Implementing a Private Rendering Mechanism

The OS/2 2.0 Programming Guide Volume II gives some advice on use of the various messages available to implement a private rendering mechanism, and also some guidelines on how such a rendering mechanism should be documented. This section illustrates the implementation of a simple rendering mechanism, by explaining the definition of the DRM_SHAREMEM rendering mechanism used by the examples earlier in this chapter.

A rendering mechanism is necessary to pass the customer record data used in the examples, since the CUSTOMER structure that contains this data is too large to be contained within the DRAGITEM structure. It is therefore necessary, after a drop has occurred, for the source program to make the relevant data available to the target, in a format which is understood by and accessible to both the source and the target. In the examples, a named shared memory object is used to transfer the data; hence the name DRM_SHAREMEM used for the rendering mechanism.

The DRM_SHAREMEM rendering mechanism operates as follows:

  • The source window stores a pointer to the customer record being dragged in the ulItemID field of the DRAGITEM structure. This field is defined as ULONG, but it can be used in any way that is meaningful to the source window to identify the item being dragged. A pointer to the customer record is a convenient way to do this.

  • The target window, on receiving a DM_DROP message, allocates a named shared memory object with a name of its choice. It then sends a DM_RENDER message to the source window, passing the name of the memory object in the hstrRenderToName field of the DRAGTRANSFER structure, and indicating whether it requires a copy (DO_COPY) or a move (DO_MOVE) to take place, using the usOperation field of the DRAGTRANSFER structure.

  • When the source window receives the DM_RENDER message, it obtains access to the shared memory object and places the customer record in that object. The source window knows which customer record to copy, since the DRAGITEM structure, which includes a pointer to the customer record, is passed along with the DRAGTRANSFER structure.

    Finally, if a move operation was requested by the target, the source window deletes the customer record from its own data.

  • On receiving a TRUE return code from the DM_RENDER message, indicating that the data was successfully rendered, the target window copies the data out of the shared memory object, and uses it in whatever way it chooses.

    It should be stressed that this is a very simple rendering mechanism. However, it illustrates the general structure of such mechanisms, and their impact on the contents of fields in the DRAGITEM and DRAGTRANSFER structures.

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