CODEWITHSUNDEEP
objective-cselector
Joe C
Joe C

Reputation: 2834

In Objective-c is performSelector executed inline or is it posted for execution as a different event?

There are plenty of reasons why this matters. Here's a simple example if you weren't using ARC.

[instance performSelector:selector withObject:objectA];
[objectA release];    // Did the selector actually finish executing
                      // in the line above so everyone's done with objectA
                      // or did the selector merely get scheduled in the line
                      // above, and is yet to execute, so objectA had better
                      // not be released yet?

I've done some research and context clues seem to point to selector getting done inline. But I haven't seen any definitive statement anywhere I've looked, that states it gets executed inline.

Upvotes: 1

Views: 160

Answers (2)

newacct
newacct

Reputation: 122439

Did the selector actually finish executing in the line above so everyone's done with objectA or did the selector merely get scheduled in the line above, and is yet to execute, so objectA had better not be released yet?

It doesn't matter, for memory management purposes. Memory management in Cocoa is local -- you only need to care what your function does; you don't need to know, and shouldn't care, what other functions do internally, to manage memory correctly (ignoring retain cycles). As long as you're done with your owning reference, you should release it. It doesn't matter to you if other people are done with it.

This is because, according to Cocoa memory management rules, any function which needs to store it for use beyond the function call is required to retain it and release it when it's done (because the function cannot assume that the object lives beyond the calling scope otherwise). Any function which uses an argument asynchronously (e.g. performSelector:withObject:afterDelay: etc.) does indeed retain the object and release it when it's done.

Alternately, think about it this way: How does ARC work? How does ARC know whether a function uses its argument synchronously or asynchronously? It doesn't. There is no annotation that tells the compiler whether a function uses something synchronously or asynchronously. Yet ARC does it correctly. That means you can too, without knowing whether the function uses its argument synchronously or asynchronously.

Upvotes: 0

Bryan Chen
Bryan Chen

Reputation: 46598

performSelector:withObject: is executed synchronously (block until the method finished).

Use performSelector:withObject:afterDelay: to execute method asynchronously on main thread (return immediately and execute later).

Use performSelectorInBackground:withObject: to execute method asynchronously on background thread (return immediately and execute on different thread).

Note: performSelector and its friends should be avoid because it is undefined behaviour if you use them on method with incompatible method signature.

The aSelector argument should identify a method that takes no arguments. For methods that return anything other than an object, use NSInvocation.

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/index.html#//apple_ref/occ/intfm/NSObject/performSelector:

Upvotes: 2

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