suppose i call with
[self methodname]
and other with
[self performSelector:#selector(methodname) withObject:nil];
No difference whatsoever.
Straight from the documentation of performSelector:
The performSelector: method is equivalent to sending an aSelector message directly to the receiver. For example, all three of the following messages do the same thing:
id myClone = [anObject copy];
id myClone = [anObject performSelector:#selector(copy)];
id myClone = [anObject performSelector:sel_getUid("copy")];
While there's no difference in the specific case, however, the reason why performSelector: exists is that it allows invoking an arbitrary selector that may not be available at compile time, as discussed in the doc:
However, the performSelector: method allows you to send messages that aren’t determined until runtime. A variable selector can be passed as the argument:
SEL myMethod = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector:myMethod];
The considerations above also apply to the two variants performSelector:withObject:, performSelector:withObject:withObject:.
Please also note that this doesn't hold true for another set of methods, namely
performSelector:withObject:afterDelay:
performSelector:withObject:afterDelay:inModes:
performSelectorOnMainThread:withObject:waitUntilDone:
performSelectorOnMainThread:withObject:waitUntilDone:modes:
performSelector:onThread:withObject:waitUntilDone:
performSelector:onThread:withObject:waitUntilDone:modes:
performSelectorInBackground:withObject:
Further info here: Does performSelector perform right away or is it scheduled to be performed?
[self methodname]` is shorter and easier to read, write and comprehend.
[self performSelector:#selector(methodname) withObject:nil]` makes it possible to execute arbitrary selectors. If you save the selector in a variable, then you can execute it later on without knowing the method you invoke.
//Self works like this in oops and self works as setter for your class. It also indicates that u r using getter and setter method.
Related
Let me start off by saying I am new to Objective C.
I am getting the error
atusMenuApp[24288:303] -[__NSCFConstantString createListItem]: unrecognized selector sent to instance 0x100002450
Here is my code:
selector = [NSMutableArray arrayWithObjects: #"nvda", #"aapl", #"goog", nil];
[selector makeObjectsPerformSelector:#selector(createListItem:) withObject:self];
- (void)createListItem:(NSString *)title {
//do some stuff
}
Now I have done plenty of looking around and it seems like the biggest reason for this issue is the addition of or lack of the :however I do believe I properly have that in place. Maybe I do not understand the use of makeObjectsPerformSelector very well as after look up the doc on it I found:
Sends to each object in the array the message identified by a given selector, starting with the first object and continuing through the array to the last object.
Any help would be great, Thanks!
[Only if you read the documentation (or thought a bit about why a method is named this way and not that), or even made the effort trying to understand the error message...]
The makeObjectsPerformSelector:withObject: method of NSArray does what it suggests it does: it makes the objects of the array perform the selector, that can have an optional argument. So
[selector makeObjectsPerformSelector:#selector(createListItem:) withObject:self];
will send the createListItem: message to every single NSString object in the selector array and pass in self as its argument. It won't perform the selector on self passing in the object. I. e., what you have is equivalent to
for (NSString *obj in selector) {
[obj createListItem:self];
}
Obviously, instead of this, you want the following:
for (NSString *obj in selector) {
[self createListItem:obj];
}
You don't even need that nasty method for this. A nice fast enumeration for loop will do it.
First you make an array of NSStrings. Then, you send them all the message createListItem. That's all fine and dandy, but NSString doesn't have any method called createListItem; just because you've defined an instance method called createListItem doesn't mean every instance of every class can use it. Only the class who's implementation file has the definition will be able to handle the message. For instance, I can't make a list of Car instances, then define the method fly in another class called Helicopter's implementation and expect to be able to call fly on an instance of Car; only Helicopter can use it. I recommend you read a good book on Objective-C and further familiarize yourself with classes, instances and instance methods.
You misunderstood the method.
It will call the method createListItem: with argument self over every object of the NSArray.
So the resulting call would be something like:
[#"nvda" createListItem:self];
...
Clearly that method doesn't exist for a NSString and there goes your exception.
If you need to apply a method of self to every object inside your array, simply loop through it.
MyThreadRun method is invoked from MyMethod like this
NSArray* args = [NSArray arrayWithObjects:arg1, target, NSStringFromSelector(mySelector), nil];
NSThread* mythread= [[[NSThread alloc] initWithTarget:self selector: #selector(MyThreadRun:) object:args] autorelease];
[MyThreadRun start];
In the end of MyThreadRun, I try to invoke a function in the class which has called MyMethod to initiate the thread to begin with, like this:
NSObject* callbackTarget = [args objectAtIndex:1];
NSString* selector = [args objectAtIndex:2];
[callbackTarget performSelector:NSSelectorFromString(selector) withObject:calculationResult afterDelay:0];
I have a break point on the method that selector is pointing at, and it is never hit.
