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CHAPTER 7 FEATURES OF A .NET CLASS
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Here is the output for Listing 7-12: Return Return Return Return value value value value was was was was R::h R::h R::f R::h
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The output shows us that, in reality, the last function added is the one whose value is returned. But since this is implementation-defined, we should heed the warning and always use a manual walk of the invocation list with these delegates. Using GetInvocationList is also useful if exceptions might be thrown by the functions called through the delegate. If one delegate function throws an exception, other target functions may never execute. Walking through the invocation list manually enables you to wrap each invocation in a try/catch block, giving you more control over the functions that are invoked. Listing 7-13 demonstrates this technique. Listing 7-13. Manually Walking Through an Invocation List // delegate_with_exceptions.cpp using namespace System; delegate String^ MyDelegate(); ref class R { public: String^ f() { throw gcnew Exception(); return "R::f"; } String^ g() { return "R::g"; } String^ h() { return "R::h"; } }; int main() { MyDelegate^ d; R^ r = gcnew R(); d = gcnew MyDelegate(r, &R::f); d = safe_cast<MyDelegate^>(d->Combine(d, gcnew MyDelegate(r, &R::g))); d = safe_cast<MyDelegate^>(d->Combine(d, gcnew MyDelegate(r, &R::h))); for each (MyDelegate^ del in d->GetInvocationList()) {
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try { String^ s = del->Invoke(); Console::WriteLine("Return value was {0}", s); } catch(Exception^) { // Handle the exception. } } } The output of Listing 7-13 is shown here: Return value was R::g Return value was R::h Without the try/catch, g and h would never have been called.
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Asynchronous Delegates
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If the function you are calling via a delegate takes a long time to execute, you may want your code to perform other work while the called function is executing asynchronously on another thread. The .NET Framework provides support for calling delegates asynchronously, using a worker thread to call the function indicated by the delegate and allowing the initiating thread to continue with other work. Instead of using the Invoke method, use the BeginInvoke method to initiate the function call, and later in your code, call EndInvoke to retrieve the result. A variety of design patterns may be used. If you simply have a few other tasks to complete, you can perform those tasks and then simply wait for the result by calling EndInvoke. When EndInvoke is called before the worker thread has completed its work, execution on the main thread will block waiting for the function to complete. You can also poll the secondary thread, enabling you to continue working and keep checking the secondary thread until it s done. Another design pattern allows you to set up a callback function that is called when the function called by the delegate completes. The BeginInvoke has a signature that is determined by the delegate declaration. BeginInvoke has the same parameters as the usual Invoke function, plus two additional parameters: the first is an AsyncCallback class and the second is the delegate. EndInvoke has only one parameter of type IAsyncResult. So, for example if you have a delegate like this one: delegate void MyDelegate(R^ r); the invoke methods have the following signatures: AsyncResult^ BeginInvoke(R^, AsyncCallback^, MyDelegate^ ); void EndInvoke(IAsyncResult^);
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The classes AsyncCallback and AsyncResult and the associated interface IAsyncResult provide the methods needed to implement these designs, such as providing a way to check on whether the function has completed. The BeginInvoke function returns an object of type AsyncResult. Listing 7-14 shows an example. Listing 7-14. Checking Function Completion // async_delegates.cpp using namespace System; using namespace System::Threading; ref class R { public: property String^ Value; R(String^ s) { Value = s; } }; delegate void QueryFunc(String^, R^); ref class Document { IAsyncResult^ result; R^ m_r; public: Document(String^ s) { m_r = gcnew R(s); } // Query the database. void Query(String^ queryString, R^ r) { // Execute a long query. r->Value = "New Value"; } void InitiateQuery(String^ queryString) { QueryFunc^ qf = gcnew QueryFunc(this, &Document::Query); Console::WriteLine(m_r->Value); result = qf->BeginInvoke(queryString, m_r, gcnew AsyncCallback(this, &Document::ProcessResult), qf); }
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