Clean up the link preview cache.

Signal/src/ViewControllers/ConversationView/ConversationInputToolbar.m

@@ -878,6 +878,10 @@ const CGFloat kMaxTextViewHeight = 98;
     [self ensureShouldShowVoiceMemoButtonAnimated:YES doLayout:YES];
     [self updateHeightWithTextView:textView];
     [self updateInputLinkPreview];
+
+    if (textView.text.ows_stripped.length < 1) {
+        [OWSLinkPreview clearLinkPreviewCache];
+    }
 }
 
 - (void)textViewDidChangeSelection:(UITextView *)textView

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -2752,6 +2752,8 @@ typedef enum : NSUInteger {
         AudioServicesPlaySystemSound(soundId);
     }
     [self.typingIndicators didSendOutgoingMessageInThread:self.thread];
+
+    [OWSLinkPreview clearLinkPreviewCache];
 }
 
 #pragma mark UIDocumentMenuDelegate

Signal/src/ViewControllers/HomeView/HomeViewController.m

@@ -709,6 +709,8 @@ typedef NS_ENUM(NSInteger, HomeViewControllerSection) {
     [self.searchResultsController viewDidAppear:animated];
 
     self.hasEverAppeared = YES;
+
+    [OWSLinkPreview clearLinkPreviewCache];
 }
 
 - (void)viewDidDisappear:(BOOL)animated

SignalServiceKit/src/Messages/Interactions/OWSLinkPreview.swift

@@ -553,6 +553,13 @@ public class OWSLinkPreview: MTLModel {
         }
     }
 
+    @objc
+    public class func clearLinkPreviewCache() {
+        return serialQueue.async {
+            linkPreviewDraftCache.removeAllObjects()
+        }
+    }
+
     @objc
     public class func tryToBuildPreviewInfoObjc(previewUrl: String?) -> AnyPromise {
         return AnyPromise(tryToBuildPreviewInfo(previewUrl: previewUrl))