diff --git a/GoogleDataLogger/GoogleDataLogger/Classes/GDLRegistrar.m b/GoogleDataLogger/GoogleDataLogger/Classes/GDLRegistrar.m
index 8520dbfd17f..d88508bde0f 100644
--- a/GoogleDataLogger/GoogleDataLogger/Classes/GDLRegistrar.m
+++ b/GoogleDataLogger/GoogleDataLogger/Classes/GDLRegistrar.m
@@ -16,6 +16,8 @@
 
 #import "GDLRegistrar.h"
 
+#import "GDLRegistrar_Private.h"
+
 @implementation GDLRegistrar
 
 + (instancetype)sharedInstance {
@@ -27,13 +29,22 @@ + (instancetype)sharedInstance {
   return sharedInstance;
 }
 
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _logTargetToPrioritizer = [[NSMutableDictionary alloc] init];
+    _logTargetToBackend = [[NSMutableDictionary alloc] init];
+  }
+  return self;
+}
+
 - (void)registerBackend:(id<GDLLogBackend>)backend forLogTarget:(NSInteger)logTarget {
-  // TODO
+  self.logTargetToBackend[@(logTarget)] = backend;
 }
 
 - (void)registerLogPrioritizer:(id<GDLLogPrioritizer>)prioritizer
                   forLogTarget:(NSInteger)logTarget {
-  // TODO
+  self.logTargetToPrioritizer[@(logTarget)] = prioritizer;
 }
 
 @end
diff --git a/GoogleDataLogger/GoogleDataLogger/Classes/Private/GDLRegistrar_Private.h b/GoogleDataLogger/GoogleDataLogger/Classes/Private/GDLRegistrar_Private.h
index 0ee36e16a61..c05b5cdfe5e 100644
--- a/GoogleDataLogger/GoogleDataLogger/Classes/Private/GDLRegistrar_Private.h
+++ b/GoogleDataLogger/GoogleDataLogger/Classes/Private/GDLRegistrar_Private.h
@@ -19,9 +19,9 @@
 @interface GDLRegistrar ()
 
 /** A map of logTargets to backend implementations. */
-@property(nonatomic) NSDictionary<NSNumber *, id<GDLLogBackend>> *logTargetToBackend;
+@property(nonatomic) NSMutableDictionary<NSNumber *, id<GDLLogBackend>> *logTargetToBackend;
 
 /** A map of logTargets to prioritizer implementations. */
-@property(nonatomic) NSDictionary<NSNumber *, id<GDLLogPrioritizer>> *logTargetToPrioritizer;
+@property(nonatomic) NSMutableDictionary<NSNumber *, id<GDLLogPrioritizer>> *logTargetToPrioritizer;
 
 @end
diff --git a/GoogleDataLogger/GoogleDataLogger/Classes/Public/GDLLogBackend.h b/GoogleDataLogger/GoogleDataLogger/Classes/Public/GDLLogBackend.h
index 8c17ec57a65..e4638ee54b0 100644
--- a/GoogleDataLogger/GoogleDataLogger/Classes/Public/GDLLogBackend.h
+++ b/GoogleDataLogger/GoogleDataLogger/Classes/Public/GDLLogBackend.h
@@ -16,11 +16,13 @@
 
 #import <Foundation/Foundation.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 /** A convenient typedef to define the block to be called upon completion of an upload to the
  * backend.
  */
-typedef void (^GDLBackendCompletionBlock)(NSSet<NSURL *> *successfulUploads,
-                                          NSSet<NSURL *> *unsuccessfulUploads);
+typedef void (^GDLBackendCompletionBlock)(NSSet<NSURL *> *_Nullable successfulUploads,
+                                          NSSet<NSURL *> *_Nullable unsuccessfulUploads);
 
 /** This protocol defines the common interface for logging backend implementations. */
 @protocol GDLLogBackend <NSObject>
@@ -37,3 +39,5 @@ typedef void (^GDLBackendCompletionBlock)(NSSet<NSURL *> *successfulUploads,
 - (void)uploadLogs:(NSSet<NSURL *> *)logFiles onComplete:(GDLBackendCompletionBlock)onComplete;
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/GoogleDataLogger/GoogleDataLogger/Classes/Public/GoogleDataLogger.h b/GoogleDataLogger/GoogleDataLogger/Classes/Public/GoogleDataLogger.h
index 091625895eb..3c1ac580835 100644
--- a/GoogleDataLogger/GoogleDataLogger/Classes/Public/GoogleDataLogger.h
+++ b/GoogleDataLogger/GoogleDataLogger/Classes/Public/GoogleDataLogger.h
@@ -14,6 +14,10 @@
  * limitations under the License.
  */
 
+#import "GDLLogBackend.h"
 #import "GDLLogEvent.h"
+#import "GDLLogPrioritizer.h"
+#import "GDLLogProto.h"
 #import "GDLLogTransformer.h"
 #import "GDLLogger.h"
+#import "GDLRegistrar.h"
diff --git a/GoogleDataLogger/Tests/GDLClockTest.m b/GoogleDataLogger/Tests/GDLClockTest.m
new file mode 100644
index 00000000000..09f43daf5df
--- /dev/null
+++ b/GoogleDataLogger/Tests/GDLClockTest.m
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <XCTest/XCTest.h>
+
+#import "GDLClock.h"
+
+@interface GDLClockTest : XCTestCase
+
+@end
+
+@implementation GDLClockTest
+
+/** Tests the default initializer. */
+- (void)testInit {
+  XCTAssertNotNil([[GDLClockTest alloc] init]);
+}
+
+@end