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Add explanations of session history structure
== Browsers, Frames, and Document Navigation ==
=== Docshell and Session History ===
The user of a Web browser can change the page shown in that browser in
One of the most interfaces docshell implemented is <code>[https://dxr.mozilla.org/mozilla-central/source/docshell/base/nsIWebNavigation.idl nsIWebNavigation]</code>. It defines major functions of a browsing context, such as <code>loadURI</code> / <code>goBack</code> / <code>goForward</code> and <code>reload</code>. In single process configuration of desktop Firefox, <code><xul:browser></code> (which represents a tab) operates on docshell through <code>nsIWebNavigation</code>, as shown in the right figure.
* code: mozilla/docshell/
* bugzilla: Core::Document Navigation
* documentation: [[DocShell:Home Page]]
=== Session History ===
In order to keep the session history of subframes after the root document is unloaded and docshells of subframes are destroyed, only the root docshell of a docshell tree manages the session history (this does not match the conceptual model in the HTML5 spec and may be subject to change).
As an example to explain the structure of session history implementation in Gecko, consider there's a tab A, which loads document 1; in document 1, there are iframe B & C, which loads document 2 & 3, respectively. Later, an user navigates iframe B to document 4. The following figures show the conceptual model of the example, and the corresponding session history structure.
{|
| [[File:BrowsingContextExample1.png|thumb|190px|Conceptual model representation; color denotes active documents]]
| [[File:SHistoryExample1.png|thumb|180px|Session history structure; color denotes current transaction]]
|}
In Gecko, a [https://dxr.mozilla.org/mozilla-central/source/docshell/shistory/nsSHistory.h session history] object holds a list of [https://dxr.mozilla.org/mozilla-central/source/docshell/shistory/nsSHTransaction.h transactions] (denoted as T1 & T2 in the figure); each transaction points to a tree of [https://dxr.mozilla.org/mozilla-central/source/docshell/shistory/nsSHEntry.h entries]; each entry records the docshell it associated to, and a URL. Now that if we navigate the tab to a different root document 5, which includes an iframe D with document 6, then it becomes:
{|
| [[File:BrowsingContextExample2.png|thumb|275px|Conceptual model representation]]
| [[File:SHistoryExample2.png|thumb|250px|Session history structure]]
|}
=== Embedding ===
