IE 4.0 : Discarable Browser Property

This document describes one of new features of IE 4.0, which is called "discardable browser property", which allows ActiveX controls or documents to pass some information across pages or cache some data. This mechanism will be used in following cases:

• An ActiveX control in an HTML page gathers some data (such as a shopping list) from the user and passes it to another control in the subsequent pages.
• Two ActiveX controls in two different HTML pages wants to coordinate and play a music or an animation continuously when the user browse from one page to anotherl.
• An ActiveX control which retrieves some large data asynchronously from the server wants to cache it so that it does not have to download the same data again when the user visits the same or similar page.

How it works

IE 3.0 has a capability to store arbitrary data or objects (VARIANT) as properties of each browser window. Any programs which can access IWebBrowserApp interface can call its PutProperty and GetProperty member to store and retrieve those properties. This mechanism was not useful for ActiveX controls or documents because IE 3.0 browser did not have any capability to "expire" them. Once properties are added, it stays until the browser window is closed. To solve this problem, we added this mechanism which allows programmers to add discardable properties, which will be discarded automatically by the browser if it's not accessed for a certain amount of time.

In order to add properties as discardable, programmers need to package the data into a COM object which supports IDiscardableBrowserProperty interface. IDiscardableBrowserProperty interface is a dummy interface which has no additional member to IUnknown. It does not even have the interface definition. The programmer just need to responds to QueryInteraface(IID_IDiscardableBrowserProperty) and returns its IUnknown pointer.

When an ActiveX control store such a COM object as a discadable property, it should call PutProperty method with a GUID string to make it sure that the name is unique. When the same type of ActiveX control on the following page need to get that data, it should call GetProperty method with the same GUID string. If it's not expired, it will successfully return the COM object that is added by the first control.