Developers are often confused by what a callback is because of the name of the damned thing.
A callback function is a function which is:
- accessible by another function, and
- is invoked after the first function if that first function completes
A nice way of imagining how a callback function works is that it is a function that is "called at the back" of the function it is passed into.
Maybe a better name would be a "call after" function.
This construct is very useful for asynchronous behaviour where we want an activity to take place whenever a previous event completes.
Pseudocode:
// A function which accepts another function as an argument
// (and will automatically invoke that function when it completes - note that there is no explicit call to callbackFunction)
funct printANumber(int number, funct callbackFunction) {
printout("The number you provided is: " + number);
}
// a function which we will use in a driver function as a callback function
funct printFinishMessage() {
printout("I have finished printing numbers.");
}
// Driver method
funct event() {
printANumber(6, printFinishMessage);
}
Result if you called event():
The number you provided is: 6
I have finished printing numbers.
The order of the output here is important. Since callback functions are called afterwards, "I have finished printing numbers" is printed last, not first.
Callbacks are so-called due to their usage with pointer languages. If you don't use one of those, don't labour over the name 'callback'. Just understand that it is just a name to describe a method that's supplied as an argument to another method, such that when the parent method is called (whatever condition, such as a button click, a timer tick etc) and its method body completes, the callback function is then invoked.
Some languages support constructs where multiple callback function arguments are supported, and are called based on how the parent function completes (i.e. one callback is called in the event that the parent function completes successfully, another is called in the event that the parent function throws a specific error, etc).
A method group is the name for a set of methods (that might be just one) - i.e. in theory the ToString method may have multiple overloads (plus any extension methods): ToString(), ToString(string format), etc - hence ToString by itself is a "method group".
It can usually convert a method group to a (typed) delegate by using overload resolution - but not to a string etc; it doesn't make sense.
Once you add parentheses, again; overload resolution kicks in and you have unambiguously identified a method call.
Best Solution
The fact that you have multiple forms operating on the same data means that a better option is to encapsulate that data in a set of "model" classes that can handle both handing out information to your forms and persisting any changes to storage as necessary.
The advantage of this is when you have multiple forms that need to deal with the same data, you can publish callbacks on the model objects for change notification. Each form subscribes to the events in the model that it cares about and it means any number of forms can manipulate your model and all the forms can maintain current state by reacting to notifications.
When does this way you don't care about which forms are manipulating the data and you don't need to pass anything more than the model class when launching a new form. Likewise, when a form requests a save, all the forms can update the state so they don't show the pending change.