Add u0_constructor kwargs

Author: YingboMa

docs/src/basics/AbstractSystem.md

@@ -56,10 +56,12 @@ The values which are common to all `AbstractSystem`s are:
 Optionally, a system could have:
 
   - `observed(sys)`: All observed equations of the system and its subsystems.
+  - `independent_variables(sys)`: The independent variables of a system.
+  - `defaults(sys)`: A `Dict` that maps variables/parameters into their default values for the system and its subsystems.
   - `get_observed(sys)`: Observed equations of the current-level system.
   - `get_continuous_events(sys)`: `SymbolicContinuousCallback`s of the current-level system.
-  - `get_defaults(sys)`: A `Dict` that maps variables into their default values.
-  - `independent_variables(sys)`: The independent variables of a system.
+  - `get_defaults(sys)`: A `Dict` that maps variables into their default values
+    for the current-level system.
   - `get_noiseeqs(sys)`: Noise equations of the current-level system.
   - `get_metadata(sys)`: Any metadata about the system or its origin to be used by downstream packages.
 

docs/src/basics/FAQ.md

@@ -129,3 +129,19 @@ julia> ModelingToolkit.missing_variable_defaults(sys, [1,2,3])
  x2ˍtt(t) => 2
  x3ˍtt(t) => 3
 ```
+
+## Change the state vector type
+
+Use the `u0_constructor` keyword argument to map an array to the desired
+container type. For example:
+
+```
+using ModelingToolkit, StaticArrays
+@variables t
+sts = @variables x1(t)=0.0
+D = Differential(t)
+eqs = [D(x1) ~ 1.1 * x1]
+@named sys = ODESystem(eqs, t)
+sys = structural_simplify(sys)
+prob = ODEProblem{false}(sys, [], (0,1); u0_constructor = x->SVector(x...))
+```

src/systems/diffeqs/abstractodesystem.jl

@@ -797,6 +797,7 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
     use_union = true,
     tofloat = true,
     symbolic_u0 = false,
+    u0_constructor = identity,
     kwargs...)
     eqs = equations(sys)
     dvs = states(sys)
@@ -809,6 +810,9 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
         tofloat,
         use_union,
         symbolic_u0)
+    if u0 !== nothing
+        u0 = u0_constructor(u0)
+    end
 
     p, split_idxs = split_parameters_by_type(p)
     if p isa Tuple

test/odesystem.jl

@@ -438,6 +438,8 @@ prob = ODEProblem(sys)
 sol = solve(prob, Tsit5())
 @test sol.t[end] == tspan[end]
 @test sum(abs, sol[end]) < 1
+prob = ODEProblem{false}(sys; u0_constructor = x -> SVector(x...))
+@test prob.u0 isa SVector
 
 # check_eqs_u0 kwarg test
 @parameters t