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performSelector may cause a leak because its selector is unknown

I'm getting the following warning by the ARC compiler:

"performSelector may cause a leak because its selector is unknown".

Here's what I'm doing:

[_controller performSelector:NSSelectorFromString(@"someMethod")];

Why do I get this warning? I understand the compiler can't check if the selector exists or not, but why would that cause a leak? And how can I change my code so that I don't get this warning anymore?

The name of the variable is dynamic, it depends on a lot of other things. There's the risk that I call something that doesn't exist, but that's not the problem.
@matt why would calling a method dynamically on an object be bad practice? Isn't the whole purpose of NSSelectorFromString() to support this practice?
You should/could also test [_controller respondsToSelector:mySelector] before setting it via performSelector:
@mattacular Wish I could vote down: "That... is bad practice."
If you know the string is a literal, just use @selector() so the compiler can tell what the selector name is. If your actual code is calling NSSelectorFromString() with a string that’s constructed or provided at runtime, then you must use NSSelectorFromString().

w
wbyoung

Solution

The compiler is warning about this for a reason. It's very rare that this warning should simply be ignored, and it's easy to work around. Here's how:

if (!_controller) { return; }
SEL selector = NSSelectorFromString(@"someMethod");
IMP imp = [_controller methodForSelector:selector];
void (*func)(id, SEL) = (void *)imp;
func(_controller, selector);

Or more tersely (though hard to read & without the guard):

SEL selector = NSSelectorFromString(@"someMethod");
((void (*)(id, SEL))[_controller methodForSelector:selector])(_controller, selector);

Explanation

What's going on here is you're asking the controller for the C function pointer for the method corresponding to the controller. All NSObjects respond to methodForSelector:, but you can also use class_getMethodImplementation in the Objective-C runtime (useful if you only have a protocol reference, like id<SomeProto>). These function pointers are called IMPs, and are simple typedefed function pointers (id (*IMP)(id, SEL, ...))1. This may be close to the actual method signature of the method, but will not always match exactly.

Once you have the IMP, you need to cast it to a function pointer that includes all of the details that ARC needs (including the two implicit hidden arguments self and _cmd of every Objective-C method call). This is handled in the third line (the (void *) on the right hand side simply tells the compiler that you know what you're doing and not to generate a warning since the pointer types don't match).

Finally, you call the function pointer2.

Complex Example

When the selector takes arguments or returns a value, you'll have to change things a bit:

SEL selector = NSSelectorFromString(@"processRegion:ofView:");
IMP imp = [_controller methodForSelector:selector];
CGRect (*func)(id, SEL, CGRect, UIView *) = (void *)imp;
CGRect result = _controller ?
  func(_controller, selector, someRect, someView) : CGRectZero;

Reasoning for Warning

The reason for this warning is that with ARC, the runtime needs to know what to do with the result of the method you're calling. The result could be anything: void, int, char, NSString *, id, etc. ARC normally gets this information from the header of the object type you're working with.3

There are really only 4 things that ARC would consider for the return value:4

Ignore non-object types (void, int, etc) Retain object value, then release when it is no longer used (standard assumption) Release new object values when no longer used (methods in the init/ copy family or attributed with ns_returns_retained) Do nothing & assume returned object value will be valid in local scope (until inner most release pool is drained, attributed with ns_returns_autoreleased)

The call to methodForSelector: assumes that the return value of the method it's calling is an object, but does not retain/release it. So you could end up creating a leak if your object is supposed to be released as in #3 above (that is, the method you're calling returns a new object).

For selectors you're trying to call that return void or other non-objects, you could enable compiler features to ignore the warning, but it may be dangerous. I've seen Clang go through a few iterations of how it handles return values that aren't assigned to local variables. There's no reason that with ARC enabled that it can't retain and release the object value that's returned from methodForSelector: even though you don't want to use it. From the compiler's perspective, it is an object after all. That means that if the method you're calling, someMethod, is returning a non object (including void), you could end up with a garbage pointer value being retained/released and crash.

