutableDictionary* sectionData;
}
#property (nonatomic, retain) NSMutableDictionary* sectionData;
+(OAI_AnimationManager* )sharedAnimationManager;
- (void) checkToggleStatus : (UIView* ) thisSection;
#end
.m file
#import "OAI_AnimationManager.h"
#implementation OAI_AnimationManager
#synthesize sectionData;
+(OAI_AnimationManager *)sharedAnimationManager {
static OAI_AnimationManager* sharedAnimationManager;
#synchronized(self) {
if (!sharedAnimationManager)
sharedAnimationManager = [[OAI_AnimationManager alloc] init];
return sharedAnimationManager;
}
}
- (void) checkToggleStatus : (UIView* ) thisSection {
//get the section data dictionary
NSLog(#"%#", sectionData);
}
#end
You'll see in the .h file I added a NSMutableDictionary and am using #property/#synthesize for it's getter and setter.
In my ViewController I instantiate the animation manager as well as a series of subclasses of UIView called Section. With each one I store the data (x/y w/h, title, etc.) in a dictionary and pass that to the dictionary delcared in animation manager. In the Section class I also instantiate animation manager and add a UITapGestureRecognizer which calls a method, which passes along which section was tapped to a method (checkToggleStatus) in animation manager.
As you can I see in the method I am just logging sectionData. Problem is I am getting null for the value.
Maybe my understanding of singletons is wrong. My assumption was the class would only be instantiated once, if it was already instantiated then that existing object would be returned.
I do need all the other Section classes data as if one animates others animate in response and I can get around it by passing the tapped Section to the animation manager and doing [[Section superview] subviews] and then looping and getting the data from each that way but it seems redundant since that data is available in the ViewController when they are created.
Am I doing something wrong in trying to transfer that data? Is there a better solution? I am open to suggestions and criticisms.
Thanks
h file
#interface OAI_AnimationManager : NSObject
#property (nonatomic, retain) NSMutableDictionary* sectionData;
+(OAI_AnimationManager* )sharedAnimationManager;
- (void) checkToggleStatus : (UIView* ) thisSection;
#end
m file
static OAI_AnimationManager* _sharedAnimationManager;
#implementation OAI_AnimationManager
#synthesize sectionData = _sectionData;
+(OAI_AnimationManager *)sharedAnimationManager {
#synchronized(self) {
if (!_sharedAnimationManager) {
_sharedAnimationManager = [[OAI_AnimationManager alloc] init];
}
}
return _sharedAnimationManager;
}
- (void) checkToggleStatus : (UIView* ) thisSection {
//get the section data dictionary
NSLog(#"%#", _sectionData);
}
#end
Notice I moved your sectionData variable from the header and moved it to the implementation file. A while back, they changed it to where you can synthesize properties and specify their instance variable names along side it... hence:
sectionData = _sectionData;
I also added and underscore to the instance variable... this is a universal convention for private variables and it also will throw a compile error now if you try to type just sectionData as you did in the return statement of checkToggleStatus:. Now you either have to type self.sectionData or _sectionData.
You didn't include the code that creates an instance of your dictionary but I bet you didn't set it as self.sectionData = [[NSDictionary alloc] init] which means it would not retain the value and you would get null the next time you called it. Classic memory management mistake... I know it well because I learned the hard way hehehe
I'm not sure I understood how alloc and retain work.
Recently I discovered that the NSString properties were not retained and I had to add [myString copy] when I set them. Which makes me wonder if I misunderstood the whole way of using retain/alloc
Please, may someone tell me if I'm doing it correctly? I read a lot and had a look on open source projects, this let me thing that I may have been wrong since the beginning.
Here is my way of doing it:
/**** VIEW.h *****/
#import "MyClass.h"
#interface MyViewController : UIViewController {
//Is the following line really necessary?
MyClass *myObject;
}
#property (nonatomic, retain) MyClass *myObject;
- (void)defineObject;
#end
.
/**** VIEW.m *****/
#import "VIEW.h"
#implementation MyViewController
#dynamic myObject;
- (void)viewDidLoad
{
[super viewDidLoad];
[self defineObject];
NSLog(#"My object's name is: %#", myObject.name);
}
- (void)defineObject
{
//Here particularly, Why doesn't it work without both alloc and init
//shouldn't "#property (nonatomic, retain) MyClass *myObject;" have done that already?
myObject = [[MyClass alloc] initPersonalised];
[myObject setName:#"my name"];
}
.
