An application can suspend transactions to its DDE callback function—either those transactions associated with a specific conversation handle or all transactions regardless of the conversation handle. This is useful when an application receives a transaction that requires lengthy processing. In this case, an application can return CBR_BLOCK to suspend future transactions associated with that transaction's conversation handle, leaving the application free to process other conversations.
When processing is complete, the application calls the DdeEnableCallback function to resume transactions associated with the suspended conversation. Calling DdeEnableCallback causes the DDEML to resend the transaction that resulted in the application suspending the conversation. Therefore, the application should store the result of the transaction in such a way that it can obtain and return the result without reprocessing the transaction.
An application can suspend all transactions associated with a specific conversa-tion handle by specifying the handle and the EC_DISABLE flag in a call to the DdeEnableCallback function. By specifying a NULL handle, an application can suspend all transactions for all conversations.
When a conversation is suspended, the DDEML saves transactions for the conversation in a transaction queue. When the application reenables the conversation, the DDEML removes the saved transactions from the queue, passing each transaction to the appropriate callback function. Even though the capacity of the transaction queue is large, an application should reenable a suspended conversation as soon as possible to avoid losing transactions.
An application can resume usual transaction processing by specifying the EC_ENABLEALL flag in the DdeEnableCallback function. For a more controlled resumption of transaction processing, the application can specify the EC_ENABLEONE flag. This removes one transaction from the transaction queue and passes it to the appropriate callback function; after the single transaction is processed, any conversations are again disabled.