财产与“自我”有什么关系，反之亦然？

Question

My question is two-part:

First , Say I have a class:

MyClass.h

@interface MyClass: NSObject

-(id)initWithName:(NSString*)name;

@property(nonatomic, strong) NSString *name;

@end

MyClass.m

@implementation MyClass

-(id)initWithName:(NSString*)name
{
    if (self = [super init])
    {
        self.name = name;
    }
    return self;
}

@end

My question: I know that self will hold the name property strongly. But how will the name property relate to self? What I mean is that I can access name as self.name but while class instantiation, how is the children of self (which in this case is name) related to self? I am imagining the structure of the class as a tree, with the parent holding strong reference to the children and the children holding a weak reference to the parent. I want to know if I am thinking about it correctly or not. My guess is it will be a weak relationship.

Second , if I add a method which has a block that references the name property. So my updated implementation of MyClass.m is:

MyClass.m

@implementation MyClass

-(id)initWithName:(NSString*)name
{
    if (self = [super init])
    {
        self.name = name;
    }
    return self;
}

-(void)doSomeStuff
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        self.name = @"Name changed inside block";
}];
}

@end

My second question is: I am not referencing self directly inside my block. So, I guess there is no retain cycle here. But I am referencing name property which is held by self. So does this create a retain cycle?

Answer 1

First: The name property holds no relationship to MyClass , weak or otherwise. (That is, if you pass name to some arbitrary method, it doesn't carry any reference to the MyClass instance where it was a property.)

Second: Since you're simply executing the block rather than storing it, I don't see an opportunity for a retain cycle.

Answer 2

1: The MyClass instance has retained the name property, name property itself has no idea what is MyClass and therefore , there is nothing referring from String-name to the MyClass itself.

2: In the following code

-(void)doSomeStuff
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        self.name = @"Name changed inside block";
}];
}

self.name = @"Name changed inside block"; is the same as [self setName:@"Name changed inside block"];

So you are actually retaining MyClass instance inside Block and then executing its method to update the name , ( block needs pointer to this Name to change it , right ? block retains class object which contains this property ) , you are not retaining the property name itself.

Answer 3

I know that self will hold the name property strongly. But how will the name property relate to self ?

Each property will have a backing instance variable , conventionally named the same as the property with a leading underscore and a getter and/or setter method. There is no relationship; the property generally makes the class instance larger (due to additional instance variable) and the class larger (due to additional methods).

I am not referencing self directly inside my block. So, I guess there is no retain cycle here. But I am referencing name property which is held by self. So does this create a retain cycle?

Yes you are referencing self directly, so a retain cycle is possible. However a retain cycle can only happen under certain circumstances, and it's often just safer to avoid this by creating a weak reference to self and using that within the block.

Answer 4

But how will the name property relate to self? My guess is it will be a weak relationship.

The name property does not have a reference back to self , so the attributes strong and weak don't apply. An object instance is just a collection of instance variables, gathered into a struct. When we talk about object memory management, we are talking about the memory containing that struct. It doesn't make sense to say that a property (really the instance variable backing the property) has a reference to anything. It is simply one part of what self is.

Answer 5

My question: I know that self will hold the name property strongly. But how will the name property relate to self? What I mean is that I can access name as self.name but while class instantiation, how is the children of self (which in this case is name) related to self? I am imagining the structure of the class as a tree, with the parent holding strong reference to the children and the children holding a weak reference to the parent. I want to know if I am thinking about it correctly or not. My guess is it will be a weak relationship.

The children can have a reference to the parent and then it should be weak for the reasons you mentioned. But NSString instances does not have such an up-reference. So there cannot be a retain cycle.

In general it is up to you to manage such inverse relationship. (Core Data does it automatically in its default setters,if you insert a inverse relationship.) Nothing is done automatically, no definition of an up-reference, no setting of a up-reference.

My second question is: I am not referencing self directly inside my block. So, I guess there is no retain cycle here. But I am referencing name property which is held by self. So does this create a retain cycle?

You refer self inside the block, because you use it. Period.

But a retain cycle needs two references. As long as self is used inside the block, but the block is not stored in a property of self (directly or indirectly) no retain cycle can occur.