Additional Arguments

One consideration is that this is the same warning will occur with performSelector:withObject: and you could run into similar problems with not declaring how that method consumes parameters. ARC allows for declaring consumed parameters, and if the method consumes the parameter, you'll probably eventually send a message to a zombie and crash. There are ways to work around this with bridged casting, but really it'd be better to simply use the IMP and function pointer methodology above. Since consumed parameters are rarely an issue, this isn't likely to come up.

Static Selectors

Interestingly, the compiler will not complain about selectors declared statically:

[_controller performSelector:@selector(someMethod)];

The reason for this is because the compiler actually is able to record all of the information about the selector and the object during compilation. It doesn't need to make any assumptions about anything. (I checked this a year a so ago by looking at the source, but don't have a reference right now.)

Suppression

In trying to think of a situation where suppression of this warning would be necessary and good code design, I'm coming up blank. Someone please share if they have had an experience where silencing this warning was necessary (and the above doesn't handle things properly).

More

It's possible to build up an NSMethodInvocation to handle this as well, but doing so requires a lot more typing and is also slower, so there's little reason to do it.

History

When the performSelector: family of methods was first added to Objective-C, ARC did not exist. While creating ARC, Apple decided that a warning should be generated for these methods as a way of guiding developers toward using other means to explicitly define how memory should be handled when sending arbitrary messages via a named selector. In Objective-C, developers are able to do this by using C style casts on raw function pointers.

With the introduction of Swift, Apple has documented the performSelector: family of methods as "inherently unsafe" and they are not available to Swift.

Over time, we have seen this progression:

Early versions of Objective-C allow performSelector: (manual memory management) Objective-C with ARC warns for use of performSelector: Swift does not have access to performSelector: and documents these methods as "inherently unsafe"

The idea of sending messages based on a named selector is not, however, an "inherently unsafe" feature. This idea has been used successfully for a long time in Objective-C as well as many other programming languages.

1 All Objective-C methods have two hidden arguments, self and _cmd that are implicitly added when you call a method.

2 Calling a NULL function is not safe in C. The guard used to check for the presence of the controller ensures that we have an object. We therefore know we'll get an IMP from methodForSelector: (though it may be _objc_msgForward, entry into the message forwarding system). Basically, with the guard in place, we know we have a function to call.

3 Actually, it's possible for it to get the wrong info if declare you objects as id and you're not importing all headers. You could end up with crashes in code that the compiler thinks is fine. This is very rare, but could happen. Usually you'll just get a warning that it doesn't know which of two method signatures to choose from.

4 See the ARC reference on retained return values and unretained return values for more details.


@wbyoung If your code solves the retaining problem, I wonder why performSelector: methods aren’t implemented this way. They have strict method signature (returning id, taking one or two ids), so no primitive types need to be handled.
@Andy the argument is handled based on the definition of the method's prototype (it won't be retained/released). The concern is mostly based on the return type.
The "Complex Example" gives an error Cannot initialize a variable of type 'CGRect (*)(__strong id, SEL, CGRect, UIView *__strong)' with an rvalue of type 'void *' when using latest Xcode. (5.1.1) Still, I learned a lot!
void (*func)(id, SEL) = (void *)imp; does not compile, I have replaced it with void (*func)(id, SEL) = (void (*)(id, SEL))imp;
change void (*func)(id, SEL) = (void *)imp; to <…> = (void (*))imp; or <…> = (void (*) (id, SEL))imp;
S
Scott Thompson

In the LLVM 3.0 compiler in Xcode 4.2 you can suppress the warning as follows:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    [self.ticketTarget performSelector: self.ticketAction withObject: self];
#pragma clang diagnostic pop

If you're getting the error in several places, and want to use the C macro system to hide the pragmas, you can define a macro to make it easier to suppress the warning:

#define SuppressPerformSelectorLeakWarning(Stuff) \
    do { \
        _Pragma("clang diagnostic push") \
        _Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
        Stuff; \
        _Pragma("clang diagnostic pop") \
    } while (0)

