feat: add ContinuousCLock

A ContinuousClock ticks at every solver step

Author: baggepinnen

src/clock.jl

@@ -124,3 +124,23 @@ function Base.:(==)(c1::Clock, c2::Clock)
 end
 
 is_concrete_time_domain(x) = x isa Union{AbstractClock, Continuous}
+
+"""
+    ContinuousClock <: AbstractClock
+    ContinuousClock([t]; dt)
+
+A clock that ticks at each solver step. This clock does generally not have equidistant tick intervals, instead, the tick interval depends on the adaptive step-size slection of the continuous solver, as well as any continuous event handling. If adaptivity of the solver is turned off and there are no continuous events, the tick interval will be given by the fixed solver time step `dt`.
+"""
+struct ContinuousClock <: AbstractClock
+    "Independent variable"
+    t::Union{Nothing, Symbolic}
+    "Period"
+    ContinuousClock(t::Union{Num, Symbolic}) = new(value(t))
+end
+ContinuousClock() = ContinuousClock(nothing)
+
+sampletime(c) = nothing
+Base.hash(c::ContinuousClock, seed::UInt) = seed ⊻ 0x953d7b9a18874b91
+function Base.:(==)(c1::ContinuousClock, c2::ContinuousClock)
+    ((c1.t === nothing || c2.t === nothing) || isequal(c1.t, c2.t))
+end

src/systems/diffeqs/abstractodesystem.jl

@@ -1053,6 +1053,10 @@ function DiffEqBase.ODEProblem{iip, specialize}(sys::AbstractODESystem, u0map =
         discrete_cbs = map(affects, clocks, svs) do affect, clock, sv
             if clock isa Clock
                 PeriodicCallback(DiscreteSaveAffect(affect, sv), clock.dt)
+            elseif clock isa ContinuousClock
+                affect = DiscreteSaveAffect(affect, sv)
+                DiscreteCallback(Returns(true), affect,
+                    initialize = (c, u, t, integrator) -> affect(integrator))
             else
                 error("$clock is not a supported clock type.")
             end

test/clock.jl

@@ -477,3 +477,42 @@ y = res.y[:]
     # y ~ x(k-1) + kp * u
     # Instead. This should be equivalent to the above, but gve me an error when I tried
 end
+
+## Test continuous clock
+
+c = ModelingToolkit.ContinuousClock(t)
+
+@mtkmodel Counter begin
+    @variables begin
+        count(t) = 0
+        u(t) = 0
+    end
+    @equations begin
+        count(k + 1) ~ Sample(c)(u)
+    end
+end
+
+@mtkmodel FirstOrder begin
+    @variables begin
+        x(t) = 0
+    end
+    @equations begin
+        D(x) ~ -x + sin(t)
+    end
+end
+
+@mtkmodel FirstOrderWithStepCounter begin
+    @components begin
+        counter = Counter()
+        fo = FirstOrder()
+    end
+    @equations begin
+        counter.u ~ fo.x
+    end
+end
+
+@mtkbuild model = FirstOrderWithStepCounter()
+prob = ODEProblem(model, [], (0.0, 10.0))
+sol = solve(prob, Tsit5(), kwargshandle = KeywordArgSilent)
+
+@test sol.prob.kwargs[:disc_saved_values][1].t == sol.t[1:2:end] # Test that the discrete-tiem system executed at every step of the continuous solver. The solver saves each time step twice, one state value before discrete affect and one after.