After forking/cloning the source code repository from https://github.com/citrusframework/citrus-simulator you must build the application locally with Maven at least once:
mvn clean install
Add the -DskipFrontend=false parameter to include the simulator-ui into the build.
This will compile all classes and generate constructors as well as getters and setters using Project Lombok.
You will have to install Project Lombok in your IDE for an active development!
You can start any of the sample simulator projects using maven:
mvn -pl simulator-samples/sample-rest spring-boot:run
Use the angular-cli dev-server for active development or a short round trip.
mvn -pl simulator-ui package -Pdevelopment
This will automatically compile and watch the source files (e.g. *.js, *.ts, *.css). They will be copied to the Maven target folder too. The running spring-boot application is able to automatically grab the newly compiled sources. Just go to the browser and hit refresh to see the changes. If you change server Java sources spring-boot automatically restarts the web application so you may just hit refresh in your browser, too.
The development server is running on its own port 4200 (http://localhost:4200). To avoid cors issues an api proxy to the backend is provided out of the box. You can configure the proxy settings in proxy.conf.json.
Make sure to start one of the sample projects in order to have a server running which the UI can connect to.