You can use the macro like this:

SuppressPerformSelectorLeakWarning(
    [_target performSelector:_action withObject:self]
);

If you need the result of the performed message, you can do this:

id result;
SuppressPerformSelectorLeakWarning(
    result = [_target performSelector:_action withObject:self]
);

This method can cause memory leaks when the optimization is set to anything other than None.
@Eric No it cannot, unless you're invoking funny methods like "initSomething" or "newSomething" or "somethingCopy".
@Julian That does work, but that turns off the warning for the entire file – you might not need or want that. Wrappping it with the pop and push-pragmas are much cleaner and more secure.
All this does is it silences up the compiler. This does not solve the problem. If the selector doesn't exist you're pretty much screwed.
This should be used only when wrapped by an if ([_target respondsToSelector:_selector]) { or similar logic.
b
brainjam

My guess about this is this: since the selector is unknown to the compiler, ARC cannot enforce proper memory management.

In fact, there are times when memory management is tied to the name of the method by a specific convention. Specifically, I am thinking of convenience constructors versus make methods; the former return by convention an autoreleased object; the latter a retained object. The convention is based on the names of the selector, so if the compiler does not know the selector, then it cannot enforce the proper memory management rule.

If this is correct, I think that you can safely use your code, provided you make sure that everything is ok as to memory management (e.g., that your methods do not return objects that they allocate).


Thanks for the answer, I'll look more into this to see what is going on. Any idea on how I can bypass the warning though and make it disappear? I would hate to have the warning sitting in my code forever for what is a safe call.
So I got confirmation from somebody at Apple in their forums that this is indeed the case. They'll be adding a forgotten override to allow people to disable this warning in future releases. Thanks.
This answer raises some questions, like if ARC tries to make determinations on when to release something based upon convention and method names, then how is it "reference counting"? The behavior you describe sounds only marginally better than completely arbitrary, if ARC is assuming the code follows a certain convention as opposed to actually keeping track of the references no matter what convention is followed.
ARC automates the process of adding retains and releases at compile. It is not garbage collection (which is why it is so incredibly fast and low overhead). It is not arbitrary at all. The default rules are based on well-established ObjC conventions that have been consistently applied for decades. This avoids the need to explicitly add an __attribute to every method explaining its memory management. But it also makes it impossible for the complier to properly handle this pattern (a pattern which used to be very common, but has been replaced with more robust patterns in recent years).
So we can no longer have an ivar of type SEL and assign different selectors depending on the situation? Way to go, dynamic language...
0
0xced

In your project Build Settings, under Other Warning Flags (WARNING_CFLAGS), add
-Wno-arc-performSelector-leaks

Now just make sure that the selector you are calling does not cause your object to be retained or copied.


Note you can add the same flag for specific files rather than the entire project. If you look under Build Phases->Compile Sources, you can set per file Compiler Flags (just like you want to do for excluding files from ARC). In my project just one file should use selectors this way, so I just excluded it and left the others.
T
Top-Master

As a workaround until the compiler allows overriding the warning, you can use the runtime.

You need header:

#import <objc/message.h>

Then try below:

// For strict compilers.
((id(*)(id,SEL))objc_msgSend)(_controller, sel_getUid("someMethod"));

OR

// Old answer's code:
objc_msgSend(_controller, NSSelectorFromString(@"someMethod"));

Instead of:

[_controller performSelector:NSSelectorFromString(@"someMethod")];

