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diff --git a/dom/ipc/TabContext.h b/dom/ipc/TabContext.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_TabContext_h
+#define mozilla_dom_TabContext_h
+
+#include "mozIApplication.h"
+#include "nsCOMPtr.h"
+#include "mozilla/BasePrincipal.h"
+#include "nsPIDOMWindow.h"
+#include "nsPIWindowRoot.h"
+
+namespace mozilla {
+namespace dom {
+
+class IPCTabContext;
+
+/**
+ * TabContext encapsulates information about an iframe that may be a mozbrowser
+ * or mozapp.  You can ask whether a TabContext corresponds to a mozbrowser or
+ * mozapp, get the app that contains the browser, and so on.
+ *
+ * TabParent and TabChild both inherit from TabContext, and you can also have
+ * standalone TabContext objects.
+ *
+ * This class is immutable except by calling one of the protected
+ * SetTabContext*() methods (and those methods can only be called once).  See
+ * also MutableTabContext.
+ */
+class TabContext
+{
+public:
+  TabContext();
+
+  /* (The implicit copy-constructor and operator= are fine.) */
+
+  /**
+   * Generates IPCTabContext of type BrowserFrameIPCTabContext or
+   * AppFrameIPCTabContext from this TabContext's information.
+   */
+  IPCTabContext AsIPCTabContext() const;
+
+  /**
+   * Does this TabContext correspond to a mozbrowser?
+   *
+   * <iframe mozbrowser mozapp> and <xul:browser> are not considered to be
+   * mozbrowser elements.
+   *
+   * If IsMozBrowserElement() is true, HasOwnApp() and HasAppOwnerApp() are
+   * guaranteed to be false.
+   *
+   * If IsMozBrowserElement() is false, HasBrowserOwnerApp() is guaranteed to be
+   * false.
+   */
+  bool IsMozBrowserElement() const;
+
+  /**
+   * Does this TabContext correspond to an isolated mozbrowser?
+   *
+   * <iframe mozbrowser mozapp> and <xul:browser> are not considered to be
+   * mozbrowser elements.  <iframe mozbrowser noisolation> does not count as
+   * isolated since isolation is disabled.  Isolation can only be disabled by
+   * chrome pages.
+   */
+  bool IsIsolatedMozBrowserElement() const;
+
+  /**
+   * Does this TabContext correspond to a mozbrowser or mozapp?  This is
+   * equivalent to IsMozBrowserElement() || HasOwnApp().  Returns false for
+   * <xul:browser>, which is neither a mozbrowser nor a mozapp.
+   */
+  bool IsMozBrowserOrApp() const;
+
+  /**
+   * OwnAppId() returns the id of the app which directly corresponds to this
+   * context's frame.  GetOwnApp() returns the corresponding app object, and
+   * HasOwnApp() returns true iff GetOwnApp() would return a non-null value.
+   *
+   * If HasOwnApp() is true, IsMozBrowserElement() is guaranteed to be
+   * false.
+   */
+  uint32_t OwnAppId() const;
+  already_AddRefed<mozIApplication> GetOwnApp() const;
+  bool HasOwnApp() const;
+
+  /**
+   * BrowserOwnerAppId() gets the ID of the app which contains this browser
+   * frame.  If this is not a mozbrowser frame (if !IsMozBrowserElement()), then
+   * BrowserOwnerAppId() is guaranteed to return NO_APP_ID.
+   *
+   * Even if we are a browser frame, BrowserOwnerAppId() may still return
+   * NO_APP_ID, if this browser frame is not contained inside an app.
+   */
+  uint32_t BrowserOwnerAppId() const;
+  already_AddRefed<mozIApplication> GetBrowserOwnerApp() const;
+  bool HasBrowserOwnerApp() const;
+
+  /**
+   * AppOwnerAppId() gets the ID of the app which contains this app frame.  If
+   * this is not an app frame (i.e., if !HasOwnApp()), then AppOwnerAppId() is
+   * guaranteed to return NO_APP_ID.
