Ensure gif cells reload when app becomes active or network becomes available.

// FREEBIE

Signal/src/Signal-Bridging-Header.h

@@ -49,6 +49,7 @@
 #import <JSQMessagesViewController/UIColor+JSQMessages.h>
 #import <JSQSystemSoundPlayer.h>
 #import <PureLayout/PureLayout.h>
+#import <Reachability/Reachability.h>
 #import <SignalServiceKit/AppVersion.h>
 #import <SignalServiceKit/Asserts.h>
 #import <SignalServiceKit/Contact.h>

Signal/src/ViewControllers/GifPicker/GifPickerCell.swift

@@ -77,7 +77,7 @@ class GifPickerCell: UICollectionViewCell {
         clearFullAssetRequest()
     }
 
-    private func ensureCellState() {
+    public func ensureCellState() {
         guard isCellVisible else {
             // Cancel any outstanding requests.
             clearAssetRequests()

Signal/src/ViewControllers/GifPicker/GifPickerViewController.swift

@@ -27,6 +27,8 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
 
     var imageInfos = [GiphyImageInfo]()
 
+    var reachability: Reachability?
+
     private let kCellReuseIdentifier = "kCellReuseIdentifier"
 
     // MARK: Initializers
@@ -57,6 +59,38 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
         self.layout.delegate = self
     }
 
+    deinit {
+        NotificationCenter.default.removeObserver(self)
+    }
+
+    func didBecomeActive() {
+        AssertIsOnMainThread()
+
+        Logger.info("\(self.TAG) \(#function)")
+
+        // Prod cells to try to load when app becomes active.
+        ensureCellState()
+    }
+
+    func reachabilityChanged() {
+        AssertIsOnMainThread()
+
+        Logger.info("\(self.TAG) \(#function)")
+
+        // Prod cells to try to load when connectivity changes.
+        ensureCellState()
+    }
+
+    func ensureCellState() {
+        for cell in self.collectionView.visibleCells {
+            guard let cell = cell as? GifPickerCell else {
+                owsFail("\(TAG) unexpected cell.")
+                return
+            }
+            cell.ensureCellState()
+        }
+    }
+
     // MARK: View Lifecycle
 
     override func viewDidLoad() {
@@ -71,6 +105,16 @@ class GifPickerViewController: OWSViewController, UISearchBarDelegate, UICollect
                                                       comment: "Title for the 'gif picker' dialog.")
 
         createViews()
+
+        reachability = Reachability.forInternetConnection()
+        NotificationCenter.default.addObserver(self,
+                                               selector:#selector(reachabilityChanged),
+                                               name:NSNotification.Name.reachabilityChanged,
+                                               object:nil)
+        NotificationCenter.default.addObserver(self,
+                                               selector:#selector(didBecomeActive),
+                                               name:NSNotification.Name.UIApplicationDidBecomeActive,
+                                               object:nil)
     }
 
     override func viewDidAppear(_ animated: Bool) {

Signal/src/network/GifDownloader.swift

@@ -269,11 +269,18 @@ extension URLSessionTask {
                 self.startRequestIfNecessary()
                 return
             }
+            guard UIApplication.shared.applicationState == .active else {
+                // If app is not active, fail the asset request.
+                self.assetRequestDidFail(assetRequest:assetRequest)
+                self.startRequestIfNecessary()
+                return
+            }
+
             self.activeAssetRequests.insert(assetRequest)
 
             if let asset = self.assetMap.get(key:assetRequest.rendition.url) {
-                // Deferred cache hit, avoids re-downloading assets already in the
-                // asset cache.
+                // Deferred cache hit, avoids re-downloading assets that were
+                // downloaded while this request was queued.
 
                 self.assetRequestDidSucceed(assetRequest : assetRequest, asset: asset)
                 return
@@ -297,6 +304,10 @@ extension URLSessionTask {
 
         // Prefer the first "high" priority request, 
         // fall back to the first "low" priority request.
+        //
+        // TODO: We could refine this logic to defer requests if
+        //       there is already an active asset request with the
+        //       same URL.
         for priority in [GiphyRequestPriority.high, GiphyRequestPriority.low] {
             for (assetRequestIndex, assetRequest) in assetRequestQueue.enumerated() {
                 if assetRequest.priority == priority {