Add test cases demonstrating the external registration.

Author: dpad

test/function_registration.jl

@@ -0,0 +1,59 @@
+# TEST: Function registration in a module.
+# ------------------------------------------------
+module MyModule
+    using ModelingToolkit, DiffEqBase, LinearAlgebra, Test
+    @parameters t x
+    @variables u(t)
+    @derivatives Dt'~t
+
+    function do_something(a)
+        a + 10
+    end
+    @register do_something(a)
+
+    eq  = Dt(u) ~ do_something(x)
+    sys = ODESystem([eq], t, [u], [x])
+    fun = ODEFunction(sys)
+
+    @test fun([0.5], [5.0], 0.) == [15.0]
+end
+
+# TEST: Function registration in a nested module.
+# ------------------------------------------------
+module MyModule2
+    module MyNestedModule
+        using ModelingToolkit, DiffEqBase, LinearAlgebra, Test
+        @parameters t x
+        @variables u(t)
+        @derivatives Dt'~t
+
+        function do_something_2(a)
+            a + 20
+        end
+        @register do_something_2(a)
+
+        eq  = Dt(u) ~ do_something_2(x)
+        sys = ODESystem([eq], t, [u], [x])
+        fun = ODEFunction(sys)
+
+        @test fun([0.5], [3.0], 0.) == [23.0]
+    end
+end
+
+# TEST: Function registration outside any modules.
+# ------------------------------------------------
+using ModelingToolkit, DiffEqBase, LinearAlgebra, Test
+@parameters t x
+@variables u(t)
+@derivatives Dt'~t
+
+function do_something_3(a)
+    a + 30
+end
+@register do_something_3(a)
+
+eq  = Dt(u) ~ do_something_3(x)
+sys = ODESystem([eq], t, [u], [x])
+fun = ODEFunction(sys)
+
+@test fun([0.5], [7.0], 0.) == [37.0]

test/runtests.jl

@@ -20,6 +20,7 @@ using SafeTestsets, Test
 @safetestset "Lowering Integration Test" begin include("lowering_solving.jl") end
 @safetestset "Test Big System Usage" begin include("bigsystem.jl") end
 @safetestset "Depdendency Graph Test" begin include("dep_graphs.jl") end
+@safetestset "Function Registration Test" begin include("function_registration.jl") end
 #@testset "Latexify recipes Test" begin include("latexify.jl") end
 @testset "Distributed Test" begin include("distributed.jl") end
 @testset "Array of Array Test" begin include("build_function_arrayofarray.jl") end