ARC recognizes Cocoa conventions and then adds retains and releases based on those conventions. Because C does not follow those conventions, ARC forces you to use manual memory management techniques. If you create a CF object, you must CFRelease() it. If you dispatch_queue_create(), you must dispatch_release(). Bottom line, if you want to avoid the ARC warnings, you can avoid them by using C objects and manual memory management. Also, you can disable ARC on a per-file basis by using the -fno-objc-arc compiler flag on that file.
Not without casting, you can't. Varargs is not the same as an explicitly typed argument list. It'll generally work by coincidence, but I don't consider "by coincidence" to be correct.
Don't do that, [_controller performSelector:NSSelectorFromString(@"someMethod")]; and objc_msgSend(_controller, NSSelectorFromString(@"someMethod")); are not equivalent! Have a look at Method Signature Mismatches and A big weakness in Objective-C's weak typing they are explaining the problem in depth.
@0xced In this case, it's fine. objc_msgSend will not create a method signature mismatch for any selector that would have worked correctly in performSelector: or its variants since they only ever take objects as parameters. As long as all your parameters are pointers (incl. objects), doubles and NSInteger/long, and your return type is void, pointer or long, then objc_msgSend will work correctly.
ObjC does not have function overloading like c++ has. So even thought mikeash's website is expressing real concerns, you should get a compiler warning when you try to overload (not meaning override - in case someone mixes those words) methods that can not be overloaded because of ObjC.
B
Barlow Tucker

To ignore the error only in the file with the perform selector, add a #pragma as follows:

#pragma clang diagnostic ignored "-Warc-performSelector-leaks"

This would ignore the warning on this line, but still allow it throughout the rest of your project.


I gather that you can also turn the warning back on immediately after the method in question with #pragma clang diagnostic warning "-Warc-performSelector-leaks". I know if I turn off a warning, I like to turn it back on at the soonest possible moment, so I don't accidentally let another unanticipated warning slip by. It's unlikely that this is a problem, but it's just my practice whenever I turn off a warning.
You can also restore your previous compiler configuration state by using #pragma clang diagnostic warning push before you make any changes and #pragma clang diagnostic warning pop to restore the previous state. Useful if you are turning off loads and don't want to have lots of re-enable pragma lines in your code.
It will only ignore the following line?
m
matt

Strange but true: if acceptable (i.e. result is void and you don't mind letting the runloop cycle once), add a delay, even if this is zero:

[_controller performSelector:NSSelectorFromString(@"someMethod")
    withObject:nil
    afterDelay:0];

This removes the warning, presumably because it reassures the compiler that no object can be returned and somehow mismanaged.


Do you know if this actually resolves the related memory management issues, or does it have the same issues but Xcode isn't smart enough to warn you with this code?
This is semantically not the same thing! Using performSelector:withObject:AfterDelay: will perform the selector in the next run of the runloop. Therefore, this method returns immediately.
@Florian Of course it's not the same! Read my answer: I say if acceptable, because the result is void and the runloop cycles. That's the first sentence of my answer.
s
syvex

Here is an updated macro based on the answer given above. This one should allow you to wrap your code even with a return statement.

#define SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(code)                        \
    _Pragma("clang diagnostic push")                                        \
    _Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"")     \
    code;                                                                   \
    _Pragma("clang diagnostic pop")                                         \


SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(
    return [_target performSelector:_action withObject:self]
);

return doesn't have to be inside the macro; return SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING([_target performSelector:_action withObject:self]); also works and looks saner.
B
Benedict Cohen

This code doesn't involve compiler flags or direct runtime calls:

SEL selector = @selector(zeroArgumentMethod);
NSMethodSignature *methodSig = [[self class] instanceMethodSignatureForSelector:selector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSig];
[invocation setSelector:selector];
[invocation setTarget:self];
[invocation invoke];

NSInvocation allows multiple arguments to be set so unlike performSelector this will work on any method.


Do you know if this actually resolves the related memory management issues, or does it have the same issues but Xcode isn't smart enough to warn you with this code?
You could say it solves the memory management issues; but this is because it basically lets you specify the behavior. For example, you can choose to let invocation retain the arguments or not. To my current knowledge, it attempts to fix the signature mismatch problems that might appear by trusting that you know what you are doing and don't provide it with incorrect data. I'm not sure if all the checks can be performed at runtime. As mentiones in another comment, mikeash.com/pyblog/… nicely explaines what mismatches can do.
C
Chris Prince

Well, lots of answers here, but since this is a little different, combining a few answers I thought I'd put it in. I'm using an NSObject category which checks to make sure the selector returns void, and also suppresses the compiler warning.

