зеркало из https://github.com/mozilla/pjs.git
Useful comments contributed by rginda@ndcico.com
This commit is contained in:
mang%subcarrier.org
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9fdbc7ced3
Коммит
00ff287e96
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@ -29,21 +29,56 @@
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#include "plstr.h"
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#include "stdio.h"
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/**
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* SampleImpl is an implementation of the nsISample interface. In XPCOM,
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* there can be more than one implementation of an given interface. Class
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* IDs (CIDs) uniquely identify a particular implementation of an interface.
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* Interface IDs (IIDs) uniquely identify an interface.
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*/
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class SampleImpl : public nsISample
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{
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public:
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SampleImpl();
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virtual ~SampleImpl();
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/**
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* This macro expands into a declaration of the nsISupports interface.
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* Every XPCOM component needs to implement nsISupports, as it acts
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* as the gateway to other interfaces this component implements. You
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* could manually declare QueryInterface, AddRef, and Release instead
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* of using this macro, but why?
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*/
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// nsISupports interface
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NS_DECL_ISUPPORTS
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/**
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* NS_IMETHOD expands to the standard XPCOM return type. XPCOM methods
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* should never return any other type. The return value is used
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* behind the scenes by the XPConnect runtime to figure out if the call
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* failed in any way.
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* These methods were generated by "attribute string Value" in
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* nsISample.idl. When reflected into JavaScript, XPCOM will use these
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* calls as Getter/Setter ops, so that they can be called transparently
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* as "sample.Value='foo';" and "var val = sample.Value"
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*/
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// nsISample interface
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NS_IMETHOD GetValue(char * *aValue);
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NS_IMETHOD SetValue(char * aValue);
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/**
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* The const came from the "in" specifier in nsISample.idl. "in"
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* specifies that the value of this parameter is used only for input,
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* this method is not allowed to modify the contents of the buffer.
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*/
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NS_IMETHOD WriteValue(const char *aPrefix);
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/**
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* nsISample.idl specifies all of it's string types as string, instead
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* of wstring (wide string), the Unicode type. If the world were a
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* perfect place, all normal strings in XPCOM interfaces would be unicode.
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* If this type had been specified as wstring, it would appear as
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* PRUnichar * in C++, which is the NSPR type for unicode characters.
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*/
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NS_IMETHOD Poke(const char* aValue);
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private:
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@ -52,6 +87,20 @@ private:
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////////////////////////////////////////////////////////////////////////
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/**
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* This is the static constructor for the sample component. Notice that
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* the prototype for this function is included in the {C++ ... } section
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* of nsISample.idl. This prototype is not actually part of the nsISample
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* interface, it only gets included, verbatim, in nsISample.h.
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* This is so that the factory for this class (nsSampleFactory.cpp)
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* can create a nsSample object. Normally you would expect to use
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* "SampleImpl s = new SampleImpl();" to create the object, the catch here
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* is that SampleImpl is not declared anywhere except in this file, so the
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* factory has no idea what a SampleImpl is. Instead, this static function's
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* prototype is declared in in nsISample.h (generated from nsISample.idl),
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* which any nsISample factory would require for the declaration of
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* nsISample anyway.
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*/
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nsresult
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NS_NewSample(nsISample** aSample)
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{
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@ -63,31 +112,59 @@ NS_NewSample(nsISample** aSample)
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if (! *aSample)
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return NS_ERROR_OUT_OF_MEMORY;
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/**
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* XPCOM automatically frees up memory used by objects when they are
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* no longer in use. It determines that an object is no longer in use
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* by checking how many unique, owning references there are to it.
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* Unfortunately, there is no automatic procedure for determining
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* what an owning reference is. Ownership is determined by conventions,
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* and you must be very careful to adhere to these conventions, or you
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* will forever be plagued by circular dependancies, and memory leaks.
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* The first rule of ownership is, "If You Created It, You Own It"
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* The other part of this convention is, when you create a new
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* object, the factory has already added you as an owning reference.
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* It is the clients responsibility to call Release() when it is finished
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* using the object.
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* NS_ADDREF() takes care of calling AddRef on the nsISupports interface
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* of the object you pass it.
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*/
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NS_ADDREF(*aSample);
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return NS_OK;
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}
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////////////////////////////////////////////////////////////////////////
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SampleImpl::SampleImpl() : mValue(nsnull)
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{
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NS_INIT_REFCNT();
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mValue = PL_strdup("initial value");
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}
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SampleImpl::~SampleImpl()
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{
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if (mValue)
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PL_strfree(mValue);
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}
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/**
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* NS_IMPL_ISUPPORTS expands to a simple implementation of the nsISupports
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* interface. This includes a proper implementation of AddRef, Release,
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* and QueryInterface. If this class supported more interfaces than just
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* nsISupports,
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* you could use NS_IMPL_ADDREF() and NS_IMPL_RELEASE() to take care of the
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* simple stuff, but you would have to create QueryInterface on your own.
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* nsSampleFactory.cpp is an example of this approach.
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* Notice that the second parameter to the macro is the static IID accessor
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* method, and NOT the #defined IID.
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*/
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NS_IMPL_ISUPPORTS(SampleImpl, nsISample::GetIID());
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/**
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* Notice that in the protoype for this function, the NS_IMETHOD macro was
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* used to declare the return type. For the implementation, the return
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* type is declared by NS_IMETHODIMP
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*/
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NS_IMETHODIMP
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SampleImpl::GetValue(char** aValue)
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{
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@ -96,6 +173,19 @@ SampleImpl::GetValue(char** aValue)
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return NS_ERROR_NULL_POINTER;
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if (mValue) {
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/**
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* GetValue's job is to return data known by an instance of
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* SampleImpl to the outside world. If we were to simply return
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* a pointer to data owned by this instance, and the client were to
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* free it, bad things would surely follow.
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* On the other hand, if we create a new copy of the data for our
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* client, and it turns out that client is implemented in JavaScript,
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* there would be no way to free the buffer. The solution to the
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* buffer ownership problem is the nsAllocator singleton. Any buffer
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* returned by an XPCOM method should be allocated by the nsAllocator.
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* This convention lets things like JavaScript reflection do their
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* job, and simplifies the way C++ clients deal with returned buffers.
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*/
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*aValue = (char*) nsAllocator::Alloc(PL_strlen(mValue) + 1);
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if (! *aValue)
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return NS_ERROR_NULL_POINTER;
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@ -108,7 +198,6 @@ SampleImpl::GetValue(char** aValue)
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return NS_OK;
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}
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NS_IMETHODIMP
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SampleImpl::SetValue(char* aValue)
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{
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@ -120,6 +209,12 @@ SampleImpl::SetValue(char* aValue)
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PL_strfree(mValue);
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}
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/**
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* Another buffer passing convention is that buffer's passed INTO your
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* object ARE NOT YOURS. Keep your hands off them, unless they are
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* declared "in out". If you want to keep the value for posterity,
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* you will have to make a copy of it.
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*/
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mValue = PL_strdup(aValue);
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return NS_OK;
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}
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