为代表提供“强"参考是否可以? [英] Is it ever Ok to have a 'strong' reference for a delegate?
问题描述
我有一个从 URL 检索 JSON 并通过协议/委托模式返回数据的类.
I have a class that retrieves JSON from a URL and returns the data via the protocol/delegate pattern.
MRDelegateClass.h
#import <Foundation/Foundation.h>
@protocol MRDelegateClassProtocol
@optional
- (void)dataRetrieved:(NSDictionary *)json;
- (void)dataFailed:(NSError *)error;
@end
@interface MRDelegateClass : NSObject
@property (strong) id <MRDelegateClassProtocol> delegate;
- (void)getJSONData;
@end
请注意，我将 strong 用于我的委托属性.稍后会详细介绍...
Note that I'm using strong for my delegate property. More about that later...
我正在尝试编写一个以基于块的格式实现 getJSONData 的包装器"类.
I am trying to write a 'wrapper' class that implements getJSONData in a block-based format.
MRBlockWrapperClassForDelegate.h
#import <Foundation/Foundation.h>
typedef void(^SuccessBlock)(NSDictionary *json);
typedef void(^ErrorBlock)(NSError *error);
@interface MRBlockWrapperClassForDelegate : NSObject
+ (void)getJSONWithSuccess:(SuccessBlock)success orError:(ErrorBlock)error;
@end
MRBlockWrapperClassForDelegate.m
#import "MRBlockWrapperClassForDelegate.h"
#import "MRDelegateClass.h"
@interface DelegateBlock:NSObject <MRDelegateClassProtocol>
@property (nonatomic, copy) SuccessBlock successBlock;
@property (nonatomic, copy) ErrorBlock errorBlock;
@end
@implementation DelegateBlock
- (id)initWithSuccessBlock:(SuccessBlock)aSuccessBlock andErrorBlock:(ErrorBlock)aErrorBlock {
self = [super init];
if (self) {
_successBlock = aSuccessBlock;
_errorBlock = aErrorBlock;
}
return self;
}
#pragma mark - <MRDelegateClass> protocols
- (void)dataRetrieved:(NSDictionary *)json {
self.successBlock(json);
}
- (void)dataFailed:(NSError *)error {
self.errorBlock(error);
}
@end
// main class
@interface MRBlockWrapperClassForDelegate()
@end
@implementation MRBlockWrapperClassForDelegate
+ (void)getJSONWithSuccess:(SuccessBlock)success orError:(ErrorBlock)error {
MRDelegateClass *delegateClassInstance = [MRDelegateClass new];
DelegateBlock *delegateBlock = [[DelegateBlock alloc] initWithSuccessBlock:success andErrorBlock:error];
delegateClassInstance.delegate = delegateBlock; // set the delegate as the new delegate block
[delegateClassInstance getJSONData];
}
@end
我最近才进入 Objective-C 世界(只生活在 ARC 时代，并且仍然接受块)并且无可否认，我对内存管理的理解是在事物的狭隘方面.
I've come to the objective-c world relatively recently (only lived in ARC times, and still coming to terms with blocks) and admittedly my understanding of memory management is on the slimmer side of things.
此代码似乎工作正常，但前提是我的委托为 strong.我知道我的委托应该 weak 以避免潜在的保留周期.查看工具，我发现分配不会随着持续调用而继续增长.但是，我认为最佳实践"是使用 weak 委托.
This code seems to work fine, but only if I have my delegate as strong. I understand that my delegate should be weak to avoid potential retain-cycles. Looking in instruments, I find that allocations do not continue to grow with continued calls. However, I believe 'best practice' is to have weak delegates.
问题
Q1) 拥有 strong 代表是否确定"
Q1) is it ever 'ok' to have strong delegates
Q2) 我如何实现基于块的包装器，将底层类的委托保留为 weak 委托(即防止 *delegateBlock 在接收协议方法之前被释放)?
Q2) how could I implement the block-based wrapper leaving the delegate of the underlying class as weak delegate (ie. prevent the *delegateBlock from being deallocated before it receives the protocol methods)?
推荐答案
Q1 - 是的.正如您指出的，委托属性很弱是一个建议，以帮助避免保留循环.因此，拥有强大的委托本身并没有错本身，但是如果您班级的客户期望它很弱，您可能会让他们感到惊讶.更好的方法是让委托保持弱引用，并让服务器端(具有委托属性的类)在需要的时候在内部保持强引用.正如@Scott 指出的那样，Apple 文档为 NSURLConnection 做了这件事.当然，这种方法并不能解决您的问题 - 您希望服务器为您保留委托......
Q1 - Yes. As you point out yourself having delegate properties being weak is a recommendation to help avoid retain cycles. So there is nothing wrong per se with having a strong delegate, but if the clients of your class expect it to be weak you may cause them surprises. The better approach is to keep the delegate weak and for the server side (the class with the delegate property) to keep a strong reference internally for those periods it needs one. As @Scott points out Apple documents doing this for NSURLConnection. Of course that approach doesn't solve your issue - where you want the server to retain the delegate for you...
Q2 - 从客户端来看，问题是如何让委托保持活动状态，只要对它有弱引用的服务器需要它.这个问题有一个标准的解决方案，叫做关联对象.简而言之，Objective-C 运行时本质上允许对象的键集合与另一个对象相关联，以及说明该关联应持续多长时间的关联策略.要使用此机制，您只需选择自己的唯一密钥，该密钥类型为 void * - 即一个 地址.以下代码大纲以 NSOpenPanel 为例展示了如何使用它:
Q2 - Looked at from the client side the problem is how to keep a delegate alive as long as a server with a weak reference to it requires it. There is a standard solution to this problem called associated objects. In brief the Objective-C runtime essentially allows a key-collection of objects to be associated with another object, along with an association policy which states how long that association should last. To use this mechanism you just need to pick your own unique key, which is of type void * - i.e. an address. The following code outline shows how to use this using NSOpenPanel as an example:
#import <objc/runtime.h> // import associated object functions
static char myUniqueKey; // the address of this variable is going to be unique
NSOpenPanel *panel = [NSOpenPanel openPanel];
MyOpenPanelDelegate *myDelegate = [MyOpenPanelDelegate new];
// associate the delegate with the panel so it lives just as long as the panel itself
objc_setAssociatedObject(panel, &myUniqueKey, myDelegate, OBJC_ASSOCIATION_RETAIN);
// assign as the panel delegate
[panel setDelegate:myDelegate];
关联策略OBJC_ASSOCIATION_RETAIN 将保留传入的对象(myDelegate)与它关联的对象(panel)然后释放它.
The association policy OBJC_ASSOCIATION_RETAIN will retain the passed in object (myDelegate) for as long as the object it is associated with (panel) and then release it.
采用这种解决方案避免了使委托属性本身变得强大，并允许客户端控制是否保留委托.如果您也正在实现服务器，您当然可以提供一种方法来执行此操作，也许是 associatedDelegate:?，以避免客户端需要定义密钥并调用 objc_setAssociatedObject 本身.(或者，您可以使用类别将其添加到现有类中.)
Adopting this solution avoids making the delegate property itself strong and allows the client to control whether the delegate is retained. If you are also implementing the server you can of course provide a method to do this, maybe associatedDelegate:?, to avoid the client needing to define the key and call objc_setAssociatedObject itself. (Or you can add it to an existing class using a category.)
HTH.
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