Auto complete for dependencies for pom.xml

[Eskibear]

The upstream issue is #47

An important feature to improve pom.xml editing experience is dependency auto-completion.
The basic use cases are:

1. For <groupId>[cursor]</groupId>, show a list of available group ids, e.g. from local maven repository.
2. For <artifactId>[cursor]</artifactId>, show a list of available artifact ids according to the value of groupId.

[Eskibear]

Experiences in Java IDEs

Eclipse photon

I don't see any completion for groupId or artifactId.

IntelliJ IDEA 2018.3

• For groupId, it always provides a list of all group ids which contains the keyword you've typed in.
• For artifactId, it lists artifacts of the given group id.
• For version, it lists versions in the same way above, together with two extra items ("RELEASE" and "LATEST")

Note that all the completion items are from local Maven repository. Every time you open a Maven project, it takes some time to update the Maven indices of your local repository. If you don't have an artifact installed into your local repository, it won't appear in the completion list.

[Eskibear]

We need context info of current active pom.xml. E.g. we need to know current cursor position, and if it's located in an artifactId node, the value of groupId is required to calculate available artifacts. And if it's located elsewhere, we don't need to provide the result.

1. provideCompletionItems has the cursor position as a param, so it's easy to get the whole text before the cursor.
2. xml-zero-lexer is a lexer which can tokenize the xml file into an array of tokens, so it's feasible to tell which node the cursor is in.

[Eskibear]

The current implementation requires to find all pom files in local Maven Repo, which introduces extra CPU & IO overload. It depends on the size of the local Maven repo, and I'm not sure whether it will lead to performance deterioration. So I think we can disable it by default, and add a flag in settings to enable the feature.

[Eskibear]

Artifact info from local maven repository can only provide limited help. Imagine a user who wants to add a some dependencies related with "docker", without installing it into local maven repository first, the users can not get related suggestions.

So next step is to retrieve information from Maven central repo in real time. By the REST API provided, there's big chance to improve the experience.

[Eskibear]

With PR #221 , it's able to consume data from Maven Central repository.
With PR #222 , it improves performance of fetching data from Maven local repository.

TODO

• Refine sorting of completion items, e.g. versions should be sorted semantically
• Use effective local repository instead of the hard-coded ~/.m2/repository