How to I pass @selector as a parameter?

Question

For the method:

[NSThread detachNewThreadSelector:@selector(method:) toTarget:self withObject:(id)SELECTOR];

How do I pass in a @selector? I tried casting it to (id) to make it compile, but it crashes in runtime.

More specifically, I have a method like this:

+(void)method1:(SEL)selector{
[NSThread detachNewThreadSelector:@selector(method2:) toTarget:self withObject:selector];   
}

It crashes. How do I pass in the selector without crashing, so that the new thread can call the selector when the thread is ready?

Chuck · Accepted Answer · 2011-05-26T17:56:51.433

The problem here isn't passing a selector to a method, per se, but passing a selector where an object is expected. To pass a non-object value as an object, you can use NSValue. In this case, you'll need to create a method that accepts an NSValue and retrieves the appropriate selector. Here's an example implementation:

@implementation Thing
- (void)method:(SEL)selector {
    // Do something
}

- (void)methodWithSelectorValue:(NSValue *)value {
    SEL selector;

    // Guard against buffer overflow
    if (strcmp([value objCType], @encode(SEL)) == 0) {
        [value getValue:&selector];
        [self method:selector];
    }
}

- (void)otherMethodShownInYourExample {
    SEL selector = @selector(something);
    NSValue *selectorAsValue = [NSValue valueWithBytes:&selector objCType:@encode(SEL)];
    [NSThread detachNewThreadSelector:@selector(methodWithSelectorValue:) toTarget:self withObject:selectorAsValue];
}
@end

@cstack: If you look at the question, spawning a new thread is what he OP was trying to do. So I used the same task in my example. But this technique isn't in any way specific to spawning a new thread. — Chuck, Sep 07 '12 at 15:31

score 43 · Answer 2 · answered Apr 23 '11 at 01:58

43

You can convert between selectors and string objects using the NSStringFromSelector() and NSSelectorFromString() functions. So you can just pass string objects instead.

Alternately, if you don't want to change your methods, you can create an NSInvocation to create an invocation for your method call (since it can set up invocations with non-object arguments), and then to call it do [NSThread detachNewThreadSelector:@selector(invoke) toTarget:myInvocation withObject:nil];

answered Apr 23 '11 at 01:58

user102008

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This is the best answer of the lot, wish i could get more people to upvote this one. – Praveen S Dec 17 '12 at 15:04
the is the easier approach - it should be the chosen answer in my opinion – roocell Apr 24 '13 at 23:29

BJ Homer · Answer 3 · 2010-04-13T14:02:29.730

4

Use NSValue, like so:

+(void)method1:(SEL)selector {
    NSValue *selectorValue = [NSValue value:&selector withObjCType:@encode(SEL)];
    [NSThread detachNewThreadSelector:@selector(method2:) 
                             toTarget:self 
                           withObject:selectorValue];
}

NSValue is intended as an object-wrapper for arbitrary non-object types.

edited Apr 13 '10 at 14:02

answered May 31 '09 at 21:14

BJ Homer

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I believe you are missing an "&" sign before "selector" so that it ends up being "... value:&selector ..." instead of "... value:selector ...". – Senseful Apr 13 '10 at 07:54

score 2 · Answer 4 · edited May 23 '17 at 10:31

2

Please see: passing a method as an argument

edited May 23 '17 at 10:31

Community

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answered May 31 '09 at 20:54

ennuikiller

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score 0 · Answer 5 · answered May 31 '09 at 20:55

If you don't want to specify an object just use nil.

[NSThread detachNewThreadSelector:@selector(method:) toTarget:self withObject:nil];

If you need to pass an object to the selector it would look something like this.

Here I'm passing a string to the method "setText".

NSString *string = @"hello world!";
 [NSThread detachNewThreadSelector:@selector(setText:) toTarget:self withObject:string];


-(void)setText:(NSString *)string {
    [UITextField setText:string]; 
}

How to I pass @selector as a parameter?

5 Answers5

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