Add getDefaultTag() so a Tree can have its own custom tag generation logic

timber/src/main/java/timber/log/Timber.java

@@ -403,10 +403,20 @@ String getTag() {
       String tag = explicitTag.get();
       if (tag != null) {
         explicitTag.remove();
+      } else {
+        tag = getDefaultTag();
       }
       return tag;
     }
 
+    /**
+     * If no tag is explicitly set, then the tree will fall back to this tag.
+     * @return The default tag for this tree
+     */
+    @Nullable protected String getDefaultTag() {
+      return null;
+    }
+
     /** Log a verbose message with optional format args. */
     public void v(String message, Object... args) {
       prepareLog(Log.VERBOSE, null, message, args);
@@ -587,7 +597,7 @@ protected abstract void log(int priority, @Nullable String tag, @NotNull String
   public static class DebugTree extends Tree {
     private static final int MAX_LOG_LENGTH = 4000;
     private static final int MAX_TAG_LENGTH = 23;
-    private static final int CALL_STACK_INDEX = 5;
+    private static final int CALL_STACK_INDEX = 6;
     private static final Pattern ANONYMOUS_CLASS = Pattern.compile("(\\$\\d+)+$");
 
     /**
@@ -612,11 +622,7 @@ protected String createStackElementTag(@NotNull StackTraceElement element) {
       return tag.substring(0, MAX_TAG_LENGTH);
     }
 
-    @Override final String getTag() {
-      String tag = super.getTag();
-      if (tag != null) {
-        return tag;
-      }
+    @Override protected String getDefaultTag() {
 
       // DO NOT switch this to Thread.getCurrentThread().getStackTrace(). The test will pass
       // because Robolectric runs them on the JVM but on Android the elements are different.

timber/src/test/java/timber/log/TimberTest.java

@@ -502,11 +502,55 @@ protected String formatMessage(@NotNull String message, @NotNull Object[] args)
         .hasDebugMessage("TimberTest", "Test formatting: Test message logged. 100");
   }
 
+
+  @Test public void logsWithCustomTag() {
+    Timber.plant(new Timber.DebugTree() {
+      @Override
+      protected String getDefaultTag() {
+        return "CUSTOMTAG";
+      }
+    });
+
+    Timber.d("Test with custom tag");
+    assertLog().hasDebugMessage("CUSTOMTAG", "Test with custom tag");
+
+  }
+
+  @Test public void logsWithCustomTagOverridden() {
+    Timber.plant(new Timber.DebugTree() {
+      @Override protected String getDefaultTag() {
+        return "CUSTOMTAG";
+      }
+    });
+
+    Timber.tag("NewTag").d("Tag manually set");
+
+    assertLog().hasDebugMessage("NewTag", "Tag manually set");
+  }
+
+  @Test public void logsWithMultipleTreesMultipleTags() {
+    Timber.plant(new Timber.DebugTree() {
+      @Override protected String getDefaultTag() {
+        return "CUSTOMTAG";
+      }
+    });
+
+    Timber.plant(new Timber.DebugTree() {
+      @Override protected String getDefaultTag() {
+        return "DIFFERENTTAG";
+      }
+    });
+
+    Timber.d("multiple tags");
+
+    assertLog().hasDebugMessage("CUSTOMTAG", "multiple tags")
+        .hasDebugMessage("DIFFERENTTAG", "multiple tags");
+  }
+
   @Test public void nullArgumentObjectArray() {
     Timber.plant(new Timber.DebugTree());
     Timber.v("Test", (Object[]) null);
-    assertLog()
-        .hasVerboseMessage("TimberTest", "Test")
+    assertLog().hasVerboseMessage("TimberTest", "Test")
         .hasNoMoreMessages();
   }