+   *
+   * Even if we are an app frame, AppOwnerAppId() may still return NO_APP_ID, if
+   * this app frame is not contained inside an app.
+   */
+  uint32_t AppOwnerAppId() const;
+  already_AddRefed<mozIApplication> GetAppOwnerApp() const;
+  bool HasAppOwnerApp() const;
+
+  /**
+   * OwnOrContainingAppId() gets the ID of this frame, if HasOwnApp().  If this
+   * frame does not have its own app, it gets the ID of the app which contains
+   * this frame (i.e., the result of {Browser,App}OwnerAppId(), as applicable).
+   */
+  uint32_t OwnOrContainingAppId() const;
+  already_AddRefed<mozIApplication> GetOwnOrContainingApp() const;
+  bool HasOwnOrContainingApp() const;
+
+  /**
+   * OriginAttributesRef() returns the DocShellOriginAttributes of this frame to
+   * the caller. This is used to store any attribute associated with the frame's
+   * docshell, such as the AppId.
+   */
+  const DocShellOriginAttributes& OriginAttributesRef() const;
+
+  /**
+   * Returns the presentation URL associated with the tab if this tab is
+   * created for presented content
+   */
+  const nsAString& PresentationURL() const;
+
+  UIStateChangeType ShowAccelerators() const;
+  UIStateChangeType ShowFocusRings() const;
+
+protected:
+  friend class MaybeInvalidTabContext;
+
+  /**
+   * These protected mutator methods let you modify a TabContext once.  Further
+   * attempts to modify a given TabContext will fail (the method will return
+   * false).
+   *
+   * These mutators will also fail if the TabContext was created with anything
+   * other than the no-args constructor.
+   */
+
+  /**
+   * Set this TabContext to match the given TabContext.
+   */
+  bool SetTabContext(const TabContext& aContext);
+
+  /**
+   * Set the tab context's origin attributes to a private browsing value.
+   */
+  void SetPrivateBrowsingAttributes(bool aIsPrivateBrowsing);
+
+  /**
+   * Set the TabContext for this frame. This can either be:
+   *  - an app frame (with the given own app) inside the given owner app. Either
+   *    apps can be null.
+   *  - a browser frame inside the given owner app (which may be null).
+   *  - a non-browser, non-app frame. Both own app and owner app should be null.
+   */
+  bool SetTabContext(bool aIsMozBrowserElement,
+                     bool aIsPrerendered,
+                     mozIApplication* aOwnApp,
+                     mozIApplication* aAppFrameOwnerApp,
+                     UIStateChangeType aShowAccelerators,
+                     UIStateChangeType aShowFocusRings,
+                     const DocShellOriginAttributes& aOriginAttributes,
+                     const nsAString& aPresentationURL);
+
+  /**
+   * Modify this TabContext to match the given TabContext.  This is a special
+   * case triggered by nsFrameLoader::SwapWithOtherRemoteLoader which may have
+   * caused the owner content to change.
+   *
+   * This special case only allows the field `mIsMozBrowserElement` to be
+   * changed.  If any other fields have changed, the update is ignored and
+   * returns false.
+   */
+  bool UpdateTabContextAfterSwap(const TabContext& aContext);
+
+  /**
+   * Whether this TabContext is in prerender mode.
+   */
+  bool mIsPrerendered;
+
+private:
+  /**
+   * Has this TabContext been initialized?  If so, mutator methods will fail.
+   */
+  bool mInitialized;
+
+  /**
+   * Whether this TabContext corresponds to a mozbrowser.
+   *
+   * <iframe mozbrowser mozapp> and <xul:browser> are not considered to be
+   * mozbrowser elements.
+   */
+  bool mIsMozBrowserElement;
+
+  /**
+   * This TabContext's own app.  If this is non-null, then this
+   * TabContext corresponds to an app, and mIsMozBrowserElement must be false.
+   */
+  nsCOMPtr<mozIApplication> mOwnApp;
+
+  /**
+   * This TabContext's containing app.  If mIsMozBrowserElement, this
+   * corresponds to the app which contains the browser frame; otherwise, this
+   * corresponds to the app which contains the app frame.