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "Debug.h" // not given; just an assert

@interface NSObject (Extras)

// Enforce the rule that the selector used must return void.
- (void) performVoidReturnSelector:(SEL)aSelector withObject:(id)object;
- (void) performVoidReturnSelector:(SEL)aSelector;

@end

@implementation NSObject (Extras)

// Apparently the reason the regular performSelect gives a compile time warning is that the system doesn't know the return type. I'm going to (a) make sure that the return type is void, and (b) disable this warning
// See http://stackoverflow.com/questions/7017281/performselector-may-cause-a-leak-because-its-selector-is-unknown

- (void) checkSelector:(SEL)aSelector {
    // See http://stackoverflow.com/questions/14602854/objective-c-is-there-a-way-to-check-a-selector-return-value
    Method m = class_getInstanceMethod([self class], aSelector);
    char type[128];
    method_getReturnType(m, type, sizeof(type));

    NSString *message = [[NSString alloc] initWithFormat:@"NSObject+Extras.performVoidReturnSelector: %@.%@ selector (type: %s)", [self class], NSStringFromSelector(aSelector), type];
    NSLog(@"%@", message);

    if (type[0] != 'v') {
        message = [[NSString alloc] initWithFormat:@"%@ was not void", message];
        [Debug assertTrue:FALSE withMessage:message];
    }
}

- (void) performVoidReturnSelector:(SEL)aSelector withObject:(id)object {
    [self checkSelector:aSelector];

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    // Since the selector (aSelector) is returning void, it doesn't make sense to try to obtain the return result of performSelector. In fact, if we do, it crashes the app.
    [self performSelector: aSelector withObject: object];
#pragma clang diagnostic pop    
}

- (void) performVoidReturnSelector:(SEL)aSelector {
    [self checkSelector:aSelector];

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    [self performSelector: aSelector];
#pragma clang diagnostic pop
}

@end

Should 'v' be replaced by _C_VOID? _C_VOID is declared in .
P
Patrick Perini

For posterity's sake, I've decided to throw my hat into the ring :)

Recently I've been seeing more and more restructuring away from the target/selector paradigm, in favor of things such as protocols, blocks, etc. However, there is one drop-in replacement for performSelector that I've used a few times now:

[NSApp sendAction: NSSelectorFromString(@"someMethod") to: _controller from: nil];

These seem to be a clean, ARC-safe, and nearly identical replacement for performSelector without having to much about with objc_msgSend().

Though, I have no idea if there is an analog available on iOS.


Thanks for including this.. It is available in iOS: [[UIApplication sharedApplication] sendAction: to: from: forEvent:]. I looked into it once, but it kind of feels awkward to use a UI-related class in the middle of your domain or service just to do a dynamic call.. Thanks for including this though!
Ew! It'll have more overhead (since it needs to check whether the method is available and walk up the responder chain if it isn't) and have different error behaviour (walking up the responder chain and returning NO if it can't find anything which responds to the method, instead of simply crashing). It also doesn't work when you want the id from -performSelector:...
@tc. It doesn't "walk up the responder chain" unless to: is nil, which it isn't. It just goes straight to the targeted object with no checking beforehand. So there isn't "more overhead". It's not a great solution, but the reason you give isn't the reason. :)
C
Community

Matt Galloway's answer on this thread explains the why:

Consider the following: id anotherObject1 = [someObject performSelector:@selector(copy)]; id anotherObject2 = [someObject performSelector:@selector(giveMeAnotherNonRetainedObject)]; Now, how can ARC know that the first returns an object with a retain count of 1 but the second returns an object which is autoreleased?

It seems that it is generally safe to suppress the warning if you are ignoring the return value. I'm not sure what the best practice is if you really need to get a retained object from performSelector -- other than "don't do that".


