217 lines
8.6 KiB
Plaintext
217 lines
8.6 KiB
Plaintext
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COM 'Instance's (and explorer band categories)
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980305
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andyp
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WARNING WARNING WARNING -- v. early draft of doc in progress, likely to
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have many errors/omissions.
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- abstract
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...
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- contents
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...
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- overview
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COM has a (core!) notion of a CLSID for the code that implements an object
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but no similar notion for a particular instance of that object. thus every
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client must write custom code for each instance.
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we provide a generalization which makes creation and initialization of such
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instances easy and consistent.
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- motivation
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an e.g. of usage might be a browser band implementation. the code for
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such a band is identical no matter what URL one happens to initialize
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it to. however it is quite useful to be able to install many different
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browser bands, each pointing to a different URL, by writing only
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registry 'goo' (vs. writing custom code, however simple it may be,
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to do the same thing).
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- instance = code + data
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an 'INSTID' is a GUID that identifies two things:
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- the CLSID for the code for an object
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- data for an IPersistXxx iface to initialize an instance of the object
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- creation
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INSTID's are fully hidden from the client. that is, there is no special
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API to create an instance. one simply does a CoCreateInstance, and if the
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CLSID happens to actually be an INSTID, we quietly do everything that is
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necessary to create and initialize the instance.
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- code
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the code for an 'instance' is exactly a CLSID.
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- initialization
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initialization is done from an IPersistXxx iface. we try to load 1st from
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an IPropertyBag, next from an IPersistStream, and finally from ???
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all of these ifaces are created using our various registry-based
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implementations. e.g. CRegStrPropertyBag (IPropertyBag on top of
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a registry key), OpenRegStream (IPersistStream on top of a registry
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key), etc.
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NYI: only IPersistPropertyBag is implemented. this makes (some) sense,
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since we're trying to provide a registry-goo-only method for 'coding',
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and a property bag is the main string-based COM IPersistXxx mechanism.
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- installation
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for convenience, we also provide code to create the registry goo (why?).
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CRegStrPropBag *InstallInstAndBag(LPTSTR pszInst, LPTSTR pszName, LPTSTR pszClass)
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...
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- registry goo
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the key looks as follows:
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subkey value(s)
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------ --------
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HKCR/CLSID/
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{instid}/ @=...description...
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InProcServer
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@=...path...
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ThreadingModel=...etc....
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Instance
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CLSID={clsid}
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InitPropertyBag
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name1=value1
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...
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the InProcServer should point to browseui.dll, which is where we've
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implemented the generic support code for InstIDs. (if the idea proves
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useful enough, perhaps COM will pick it up as a standard part of the
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CoCreateInstance API).
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- implementation
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here's how it works...
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the INSTID points to our DLL which implements inst.cpp. DllGetClassObject
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goes thru its usual loop. if it fails, it tries to create a
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CInstClassFactory for the given CLSID (INSTID). creation looks for and
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caches the magic 'Instance' subkey (and fails if it's not found). then
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when the ::CI method is called it gets the appropriate keys/values, does
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the CCI, creates the IPropertyBag (etc.), and does the ::Load.
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- perf
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from the implementation details, it should be clear that a 'normal'
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CCI goes thru exactly the same code path as before. we intentionally
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keep this code path exactly the same cost.
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the only time our INSTID code is hit at all is if the vanilla CCI
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fails (due to it not being in our sccls.c table). when that happens,
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we look for the 'Instance' subkey. if that fails, we fail the entire
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CCI (w/ the only added cost for that failure case being the 1 extra
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RegOpenKey call). we intentionally keep this code path as close to
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0 extra cost as possible.
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if that succeeds, we open/read several other keys, do the 'real' CCI,
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and do the initialization. again, the only extra cost (vs. the equivalent
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custom code) is the extra registry operations, which we try to keep
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cheap.
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to keep the extra registry operations cheap we do 'relative' opens
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as we work our way down the registry.
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BUGBUG what about cost of CInstClassFactory?