If I hard code the method name, like this
[callbackTarget updateWithResult:calculationResult]
it works fine.
What is there I need to know about performSelector?
The context where performSelector:withObject:afterDelay: is getting invoked is the culprit. Here's what's going on.
Some members of the performSelector... family, like this one, don't perform the selector right away; they queue up an invocation on the current run loop, so that it happens after your fn returns, the next go-round of the run loop. According to apple: "Specifying a delay of 0 does not necessarily cause the selector to be performed immediately. The selector is still queued on the thread’s run loop and performed as soon as possible."
Normally this is fine and expected. But your code is calling it on a thread that you started manually... and such threads don't keep their run loop going repeatedly the way the main thread does. They invoke the selector specified at creation once, and exit. So: your code queues up an invocation of your callback selector, but then the thread exits; and its run loop is thrown away without ever running... so your queued invocation never happens.
What you probably need is performSelectorOnMainThread:withObject:waitUntilDone:, since you may want the callback to happen on the thread that invoked the MyMethod method in the first place, which is presumably the main thread.
More generally, threading is very tricky stuff. I highly recommend checking out NSOperationQueue, NSBlockOperation, and related techniques - they can remove a great deal of the pain.
Is
NSString* selector = [args objectAtIndex:2];
equal to updateWithResult or updateWithResult:?
They're two different methods. You want the one with the colon.
Remove the "afterDelay:0" and your code works. Also "[MyThreadRun start]" should be "[mythread start]".
Let's say I have a method called foo. What's the difference between:
[self foo];
and
[self performSelector:#selector(foo)];
Are they the same? The first one seems so much easier, so why would you ever want to use the second one?
From the docs:
The performSelector: method is equivalent to sending an aSelector message directly to the receiver. For example, all three of the following messages do the same thing:
id myClone = [anObject copy];
id myClone = [anObject performSelector:#selector(copy)];
id myClone = [anObject performSelector:sel_getUid("copy")];
However, the performSelector: method allows you to send messages that aren’t determined until runtime. A variable selector can be passed as the argument:
SEL myMethod = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector:myMethod];
Well, say you were getting a SEL from somewhere else, and you just wanted to execute it, you'd use this method.
Otherwise, yeah, you'd usually want to use the first example.
The first sends a message to an object and so does the second. In almost all cases, you should use the first. It's quicker and clearer. However, the second has its uses. For instance, you can use it in a situation where you would need to supply a call back.
Another powerful use is in combination with NSSelectorFromString(). You can literally decide what message to use at run time based on a string in a configuration file or from user input (don't forget to validate it!). You can even build selector names using NSString -stringWithFormat: etc. For instance, this parser kit uses the technique to inform the program when parser rules have been matched.
Is it possible to use #selector and performSelector: (or similar) with methods using variable arguments list?
I'm writing a class that can be assigned a delegate to override the default behavior. In the presence of a delegate select method calls made on an instance of that class will be forward to the same corresponding delegate method, some which use variable argument lists.
So, for instance, I need to be able to create retrieve SEL reference and message the delegate object with a method such as this:
- (void)logEventWithFormat:(NSString *)format, ... {
va_list argList;
id del = self.delegate;
if (del != nil &&
[del conformsToProtocol:#protocol(AProtocolWithOptionalMethods)] &&
[del respondsToSelector:#selector(logEventWithFormat:)])
{
// Perform selector on object 'del' with 'argList'
}
}
I am assuming this is not possible, hence the similar method declaration in the Foundation framework - in NSString:
- (id)initWithFormat:(NSString*)format, ...;
and
- (id)initWithFormat:(NSString *)format arguments:(va_list)argList;
I assume that the protocol I wish to delegate to should suggest the implementation of:
- (void)logEventWithFormat:(NSString *)format arguments:(va_list)argList;
so I the selector #selector(logEventWithFormat:arguments:) can be used an called with:
[del performSelector:#selector(logEventWithFormat:arguments:)
withObject:format
withObject:argList];
I just wondered if I was missing something or going the long way around to achieve what I'm trying to?
You can pass anything you want into the runtime function objc_msgSend.
objc_msgSend(del, #selector(logEventWithFormat:arguments:), format, argList);
It's the most powerful way of sending a manually constructed message.
However, it's not clear that you need to perform the invocation this way. As KennyTM pointed out, in the code you have, you could invoke the method directly.
You can't use -performSelector:withObject:withObject: because va_list simply isn't an "object". You need to use NSInvocation.
