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 N4JS Design Specification

4. Tips and Tricks

In this chapter we collect some coding hints and guidelines on how to properly use the APIs of Eclipse, EMF, Xtext and other dependencies we are using, as well as our own utilities and helpers.

This chapter is only about coding; add information on things like Eclipse setup or Maven/Jenkins to one of the preceding chapters. Similarly, this chapter is intended to provide just a quick overview, check-list and reminder; add detailed information and diagrams to one of the succeeding chapters.

4.1. Naming

• The internal handling of N4JS project names is non-trivial (due to the support for npm scopes), see API documentation of ProjectDescriptionUtils#isProjectNameWithScope(String) for a detailed overview. In short:

• IN4JSProject#getProjectName() and IProject#getName() return different values!

• Avoid using the Eclipse project name, i.e. the return value of IProject#getName(), as far as possible (only use it in UI code when actually dealing with what is shown in the Eclipse UI).

• The last segment of an URI or path pointing to an N4JS project is not always the project name; use utilities in ProjectDescriptionUtils instead, e.g. #deriveN4JSProjectNameFromURI()! (However, given an URI or path pointing to a file inside an N4JS project, you can use its last segment to obtain the file name.)

4.2. Logging

In many situations developer needs to use some kind of logging. When in need, follow these rules:

1. Use org.apache.log4j.Logger; for logging. Other logging utilities (like java built in logger) are not configured.

2. do not use System.out nor Sysetem.err for logging. It is ok to use it for debugging purposes, but those calls should never be merged to master. (with exception of headless compiler, which uses them explicitly)

3. There is central logger configuration in org.eclipse.n4js.utils.logging (and org.eclipse.n4js.utils.logging) that should be used

1. log4j.xml used for production

2. log4j_tests.xml used when running tests

4. in Eclipse run configurations logger has to be set properly, e.g. log4j.configuration=file:${workspace_loc:org.eclipse.n4js.utils.logging/log4j_tests.xml} 5. in maven configurations logger has to be set separately, e.g. -Dlog4j.configuration="file:${basedir}/../../plugins/org.eclipse.n4js.utils.logging/log4j_tests.xml

4.3. Cancellation Handling

At various occasions, Xtext provides an instance of class CancelIndicator to allow our code to handle cancellation of long-running task.

Some things to keep in mind:

• whenever a CancelIndicator is available any code that might not return immediately should implement proper cancellation handling (as explained in the next items).

• most importantly: reacting to a cancellation by returning early from a method is an anti-pattern that leads to problems (client code might continue work on a canceled and thus invalid state); instead: throw an OperationCanceledException!

• don’t use CancelIndicator#isCanceled() for cancellation handling, except in certain special cases. A valid exception case might be during logging to show a message like "operation was canceled".

• instead, inject the Xtext service called OperationCanceledManager and invoke its method #checkCanceled(), passing-in the cancel indicator (this method is null-safe; it will throw an OperationCanceledException in case a cancellation has occurred). Don’t directly create and throw an OperationCanceledException yourself.

• use the other methods provided by OperationCanceledManager when appropriate (see code of that class for details).

• in try/catch blocks, when catching exceptions of a super type of OperationCanceledException, be sure to not suppress cancellation exceptions. For example:

// Java code
@Inject private OperationCanceledManager operationCanceledManager;
/** Returns true on success, false otherwise. */
public boolean doSomething(CancelIndicator ci) {
try {
// do something that might be canceled
return true;
} catch(Exception e) {
operationCanceledManager.propagateIfCancelException(e); // <- IMPORTANT!
return false;
}
}

Try/finally blocks, on the other hand, do not need any special handling.

• a cancel indicator can also be stored in the rule environment (see RuleEnvironmentExtensions#addCancelIndicator()). This means:

• if you create a rule environment completely from scratch and you have a cancel indicator at hand, add it to the rule environment via RuleEnvironmentExtensions#addCancelIndicator() (not required when using RuleEnvironmentExtensions#wrap() for deriving a rule environment from an existing one).

• if you have a rule environment available, be sure to use its cancel indicator in long-running operations, i.e. with code like:

// Xtend code
import static extension org.eclipse.n4js.typesystem.utils.RuleEnvironmentExtensions.*
class C {
@Inject private OperationCanceledManager operationCanceledManager;
def void doSomething() {
for(a : aLotOfStuff) {
operationCanceledManager.checkCanceled(G.cancelIndicator);
// main work ...
}
}

4.4. Caching

• Caching of external libraries (implemented in ExternalProjectMappings)

• update only using EclipseExternalLibraryWorkspace#updateState()

• always mind that the diff of current state and cached state is a necessary information for cleaning dependencies of removed npms

• see EclipseExternalIndexSynchronizer#synchronizeNpms() for implementation

• updating also happens when external root locations change (see ExternalIndexUpdater)

• Caching of user workspace projects (implemented in MuliCleartriggerCache)

• caches only some project information and should be refactored along with Core, Model and EclipseBasedN4JSWorkspace

4.5. Dependency Injection

There are some things to keep in mind when using dependency injection in the context of Xtext. This is a longer topic and it is discussed in the appendix Xtext Injection.

4.6. Miscellaneous

• Resource load states: when an N4JS/N4JSD file is loaded, a certain sequence of processing is triggered (parsing, linking, validation, etc.) and thus an N4JSResource transitions through a sequence of "load states". For details, see N4JS Resource Load States.