[Android] impossible to use newGuid() between jni calls

[thedilletante]

Hi! Thank you for the great library again.
I faced the issue that newGuid() could not be called between separate jni calls or from different threads. It seems, AndroidGuidInfo.uuidClass must be global reference to allow such calls. Also, JniEnv * variable should be passed from the current context into newGuid() to operate with jni functions to prevent context mismatch.
Is it appropriate to me to open PR with implementation of this behaviour?

[graeme-hill]

You're right that the global state will screw you up on multiple threads. You could work around it by sending all the guid creation calls through the same thread like you would do in a lot of java libs when updating a GUI.

I think this is a loss of functionality from a recent change. When you used to have to create a GuidGenerator you could pass in the JNIEnv * so that would allow you to make one generator per thread each with its own JNIEnv *. The reason I made the change is that I wanted to keep it simple on non-android platforms and I wanted to avoid adding any params to newGuid because it should be used the same on every platform.

Since this lib already requires c++11 maybe we can just use thread_local (I haven't used it before so not 100% on how it works). Something like change this:

AndroidGuidInfo androidInfo;

to this:

thread_local AndroidGuidInfo androidInfo;

and then require that you call initJni once per thread instead of once globally.

[graeme-hill]

I can try this out myself when I get time (not sure when that will be) but I will also accept a PR as long as it doesn't make any change to non-android usage (like the change I suggested above).

[thedilletante]

Thank you for quick response and detailed advice.
I fully agree with you that a call to newGuid() should look the same on every platforms to simplify the usage.
I opened PR to support multithread invocations (#24). However, I decided not to use thread_local feature (though it would work perfectly) cause I think it would make the usage complicated - the user would have to care of only call to initJni() for every thread to use newGuid(). So, I implemented newGuid() overloading with JNIEnv pointer as an argument that should be called with appropriate pointer to the context if iniJni() was called from different thread.
I didn't manage to add reviewers to this PR and link it to this issue, so it would be good if you advise me how to do that.