I have a bunch of properties on my model object: ModelObject declared as follows:
#property (atomic, strong, readonly) LotsOfProps *lotsOfProps;
Inside this object there are properties like this:
LotsOfProps {
#property (nonatomic, strong) AProp prop;
#property (nonatomic, strong) MyProp myProp;
}
I do some navigation between different collectionViewCells each of which is backed by a model: Model. On first navigation the properties are present on LotsOfProps and when I navigate away and back to collection view backed-model and check the properties on LotsOfProps I see that they're all nil. I've declared encodeWithCoder, decodeWIthCoder on Model and LotsOfProps and from my understanding atomic already auto generates synchronize methods so even if it's accessed on different threads it should be thread safe.
Any ideas why all of the properties are nil on my object LotsOfProps? I've never seen an issue like this before hence if there's any more information I can provide please let me know!
Related
Can I replace an property with new one using some obj-c runtime features.
So I have a class A which contains a property:
#property (nonatomic, strong) Status *status;
So I want to inherit from this class like ClassB : ClassA and have ability to switch original #property (nonatomic, strong) Status *status; to my new property like #property (nonatomic, assign) NSInteger status;
So the reasone why I needed because I don't want to have a full copy of class A which contains 20 properties, so I just want to inherit from it and replace one with needed type.
Not sure if this possible, but I know something like swizzling and some obj-c runtime features can make a magic in the code.
I'm new to objective-C, so apologies if this is repeated somewhere. I have a category(?) that is something like:
inside SomeClass.h:
#interface SomeClass (SomeCategory) <SomeDelegate>
#property (nonatomic, retain) id somePublicProperty;
#property (nonatomic, retain) id someProperty; // <-- i want to move this to "private"
#end
and now in my SomeClass.m, all i have is:
#implementation SomeClass (SomeCategory)
// dynamic setters/getters here for someProperty.
#end
I think the someProperty is public. how do i make this "private"? (in other words, how do i syntactically put this in the .m file? i tried to use
#interface SomeClass (SomeCategory) {
#property (nonatomic, retain) somePrivateProperty;
}
#end
but it just complains that i have duplicate definition of the category. how do i do this correctly?
In your .h file, you should not give the category. Just use:
#interface SomeClass : SomeBaseClass < SomeDelegate>
#property (nonatomic, retain) id somePublicProperty;
#end
In your .m file, define your private property inside a class extension:
#interface SomeClass ()
#property (nonatomic, retain) id somePrivateProperty;
#end
A class extension is not a like category in that it allows you to extend an interface as well as add new storage to your class.
In a class category, you can define new properties, but no storage will be allocated for it, so you have to do it by hand:
#interface SomeClass (SomeBaseCategory)
#property (nonatomic, retain) id somePrivateProperty;
#end
#implementation SomeClass {
id _somePrivateProperty;
}
- (void)setSomePrivateProperty:(id)property {
_somePrivateProperty = property;
}
- (id)somePrivateProperty {
return _somePrivateProperty;
}
#end
Otherwise your app will crash.
In any case, keep in mind that given the dynamic nature of Objective-C, your property will never be fully private, since you can always send a message to an Objective-C object through objc_msgsend and thus set or read the property value.
EDIT:
If you do not have the source code for a class implementation, you cannot define a class extension (as per source linked above).
In this case, you could use object association to define properties.
Just add the category definition in the .m file OUTSIDE the implementation block
Like so:
#interface MyClass (MyCategory)
#property (assign) BOOL myPrivateProperty;
#end
#implementation MyClass
...
#end
Categories are best used for adding capability to code you do not own and cannot change. Adding properties via categories is not impossible, but is much more difficult.
Class Extensions are best used for keeping properties your object needs, but are not intended to be public.
If you do truly need to add properties to this object, the way to do it is with the Objective-C runtime's associated objects
There's an excellent writeup of when/how to use them here
I made few classes via Core Data. And I need some additional #propertys for one of that classes in runtime. This #propertys are responsible for download progress and I don't want to store them in Core Data DB. I tried to use a separate extension class:
#interface MyClass ()
{
CGFloat _downloadProgress;
NSInteger _downloadErrorCounter;
BOOL _downloadAllStuff;
BOOL _downloadUserCanceled;
}
#property (nonatomic, assign) CGFloat downloadProgress;
#property (nonatomic, assign) NSInteger downloadErrorCounter;
#property (nonatomic, assign) BOOL downloadAllStuff;
#property (nonatomic, assign) BOOL downloadUserCanceled;
#end
But private variables are not visible out of MyClass, and #propertys compile all right, but in runtime i get -[MyClass setDownloadErrorCounter:]: unrecognized selector sent to instance.
Can anyone suggest me some solution?
The easiest solution (if you don't want to modify the Xcode generated class files) is to add the properties to the Core Data model and define the
properties as transient. Transient properties are not saved to the store file.
Another option is to use a tool like "mogenerator", which generates two class files for each
entity, one for the Core Data properties (which is overwritten if the the model changes),
and one for your custom properties (which is not overwritten).
