Obfuscate private fields using ProGuard

Question

I'm using ProGuard in AndroidStudio 1.2.1.1 with Gradle 1.2.3.

My Gradle's release build is configured like so:

minifyEnabled true
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
shrinkResources true

I would like the private fields of classes to be obfuscated.

Here is my proguard config file (after many tries) as of now:

-allowaccessmodification
-dontskipnonpubliclibraryclasses
-dontskipnonpubliclibraryclassmembers
-renamesourcefileattribute SourceFile
-keepattributes SourceFile,LineNumberTable
-repackageclasses ''
-verbose
[...]

But I end up, after decompiling with androdd from AndroidGuard, with:

private com.google.android.gms.common.api.GoogleApiClient googleApiClient;

I know the use of this obfuscation is limited, but I would like googleApiClient to be renamed by ProGuard. How to do so?

Here is the refcard.

Is there any way to do the opposite of -keepclassmembernames?

Answer 1

Getting from this: How to tell ProGuard to keep private fields without specifying each field

According to ProGuard documenation the wildcard matches any field.

Top of that, You can use negators (!). (http://proguard.sourceforge.net/#manual/usage.html)

Attribute names can contain ?, *, and ** wildcards, and they can be preceded by the ! negator.

I am not so experienced in this field, so rather it is a guess, but easier to write in a new comment. Something like this should do the job (NOT TESTED):

-keepclassmembers class * { //should find all classes 
    !private <fields>;    
    <methods>;
    <init>; //and keep every field, method, constructor apart from private fields
}

May be you can use like this, but do not sure how it works with a negator first:

-keepclassmembers class * { //should find all classes 
    !private <fields>;    
    *; //should exclude everything except private fields, which should be obfuscated.
}