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1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 // This class assists you in dealing with a specific situation when managing
6 // ownership between a C++ object and a GTK widget.  It is common to have a
7 // C++ object which encapsulates a GtkWidget, and that widget is exposed from
8 // the object for use outside of the class.  In this situation, you commonly
9 // want the GtkWidget's lifetime to match its C++ object's lifetime.  Using an
10 // OwnedWigetGtk will take ownership over the initial reference of the
11 // GtkWidget, so that it is "owned" by the C++ object.  Example usage:
12 //
13 // class FooViewGtk() {
14 //  public:
15 //   FooViewGtk() { }
16 //   ~FooViewGtk() { }
17 //   void Init() { vbox_.Own(gtk_vbox_new()); }
18 //   GtkWidget* widget() { return vbox_.get() };  // Host my widget!
19 //  private:
20 //   OwnedWidgetGtk vbox_;
21 // };
22 //
23 // This design will ensure that the widget stays alive from the call to Own()
24 // until the call to Destroy().
25 //
26 // - Details of the problem and OwnedWidgetGtk's solution:
27 // In order to make passing ownership more convenient for newly created
28 // widgets, GTK has a concept of a "floating" reference.  All GtkObjects (and
29 // thus GtkWidgets) inherit from GInitiallyUnowned.  When they are created, the
30 // object starts with a reference count of 1, but has its floating flag set.
31 // When it is put into a container for the first time, that container will
32 // "sink" the floating reference, and the count will still be 1.  Now the
33 // container owns the widget, and if we remove the widget from the container,
34 // the widget is destroyed.  This style of ownership often causes problems when
35 // you have an object encapsulating the widget.  If we just use a raw
36 // GtkObject* with no specific ownership management, we push the widget's
37 // ownership onto the user of the class.  Now the C++ object can't depend on
38 // the widget being valid, since it doesn't manage its lifetime.  If the widget
39 // was removed from a container, removing its only reference, it would be
40 // destroyed (from the C++ object's perspective) unexpectantly destroyed.  The
41 // solution is fairly simple, make sure that the C++ object owns the widget,
42 // and thus it is also responsible for destroying it.  This boils down to:
43 //   GtkWidget* widget = gtk_widget_new();
44 //   g_object_ref_sink(widget);  // Claim the initial floating reference.
45 //   ...
46 //   gtk_destroy_widget(widget);  // Ask all code to destroy their references.
47 //   g_object_unref(widget);  // Destroy the initial reference we had claimed.
48 
49 #ifndef CHROME_BROWSER_UI_GTK_OWNED_WIDGET_GTK_H_
50 #define CHROME_BROWSER_UI_GTK_OWNED_WIDGET_GTK_H_
51 #pragma once
52 
53 #include "base/basictypes.h"
54 
55 typedef struct _GtkWidget GtkWidget;
56 
57 class OwnedWidgetGtk {
58  public:
59   // Create an instance that isn't managing any ownership.
OwnedWidgetGtk()60   OwnedWidgetGtk() : widget_(NULL) { }
61   // Create an instance that owns |widget|.
OwnedWidgetGtk(GtkWidget * widget)62   explicit OwnedWidgetGtk(GtkWidget* widget) : widget_(NULL) { Own(widget); }
63 
64   ~OwnedWidgetGtk();
65 
66   // Return the currently owned widget, or NULL if no widget is owned.
get()67   GtkWidget* get() const { return widget_; }
68   GtkWidget* operator->() const { return widget_; }
69 
70   // Takes ownership of a widget, by taking the initial floating reference of
71   // the GtkWidget. It is expected that Own() is called right after the widget
72   // has been created, and before any other references to the widget might have
73   // been added. It is valid to never call Own(), in which case Destroy() will
74   // do nothing. If Own() has been called, you must explicitly call Destroy().
75   void Own(GtkWidget* widget);
76 
77   // You may call Destroy() after you have called Own(). Calling Destroy()
78   // will call gtk_widget_destroy(), and drop our reference to the widget.
79   // Destroy() is also called in this object's destructor.
80   // After a call to Destroy(), you may call Own() again. NOTE: It is expected
81   // that after gtk_widget_destroy we will be holding the only reference left
82   // on the object. We assert this in debug mode to help catch any leaks.
83   void Destroy();
84 
85  private:
86   GtkWidget* widget_;
87 
88   DISALLOW_COPY_AND_ASSIGN(OwnedWidgetGtk);
89 };
90 
91 #endif  // CHROME_BROWSER_UI_GTK_OWNED_WIDGET_GTK_H_
92