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- example
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to continue w/ our browser band e.g., here's the registry 'goo' for
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such a band:
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HKCR/CLSID/
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{77777777-7777-7777-7777-777777777777}
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InProcServer
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@="%systemdir%/browseui.dll"
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ThreadingModel="Apartment"
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Instance
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CLSID=Clsid_BrowserBand
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InitPropertyBag
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Url="http://www.nytimes.com"
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... other properties ...
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a CCI(7777, ...) will do:
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- create an uninitialized instance of the object by doing
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CCI(Clsid_BrowserBand, ...);
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- create an IPropertyBag for the 'InitPropertyBag' registry data (using
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our CRegStrPropertyBag implementation)
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- call punk->IPB::Load to load the IPropertyBag into the punk
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- we now have an initialized instance of the object (we're done!)
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- gotcha's and subtleties
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- gotcha: GetCLSID et. al.
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while each INSTID is unique and will create a separate instance initialized
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w/ the appropriate data, once the object has been created there is no
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(standard) way to distinguish it from any other instance of the same class
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(code).
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e.g. two browsers, one pointing at www.nytimes.com and the other at
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www.wsj.com, actually just look like two generic browsers.
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thus (e.g.)
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- IPS::GetCLSID gives the same CLSID (not different INSTID!) for both
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of them,
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- OleSaveToStream saves out the same CLSID and the (different) URL's
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- a subsequent OleLoadFromStream will use the same CLSID and the
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(different URL's)
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while at 1st this might seem odd, it actually makes a lot of sense. INSTID's
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are just a convenient standard 'packaging' of existing COM mechanisms.
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if one does a 'classic' CCI of two CLSID_BrowserBand's, initializes them
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to point to two different URLs, saves each w/ OleSaveToStream (presumably
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to two different streams), and later reloads each w/ OleLoadFromStream
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(again from the different streams), one will get *exactly* the same
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behavior as w/ the above INSTID e.g.
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in fact doing anything else (e.g. saving the object out w/ its INSTID)
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would be wrong (or at least inefficient). consider: the user may have
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changed various properties (e.g. navigated to a new URL). when we save
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it, the object saves the properties that exactly represent its current
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state. but if we create it by INSTID, we'd init it to a *different* set
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of properties (which we'd then blast w/ the IPB::Load call).
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moreover doing so would be impossible, since neither the object nor the
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system even know what the INSTID is once the CCI is complete.
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that said, one does need to be a bit careful.
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- gotcha: find
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the fact that IPS::GetCLSID returns the code CLSID rather than the
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object INSTID also complicates uniquely identifying the object in
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one's code. e.g. in our explorer bar implementation, each menu
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item has a unique CLSID or INSTID. the 1st time one clicks on a menu
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item, we just call CCI, add the band to the bar, show the band, and
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hide all other bands. so far so good.
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but now consider what happens steady-state when one re-clicks on a
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previously created menu item. while the menu item still knows they
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have different INSTID's, we have no way to ask the each object (when
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enumerating our list of bands) if it came from the INSTID.
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to solve this we use IE's Get/PutProperty mechanism to store a
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<name,value> pair, in this case <instIdBand,punkBand>.
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- gotcha: client dependencies
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since we're providing inst.cpp (vs. ole32.dll), the InProcServer for each
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INSTID must point to our implementation (ie4:shdocvw, ie5:browseui).
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however while the code 'belongs' to us, the INSTIDs in question 'belong'
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to the client app. that is, 3rd-party apps will point *their* INSTIDs at
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*our* DLL. this means that their selfreg.inx will contain knowledge of
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where we implement our support routines.
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this in turn means that we can't move our implementation (or rather when
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we do, we need to somehow forward it so that old code continues to work).
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use a treat-as or somesuch so that old code continues to work).
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BUGBUG we need to do this for ie5 since we've moved stuff!
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- links
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docs/inst.txt this document
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browseui/inst.cpp CCI support code
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browseui/stream.cpp CRegStrPropertyBag
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- appendix: 777a.reg
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here's the exact registry 'goo'
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#include 777a.reg
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