NSInvocation help

[Esko2300]

Can someone tell me any good websites or books that explain the NSInvocation class, preferably ones with good examples about how to implement this class and situations why i would use them. Thanks

[nobre84]

I use NSInvocation on situations where I need to perform a method on another thread, and that method takes a number of arguments, instead of just one. Normally you would need to wrap your arguments in some way, then unwrap when running the thread. With NSInvocation, you can prepare an invocation to the method you need regardless of its signature, and then invoke it on any thread.

Matt Gallager made a nice add-on category that makes the construction of an NSInvocation as simple as sending it the message itself, with the arguments you want.
Cocoa with Love: Construct an NSInvocation for any message, just by sending

[Esko2300]

I thank you for your response but i have one more question. What i want to accomplish is this. I want the user to press a button on the screen and when the user does a rectangle with grow on the screen to a certian height. After that rectangle grows i want another and then another to grow on the screen and so forth. Would using NSinvocation be a good means about doing this? I am trying to teach myself about NSInvocation and this is the first thing that came to my mind on how to go about doing it since i want the frist rectangle to say hey im done to the second rectangle. Thanks

[nobre84]

I don't think it would be necessary to use NSInvocation for that... You could just use animation blocks and set up a selector to be performed at the end which would start it again...
Begin an animation block, set self as the delegate, then respond to this method to handle the repetition:

-(void)animationDidStop:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context

[Esko2300]

YA but i have to use NSInvocation, i have to show a potential job i understand it and this is the only way i can think about using it and displaying my knowledge of it, is there any other suggestions you have in order for me to implement this feature?

[nobre84]

Aside from my first example with threads, I know the NSUndoManager class capabilities are based on NSInvocation ...
The example you first thought wouldn't really be a good usage scenario for it I guess.

[Duncan C]

Originally Posted by Esko2300

YA but i have to use NSInvocation, i have to show a potential job i understand it and this is the only way i can think about using it and displaying my knowledge of it, is there any other suggestions you have in order for me to implement this feature?

NSInvocation lets you invoke a method that takes an arbitrary number of arguments, where each argument is an arbitrary type.

For instance, the normal timer methods let you send a message when the timer goes off, but that message includes one and only one parameter: the timer object. you can't use a timer to call some arbitrary method with non-object parameters. Instead, you have to use a timer to invoke a "glue" method that then invokes the other method.

If you create an NSInvocation, you can use the scheduledTimerWithTimeInterval:invocation:repeats: form of a timer, which calls the method described in the invocation object. The invocation is set up in advance with all the parameters and their types.

The trick with NSInvocation is to ask an object for it's method signature using code like this:

NSMethodSignature* theMethodSig = [someObject instanceMethodSignatureForSelector: someSelector];

That lets you cheat a little bit, and not worry about building your own method signature. You then just plug in the signature and parameters into the invocation.

[Esko2300]

Thanks for both your explanation's im starting to understand why i would exactly use NSInvocation in my programs. Another question though how would i go about using the [invocation setReturnType:&r] and [invocation getReturnType:&t]?

what im trying to do is grow a rectangle after a certian time has elapsed half a second to be exact and when i get to the height i want it will fire off another rectangle. and so on and so forth. But im getting the function to fire off but im not getting the return value of my function which is supposed to update the values of the rectangle. thisis how im doing it


//when the user presses a button

#pragma mark My Actions
-(IBAction)buttonPressed:(UIButton*)sender{
	rect1 = CGRectMake(rectXPos, rectYPos, rectWidth, rectHeight);	
	view1 = [[UIView alloc]initWithFrame:rect1];
	[view1 setBackgroundColor:[UIColor redColor]];
	[self.view addSubview:view1];	

	signature = [self.class instanceMethodSignatureForSelector:@selector(growRectangle:withRect:)];
	invocation = [NSInvocation invocationWithMethodSignature:signature];
	[invocation setTarget:self];
	[invocation setSelector:@selector(growRectangle:withRect:)];
	[invocation setArgument:&view1 atIndex:2];
	[invocation setArgument:&rect1 atIndex:3];
	[invocation setReturnValue:&rect1];
	
	[NSTimer scheduledTimerWithTimeInterval:.5 invocation:invocation repeats:TRUE];
}

//and this is the function

#pragma mark My Functions
-(CGRect)growRectangle:(UIView*)v withRect:(CGRect)r{
	
	if(r.size.height>-180){
		r.size.height-=20;
		v.frame=r;
		//would i use getReturnValue here? when i do nothing changes in //my rect1
                [invocation getReturnValue:&r];

	}
	else{
		rect2 = CGRectMake(rectXPos+50, rectYPos, rectWidth, rectHeight);
		view2 = [[UIView alloc]initWithFrame:rect2];
		[view2 setBackgroundColor:[UIColor greenColor]];
		[self.view addSubview:view2];
		
		[invocation setSelector:@selector(growRectangle2:withRect:)];
		[invocation setArgument:&view2 atIndex:2];
		[invocation setArgument:&rect2 atIndex:3];
		
	}
	return r;
}

[Duncan C]

Originally Posted by Esko2300

Thanks for both your explanation's im starting to understand why i would exactly use NSInvocation in my programs. Another question though how would i go about using the [invocation setReturnType:&r] and [invocation getReturnType:&t]?

You should not need to use setReturnType, because the two statements

instanceMethodSignatureForSelector
and
invocationWithMethodSignature:
should get everything they need to know about the method (number and types of parameters, type of method return type, if any). You pass that method signature to the method invocationWithMethodSignature to create your invocation object.

As to how to get the return type: Why do you need it? It seems to me that if you're firing off a method using a timer, the return value will simply be ignored. You're not calling the method in the normal way, where you can evaluate the result after the method call and decide what to do next. The timer will fire off the method that's described in the invocation, and then queue up another call (if the timer is a repeating timer.)

I am no expert on invocations, mind. The only time I could think of that you would use the result of a method you invoked with an NSInvocation is if you invoked the method synchronously and waited around for the result to be returned to you.

I just looked at your growRectangle method. It looks to me like you are missing the point of NSInvocation objects completely.

An NSInvocation lets you turn a method call and it's parameters into an object. You can then use the invocation object to invoke that method at a later time.

The invocation object is used to call a method, but in the called method, everything seems normal. You're not aware that there was an NSInvocation object involved. Your method got called with it's normal parameters, and returns whatever value it normally returns. You don't have access to the invocation object inside your called method, and have no reason to want to.

Thus, your growRectangle method should not make any references to an invocation object. You won't have a pointer to it, and should not have a pointer to it. It may even be deallocated by the time your growRectangle method gets called.

[xerxes235]

Hamidreza Vakilian | NSInvocation Tutorial Part2: Compelling Techniques