@Override
    public CharSequence flatten(long timeMillis, int logLevel, String tag, String message) {
        String flattenedLog = pattern;
        for (ParameterFiller parameterFiller : parameterFillers) {
            flattenedLog = parameterFiller.fill(flattenedLog, timeMillis, logLevel, tag, message);
        }
        return flattenedLog;
    }
Try to create a message filler if the given parameter is a message parameter.

@return created message filler, or null if the given parameter is not a message parameter
android/android_framework/base/src/main/java/com/elvishew/xlog/flattener/PatternFlattener.java