Stale content (#184)

* Let Session know it needs to reload

It is helpful to let other Session instances know that data on the
server changed and what they are rendering might be out of date. This is
mostly useful when there are tabs.

* Public functions to clear snapshot cache or reload

Source/Session/Session.swift

@@ -14,6 +14,8 @@ public class Session: NSObject {
     private lazy var bridge = WebViewBridge(webView: webView)
     private var initialized = false
     private var refreshing = false
+    private var isShowingStaleContent = false
+    private var isSnapshotCacheStale = false
 
     /// Automatically creates a web view with the passed-in configuration
     public convenience init(webViewConfiguration: WKWebViewConfiguration? = nil) {
@@ -91,6 +93,18 @@ public class Session: NSObject {
         bridge.clearSnapshotCache()
     }
 
+    // MARK: Caching
+
+    /// Clear the snapshot cache the next time the visitable view appears.
+    public func markSnapshotCacheAsStale() {
+        isSnapshotCacheStale = true
+    }
+
+    /// Reload the `Session` the next time the visitable view appears.
+    public func markContentAsStale() {
+        isShowingStaleContent = true
+    }
+
     // MARK: Visitable activation
 
     private var activatedVisitable: Visitable?
@@ -228,6 +242,12 @@ extension Session: VisitableDelegate {
         } else if visitable !== topmostVisit.visitable {
             // Navigating backward
             visit(visitable, action: .restore)
+        } else if isShowingStaleContent {
+            reload()
+            isShowingStaleContent = false
+        } else if isSnapshotCacheStale {
+            clearSnapshotCache()
+            isSnapshotCacheStale = false
         }
     }