NSInvocation for Dummies?

Question

How exactly does NSInvocation work? Is there a good introduction?

I’m specifically having issues understanding how the following code (from Cocoa Programming for Mac OS X, 3rd Edition) works, but then also be able to apply the concepts independently of the tutorial sample. The code:

- (void)insertObject:(Person *)p inEmployeesAtIndex:(int)index
{
    NSLog(@"adding %@ to %@", p, employees);
    // Add inverse of this operation to undo stack
    NSUndoManager *undo = [self undoManager];
    [[undo prepareWithInvocationTarget:self] removeObjectFromEmployeesAtIndex:index];
    if (![undo isUndoing])
        [undo setActionName:@"Insert Person"];
    
    // Finally, add person to the array
    [employees insertObject:p atIndex:index];
}

- (void)removeObjectFromEmployeesAtIndex:(int)index
{
    Person *p = [employees objectAtIndex:index];
    NSLog(@"removing %@ from %@", p, employees);
    // Add inverse of this operation to undo stack
    NSUndoManager *undo = [self undoManager];
    [[undo prepareWithInvocationTarget:self] insertObject:p
                                       inEmployeesAtIndex:index];
    if (![undo isUndoing])
        [undo setActionName:@"Delete Person"];
    
    // Finally, remove person from array
    [employees removeObjectAtIndex:index];
}

I get what it’s trying to do. (BTW, employees is an NSArray of a custom Person class.)

Being a .NET guy, I try to associate unfamiliar Obj-C and Cocoa concepts to roughly analogous .NET concepts. Is this similar to .NET’s delegate concept, but untyped?

This isn’t 100% clear from the book, so I’m looking for something supplemental from real Cocoa/Obj-C experts, again with the goal that I understand the fundamental concept beneath the simple(-ish) example. I'm really looking to be able to independently apply the knowledge -- up until chapter 9, I was having no difficulty doing that. But now ...

Thuggee

Answer 1

According to Apple's NSInvocation class reference:

An NSInvocation is an Objective-C message rendered static, that is, it is an action turned into an object.

And, in a little more detail:

The concept of messages is central to the objective-c philosophy. Any time you call a method, or access a variable of some object, you are sending it a message. NSInvocation comes in handy when you want to send a message to an object at a different point in time, or send the same message several times. NSInvocation allows you to describe the message you are going to send, and then invoke it (actually send it to the target object) later on.

---

For example, let's say you want to add a string to an array. You would normally send the addObject: message as follows:

[myArray addObject:myString];

---

Now, let's say you want to use NSInvocation to send this message at some other point in time:

First, you would prepare an NSInvocation object for use with NSMutableArray's addObject: selector:

NSMethodSignature * mySignature = [NSMutableArray
    instanceMethodSignatureForSelector:@selector(addObject:)];
NSInvocation * myInvocation = [NSInvocation
    invocationWithMethodSignature:mySignature];

Next, you would specify which object to send the message to:

[myInvocation setTarget:myArray];

Specify the message you wish to send to that object:

[myInvocation setSelector:@selector(addObject:)];

And fill in any arguments for that method:

[myInvocation setArgument:&myString atIndex:2];

Note that object arguments must be passed by pointer. Thank you to Ryan McCuaig for pointing that out, and please see Apple's documentation for more details.

At this point, myInvocation is a complete object, describing a message that can be sent. To actually send the message, you would call:

[myInvocation invoke];

This final step will cause the message to be sent, essentially executing [myArray addObject:myString];.

Think of it like sending an email. You open up a new email (NSInvocation object), fill in the address of the person (object) who you want to send it to, type in a message for the recipient (specify a selector and arguments), and then click "send" (call invoke).

See Using NSInvocation for more information. See Using NSInvocation if the above is not working.

---

NSUndoManager uses NSInvocation objects so that it can reverse commands. Essentially, what you are doing is creating an NSInvocation object to say: "Hey, if you want to undo what I just did, send this message to that object, with these arguments". You give the NSInvocation object to the NSUndoManager, and it adds that object to an array of undoable actions. If the user calls "Undo", NSUndoManager simply looks up the most recent action in the array, and invokes the stored NSInvocation object to perform the necessary action.

See Registering Undo Operations for more details.

