Invoking Actions
The IUPnPService::InvokeAction method allows actions to be invoked on Service objects. This method has two input parameters: the name of an action and an array of input arguments to that action. The method has two parameters:
- Parameter one — an input/output parameter: an array of output arguments for that action.
- Parameter two — an output parameter: a return value.
The method causes the action to be invoked on the device. The device generates event notifications if the action causes state variables of the device to change.
VBScript Example
The following VBScript code example invokes two actions on a Service object. The first action, GetTrackInfo, takes one input argument, a track number. The GetTrackInfo action returns the track length as the return value. The action also has one output argument, which contains the track title if the method returns success.
After invoking the GetTrackInfo action, the example invokes the Play action, which takes no arguments. However, because the syntax of InvokeAction requires an array of both input and output arguments, the example must create an empty array, emptyArgs, with no elements. The example passes this array to InvokeAction for both the input and output arguments, along with the name of the action.
Dim returnVal
Dim outArgs(1)
Dim args(1)
args(0) = 3
returnVal = service.InvokeAction("GetTrackInfo", args, outArgs)
'return Val now contains the track length
'and outArgs(0) contains the track title
Dim emptyArgs(0)
returnVal = service.InvokeAction("Play", emptyArgs, emptyArgs)
'returnVal indicates if the action was successful
C++ Example
The following example defines a C++ function that invokes an action with no arguments. Because InvokeAction requires a SAFEARRAY of arguments to be passed in, this example creates an empty SAFEARRAY. Since this action does not return a value or have output arguments, this example ignores the last two VARIANT values passed to InvokeAction.
The device generates event notifications if the action causes state variables of the device to change.
#include <windows.h>
#include <upnp.h>
#include <iostream>
#include <iomanip>
#pragma comment(lib, "oleaut32.lib")
using namespace std;
void InvokePlay(IUPnPService * pService)
{
HRESULT hr;
BSTR bstrActionName;
bstrActionName = SysAllocString(L"Play");
if (bstrActionName)
{
SAFEARRAYBOUND rgsaBound[1];
SAFEARRAY * psa = NULL;
rgsaBound[0].lLbound = 0;
rgsaBound[0].cElements = 0;
psa = SafeArrayCreate(VT_VARIANT, 1, rgsaBound);
if (psa)
{
LONG lStatus;
VARIANT varInArgs;
VariantInit(&varInArgs);
varInArgs.vt = VT_VARIANT | VT_ARRAY;
V_ARRAY(&varInArgs) = psa;
hr = pService->InvokeAction(bstrActionName,
varInArgs,
NULL,
NULL);
if (SUCCEEDED(hr))
{
wcout << L"Action invoked successfully\n";
}
else
{
wcerr << L"Failed to invoke action - HRESULT 0x"
<< setbase(16)
<< hr << L"\n";
}
SafeArrayDestroy(psa);
}
else
{
wcerr << L"Failed to create safe array\n";
}
SysFreeString(bstrActionName);
}
else
{
wcerr << L"Failed to allocate action name string\n";
}
}
The following example invokes the fictitious GetTrackInfo action. It takes a track number as an argument, returns the track length as a return value, and returns the track title in an output argument. The code is similar to the previous example, except that instead of creating an empty SAFEARRAY of input arguments, this example inserts a VARIANT that contains the track number. If InvokeAction returns success, this example examines the return value and the array of output arguments.
#include <windows.h>
#include <upnp.h>
#include <iostream>
#include <iomanip>
#pragma comment(lib, "oleaut32.lib")
using namespace std;
void InvokeGetTrackInfo(IUPnPService * pService)
{
HRESULT hr;
BSTR bstrActionName;
bstrActionName = SysAllocString(L"GetTrackInfo");
if (bstrActionName)
{
SAFEARRAYBOUND rgsaBound[1];
SAFEARRAY * psa = NULL;
rgsaBound[0].lLbound = 0;
rgsaBound[0].cElements = 1;
psa = SafeArrayCreate(VT_VARIANT, 1, rgsaBound);
if (psa)
{
long rgIndices[1];
VARIANT varTrackNum;
rgIndices[0] = 0;
VariantInit(&varTrackNum);
varTrackNum.vt = VT_I4;
// An arbitrary track is chosen (track 3)
V_I4(&varTrackNum) = 3;
hr = SafeArrayPutElement(psa,
rgIndices,
(void *) &varTrackNum);
VariantClear(&varTrackNum);
if (SUCCEEDED(hr))
{
LONG lStatus;
VARIANT varInArgs;
VARIANT varOutArgs;
VARIANT varReturnVal;
VariantInit(&varInArgs);
VariantInit(&varOutArgs);
VariantInit(&varReturnVal);
varInArgs.vt = VT_VARIANT | VT_ARRAY;
V_ARRAY(&varInArgs) = psa;
hr = pService->InvokeAction(bstrActionName,
varInArgs,
&varOutArgs,
&varReturnVal);
if (SUCCEEDED(hr))
{
SAFEARRAY * psaOutArgs = NULL;
VARIANT varTrackTitle;
psaOutArgs = V_ARRAY(&varOutArgs);
VariantInit(&varTrackTitle);
rgIndices[0] = 0;
hr = SafeArrayGetElement(psaOutArgs,
rgIndices,
(void *)&varTrackTitle);
if (SUCCEEDED(hr))
{
wcout << L"Action invoked successfully\n"
<< L"\tTrack Length == "
<< V_I4(&varReturnVal) << L"\n"
<< L"\tTrack Title == "
<< V_BSTR(&varTrackTitle) << L"\n";
}
else
{
wcerr << L"Failed to get array element -"
<< L" HRESULT 0x"
<< setbase(16)
<< hr << L"\n";
}
VariantClear(&varTrackTitle);
VariantClear(&varReturnVal);
VariantClear(&varOutArgs);
}
else
{
wcerr << L"Failed to invoke action - HRESULT 0x"
<< setbase(16)
<< hr << L"\n";
}
}
else
{
wcerr << L"Failed to insert argument into array - "
<< L"HRESULT 0x"
<< setbase(16)
<< hr << L"\n";
}
SafeArrayDestroy(psa);
}
else
{
wcerr << L"Failed to create safe array\n";
}
SysFreeString(bstrActionName);
}
else
{
wcerr << L"Failed to allocate action name string\n";
}
}