Given that System.Windows.Input.ICommand as 2 primary methods:
interface ICommand {
void Execute(object parameters);
bool CanExecute(object parameters);
...
}
I expect CanExecute(...) to be called in the Command-supported frameworks before Execute(...) is called.
Internally to my Command Implementation, however, is there any reason to add the CanExecute(...) call inside my Execute(...) implementation?
e.g.:
public void Execute(object parameters){
if(!CanExecute(parameters)) throw new ApplicationException("...");
/** Execute implementation **/
}
This becomes relevant in my testing, as I may mock out some interfaces to support CanExecute, and have to do the same mocks when testing Execute.
Any design thoughts on this?
CanExecute will determine whether the command can be executed or not. If it returns false the button will be disabled on the interface.
ICommand is an interface between the Presentation & the BusinessLogic layer. Whenever any button is pressed on the screen, XAML has its code-behind ButtonClick event. But in MVVM architecture, there is no room for code-behind to keep the application loosely coupled, so ICommand was introduced.
No you can not use it to change the can execute state. It is an event and objects which participate in the ICommand pattern can choose to listen to this event e.g. a button may use this event to know when to re-query the commands state (by calling the can execute method) to set its enabled state.
Programmers are notoriously lazy and they will call Execute
without calling CanExecute
first.
The ICommand
interface is more often used with the WPF binding framework however it is an extremely robust and useful pattern that can be used elsewhere.
I call CanExecute
immediately from the Execute
method to validate the state of the object. It helps reduce duplicate logic and enforces the use of the CanExecute
method (why go through all the effort of whether they can call a method and not bother enforcing it?). I don't see a problem with calling CanExecute
lots of times because it should be a quick operation anyway.
I do however always document the results of calling an Execute
method if the CanExecute
method returns false
, so that the consumer knows the consequences.
I would not be as optimistic as others about adding a call to CanExecute
into Execute
implementation. What if your CanExecute
execution takes a very long time to complete? This would mean that in real-life your user will wait twice that long - once when CanExecute
is called by environment, and then when it is called by you.
You could possibly add some flags to check whether CanExecute
has already been called, but be careful to keep them always up to command state not to miss or perform unwanted CanExecute
call when the state has changed.
I would go one way or the other, but not both.
If you expect the user to call CanExecute, then don't call it in Execute. You've already set that expectation in your interface and now you have a contract with all the developers that implies this sort of interaction with ICommand.
However, if you're worried about developers not utilizing it properly (as you could rightfully be), then I would suggest removing it from the interface completely, and making it an implementation concern.
Example:
interface Command {
void Execute(object parameters);
}
class CommandImpl: ICommand {
public void Execute(object parameters){
if(!CanExecute(parameters)) throw new ApplicationException("...");
/** Execute implementation **/
}
private bool CanExecute(object parameters){
//do your check here
}
}
This way, you're contract (interface) is clear and concise, and you won't be confused about whether or not CanExecute is getting called twice.
However, if you're actually stuck with the interface because you don't control it, another solution could be to store the results and check it like this:
interface Command {
void Execute(object parameters);
bool CanExecute(object parameters);
}
class CommandImpl: ICommand {
private IDictionary<object, bool> ParametersChecked {get; set;}
public void Execute(object parameters){
if(!CanExecute(parameters)) throw new ApplicationException("...");
/** Execute implementation **/
}
public bool CanExecute(object parameters){
if (ParametersChecked.ContainsKey(parameters))
return ParametersChecked[parameters];
var result = ... // your check here
//method to check the store and add or replace if necessary
AddResultsToParametersChecked(parameters, result);
}
}
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