Or simply call
[del logEventWithFormat:format arguments:argList];
As far as I know, it can't be done. You can't use -performSelector:withObject:withObject: because as #KennyTM points out, a va_list isn't an object.
However, you also cannot use NSInvocation. The documentation straight up says so:
NSInvocation does not support
invocations of methods with either
variable numbers of arguments or union
arguments.
Since these are the two ways of invoking a method by selector, and neither seems to work, I'm going to go with the "can't be done" answer, unless you invoke the method directly and pass the va_list as an argument.
Perhaps #bbum will show up and enlighten us further. =)
I haven't done it that way before, but the simple solution I've often used is to box/unbox either an NSMutableArray or an NSMutableDictionary for the withObject parameter.
I'm still kind of new to Objective-C and I'm wondering what is the difference between the following two statements?
[object performSelector:#selector(doSomething)];
[object doSomething];
Basically performSelector allows you to dynamically determine which selector to call a selector on the given object. In other words the selector need not be determined before runtime.
Thus even though these are equivalent:
[anObject aMethod];
[anObject performSelector:#selector(aMethod)];
The second form allows you to do this:
SEL aSelector = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector: aSelector];
before you send the message.
For this very basic example in the question,
[object doSomething];
[object performSelector:#selector(doSomething)];
there is no difference in what is going to happen. doSomething will be synchronously executed by object. Only "doSomething" is a very simple method, that does not return anything, and does not require any parameters.
were it something a little more complicated, like:
(void)doSomethingWithMyAge:(NSUInteger)age;
things would get complicated, because
[object doSomethingWithMyAge:42];
can no longer be called with any variant of "performSelector", because all variants with parameters only accept object parameters.
The selector here would be "doSomethingWithMyAge:" but any attempt to
[object performSelector:#selector(doSomethingWithMyAge:) withObject:42];
simply won't compile. passing an NSNumber: #(42) instead of 42, wouldn't help either, because the method expects a basic C type - not an object.
In addition, there are performSelector variants up to 2 parameters, no more. While methods many times have many more parameters.
I have found out that although synchronous variants of performSelector:
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
always return an object, I was able to return a simple BOOL or NSUInteger too, and it worked.
One of the two main uses of performSelector is to compose dynamically the name of the method you want to execute, as explained in a previous answer. For example
SEL method = NSSelectorFromString([NSString stringWithFormat:#"doSomethingWithMy%#:", #"Age");
[object performSelector:method];
The other use, is to asynchronously dispatch a message to object, that will be executed later on the current runloop. For this, there are several other performSelector variants.
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;
- (void)performSelector:(SEL)aSelector target:(id)target argument:(id)arg order:(NSUInteger)order modes:(NSArray *)modes;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait modes:(NSArray *)array;
- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(id)arg waitUntilDone:(BOOL)wait;
- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg;
(yes I gathered them from several Foundation class categories, like NSThread, NSRunLoop and NSObject)
Each of the variants has its own special behavior, but all share something in common (at least when waitUntilDone is set to NO). The "performSelector" call would return immediately, and the message to object will only be put on the current runloop after some time.
Because of the delayed execution - naturally no return value is available form the method of the selector, hence the -(void) return value in all these asynchronous variants.
I hope I covered this somehow...
#ennuikiller is spot on. Basically, dynamically-generated selectors are useful for when you don't (and usually can't possibly) know the name of the method you'll be calling when you compile the code.
One key difference is that -performSelector: and friends (including the multi-threaded and delayed variants) are somewhat limited in that they are designed for use with methods with 0-2 parameters. For example, calling -outlineView:toolTipForCell:rect:tableColumn:item:mouseLocation: with 6 parameters and returning the NSString is pretty unwieldy, and not supported by the provided methods.
Selectors are a bit like function pointers in other languages. You use them when you don't know at compile time which method you want to call at runtime. Also, like function pointers, they only encapsulate the verb part of invocation. If the method has parameters, you will need to pass them as well.
An NSInvocation serves a similar purpose, except that it binds together more information. Not only does it include the verb part, it also includes the target object and the parameters. This is useful when you want to call a method on a particular object with particular parameters, not now but in the future. You can build an appropriate NSInvocation and fire it later.
There is another subtle difference between the two.
[object doSomething]; // is executed right away
[object performSelector:#selector(doSomething)]; // gets executed at the next runloop
Here is the excerpt from Apple Documentation
"performSelector:withObject:afterDelay:
Performs the specified selector on the current thread during the next run loop cycle and after an optional delay period. Because it waits until the next run loop cycle to perform the selector, these methods provide an automatic mini delay from the currently executing code. Multiple queued selectors are performed one after another in the order they were queued."