Suppose I have an #property declared like this:
#property (readwrite,retain) NSObject *someObject;
And I synthesize it like this:
#synthesize someObject = _someObject;
This generates getters/setters for me. Also, according to the docs, the setter will have built in thread safety code.
Now, suppose I want to add some code to the setSomeObject: method. Is there any way that I can extend the existing on from #synthesize? I want to be able to reuse the the thread safety code that it autogenerates.
You can define a synthesized "private" property, (put this in your .m file)
#interface ClassName ()
// Declared properties in order to use compiler-generated getters and setters
#property (nonatomic, strong <or whatever>) NSObject *privateSomeObject;
#end
and then manually define a getter and setter in the "public" part of ClassName (.h and #implementation part) like this,
- (void) setSomeObject:(NSObject *)someObject {
self.privateSomeObject = someObject;
// ... Additional custom code ...
}
- (NSArray *) someObject {
return self.privateSomeObject;
}
You can now access the someObject "property" as usual, e.g. object.someObject. You also get the advantage of automatically generated retain/release/copy, compatibility with ARC and almost lose no thread-safety.
What #synthesize does is equivalent to:
-(void)setSomeObject:(NSObject *)anObject {
[anObject retain];
[someObject release];
someObject = anObject;
}
or
-(void)setSomeObject:(NSObject *)anObject {
if(someObject != anObject) {
[someObject release];
someObject = [anObject retain];
}
}
so you can use this code and extend the method.
However, as you said, this code might not be thread-safe.
For thread safety, you might want to take a look at NSLock or #synchronized (thanks to unwesen for pointing this out).
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 need to enforce the initialization of an ivar in a superclass but that ivar usually can not be initialized without other data in the subclass to be initialized. The two solutions I have thought of is:
pass the required generated key for the ivar to the superclass's init method
calling a second superclass method from the subclass's init method
Here is example (contrived, non-working) code. The stringBasedOnSubclassKey ivar should be initialized to the NSString from the subclass's key method.
#interface MySuperclass : NSObject
#property (nonatomic, readonly) NSString *stringBasedOnSubclassKey;
#end
#interface MySubclass : MySuperclass
#property (nonatomic, assign, readonly) int value;
#end
#implementation MySubclass
- (instancetype)init
{
if (self = [super init]) {
_value = 30;
}
return self;
}
- (NSString *)key
{
return [NSString stringWithFormat:#"UniqueKey-%d", self.value];
}
So the question is is there a way to enforce the initialization of the stringBasedOnSubclassKey ivar using the return value of the "key" method? I don't believe I can enforce solution 1 and 2 above. These subclasses may also be created by other outside developers so the key method may be more complicated than this.
Update: I am dealing with existing subclasses of this base class so solutions limiting the changes to existing subclasses is a factor.
Write the getter for stringBasedOnSubclassKey in such a way as to force initialization of it:
- (NSString *) stringBasedOnSubclassKey {
if !(_stringBasedOnSubclassKey) {
_stringBasedOnSubclassKey = // whatever;
}
return _stringBasedOnSubclassKey;
}
And write the superclass key method to throw an exception, thus forcing the client to override it in the subclass.
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 have an interface with properties.
I would like to know the way to declare callback to reach its instance's setter or getter.
Is there a way to do it?
Sorry for my english and thx for your answers and time.
If you declared a #property for your instance variable, and then synthesized it in your implementation file, your getter and setter are automatically created for you. Example for a NSMutableArray
#interface ...
{
NSMutableArray *array;
}
#property (nonatomic, retain) NSMutableArray *array;
Then on your implementation:
#implementation ...
#synthesize array;
Once that's done, you can get and set your instance variable values by using:
Getter: self.array OR [self array]
Setter: self.array = ... OR [self setArray:...]
I am not sure if I understand your question correctly but if you are trying to get some code executed every time the setter or getter is invoked there are basically two ways to do that:
1) you can overwrite the synthesized getter and/or setter like this
Header:
#interface ...
{
NSString *example;
}
#property (nonatomic, copy) NSString *example;
Implementation:
#implementation ...
#synthesize aString
-(void)setExample:(NSString *)newExample
{
if (example != newExample)
{
[example autorelease];
example = [newExample copy];
// YOUR CODE HERE
}
}
...and similarly for the getter.
2) you can observe the variable via KVO and get a 'callback' whenever the variable changes. This, of course, only runs you code when the setter is invoked, not the getter.
What are the differences between implementing a #property with #dynamic or #synthesize?
#synthesize will generate getter and setter methods for your property.
#dynamic just tells the compiler that the getter and setter methods are implemented not by the class itself but somewhere else (like the superclass or will be provided at runtime).
