Merge pull request SciML#2962 from hersle/iv_property

Expose independent variable as a system property

Author: ChrisRackauckas

src/systems/abstractsystem.jl

@@ -1004,6 +1004,7 @@ function Base.propertynames(sys::AbstractSystem; private = false)
         has_observed(sys) && for s in get_observed(sys)
             push!(names, getname(s.lhs))
         end
+        has_iv(sys) && push!(names, getname(get_iv(sys)))
         return names
     end
 end
@@ -1062,6 +1063,13 @@ function getvar(sys::AbstractSystem, name::Symbol; namespace = !iscomplete(sys))
         end
     end
 
+    if has_iv(sys)
+        iv = get_iv(sys)
+        if getname(iv) == name
+            return iv
+        end
+    end
+
     throw(ArgumentError("System $(nameof(sys)): variable $name does not exist"))
 end
 

test/odesystem.jl

@@ -1308,3 +1308,10 @@ end
     u0_v, p_v, _ = ModelingToolkit.get_u0_p(sys, u0, p)
     @test prob.f(u0_v, p_v, 0.0) == [c_b, c_a]
 end
+
+@testset "Independent variable as system property" begin
+    @variables x(t)
+    @named sys = ODESystem([x ~ t], t)
+    @named sys = compose(sys, sys) # nest into a hierarchical system
+    @test t === sys.t === sys.sys.t
+end

test/reduction.jl

@@ -65,7 +65,7 @@ lorenz2 = lorenz(:lorenz2)
      lorenz1.u ~ lorenz2.F
      lorenz2.u ~ lorenz1.F],
     t, systems = [lorenz1, lorenz2])
-@test length(Base.propertynames(connected)) == 10
+@test length(Base.propertynames(connected)) == 10 + 1 # + 1 for independent variable
 @test isequal((@nonamespace connected.lorenz1.x), x)
 __x = x
 @unpack lorenz1 = connected