Update: Starting with Xcode 7, Xcode creates both a class and
a category for each managed object subclass, compare NSManagedObject subclass property in category. Custom properties can be added to the class
definition which is not overwritten when the model changes.
Just add
#synthesize downloadErrorCounter = _downloadErrorCounter;
...
in #implementation. Note, not #dynamic.
When trying to use the #synthesize solution i got the error:
#synthesize not allowed in a category's implementation.
Solution was to use associated objects as described in this blog: http://kaspermunck.github.io/2012/11/adding-properties-to-objective-c-categories/
MyManagedObject+Additions.h
#property (strong, nonatomic) NSString *test;
MyManagedObject+Additions.m
NSString const *key = #"my.very.unique.key";
- (void)setTest:(NSString *)test
{
objc_setAssociatedObject(self, &key, test, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (NSString *)test
{
return objc_getAssociatedObject(self, &key);
}
I am downloading a list of objects from an API to display to a user. The list has a mix of two types of objects. Imagine that they are combined books and authors, and the class definitions look like this:
#interface Book : NSObject
#property (nonatomic, strong) NSString *title;
#property (nonatomic, strong) Author *author;
#end
#interface Author : NSObject
#property (nonatomic, strong) NSString *fullName;
#property (nonatomic, weak) Book *book;
#end
Every Book can download its Author information from the API, and vice versa.
If the API gives me a Book, I can set its author property once I download it. The Author object points back to the Book through the book property, but this doesn't create an ARC Retain Cycle because the book property is weak.
However, if the API gives me an Author first, and I download its Book, the object will be deallocated once the method in which I set it returns, because the same property is weak.
I thought of a few ways around this:
Create a Content object that stores both (not viable for many-to-many relationships)
Create separate strongBook and weakBook properties, and then make a readonly property called book which checks which is set and returns that one
Those both seem messy to me, although the second option is preferable.
Is there a way to dynamically change a property from weak to strong (and vice-versa) using the Objective-C runtime?
UPDATE: I'm getting a few suggestions on how to work around the issue, which I don't have trouble coming up with myself. This question is specifically about whether there is a way to either (a) dynamically redefine #properties for a specific instance of a class, or (b) override ARC's retain/release behavior in specific circumstances (since this issue wouldn't exist in MRC).
Just a shot in the dark, but you could create the property and not specify and then use dynamic with the runtime apis. I didn't test it, but i think it should work:
//.h file
#import <Foundation/Foundation.h>
#interface SomeObject : NSObject
#property(nonatomic) NSObject *object;
#end
//.m file
#import "SomeObject.h"
#import <objc/runtime.h>
#implementation SomeObject
#dynamic object;
-(void)setObject:(NSObject *)object
{
BOOL isWeak = NO;
if(isWeak)
{
objc_setAssociatedObject(self, "object", object, OBJC_ASSOCIATION_ASSIGN);
}
else
{
objc_setAssociatedObject(self, "object", object, OBJC_ASSOCIATION_RETAIN);
}
}
-(NSObject *)object
{
return objc_getAssociatedObject(self, "object");
}
#end
For the period of the download, create a mutable dictionary to temporarily store author objects that arrive prior to the book. When a book is received, look in that array and see if the author info is there, if so attach it. When you are finished clean out the mutable array.
Super class Resource
#interface Resource : CoderObject
#property (strong, nonatomic) NSString *resourceID;
#property (assign, nonatomic) ResourceType resourceType;
#property (assign, nonatomic) DataType dataType;
#end
Subclass ViewResource
#interface ViewResource : Resource
#property (strong, nonatomic) CustomView *view;
#property (strong, nonatomic) UIViewController *viewController;
#end
In subclass ViewResource's init method how to access Resource's variable dataType? Now I'm trying to just use super.dataType = ...
Is there other ways?
You just need to use self.dataType. Your subclass has full visibility of all of the superclass properties defined in the .h file. Using self.xxx also gives you the ability to override the accessor methods if required in the future without coming back to edit all of your usage code.
Looking at your link below, fair enough. Those are all valid points. Accessors shouldn't have side effects but you can't guarantee they won't. If the property is defined the superclass then you have a couple of options:
Use self.xxx to set the property and endeavour to ensure no side effects
Call an init method on super, passing the required parameters, and set them there
Like Wain stated in his answer you have direct access to your super's class members (if they are not private).
And there is no problem calling self.property in the init method as long as your init looks like this
-(id)initAndTheNameYoWantAndMaybeSomeParameters:(NSString *)paramExample {
self = [super initNameOfAnInitMethodFromSuperClass];
//check if the init was with success
if(self != nil) {
self.myStringProp = paramExample;
//or
self.propertyFromSuper = paramExample;
}
}
Yes, you can also do stupid stuff in the initMethods (I did it before :)) ) like calling the same initMethod from inside it which was generating a recursive calling that was crashing my app. (Easy to spot this issue)