Implement Swift wrappers for Java enum types

[DougGregor]

Java enum types are encoded as Java classes with various constants for their runtime value. At present, Java2Swift imports these just like any other Java class.

We should try to produce Swift wrappers that either are enums or feel like enums, with support for switches and easy identification of cases.

[jrosen081]

Do we have an easy way to get the JNIEnvironment statically (instead of as an instance property)? The one thing I've been struggling to design about this is the way I thought of modeling this was one of the following:

1. static var on the java struct representation (but the issue is we can't get the JNIEnvironment statically I don't think)
2. instance property on JavaClass (the issue here is that from my understanding the only way to get the JavaClass is to already have an instance of the type, which kind of ruins the point)

[DougGregor]

I don't know about "easy". It looks one can get a JNIEnvironment for the currently-executing thread using the JNI_GetCreatedJavaVMs to get the JavaVM instance and then GetEnv to get the JNIEnvironment. If that works... actually, if that works, we could possibly drop the JNIEnvironment argument to initializers (or make it optional) and use this as a default.

[DougGregor]

To follow up on my comments yesterday, it actually wasn't that bad at all to introduce an operation that wraps GetEnv to get the JNI environment for the currently-executing thread. The pull request for that step is at #69.

[DougGregor]

I filed away the idea about initializers as #70

[jrosen081]

After playing around with this, enums seem to have their enum cases as static fields, so now that we have #61, we do get these for free (for example here is how java.lang.annotation.ElementType gets imported):

// Auto-generated by Java-to-Swift wrapper generator.
import JavaKit
import JavaRuntime

@JavaClass("java.lang.annotation.ElementType")
public struct ElementType {
  @JavaMethod
  public func name() -> String

  ... more things from object ...
}
extension JavaClass<ElementType> {
  @JavaStaticField
  public var TYPE: ElementType?

  @JavaStaticField
  public var FIELD: ElementType?

  @JavaStaticField
  public var METHOD: ElementType?

  @JavaStaticField
  public var PARAMETER: ElementType?

  @JavaStaticField
  public var CONSTRUCTOR: ElementType?

  @JavaStaticField
  public var LOCAL_VARIABLE: ElementType?

  @JavaStaticField
  public var ANNOTATION_TYPE: ElementType?

  @JavaStaticField
  public var PACKAGE: ElementType?

  @JavaStaticField
  public var TYPE_PARAMETER: ElementType?

  @JavaStaticField
  public var TYPE_USE: ElementType?

  @JavaStaticField
  public var MODULE: ElementType?

  @JavaStaticField
  public var RECORD_COMPONENT: ElementType?

  @JavaStaticMethod
  public func values() -> [ElementType?]

  @JavaStaticMethod
  public func valueOf(_ arg0: String) -> ElementType?
}

(notice that the RECORD_COMPONENT is defined here as one of the enum cases). I'm thinking one way to make this a bit nicer would be to make these specific fields non-nullable (and now that we can get the environment we can even create static properties on the base ElementType struct in this example). I can put up a pr to add this to make it a bit nicer to use