Uses for #dynamic are e.g. with subclasses of NSManagedObject (CoreData) or when you want to create an outlet for a property defined by a superclass that was not defined as an outlet.
#dynamic also can be used to delegate the responsibility of implementing the accessors. If you implement the accessors yourself within the class then you normally do not use #dynamic.
Super class:
#property (nonatomic, retain) NSButton *someButton;
...
#synthesize someButton;
Subclass:
#property (nonatomic, retain) IBOutlet NSButton *someButton;
...
#dynamic someButton;
Take a look at this article; under the heading "Methods provided at runtime":
Some accessors are created dynamically at runtime, such as certain ones used in CoreData's NSManagedObject class. If you want to declare and use properties for these cases, but want to avoid warnings about methods missing at compile time, you can use the #dynamic directive instead of #synthesize.
...
Using the #dynamic directive essentially tells the compiler "don't worry about it, a method is on the way."
The #synthesize directive, on the other hand, generates the accessor methods for you at compile time (although as noted in the "Mixing Synthesized and Custom Accessors" section it is flexible and does not generate methods for you if either are implemented).
As others have said, in general you use #synthesize to have the compiler generate the getters and/ or settings for you, and #dynamic if you are going to write them yourself.
There is another subtlety not yet mentioned: #synthesize will let you provide an implementation yourself, of either a getter or a setter. This is useful if you only want to implement the getter for some extra logic, but let the compiler generate the setter (which, for objects, is usually a bit more complex to write yourself).
However, if you do write an implementation for a #synthesize'd accessor it must still be backed by a real field (e.g., if you write -(int) getFoo(); you must have an int foo; field). If the value is being produce by something else (e.g. calculated from other fields) then you have to use #dynamic.
#dynamic is typically used (as has been said above) when a property is being dynamically created at runtime. NSManagedObject does this (why all its properties are dynamic) -- which suppresses some compiler warnings.
For a good overview on how to create properties dynamically (without NSManagedObject and CoreData:, see: http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtDynamicResolution.html#//apple_ref/doc/uid/TP40008048-CH102-SW1
here is example of #dynamic
#import <Foundation/Foundation.h>
#interface Book : NSObject
{
NSMutableDictionary *data;
}
#property (retain) NSString *title;
#property (retain) NSString *author;
#end
#implementation Book
#dynamic title, author;
- (id)init
{
if ((self = [super init])) {
data = [[NSMutableDictionary alloc] init];
[data setObject:#"Tom Sawyer" forKey:#"title"];
[data setObject:#"Mark Twain" forKey:#"author"];
}
return self;
}
- (void)dealloc
{
[data release];
[super dealloc];
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
NSString *sel = NSStringFromSelector(selector);
if ([sel rangeOfString:#"set"].location == 0) {
return [NSMethodSignature signatureWithObjCTypes:"v#:#"];
} else {
return [NSMethodSignature signatureWithObjCTypes:"##:"];
}
}
- (void)forwardInvocation:(NSInvocation *)invocation
{
NSString *key = NSStringFromSelector([invocation selector]);
if ([key rangeOfString:#"set"].location == 0) {
key = [[key substringWithRange:NSMakeRange(3, [key length]-4)] lowercaseString];
NSString *obj;
[invocation getArgument:&obj atIndex:2];
[data setObject:obj forKey:key];
} else {
NSString *obj = [data objectForKey:key];
[invocation setReturnValue:&obj];
}
}
#end
int main(int argc, char **argv)
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
Book *book = [[Book alloc] init];
printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);
book.title = #"1984";
book.author = #"George Orwell";
printf("%s is written by %s\n", [book.title UTF8String], [book.author UTF8String]);
[book release];
[pool release];
return 0;
}
As per the documentation:
https://developer.apple.com/library/mac/documentation/cocoa/conceptual/ObjCRuntimeGuide/Articles/ocrtDynamicResolution.html
#dynamic tells the compiler that the accessor methods are provided at runtime.
With a little bit of investigation I found out that providing accessor methods override the #dynamic directive.
#synthesize tells the compiler to create those accessors for you (getter and setter)
#property tells the compiler that the accessors will be created, and that can be accessed with the dot notation or [object message]
One thing want to add is that if a property is declared as #dynamic it will not occupy memory (I confirmed with allocation instrument). A consequence is that you can declare property in class category.
As per the Apple documentation.
You use the #synthesize statement in a class’s implementation block to tell the compiler to create implementations that match the specification you gave in the #property declaration.
You use the #dynamic statement to tell the compiler to suppress a warning if it can’t find an implementation of accessor methods specified by an #property declaration.
More info:-
https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/DeclaredProperty.html