CB-13145: proposal for Play Services Version

proposals/PlayServicesVersion_Proposal.md

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+# Play Services Version Proposal
+
+- Status: Proposed
+
+## Overview
+This is a proposal to offer an alternative solution for [CB-13145](https://issues.apache.org/jira/browse/CB-13145).
+
+## Problem
+Plugins using different versions of Google Play Services or the `+` to indicate range cause gradle to error out with the following error:
+
+```
+> Failed to apply plugin [id 'com.google.gms.google-services']
+   > For input string: "+"
+```
+
+## Potential Solution
+Currently, the proposed solution is to create an Android only preference in `config.xml` to control the version of Google Play Services for the whole project. For instance, `config.xml` would look like this:
+`<preference name = "PlayServicesVersion" value = "11.0.0"/>`
+
+Then if, for example, the `phonegap-plugin-push` wanted to include FCM libraries in `plugin.xml`, it would contain:
+`<framework src="com.google.firebase:firebase-messaging:$PlayServicesVersion"/>`
+
+If the `cordova-plugin-google-analytics` wanted to include Play Services in its `plugin.xml` it would contain:
+`<framework src="com.google.android.gms:play-services-analytics:$PlayServicesVersion"/>`
+
+These plugins would both use the same version of Google Play Services. This gets around the problem of plugins using `+` in the framework tag (which isn't supported anymore) or having the two plugins pin different versions of Play Services (gradle errors with that).
+
+## Proposal
+We want to use a generic way to define variables in `config.xml` that `plugin.xml` can use. We propose extending the variable tag. Below is the current variable tag.
+
+```
+<plugin name="cordova-plugin-device" spec="^1.1.0">
+    <variable name="MY_VARIABLE" value="my_variable_value" />
+</plugin>
+
+```
+
+The current variable tag goes in and finds `MY_VARIABLE` in `plugin.xml` of `cordova-plugin-device` and replaces it with the value.
+
+In order to extend the variable tag, we will need to:
+
+1. Allow the variable tag to be declared outside of a plugin tag (not nested/root level).
+
+```
+<variable name="MY_VARIABLE" value="my_variable_value" />
+```
+
+2. Run it against every plugin as it is getting installed.
+
+The original solution assumes that both plugin authors are using the same preference name (PlayServicesVersion) and expects the application author to add it to the `config.xml`. 
+
+The plugin should auto add the variable it needs to `config.xml` if a default isn't there. So assuming we want `<variable name="PlayServicesVersion" value="11.0.1" />` in `config.xml`, it would get auto added by `plugin.xml` if it doesn't already exist. If it does exist, don't touch it.
+
+We are still dependent on the plugin authors to use the standardized variables. So in this case, both plugin authors would have to use PlayServicesVersion in their plugin.xml
+
+
+## Links
+* https://issues.apache.org/jira/browse/CB-13145
+* http://cordova.apache.org/docs/en/latest/config_ref/index.html