Clear LRUCache in background.

Signal/src/ViewControllers/DebugUI/DebugUIContacts.m

@@ -1,5 +1,5 @@
 //
-//  Copyright (c) 2017 Open Whisper Systems. All rights reserved.
+//  Copyright (c) 2018 Open Whisper Systems. All rights reserved.
 //
 
 #import "DebugUIContacts.h"
@@ -1164,7 +1164,7 @@ NS_ASSUME_NONNULL_BEGIN
     OWSAssert(count > 0);
     OWSAssert(batchCompletionHandler);
 
-    DDLogDebug(@"createRandomContactsBatch: %zd", count);
+    DDLogDebug(@"createRandomContactsBatch: %zu", count);
 
     CNAuthorizationStatus status = [CNContactStore authorizationStatusForEntityType:CNEntityTypeContacts];
     if (status == CNAuthorizationStatusDenied || status == CNAuthorizationStatusRestricted) {
@@ -1219,7 +1219,7 @@ NS_ASSUME_NONNULL_BEGIN
                          }
                      }
 
-                     DDLogError(@"Saving fake contacts: %zd", contacts.count);
+                     DDLogError(@"Saving fake contacts: %zu", contacts.count);
 
                      NSError *saveError = nil;
                      if (![store executeSaveRequest:request error:&saveError]) {

SignalMessaging/utils/LRUCache.swift

@@ -43,12 +43,22 @@ public class LRUCache<KeyType: Hashable & Equatable, ValueType> {
                                                selector: #selector(didReceiveMemoryWarning),
                                                name: NSNotification.Name.UIApplicationDidReceiveMemoryWarning,
                                                object: nil)
+        NotificationCenter.default.addObserver(self,
+                                               selector: #selector(didEnterBackground),
+                                               name: NSNotification.Name.OWSApplicationDidEnterBackground,
+                                               object: nil)
     }
 
     deinit {
         NotificationCenter.default.removeObserver(self)
     }
 
+    @objc func didEnterBackground() {
+        SwiftAssertIsOnMainThread(#function)
+
+        clear()
+    }
+
     @objc func didReceiveMemoryWarning() {
         SwiftAssertIsOnMainThread(#function)