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Purpose
Mimetype.exe is an MFC sample that creates an ActiveX control and registers
it as the default player for a MIME type. It also illustrates how to handle
the calls that Internet Explorer makes when sending the URL of the file to
play. The sample has been built and tested with Visual C++ on Intel
platforms.
MIME Type Players
There are three situations where Internet Explorer (IE) uses MIME type
players: when IE sees an EMBED tag in HTML; when a link is accessed either
by clicking a hyperlink or by typing a URL in the address bar; and when IE
sees an OBJECT tag in HTML when there is no CLASSID attribute but there is
a MIME TYPE attribute specified.
In the case of the EMBED tag, the player associated with the MIME type of
the SRC attribute URL is looked up. If a player exists, it is created
within the current page with dimensions specified in the EMBED tag. The
SRC URL is then passed to the player.
Similarly, when a new URL is navigated to, the player associated with the
target URL is determined. If a player exists, it is loaded in the
destination page and the URL is passed to the player. The dimensions of
the player always are full size of the destination frame.
In the case of an OBJECT tag with no CLASSID attribute but with a MIME
TYPE attribute, the player associated with the MIME TYPE attribute is
looked up. If a player exists, it is created within the current page with
dimensions specified in the OBJECT tag. The SRC URL is then passed to the
player.
It is important to note that in each case the file to play is not
downloaded by Internet Explorer. It is up to the player to download the
file or interact with it in any way it sees fit.
Registering as a Player
In order for an ActiveX component to act as the default MIME type player,
it must have the following registry entries:
- HKEY_CLASSES_ROOT\MIME\DataBase\Content Type\<mime type>. This is
required only if you are creating a new MIME type. It adds the new MIME
type <mime type> to the MIME type database.
- HKEY_CLASSES_ROOT\MIME\DataBase\Content Type\<mime type> Extension =
<mime type ext>. This associates the file extension <mime type ext>
with the MIME type <mime type>. Note that Extension is a named value,
not a subkey.
- HKEY_CLASSES_ROOT\MIME\DataBase\Content Type\<mime type> CLSID =
<player CLSID>. This associates player's clsid with the MIME type <mime
type>. Note that CLSID is a named value, not a subkey.
- HKEY_CLASSES_ROOT\<mime type ext>. Registers the file extension. This
is necessary only for new file extensions. In addition, if a player can
be found directly from a MIME type, it is not necessary to register a
file extension.
- HKEY_CLASSES_ROOT\<mime type ext>
Content Type = <mime type>. This associates the MIME type <mime type>
with the file extension <mime type ext>, which is useful if the server
does not know the MIME type of a given file. IE can check the extension
and determine the MIME type from it. Notice that Content Type is a
named value, not a subkey.
- HKEY_CLASSES_ROOT\CLSID\<player clsid>. Class ID of player. This is
automatically added if you are using MFC or the ActiveX Designer
Framework to create your control.
- HKEY_CLASSES_ROOT\CLSID\<player clsid>\Control. This identifies the
player as a control that conforms to the OCX '94 specification. For
this release of Internet Explorer, MIME type players must register as
OCX '94 controls. Future versions of Internet Explorer will support
other types of ActiveX objects as players. This key is added
automatically if the control was created with MFC or the ActiveX
Designer Framework.
- HKEY_CLASSES_ROOT\CLSID\<player clsid>\EnableFullPage\<mime type
extension>. This tells Internet Explorer that this player can show full
frame. It is required to handle navigation to URLs with <mime type
extension> as their extension.
The following code is taken from the sample's DllRegisterServer method (in
Mimetype.cpp) and illustrates each of the registration steps in order:
// Create new MIME type key for our new MIME type. Only necessary for
// new mime types.
if ( ERROR_SUCCESS != RegCreateKey(HKEY_CLASSES_ROOT, pszMTSubKey,
&hkey) )
break;
// Add extension value to the MIME type key to associate .mtp files
// with the application/x-mimetype MIME type
if ( ERROR_SUCCESS != RegSetValueEx(hkey, pszMTExtVal, 0, REG_SZ,
(const BYTE *)pszMTExt, strlen(pszMTExt)) )
break;
// Add class id to associate this object with the mime type
if ( ERROR_SUCCESS != RegSetValueEx(hkey, pszMTCLSIDVal, 0, REG_SZ,
(const BYTE *)pszMTCLSID, strlen(pszMTCLSID)) )
break;
RegCloseKey(hkey);
// Register .mtp as a file extension. This is only necessary for new
// file extensions. A new player for .avi files for instance would not
// require this.
if ( ERROR_SUCCESS != RegCreateKey(HKEY_CLASSES_ROOT, pszMTExt, &hkey)
)
break;
// Add content type to associate this extension with the content type.
