Microsoft KB Archive/167230

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HOWTO: Detect When Internet Explorer Holds Controls and Pages in Memory

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Q167230

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The information in this article applies to:


 * Microsoft Internet Explorer (Programming) versions 3.0, 3.01, 3.02

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SUMMARY
Internet Explorer maintains a most-recently used memory cache of up to four viewed pages in order to optimize navigation back and forth between the same set of pages. What this means to the Web site programmer is that ActiveX controls on a cached page stay loaded in memory for as long as that page remains in the "memory cache." Likewise, ActiveX Document servers remain open as long as that page is still held in memory.

MORE INFORMATION
For most controls and documents, this is not an issue. This optimization improves performance for browsing the Web site. However, an ActiveX control might need to know when its host Web document is no longer visible. For example, an ActiveX control that plays a background sound might want to turn that sound off when its host page is no longer visible. All forms of navigation demonstrate this behavior--clicking the Back or Forward buttons, typing a URL directly, and clicking on a link.

For ActiveX documents, the recommended method for determining that a page has just been hidden is to respond to the OLECMDID_STOP command sent through the IOleCommandTarget interface. MFC ActiveX document servers make it easy to detect this event: just add an entry to your OLECMD_MAP for OLECMDID_STOP. Use this entry to map OLECMDID_STOP to a command such as ID_OLECMD_STOP, which you can handle as usual.

When the ActiveX document is refreshed from the memory cache, OLECMDID_REFRESH is sent to IOleCommandTarget::Exec. This can be handled in an analogous fashion to OLECMDID_STOP. Here is an MFC example of using both:

BEGIN_MESSAGE_MAP(CYourDoc, COleServerDoc) // ... other stuff

// Add the two following entries to your control's message map. // Then, define ID_OLECMD_STOP and ID_OLECMD_REFRESH command // IDs in resource.h file. They are for OLECMDID_STOP // and OLECMDID_REFRESH command. ON_COMMAND(ID_OLECMD_STOP, OnStop) ON_COMMAND(ID_OLECMD_REFRESH, OnRefresh) END_MESSAGE_MAP

// Add the following OLECMD_MAP to the .cpp file of your // COleServerDoc-derived class: BEGIN_OLECMD_MAP(CYourDoc, COleServerDoc) ON_OLECMD(NULL, OLECMDID_STOP, ID_OLECMD_STOP) ON_OLECMD(NULL, OLECMDID_REFRESH, ID_OLECMD_REFRESH) END_OLECMD_MAP

// Add DECLARE_OLECMD_MAP to the .h file of your // COleServerDoc-derived class, and declarations and // definitions for the following two functions: oid CYourDoc::OnStop {   // This control's page was navigated away from, but still remains in    // the memory cache. Stop playing background sounds, animations, // timers. TRACE("Stopped!\n"); }

oid CYourDoc::OnRefresh {   // This control's page was navigated to and is being displayed // again. Do whatever is necessary: start playing sounds, // animations, timers TRACE("Refreshed!\n"); }

ActiveX Controls typically do not support the IOleCommandTarget interface. Alternatively, a control can watch for the HIDE and SHOW verbs sent to IOleObject::DoVerb. An MFC ActiveX Control can use the ON_STDOLEVERB message map entry to hook into the default handling of OLEIVERB_HIDE and OLEIVERB_SHOW.

Here is an MFC example to detect that Internet Explorer is hiding an ActiveX control instead of unloading it:

BEGIN_MESSAGE_MAP(CYourOcxCtrl, COleControl) // ... other stuff

// Add the two following entries to your control's message map ON_STDOLEVERB(OLEIVERB_SHOW, OnVerbShow) ON_STDOLEVERB(OLEIVERB_HIDE, OnVerbHide) END_MESSAGE_MAP

// Add definitions and declarations for the following two functions. BOOL CYourOcxCtrl::OnVerbShow(LPMSG lpMsg, HWND hWndParent,                             LPCRECT lpRect) {   // Do what COleControl would have done in default case // taken from COleControl::XOleObject::DoVerb in CTLOBJ.CPP HRESULT hr = OnOpen(TRUE, lpMsg);     // Try in-place first

// This control's page was navigated to and is being displayed // Do whatever is necessary: start playing sounds, // animations, timers TRACE("VERB: SHOW\n"); return !(FAILED(hr)); }

BOOL CYourOcxCtrl::OnVerbHide(LPMSG lpMsg, HWND hWndParent,                             LPCRECT lpRect) {   // This control's page was navigated away from, but still remains in    // the memory cache. Stop playing background sounds, animations, // timers. TRACE("VERB: HIDE\n");

// Do what COleControl would have done in default case // taken from COleControl::XOleObject::DoVerb in CTLOBJ.CPP HRESULT hr = m_xOleInPlaceObject.UIDeactivate; return !(FAILED(hr)); }

These steps are all that is necessary to reliably detect when a page held in memory is shown or hidden. Note that it is not guaranteed in all cases that ActiveX Documents and ActiveX Controls will be held in memory. There are some complications to the scheme that Internet Explorer uses to determine when to dump a page or control or hold it in memory:


 * Controls that are scripted are always unloaded when navigating off of a page on Internet Explorer 3.01 for Windows NT.
 * ActiveX Documents specified in a frame remain in memory as expected. However, when returning to the original page with the ActiveX Document in a frame, Internet Explorer unloads that server it held in memory and then reloads that server immediately with the specified document. This is what would occur if the Back button and then the Forward button were pushed.