C
Community

@c-road provides the right link with problem description here. Below you can see my example, when performSelector causes a memory leak.

@interface Dummy : NSObject <NSCopying>
@end

@implementation Dummy

- (id)copyWithZone:(NSZone *)zone {
  return [[Dummy alloc] init];
}

- (id)clone {
  return [[Dummy alloc] init];
}

@end

void CopyDummy(Dummy *dummy) {
  __unused Dummy *dummyClone = [dummy copy];
}

void CloneDummy(Dummy *dummy) {
  __unused Dummy *dummyClone = [dummy clone];
}

void CopyDummyWithLeak(Dummy *dummy, SEL copySelector) {
  __unused Dummy *dummyClone = [dummy performSelector:copySelector];
}

void CloneDummyWithoutLeak(Dummy *dummy, SEL cloneSelector) {
  __unused Dummy *dummyClone = [dummy performSelector:cloneSelector];
}

int main(int argc, const char * argv[]) {
  @autoreleasepool {
    Dummy *dummy = [[Dummy alloc] init];
    for (;;) { @autoreleasepool {
      //CopyDummy(dummy);
      //CloneDummy(dummy);
      //CloneDummyWithoutLeak(dummy, @selector(clone));
      CopyDummyWithLeak(dummy, @selector(copy));
      [NSThread sleepForTimeInterval:1];
    }} 
  }
  return 0;
}

The only method, which causes memory leak in my example is CopyDummyWithLeak. The reason is that ARC doesn't know, that copySelector returns retained object.

https://i.stack.imgur.com/iW1Po.png


B
Ben Flynn

To make Scott Thompson's macro more generic:

// String expander
#define MY_STRX(X) #X
#define MY_STR(X) MY_STRX(X)

#define MYSilenceWarning(FLAG, MACRO) \
_Pragma("clang diagnostic push") \
_Pragma(MY_STR(clang diagnostic ignored MY_STR(FLAG))) \
MACRO \
_Pragma("clang diagnostic pop")

Then use it like this:

MYSilenceWarning(-Warc-performSelector-leaks,
[_target performSelector:_action withObject:self];
                )

FWIW, I did not add the macro. Someone added that to my response. Personally, I wouldn't use the macro. The pragma is there to work around a special case in the code and the pragmas are very explicit and direct about what's going on. I prefer to keep them in place rather than hiding, or abstracting them behind a macro, but that's just me. YMMV.
@ScottThompson That's fair. For me it's easy to search for this macro across my code base and I generally also add an un-silenced warning to deal with the underlying issue.
C
Community

Do not suppress warnings!

There are no less than 12 alternative solutions to tinkering with the compiler. While you are being clever at the time the first implementation, few engineer on Earth can follow your footsteps, and this code will eventually break.

Safe Routes:

All these solutions will work, with some degree of of variation from your original intent. Assume that param can be nil if you so desire:

Safe route, same conceptual behavior:

// GREAT
[_controller performSelectorOnMainThread:selector withObject:anArgument waitUntilDone:YES];
[_controller performSelectorOnMainThread:selector withObject:anArgument waitUntilDone:YES modes:@[(__bridge NSString *)kCFRunLoopDefaultMode]];

[_controller performSelector:selector onThread:[NSThread mainThread] withObject:anArgument waitUntilDone:YES];
[_controller performSelector:selector onThread:[NSThread mainThread] withObject:anArgument waitUntilDone:YES modes:@[(__bridge NSString *)kCFRunLoopDefaultMode]];

Safe route, slightly different behavior:

(See this response)
Use any thread in lieu of [NSThread mainThread].