// This is required and is used when the mime type is unknown and IE
// looks up associations by file extension.
if ( ERROR_SUCCESS != RegSetValueEx(hkey, pszMTContentVal, 0, REG_SZ,
(const BYTE *)pszMTContent, strlen(pszMTContent)) )
break;
RegCloseKey(hkey);
// Open the key under the control's clsid
// HKEY_CLASSES_ROOT\CLSID\<CLSID>
wsprintf(szSubKey, "%s\\%s", pszMTCLSIDVal, pszMTCLSID);
if ( ERROR_SUCCESS != RegOpenKey(HKEY_CLASSES_ROOT, szSubKey, &hkey) )
break;
// Create the EnableFullPage and extension key under this so that we
// can display files with the extension full frame in the browser.
wsprintf(szSubKey, "%s\\%s", pszMTFullPage, pszMTExt);
if ( ERROR_SUCCESS != RegCreateKey(hkey, szSubKey, &hkey1) )
break;
Receiving the URL to Play
In all cases Internet Explorer passes the URL of the file to play to the
default player via the IPropertyBag interface. That is, when Internet
Explorer launches a player, it creates an instance of IPropertyBag and
loads the "SRC" property with the URL of the file to play. Then it queries
the player for IPersistPropertyBag and sends it the IPropertyBag instance
through IPersistPropertyBag::Load.
In MFC, this manifests itself as a property exchange so you can load the
"SRC" property directly in your override of DoPropExchange. The following
code is from the sample CMimetypeCtrl::DoPropExchange method (in
mtpctl.cpp):
PX_String(pPX, "SRC", m_cstrFileName);
This gets the string property from the property bag and puts it into a
CString member.
NOTE: If you are using the ActiveX Designer Framework (basectl in the ActiveX SDK), you get an override called LoadTextState that gets called
from the framework's implementation of IPersistPropertyBag. You can call
IPropertyBag::Read to get the "SRC" property.
Downloading the File
Internet Explorer does not download the file before sending the URL to the
player. The player is responsible for retrieving the file and playing it.
There are a number of ways to handle this depending on what the player
needs to do. One way is illustrated in the sample with the
URLDownloadToCacheFile function. This function downloads the file only if
necessary and returns the name of the local cached copy of the file. It
should be noted that URLDownloadToCacheFile is a blocking function. Even
though the data is downloaded asynchronously the function does not return
until all the data is downloaded.
If complete asynchronous downloading is desired, one of the other UOS
functions, such as URLOpenStream, or perhaps general URL monikers would be
more appropriate. The following code is taken from
CMimetypeCtrl::DoPropExchange (in Mtpctl.cpp) and fills up the string
m_cstrCacheFileName with the complete path of the local file:
if ( FAILED(URLDownloadToCacheFile(
GetControllingUnknown(), // control's IUnknown
m_cstrFileName, // URL to download
m_cstrCacheFileName.GetBuffer(MAX_PATH),// buffer to fill with name
MAX_PATH, // buffer length
0, // reserved
NULL )) ) // no status information
{
AfxMessageBox("Cannot download file");
m_cstrCacheFileName = "Not Found";
return;
}
Running the Sample
Mimetype.exe is a self-extracting executable that you can expand to install
the project files for the Mimetype control. Build the sample with Visual
C++ 5.0 using the supplied Mimetype.dsw project file. The control is
automatically be registered as part of the build process. You can then load
the sample Test.htm into your browser.
The result of this should be two instances of the mimetype control loaded
in the current page. The control displays as a rectangle with the name of
the URL (Test.mtp) and the name of the local file (<path>\Test.mtp). The
sample page also contains a link. Clicking on the link brings up a new
page with the control loaded full frame displaying the same URL
information.
The reason the supplied URL and the local file path are the same is that
the URLDownloadToCacheFile function returns the full path of the supplied
URL if it is a local (file:) URL. If the SRC URL in the EMBED and OBJECT
tags or the destination of the link were a remote file (http:, ftp:), then
the local file path would contain the full path of the cached copy of the
remote file.
NOTE: The Mimetype control created by the sample project is not digitally
signed. Because of this, depending on the current IE security settings, IE
might either fail to instantiate the control or will warn before loading
the control. If the IE security settings are set to High, IE will not load
the sample control. To allow the control to be instantiated the IE security
level must be set to either Medium or Low for the zone where the control is
being loaded from. To set the security level in IE select the View menu and
then the Internet Options menu item, choose the Security tab. Select either
Medium or Low as the security level for the zone where the sample page is
coming from.