MATLAB: How to check if there is a new response in the “Call Service” block using ROS Toolbox

ROS Toolbox

How to check if there is a new response in the "Call Service" block using ROS Toolbox?

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The "Call Service" block is "blocking". That means that the service client block does not finish processing a simulation time-step until it has sent a request and received a response from the service server. So technically, any output that is not indicating it has timed out is a new response from the service server.

This is a different way of functioning than the Subscribe block, where it will output the latest message receive, but will indicate when it has gotten a new message.

What the block doesn't tell you is if the values of the response message are different from the previous response received. If you need to do that, you will need to implement some kind of checks yourself in the model.

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MATLAB: Does MATLAB pass parameters using “call by value” or “call by reference”

For MATLAB built-in data types, MATLAB always passes arguments 'by value' in the sense that any changes made to input arguments within a function are not visible to the respective variables in the caller workspace.

However, since passing huge chunks of data 'by value' is inefficient, MATLAB internally optimizes for some cases and passes by reference and only creates local copies if modifications are being made to these arguments.

In the case of passing structures or cell arrays, only the field or cell data being modified by the function will be passed "by value". When the function returns, (assuming that the modified structure or cell array is returned as an output argument of the function and are captured in the calling workspace's structure) the calling workspace's copy of the structure or cell array is replaced by the function's copy such that only the changed fields are altered. This is done to make copying more efficient. For example:

% This will modify the structure in the calling function's workspace.
largeStructure = myFunc(largeStructure);
% This will NOT modify the structure in the calling function's workspace.
myFunc(largeStructure);
% Simple function
function [myStructure] = myFunc(myStructure)
     myStructure.x = myStructure.x^2;
end

For MATLAB classes, there is support for two kinds of classes: value classes and handle classes. Value classes act as though they were "passed by value" and Handle classes act as though they were "passed by reference".

MATLAB: Do I get an error “Call to download server failed(HTTP error code: 12029)” when installing MATLAB Web App Server on R2018b Prerelease

The recommended workflow for the prereleases is to use the web app files in the offline folder, for example on my Windows system that would be :

"C:\Program Files\MATLAB\R2018b\toolbox\compiler\deploy\win64\MATLABWebAppServerSetup\offline"

There is a zipped folder which contains the admin tool for Web App Setup and management. Please note that if you try to unzip in place, Windows will likely not let you because it is in Program Files. You might have to copy the zipped folder to a writable location to unzip.

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Related Solutions

MATLAB: Does MATLAB pass parameters using “call by value” or “call by reference”

MATLAB: Do I get an error “Call to download server failed(HTTP error code: 12029)” when installing MATLAB Web App Server on R2018b Prerelease

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