// GOOD
[_controller performSelector:selector withObject:anArgument afterDelay:0];
[_controller performSelector:selector withObject:anArgument afterDelay:0 inModes:@[(__bridge NSString *)kCFRunLoopDefaultMode]];

[_controller performSelectorOnMainThread:selector withObject:anArgument waitUntilDone:NO];
[_controller performSelectorOnMainThread:selector withObject:anArgument waitUntilDone:NO];
[_controller performSelectorOnMainThread:selector withObject:anArgument waitUntilDone:NO modes:@[(__bridge NSString *)kCFRunLoopDefaultMode]];

[_controller performSelectorInBackground:selector withObject:anArgument];

[_controller performSelector:selector onThread:[NSThread mainThread] withObject:anArgument waitUntilDone:NO];
[_controller performSelector:selector onThread:[NSThread mainThread] withObject:anArgument waitUntilDone:NO modes:@[(__bridge NSString *)kCFRunLoopDefaultMode]];

Dangerous Routes

Requires some kind of compiler silencing, which is bound to break. Note that at present time, it did break in Swift.

// AT YOUR OWN RISK
[_controller performSelector:selector];
[_controller performSelector:selector withObject:anArgument];
[_controller performSelector:selector withObject:anArgument withObject:nil];

The wording is very wrong. The safe routes are not safer than dangerous at all. It is arguably more dangerous because it hides the warning implicitly.
I will fix the wording to not be insulting, but I stand by my word. The only time I find silencing warning acceptable is if I do not own the code. No engineer can safely maintain silenced code without understanding all the consequences, which would mean read this argument, and this practice is plain risky ; especially if you consider the 12, plain English, robust alternatives.
No. You didn't get my point. Using performSelectorOnMainThread is not a good way to silence the warning and it has side effects. (it doesn't solve the memory leak) The extra #clang diagnostic ignored explicitly suppress the warning in a very clear way.
True that performing a selector on a non - (void) method is the real issue.
and how do you call a selector with multiple arguments through this and be safe in the same time? @SwiftArchitect
h
honus

Because you are using ARC you must be using iOS 4.0 or later. This means you could use blocks. If instead of remembering the selector to perform you instead took a block, ARC would be able to better track what is actually going on and you wouldn't have to run the risk of accidentally introducing a memory leak.


Actually, blocks make it very easy to accidentally create a retain cycle which ARC does not solve. I still wish that there was a compiler warning when you implicitly used self via an ivar (e.g. ivar instead of self->ivar).
You mean like -Wimplicit-retain-self ?
s
supersabbath

Instead of using the block approach, which gave me some problems:

    IMP imp = [_controller methodForSelector:selector];
    void (*func)(id, SEL) = (void *)imp;

I will use NSInvocation, like this:

    -(void) sendSelectorToDelegate:(SEL) selector withSender:(UIButton *)button 

    if ([delegate respondsToSelector:selector])
    {
    NSMethodSignature * methodSignature = [[delegate class]
                                    instanceMethodSignatureForSelector:selector];
    NSInvocation * delegateInvocation = [NSInvocation
                                   invocationWithMethodSignature:methodSignature];


    [delegateInvocation setSelector:selector];
    [delegateInvocation setTarget:delegate];

    // remember the first two parameter are cmd and self
    [delegateInvocation setArgument:&button atIndex:2];
    [delegateInvocation invoke];
    }

a
arsenius

If you don't need to pass any arguments an easy workaround is to use valueForKeyPath. This is even possible on a Class object.

NSString *colorName = @"brightPinkColor";
id uicolor = [UIColor class];
if ([uicolor respondsToSelector:NSSelectorFromString(colorName)]){
    UIColor *brightPink = [uicolor valueForKeyPath:colorName];
    ...
}

D
Damon

You could also use a protocol here. So, create a protocol like so:

@protocol MyProtocol
-(void)doSomethingWithObject:(id)object;
@end

In your class that needs to call your selector, you then have a @property.

@interface MyObject
    @property (strong) id<MyProtocol> source;
@end

When you need to call @selector(doSomethingWithObject:) in an instance of MyObject, do this:

[self.source doSomethingWithObject:object];

Hey Wu, thanks, but the point of using the NSSelectorFromString is when you don't know which selector you want to call during runtime.

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