Tips for Defining System Objects
A System object™ is a specialized MATLAB® object that is optimized for iterative processing. Use System objects when you need to run an object multiple times or process data in a loop. When defining your own System object, use the following suggestions to help your System object run more quickly.
Define all one-time calculations in the
setupImplmethod and cache the results in a private property. Use the
stepImplmethod for repeated calculations.
Specify Boolean values using
If the variables in a method do not need to retain their values between calls, use local scope for those variables in that method.
Inputs and Outputs
Some methods use the
stepImplalgorithm inputs as their inputs, such as
processInputSpecificationChangeImpl. The inputs must match the order of inputs to
stepImpl, but do not need to match the number of inputs. If your implementation does not require any of the inputs to the System object, you can leave them all off.
getNumOutputsImplmethods, if you set the return argument from an object property, that object property must have the
Using ~ as an Input Argument in Method Definitions
All methods, except static methods, expect the System object handle as the first input argument. You can use any name for your System object handle. The code inserted by the MATLAB Editor menu uses
In many examples, instead of passing in the object handle,
~ is used
to indicate that the object handle is not used in the function. Using ~ instead of an object
handle prevents warnings about unused variables.
For properties that do not change, define them in as
Tunableproperties have slower access times than
Whenever possible, use the
privateattribute instead of the
publicattribute for a property. Some
publicproperties have slower access times than
If properties are accessed more than once in the
stepImplmethod, cache those properties as local variables inside the method. A typical example of multiple property access is a loop. Iterative calculations using cached local variables run faster than calculations that must access the properties of an object. When the calculations for the method complete, you can save the local cached results back to the properties of that System object. Copy frequently used tunable properties into private properties. This best practice also applies to the
For example, in this code
kis accessed multiple times in each loop iteration, but is saved to the object property only once.
function y = stepImpl(obj,x) k = obj.MyProp; for p=1:100 y = k * x; k = k + 0.1; end obj.MyProp = k; end
Default values of properties are shared across all instances of an object. Two instances of a class can access the same default value if that property has not been overwritten by either instance.
Do not use character vector comparisons or character vector-based switch statements in
stepImpl method. Instead, create a method handle in
setupImpl. This handle points to a method in the same class definition
file. Use that handle in a loop in
This example shows how to use method handles and cached local variables in a loop to
implement an efficient object. In
myMethod2 based on a character vector
comparison and assign the method handle to the
Because there is a loop in
stepImpl, assign the
pMethodHandle property to a local method handle,
myFun, and then use
myFun inside the loop.
classdef MyClass < matlab.System function setupImpl(obj) if strcmp(obj.Method, 'Method1') obj.pMethodHandle = @myMethod1; else obj.pMethodHandle = @myMethod2; end end function y = stepImpl(obj,x) myFun = obj.pMethodHandle; for p=1:1000 y = myFun(obj,x) end end end function y = myMethod1(x) y = x+1; end function y = myMethod2(x) y = x-1; end end
For System objects being included in Simulink, add the
attribute. This attribute sets all the
MutableImpl methods to return
false by default.
For information about System objects and code generation, see System Objects in MATLAB Code Generation (MATLAB Coder).