+   */
+  nsCOMPtr<mozIApplication> mContainingApp;
+
+  /*
+   * Cache of mContainingApp->GetLocalId().
+   */
+  uint32_t mContainingAppId;
+
+  /**
+   * DocShellOriginAttributes of the top level tab docShell
+   */
+  DocShellOriginAttributes mOriginAttributes;
+
+  /**
+   * The requested presentation URL.
+   */
+  nsString mPresentationURL;
+
+  /**
+   * Keyboard indicator state (focus rings, accelerators).
+   */
+  UIStateChangeType mShowAccelerators;
+  UIStateChangeType mShowFocusRings;
+};
+
+/**
+ * MutableTabContext is the same as MaybeInvalidTabContext, except the mutation
+ * methods are public instead of protected.  You can still only call these
+ * mutation methods once on a given object.
+ */
+class MutableTabContext : public TabContext
+{
+public:
+  bool SetTabContext(const TabContext& aContext)
+  {
+    return TabContext::SetTabContext(aContext);
+  }
+
+  bool
+  SetTabContext(bool aIsMozBrowserElement,
+                bool aIsPrerendered,
+                mozIApplication* aOwnApp,
+                mozIApplication* aAppFrameOwnerApp,
+                UIStateChangeType aShowAccelerators,
+                UIStateChangeType aShowFocusRings,
+                const DocShellOriginAttributes& aOriginAttributes,
+                const nsAString& aPresentationURL = EmptyString())
+  {
+    return TabContext::SetTabContext(aIsMozBrowserElement,
+                                     aIsPrerendered,
+                                     aOwnApp,
+                                     aAppFrameOwnerApp,
+                                     aShowAccelerators,
+                                     aShowFocusRings,
+                                     aOriginAttributes,
+                                     aPresentationURL);
+  }
+};
+
+/**
+ * MaybeInvalidTabContext is a simple class that lets you transform an
+ * IPCTabContext into a TabContext.
+ *
+ * The issue is that an IPCTabContext is not necessarily valid; for example, it
+ * might specify an app-id which doesn't exist.  So to convert an IPCTabContext
+ * into a TabContext, you construct a MaybeInvalidTabContext, check whether it's
+ * valid, and, if so, read out your TabContext.
+ *
+ * Example usage:
+ *
+ *   void UseTabContext(const TabContext& aTabContext);
+ *
+ *   void CreateTab(const IPCTabContext& aContext) {
+ *     MaybeInvalidTabContext tc(aContext);
+ *     if (!tc.IsValid()) {
+ *       NS_ERROR(nsPrintfCString("Got an invalid IPCTabContext: %s",
+ *                                tc.GetInvalidReason()));
+ *       return;
+ *     }
+ *     UseTabContext(tc.GetTabContext());
+ *   }
+ */
+class MaybeInvalidTabContext
+{
+public:
+  /**
+   * This constructor copies the information in aContext and sets IsValid() as
+   * appropriate.
+   */
+  explicit MaybeInvalidTabContext(const IPCTabContext& aContext);
+
+  /**
+   * Was the IPCTabContext we received in our constructor valid?
+   */
+  bool IsValid();
+
+  /**
+   * If IsValid(), this function returns null.  Otherwise, it returns a
+   * human-readable string indicating why the IPCTabContext passed to our
+   * constructor was not valid.
+   */
+  const char* GetInvalidReason();
+
+  /**
+   * If IsValid(), this function returns a reference to a TabContext
+   * corresponding to the IPCTabContext passed to our constructor.  If
+   * !IsValid(), this function crashes.
+   */
+  const TabContext& GetTabContext();
+
+private:
+  MaybeInvalidTabContext(const MaybeInvalidTabContext&) = delete;
+  MaybeInvalidTabContext& operator=(const MaybeInvalidTabContext&) = delete;
+
+  const char* mInvalidReason;
+  MutableTabContext mTabContext;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif