More getters

Author: YingboMa

docs/src/systems/AbstractSystem.md

@@ -20,17 +20,29 @@ The values which are common to all `AbstractSystem`s are:
 - `equations(sys)`: All equations that define the system and its subsystems.
 - `states(sys)`: All the states in the system and its subsystems.
 - `parameters(sys)`: All parameters of the system and its subsystems.
+- `nameof(sys)`: The name of the current-level system.
 - `get_equations(sys)`: Equations that define the current-level system.
 - `get_states(sys)`: States that are in the current-level system.
 - `get_ps(sys)`: Parameters that are in the current-level system.
 - `get_systems(sys)`: Subsystems of the current-level system.
-- `get_name(sys)` or `nameof(sys)`: The name of the current-level system.
 
-Optionally, a system could have
+Optionally, a system could have:
 
-- `get_default_u0`: A `Dict` that maps states into their default initial
+- `observed(sys)`: All observed equations of the system and its subsystems.
+- `get_observed(sys)`: Observed equations of the current-level system.
+- `get_default_u0(sys)`: A `Dict` that maps states into their default initial
   condition.
-- `get_default_p`: A `Dict` that maps parameters into their default value.
+- `get_default_p(sys)`: A `Dict` that maps parameters into their default value.
+- `independent_variable(sys)`: The independent variable of a system.
+
+Note that there's `get_iv(sys)`, but it is not advised to use, since it errors
+when the system has no field `iv`. `independent_variable(sys)` returns `nothing`
+for `NonlinearSystem`s.
+
+A system could also have caches:
+
+- `get_jac(sys)`: The Jacobian of a system.
+- `get_tgrad(sys)`: The gradient with respect to time of a system.
 
 ## Transformations
 

src/systems/abstractsystem.jl

@@ -127,7 +127,7 @@ function getname(t)
     end
 end
 
-independent_variable(sys::AbstractSystem) = getfield(sys, :iv)
+independent_variable(sys::AbstractSystem) = isdefined(sys, :iv) ? getfield(sys, :iv) : nothing
 
 function structure(sys::AbstractSystem)
     s = get_structure(sys)

src/systems/nonlinear/nonlinearsystem.jl

@@ -63,8 +63,6 @@ function NonlinearSystem(eqs, states, ps;
     NonlinearSystem(eqs, value.(states), value.(ps), observed, name, systems, default_u0, default_p, nothing)
 end
 
-independent_variable(::NonlinearSystem) = nothing
-
 function calculate_jacobian(sys::NonlinearSystem;sparse=false,simplify=false)
     rhs = [eq.rhs for eq ∈ equations(sys)]
     vals = [dv for dv in states(sys)]