gecko-dev/xpcom/sample/nsSample.cpp

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */


/**
 *
 * A sample of XPConnect. This file contains an implementation nsSample
 * of the interface nsISample.
 *
 */
#include <stdio.h>

#include "nsSample.h"
#include "nsMemory.h"

#include "nsEmbedString.h"
#include "nsIClassInfoImpl.h"
////////////////////////////////////////////////////////////////////////

nsSampleImpl::nsSampleImpl() : mValue(nullptr)
{
    mValue = (char*)nsMemory::Clone("initial value", 14);
}

nsSampleImpl::~nsSampleImpl()
{
    if (mValue)
        nsMemory::Free(mValue);
}

/**
 * NS_IMPL_ISUPPORTS1 expands to a simple implementation of the nsISupports
 * interface.  This includes a proper implementation of AddRef, Release,
 * and QueryInterface.  If this class supported more interfaces than just
 * nsISupports, 
 * you could use NS_IMPL_ADDREF() and NS_IMPL_RELEASE() to take care of the
 * simple stuff, but you would have to create QueryInterface on your own.
 * nsSampleFactory.cpp is an example of this approach.
 * Notice that the second parameter to the macro is name of the interface, and
 * NOT the #defined IID.
 *
 * The _CI variant adds support for nsIClassInfo, which permits introspection
 * and interface flattening.
 */
NS_IMPL_CLASSINFO(nsSampleImpl, nullptr, 0, NS_SAMPLE_CID)
NS_IMPL_ISUPPORTS1_CI(nsSampleImpl, nsISample)
/**
 * Notice that in the protoype for this function, the NS_IMETHOD macro was
 * used to declare the return type.  For the implementation, the return
 * type is declared by NS_IMETHODIMP
 */
NS_IMETHODIMP
nsSampleImpl::GetValue(char** aValue)
{
    NS_PRECONDITION(aValue != nullptr, "null ptr");
    if (! aValue)
        return NS_ERROR_NULL_POINTER;

    if (mValue) {
        /**
         * GetValue's job is to return data known by an instance of
         * nsSampleImpl to the outside world.  If we  were to simply return 
         * a pointer to data owned by this instance, and the client were to
         * free it, bad things would surely follow.
         * On the other hand, if we create a new copy of the data for our
         * client, and it turns out that client is implemented in JavaScript,
         * there would be no way to free the buffer.  The solution to the 
         * buffer ownership problem is the nsMemory singleton.  Any buffer
         * returned by an XPCOM method should be allocated by the nsMemory.
         * This convention lets things like JavaScript reflection do their
         * job, and simplifies the way C++ clients deal with returned buffers.
         */
        *aValue = (char*) nsMemory::Clone(mValue, strlen(mValue) + 1);
        if (! *aValue)
            return NS_ERROR_NULL_POINTER;
    }
    else {
        *aValue = nullptr;
    }
    return NS_OK;
}

NS_IMETHODIMP
nsSampleImpl::SetValue(const char* aValue)
{
    NS_PRECONDITION(aValue != nullptr, "null ptr");
    if (! aValue)
        return NS_ERROR_NULL_POINTER;

    if (mValue) {
        nsMemory::Free(mValue);
    }

    /**
     * Another buffer passing convention is that buffers passed INTO your
     * object ARE NOT YOURS.  Keep your hands off them, unless they are
     * declared "inout".  If you want to keep the value for posterity,
     * you will have to make a copy of it.
     */
    mValue = (char*) nsMemory::Clone(aValue, strlen(aValue) + 1);
    return NS_OK;
}

NS_IMETHODIMP
nsSampleImpl::Poke(const char* aValue)
{
    return SetValue((char*) aValue);
}


static void GetStringValue(nsACString& aValue)
{
    NS_CStringSetData(aValue, "GetValue");
}

NS_IMETHODIMP
nsSampleImpl::WriteValue(const char* aPrefix)
{
    NS_PRECONDITION(aPrefix != nullptr, "null ptr");
    if (! aPrefix)
        return NS_ERROR_NULL_POINTER;

    printf("%s %s\n", aPrefix, mValue);

    // This next part illustrates the nsEmbedString:
    nsEmbedString foopy;
    foopy.Append(PRUnichar('f'));
    foopy.Append(PRUnichar('o'));
    foopy.Append(PRUnichar('o'));
    foopy.Append(PRUnichar('p'));
    foopy.Append(PRUnichar('y'));
    
    const PRUnichar* f = foopy.get();
    uint32_t l = foopy.Length();
    printf("%c%c%c%c%c %d\n", char(f[0]), char(f[1]), char(f[2]), char(f[3]), char(f[4]), l);
    
    nsEmbedCString foopy2;
    GetStringValue(foopy2);

    //foopy2.AppendLiteral("foopy");
    const char* f2 = foopy2.get();
    uint32_t l2 = foopy2.Length();

    printf("%s %d\n", f2, l2);

    return NS_OK;
}