CODEWITHSUNDEEP
iosobjective-c
KudoCC
KudoCC

Reputation: 6952

Why isn't the attribute of UITableView's delegate property weak

I found this in UITableView's header file and almost every property is non-arc style though my project is using ARC.

@property (nonatomic, assign)   id <UITableViewDataSource> dataSource;
@property (nonatomic, assign)   id <UITableViewDelegate>   delegate;

Why Apple don't use weak property instead of assign, is it a backward compatibility for non-arc ? If so, why not use __has_feature(objc_arc) to distinguish ARC and non-ARC.

#if __has_feature(objc_arc)
@property (nonatomic, weak)   id <UITableViewDataSource> dataSource;
@property (nonatomic, weak)   id <UITableViewDelegate>   delegate;
#else
@property (nonatomic, assign)   id <UITableViewDataSource> dataSource;
@property (nonatomic, assign)   id <UITableViewDelegate>   delegate;
#endif

I hope delegate is weak so I don't need to set the delegate to nil when the delegate instance is deallocated.

Thanks for your help.

Edit:

I note that __has_feature(objc_arc) is wrong because I can use ARC when my deployment target is 4.3, but then I can't use weak. So the condition should be whether my deployment target is equal to 5.0 or above.

#if __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_5_0
@property (nonatomic, weak)   id <UITableViewDataSource> dataSource;
@property (nonatomic, weak)   id <UITableViewDelegate>   delegate;
#else
@property (nonatomic, assign)   id <UITableViewDataSource> dataSource;
@property (nonatomic, assign)   id <UITableViewDelegate>   delegate;
#endif

Upvotes: 6

Views: 1092

Answers (2)

Hussain Shabbir
Hussain Shabbir

Reputation: 15025

Why Apple don't use weak property instead of assign, is it a backward compatibility for non-arc ?

I have done some research and found that, the __weak is synonymous to the “assign” modifier. But only difference is in assign the delegate instance will not set as nil when it is deallocated. You normally use assign modifier for IBOutlets and delegates. Under ARC, this is replaced with __weak. However, there is a caveat. __weak requires you to deploy the app on a runtime that supports zero-ing weak references. This includes, >=(iOS 5 and Lion). Snow Leopard or iOS 4 and older operating systems don’t support zero-ing weak references. This obviously means you cannot use __weak ownership modifiers if you plan to deploy to older operating systems. So Yes, in this sensitive it is a backward compatibility for non-arc.

Upvotes: 4

Shubhendu
Shubhendu

Reputation: 1091

Note that when you have a delegate that's assign, it makes it very important to always set that delegate value to nil whenever the object is going to be deallocated - so an object should always be careful to nil out delegate references in dealloc if it has not done so elsewhere.

Thus assigning delegate, any reference left set, will be invalid after an object is deallocated (pointing at memory no longer allocated to the kind of object expected) - and thus cause a crash if you try to use it. A sign of this in the debugger is when the debugger claims some variable has a type that seems totally wrong from what the variable is actually declared as.

This might be the very reason “Why isn't the attribute of UITableView's delegate property weak “

Hope this might help you.

Upvotes: 0

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