/**** MyClass.h *****/
#interface MyClass : NSObject {
//not sure if this line is still necessary
NSString *name;
}
#property (nonatomic, retain) NSString *name;
- (id)initPersonalised;
- (void)setName:(NSString *)name;
- (NSString *)name;
#end
.
/**** MyClass.m *****/
#import "MyClass.h"
#implementation MyClass
#dynamic name;
(id)initPersonalised{
self = [super init];
name = #"Undefined";
}
- (void)setName:(NSString *)name{
self.name = [name copy];
}
- (NSString *)name{
return [self.name copy];
}
#end
I hope you can bring a bit of light, after months of programming this way, I'm less and less sure of doing it well.
This is indeed a topic that every Objective C programmer stumbles upon. There are a few things one needs to know:
Instance variable vs. property access
Within MyViewController,
myObject = xxx;
and
self.myObject = xxx;
are two different things. The first directly assigns to the instance variable and does neither release to old referenced insance nor retain the newly assigned instance. The latter one uses the property setter and thus releases the old and retains the new value.
Deallocation
Even when you have declared an implemented a property that takes care of retaining and releases the values, it won't take care of deallocation when your object (MyViewController in your case) is released. So you must explicitly release it in dealloc:
-(void) dealloc {
[myObject release];
[super dealloc];
}
Now to your code:
The snippet:
myObject = [[MyClass alloc] initPersonalised];
is perfectly okay. When you create an object, you use the pair of alloc and initXXX. The always create an instance with the reference count set to 1. So by directly assigning it to the instance variable, you create a clean constellation. I don't see no other way of creating the instance.
In MyClass you could use #synthesize name instead of #dynamic. Then the compiler would implement name and setName: automatically and you wouldn't need to do it yourself.
Finally, your missing dealloc.
Update:
If you use:
self.myObject = [[MyClass alloc] initPersonalised];
then you have a memory leak because initPesonalised sets the reference count to 1 and the setter of myObject increases it to two. If you want to use the setter, then I has to be:
MyClass* mo = [[MyClass alloc] initPersonalised];
self.myObject = [[MyClass alloc] initPersonalised];
[mo release];
It would be different if you weren't using initXXX to create a new instance. The class NSString for example has many methods called stringXXX, which create a new instance (or return a shared one) that has (conceptually) a reference count of 1 that will later automatically decreased by one. Then you better use the setter:
self.name = [NSString stringWithFormat: #"instance %d", cnt];
If you want to use copy instead of retain for your string property (which is good practice), then you can simply declare your property like this:
#property (nonatomic, copy) NSString *name;
When you then use #synthesize to implement the getter and setter, the compiler will generate them using copy instead of retain.
And NSString *name; is necessary even if you use #property and/or #synthesize to implement the property.
Alloc and init are methods that always go hand-in-hand. alloc allocates space for your object, and init initializes your object to some value. When you call alloc, you are responsible for freeing that object later. If you call copy, you are also responsible for releasing that object later. It's considered good practice to always initialize your objects right after you allocate them.
Now, to answer the questions I found in your code.
#interface MyViewController : UIViewController {
//Is the following line really necessary?
MyClass *myObject;
}
So is that line necessary? That depends. Does it make sense that your object has a MyClass as a property? This is a question only you can answer based on your design. I recommend you to study Object-Oriented Programming in more depth.
- (void)defineObject
{
//Here particularly, Why doesn't it work without both alloc and init
//shouldn't "#property (nonatomic, retain) MyClass *myObject;" have done that already?
myObject = [[MyClass alloc] initPersonalised];
[myObject setName:#"my name"];
}
Not necessarily. You are just providing a pointer to an object of the specified kind. The moment you set your property, depending on the property modifiers, your class will know what to do with MyObject.
In that way, there's no need to call [yourObject copy]. In this way your properties will be copied instead of being retained. Just don't forget to release it later in your -dealloc method, like you would with retain properties.
All in all, this is what I recommend you to study a bit more:
Object-Oriented Programming (not related to your issue, but I can tell you are not comfortable using it. Objective-C is heavily object oriented, so you want to understand OOP).
iOS Memory Management.
You can have a look at the Memory Management Guide. It will help you to better understand the alloc & retain concepts; hope this helps you.
I am new here, and already searched related articles like "Is “self” necessary?" and "Setting an Objective-C class property without using a self reference" However i still can't get a clear answer which can explain my case.