Pete

One minor correction to an otherwise excellent answer... you have to pass a pointer to objects in setArgument:atIndex:, so the arg assignment should actually read [myInvocation setArgument:&myString atIndex:2]. – Limpid

@Ryan McCuaig: Thank you for pointing that out. I've made the change and added a link to the relevant documentation. – Pete

Just to clarify Ryan's note, index 0 is reserved for "self" and index 1 is reserved for "_cmd" (see the link e.James posted for more detail). So your first argument gets placed at index 2, second argument at index 3, etc... – Brigandine

I honestly don't understand why we have to call – Collette

@haroldcampbell: what do we have to call? – Pete

I don't understand why we have to call setSelector, since we already specified the selector in mySignature. – Extractor

@Gleno: NSInvocation is quite flexible. You can actually set any selector that matches the method signature, so you don't necessarily have to use the same selector that was used to create the method signature. In this example, you could just as easily do setSelector:@selector(removeObject:), since they share the same method signature. – Pete

Aha, so only the signature is set in the init, which is created from the selector, not the selector itself. Very interesting! – Primrose

I would give 2 up votes for nice explanation with simple example. Thanks, e.James. – Luralurch

Why should one use this at all? At the 'later point in time', as you said, why not simply call [object message] instead of this voodoo? What advantage does the process of preparing the message in advance provide? – Persevering

@Cupidvogel the NSInvocation object can be stored and/or shared with other objects without those objects needing to know the implementation details. The best example is NSUndoManager: the controller that makes the "do" action knows exactly what function to call, and with what arguments for "undo", so it creates the NSInvocation object and hands that off to the NSUndoManager. NSUndoManager only has to call 'invoke'. It doesn't need to know what is actually being done in the target. – Pete

Can't you create the invocation with just NSInvocation* myInvocation = [[NSInvocation alloc] init];? Why do you need to specify the selector twice? – Lareine

Unfortunately the 'Using NSInvocation' link is no longer valid. :( – Maypole

@LogicsaurusRex Try here: web.archive.org/web/20121014210741/https://developer.apple.com/… – Laic

Answer 2

Here's a simple example of NSInvocation in action:

- (void)hello:(NSString *)hello world:(NSString *)world
{
    NSLog(@"%@ %@!", hello, world);

    NSMethodSignature *signature  = [self methodSignatureForSelector:_cmd];
    NSInvocation      *invocation = [NSInvocation invocationWithMethodSignature:signature];

    [invocation setTarget:self];                    // index 0 (hidden)
    [invocation setSelector:_cmd];                  // index 1 (hidden)
    [invocation setArgument:&hello atIndex:2];      // index 2
    [invocation setArgument:&world atIndex:3];      // index 3

    // NSTimer's always retain invocation arguments due to their firing delay. Release will occur when the timer invalidates itself.
    [NSTimer scheduledTimerWithTimeInterval:1 invocation:invocation repeats:NO];
}

When called - [self hello:@"Hello" world:@"world"]; - the method will:

• Print "Hello world!"
• Create an NSMethodSignature for itself.
• Create and populate an NSInvocation, calling itself.
• Pass the NSInvocation to an NSTimer
• The timer will fire in (approximately) 1 second, causing the method to be called again with its original arguments.
• Repeat.

In the end, you'll get a printout like so:

2010-07-11 17:48:45.262 Your App[2523:a0f] Hello world!
2010-07-11 17:48:46.266 Your App[2523:a0f] Hello world!
2010-07-11 17:48:47.266 Your App[2523:a0f] Hello world!
2010-07-11 17:48:48.267 Your App[2523:a0f] Hello world!
2010-07-11 17:48:49.268 Your App[2523:a0f] Hello world!
2010-07-11 17:48:50.268 Your App[2523:a0f] Hello world!
2010-07-11 17:48:51.269 Your App[2523:a0f] Hello world!
...

Of course, the target object self must continue to exist for the NSTimer to send the NSInvocation to it. For example, a Singleton object, or an AppDelegate which exists for the duration of the application.

---

UPDATE:

As noted above, when you pass an NSInvocation as an argument to an NSTimer, the NSTimer automatically retains all of the NSInvocation's arguments.

If you are not passing an NSInvocation as an argument to an NSTimer, and plan on having it stick around for a while, you must call its -retainArguments method. Otherwise its arguments may be deallocated before the invocation is invoked, eventually causing your code to crash. Here's how to do it:

NSMethodSignature *signature  = ...;
NSInvocation      *invocation = [NSInvocation invocationWithMethodSignature:signature];
id                arg1        = ...;
id                arg2        = ...;

[invocation setTarget:...];
[invocation setSelector:...];
[invocation setArgument:&arg1 atIndex:2];
[invocation setArgument:&arg2 atIndex:3];

[invocation retainArguments];  // If you do not call this, arg1 and arg2 might be deallocated.

[self someMethodThatInvokesYourInvocationEventually:invocation];

Brigandine

It it interesting that even though the invocationWithMethodSignature: initializer is used, you still need to call setSelector:. It seems redundant, but I just tested and it is necessary. – Misunderstood

Does this keep running in a infinite loop ? and what is _cmd – Caldeira

Answer 3

You could try just using the library here which is much nicer: http://cocoawithlove.com/2008/03/construct-nsinvocation-for-any-message.html

Loydloydie

Answer 4

I build a simple example of calling various method types with NSInvocation.

I had problems calling multiple params using obj_msgSend

https://github.com/clearbrian/NSInvocation_Runtime

Yorker