Turn any type into a state machine.
A simple example of a state machine would be as follows.
import state_machine;
struct Order
{
// Turn order into a state-machine with 3 states.
mixin StateMachine!(status, "pending", "ordered", "paid");
uint status;
}Mixing in StateMachine gives our type functions to check its current state, as well as ones to control its transitions between states.
void main()
{
Order o;
assert(o.pending is true);
assert(o.ordered is false);
// Change status to ordered.
assert(o.toOrdered() is true);
assert(o.pending is false);
assert(o.ordered is true);
}You can also define callbacks to control transitions between states, as well as update fields in response to transitions.
import state_machine;
struct Order
{
// Turn order into a state-machine with 3 states.
mixin StateMachine!(status, "pending", "ordered", "paid");
uint status;
double total = 0;
double balance = 0;
@BeforeTransition("ordered")
bool isPendingAndTotalNonZero()
{
return this.pending && total > 0;
}
@AfterTransition("ordered")
void setBalanceFromTotal()
{
balance = total;
}
@BeforeTransition("paid")
bool isOrderedAndBalanceZero()
{
return this.ordered && balance == 0;
}
}Now Order requires a non-zero total to be transition to the ordered state, and a zero balance to become paid. In addition, the balance is automatically set when transitioning from pending to ordered.
void main()
{
Order o;
assert(o.pending is true);
assert(o.toOrdered() is false);
// Requires non-zero total.
o.total = 50.00;
assert(o.toOrdered() is true);
// Balance is auto-set.
assert(o.balance == 50.00);
assert(o.toPaid() is false);
// Balance must be zero.
o.balance = 0;
assert(o.toPaid() is true);
}StateMachine operates on a given state field, which can be an int (or covariant type), a string, an enum type, or a @property of one of those 3 types. It also exposes some helpful meta-functions.
void main()
{
Order o;
assert(o.statusNames == ["pending", "ordered", "paid"]);
assert(o.statusValues == [
"pending": 0,
"ordered": 1,
"paid": 2
]);
}MIT
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