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    <p>According to <a href="http://developer.apple.com/documentation/Cocoa/Reference/Foundation/Classes/NSInvocation_Class/Reference/Reference.html" rel="noreferrer">Apple's NSInvocation class reference</a>:</p> <blockquote> <p>An <code>NSInvocation</code> is an Objective-C message rendered static, that is, it is an action turned into an object.</p> </blockquote> <p>And, in a <em>little</em> more detail:</p> <p>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. <code>NSInvocation</code> 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. <code>NSInvocation</code> allows you to <em>describe</em> the message you are going to send, and then <em>invoke</em> it (actually send it to the target object) later on.</p> <hr> <p>For example, let's say you want to add a string to an array. You would normally send the <code>addObject:</code> message as follows:</p> <pre><code>[myArray addObject:myString]; </code></pre> <hr> <p>Now, let's say you want to use <code>NSInvocation</code> to send this message at some other point in time:</p> <p>First, you would prepare an <code>NSInvocation</code> object for use with <code>NSMutableArray</code>'s <code>addObject:</code> selector:</p> <pre><code>NSMethodSignature * mySignature = [NSMutableArray instanceMethodSignatureForSelector:@selector(addObject:)]; NSInvocation * myInvocation = [NSInvocation invocationWithMethodSignature:mySignature]; </code></pre> <p>Next, you would specify which object to send the message to:</p> <pre><code>[myInvocation setTarget:myArray]; </code></pre> <p>Specify the message you wish to send to that object:</p> <pre><code>[myInvocation setSelector:@selector(addObject:)]; </code></pre> <p>And fill in any arguments for that method:</p> <pre><code>[myInvocation setArgument:&amp;myString atIndex:2]; </code></pre> <p>Note that object arguments must be passed by pointer. Thank you to <a href="https://stackoverflow.com/users/53790/ryan-mccuaig">Ryan McCuaig</a> for pointing that out, and please see <a href="http://developer.apple.com/mac/library/documentation/cocoa/reference/foundation/Classes/NSInvocation_Class/Reference/Reference.html#//apple_ref/occ/instm/NSInvocation/setArgument:atIndex:" rel="noreferrer">Apple's documentation</a> for more details.</p> <p>At this point, <code>myInvocation</code> is a complete object, describing a message that can be sent. To actually send the message, you would call:</p> <pre><code>[myInvocation invoke]; </code></pre> <p>This final step will cause the message to be sent, essentially executing <code>[myArray addObject:myString];</code>.</p> <p>Think of it like sending an email. You open up a new email (<code>NSInvocation</code> 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 <code>selector</code> and arguments), and then click "send" (call <code>invoke</code>).</p> <p>See <a href="http://developer.apple.com/DOCUMENTATION/Cocoa/Conceptual/DistrObjects/Tasks/invocations.html" rel="noreferrer">Using NSInvocation</a> for more information. See <a href="https://web.archive.org/web/20121014210741/https://developer.apple.com/library/mac/#/web/20121015003159/https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/DistrObjects/Tasks/invocations.html" rel="noreferrer">Using NSInvocation</a> if the above is not working.</p> <hr> <p><code>NSUndoManager</code> uses <code>NSInvocation</code> objects so that it can <em>reverse</em> commands. Essentially, what you are doing is creating an <code>NSInvocation</code> 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 <code>NSInvocation</code> object to the <code>NSUndoManager</code>, and it adds that object to an array of undoable actions. If the user calls "Undo", <code>NSUndoManager</code> simply looks up the most recent action in the array, and invokes the stored <code>NSInvocation</code> object to perform the necessary action.</p> <p>See <a href="http://developer.apple.com/documentation/Cocoa/Conceptual/UndoArchitecture/Tasks/RegisteringUndo.html" rel="noreferrer">Registering Undo Operations</a> for more details.</p>
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    1. COOne 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]`.
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      1. USRyan McCuaig
    2. CO@Ryan McCuaig: Thank you for pointing that out. I've made the change and added a link to the relevant documentation.
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    3. COJust 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...
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      1. USDave Gallagher
 

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