Using NSInvocation

Creating an NSInvocation object requires several steps. Consider this method of the custom class MyCalendar:

– (BOOL)updateAppointmentsForDate:(NSDate *)aDate

updateAppointmentsForDate: takes an NSDate object as its only argument, and returns YES or NO depending on whether the appointments could be updated without conflicts. The following code fragment sets up an NSInvocation object for it:

SEL theSelector;
NSMethodSignature *aSignature;
NSInvocation *anInvocation;
theSelector = @selector(updateAppointmentsForDate:);
aSignature = [MyCalendar instanceMethodSignatureForSelector:theSelector];
anInvocation = [NSInvocation invocationWithMethodSignature:aSignature];
[anInvocation setSelector:theSelector];

The first two lines get the NSMethodSignature object for the updateAppointmentsForDate: method. The last two lines actually create the NSInvocation object and set its selector. Note that the selector can be set to any selector matching the signature of updateAppointmentsForDate:. Any of these methods can be used with anInvocation:

– (BOOL)clearAppointmentsForDate:(NSDate *)aDate
– (BOOL)isAvailableOnDate:(NSDate *)aDate
– (BOOL)setMeetingTime:(NSDate *)aDate

Before being dispatched, anInvocation must have its target and arguments set:

MyCalendar *userDatebook;    /* Assume this exists. */
NSDate *todaysDate;          /* Assume this exists. */
[anInvocation setTarget:userDatebook];
[anInvocation setArgument:&todaysDate atIndex:2];

setArgument:atIndex: sets the specified argument to the value supplied. Every method has two hidden arguments, the target and selector (whose indices are 0 and 1), so the first argument that needs to be set is actually at index 2. In this case, todaysDate is the NSDate argument to updateAppointmentsForDate:.

To dispatch the NSInvocation object, send an invoke or invokeWithTarget: message. invoke only produces a result if the NSInvocation object has a target set. Once dispatched, the NSInvocation object contains the return value of the message, which getReturnValue: produces:

BOOL result;
[anInvocation invoke];
[anInvocation getReturnValue:&result];

NSInvocation does not support invocations of methods with either variable numbers of arguments or union arguments.

Saving NSInvocation Objects for Later Use

Because an NSInvocation object does not always need to retain its arguments, by default it does not do so. This can cause object arguments as well as the target to become invalid if they are automatically released. If you plan to cache an NSInvocation object or dispatch it repeatedly during the execution of your application, you should send it a retainArguments message. This method retains the target and all object arguments, and copies C strings so that they are not lost because another object frees them.

Using NSInvocation Objects with Timers

Suppose the NSInvocation object created above is being used in a time-management application that allows multiple users to set appointments for others, such as group meetings. This application might allow a user’s calendar to be automatically updated every few minutes, so that the user always knows what his schedule looks like. Such automatic updating can be accomplished by setting up NSTimer objects with NSInvocation objects.

Given the NSInvocation object above, this is as simple as invoking one NSTimer method:

[NSTimer scheduledTimerWithInterval:600

This line of code sets up an NSTimer object to dispatch anInvocation every 10 minutes (600 seconds). Note that an NSTimer object always instructs its NSInvocation object to retain its arguments; thus, you do not need to send retainArguments yourself. See Timer Programming Topics for more information on timers.