I have a simple class and my UI has two textfield and one button, here is the code:
#interface testViewController : UIViewController {
NSString *teststring_A;
NSString *teststring_B;
IBOutlet UITextField *textfield_1;
IBOutlet UITextField *textfield_2;
}
#property (nonatomic, retain) NSString *teststring_A;
#property (nonatomic, retain) NSString *teststring_B;
- (IBAction)action1:(id)sender;
- (IBAction)action2:(id)sender;
#end
#implementation testViewController
#synthesize teststring_A;
#synthesize teststring_B;
- (void)dealloc {
[super dealloc];
}
- (IBAction)action1:sender
{
teststring_A = textfield_1.text ;
NSLog(#"teststring_A in action 1 is : %#\n", teststring_A);
teststring_B = textfield_2.text ;
NSLog(#"teststring_B in action 1 is : %#\n", teststring_B);
}
- (IBAction)action2:(id)sender
{
NSLog(#"teststring_A in action 2 is : %#\n", teststring_A);
NSLog(#"teststring_B in action 2 is : %#\n", teststring_B);
}
the output is :
2010-11-19 15:32:14.827 test[419:207] teststring_A in action 1 is : 123
2010-11-19 15:32:14.829 test[419:207] teststring_B in action 1 is : 456
2010-11-19 15:32:14.927 test[419:207] teststring_A in action 2 is : 123
2010-11-19 15:32:14.929 test[419:207] teststring_B in action 2 is : {(
>
)}
And when click button, it triggers action1 first then action2. My problem is... in action2, the value of teststring_B becomes incorrect, sometimes the application even crashes. What confuses me is (1) why is the value of teststring_A correct??? (2) teststring_B is assigned by textfield_2.text which is not created with 'alloc', so suppose the pointer should exist all the time. then why teststring_B's value becomes incorrect in action2 ??? (3) in dealloc, I should release teststring_A and teststring_B, right? (i think so )
All I know is if I add 'self.', like 'self.teststring_B = textfield_2.text;' then there won't be problem. the value will be correct. So I would like to know the technical reason.
You are confusing variables and properties. Properties are backed by variables, but in reality they are methods.
Here, you define a property named teststring_B which retains anything that gets assigned to it (and releases the old value). The equivalent methods would look like this (simplified):
- (NSString *)teststring_B {
// Return the content of the variable teststring_B.
return teststring_B;
}
- (void)setTeststring_B:(NSString *)val {
// Retain new value.
[val retain];
// Release old value in variable teststring_B
[teststring_B release];
// Assign the new, retained value to variable teststring_B
teststring_B = val;
}
You can now use the property in two ways: either with [self setTeststring_B:foo]; or with self.teststring_B = foo;. The important point is that the later is just a convenient way of writing, the compiler will translate it into the first form, that is the compiler will turn the self.foo = bar; lines into [self setFoo:bar];.
Now that we have this explained, on to your crash: you've got a string value which is most likely autoreleased. Now you just plain assign it to the variable teststring_B, not the property. And you forgot to retain the value. The property would have retained that value for you.
Now the assigned value was autoreleased (it didn't know you've got a variable still pointing to it) and later a new object came to life at the exact same memory location (if you're lucky). In any case, the teststring_B variable is now not pointing to the text as you thought it would, but to some random object (or to garbage).
There are two ways to fix this:
// First retain, then release; it might be the same object
// and if you would release it first its retain count might
// drop to 0 and get cleaned up before you can retain it again.
NSString *tmp = [textfield_2.text retain];
[teststring_B release];
teststring_B = tmp;
// Better !
self.teststring_B = textfield_2.text;
Accessing the variable directly without using the self will not retain it. So when you accessing it later the variable got auto-released and makes your application crash.
so you can write
1) [self setTeststring_B:textfield_2.text]; or
2) the dot syntax self.teststring_B = textfield_2.text; or
3) teststring_b = [textfield_2.text retain]
What you are doing now is simple assignment. That can cause a crash if the object teststring_A or teststring_B point to is deallocated; this is also called a dangling reference.
The reason only simple assignment is happening is because you aren't accessing the setters through the #property semantics; you could get a retain on those NSString objects by doing self.teststring_A = textfield_1.text instead.
However, you should be using copy with NSString properties. See: NSString property: copy or retain?
In other words, you want this:
#property (nonatomic, copy) NSString *teststring_A;
#property (nonatomic, copy) NSString *teststring_B;
and this:
self.teststring_A = textfield_1.text ;
self.teststring_B = textfield_1.text ;