Sample program for OS/2 Drag/Drop facilities

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By Rick Fishman

Source Code 119Kb

DRGDROP.EXE is a sample program for OS/2 Drag/Drop facilities (aka Direct Manipulation). It is useful as a Drag/Drop monitoring program to dynamically see what is taking place during a direct manipulation operation.

DRGDROP starts out by creating 2 container windows, each being a container control which is a child of a frame window. One of the containers has some icons in it. Each icon represents a temporary file that is created at the beginning of the program and used to drag to other windows during program operation. The other container is empty. Once you drag some icons to the empty container it functions just like the container with the icons. Both containers start out at the bottom of the screen, each being half the width of the screen and 1/3 the height of the screen.

Since the icons in the first container represent actual files, these icons can be dragged and dropped on either the other DRGDROP container or other windows that support Drag/Drop.

In order to see what is going on during the Drag/Drop operation, 2 additional windows are created. I call these 'debug' windows. They are listboxes that are children of frame windows. All Drag/Drop activity is displayed in these windows. The windows are displayed on the top of the screen - one on the left and one on the right. They are tied to the container windows that are below them, i.e. any Drag/Drop activity that happens for the container below them gets output in their listbox. So the screen looks like this:

│ Container 1│               │ Container 2│
│   Debug    │               │   Debug    │
│  Window    │               │  Window    │
│            │               │            │
├────────────┘               └────────────┤
│                                         │
│                                         │
│                                         │
│                                         │
│                                         │
│   Container #1     │   Container #2     │
│      Window        │      Window        │
│                    │                    │
│                    │                    │

Of course you can move these window anywhere you like. Their positions are not 'remembered' so the next time you bring up the program they will start out as above.

Drag/Drop all centres around PM messages and data structures. In order to understand it, you must understand these messages and structures and the sequences of events that causes the messages to be generated. That's where the Debug windows come in. Whenever the container's owner gets a Drag/Drop message, it calls program functions that dump the name of the message and the contents of the passed structures to the Debug window.

Once you get the hang of Drag and Drop, it will be handy to experiment with what happens when you start changing the values you set into the Drag/Drop data structures before the drag starts. To do this, a Settings Notebook is provided.

You can bring up the Settings notebook at any time by using the context-menu keystroke (the Right Mouse Button by default, unless it was changed because you are left-handed or something like that). This context menu gives you 4 options, each will bring up the same Settings Notebook but opened to a different page. The 4 pages of the settings notebook are as follows:

DragInfo page

This page lets you alter all fields of the DRAGINFO strucure and its DRAGITEM structures. These structures are set up by source window in the Drag/Drop, i.e. the window that the Drag was started in. The only field that isn't set on this page is the RMF settings. Since there are a number of options for the RMF field, it has its own page.

RMF page

This sets up a field in the DRAGITEM structure (hstrRMF). RMF stands for Rendering Mechanism/Format. Listboxes display the available options for the Mechanism and the Format. These are not mutually exclusive options so they are multiple-select listboxes. Each listbox has an entryfield below it that lets you enter your own additional strings for the mechanism or format. Your strings, if you enter any, will be placed first in the RMF string (before the ones in the listboxes).

As you enter these strings or select listbox items, the hstrRMF field is updated to reflect your new selections. This is displayed in the 'generated RMF' MLE. This is a read-only MLE so you can't change it. If you are in the middle of one of the entryfields and want the RMF string to be generated (it is normally re-generated when you exit the entryfield), hit the 'Generate' pushbutton. This pushbutton is only enabled when changes have been made to either of the entryfields.

There is also a 'Use Manual RMF' checkbox and a 'Manual RMF' MLE. If you check this checkbox and enter a string in the MLE, that string will be used for hstrRMF instead of the generated one.

Reply page

Some Drag/Drop messages require a reply (returncode) in order to determine what will happen next. This page lets you select the reply that will be used during execution of the program. You can change the replies from any Printer or Shredder drops as well as being able to decide what will be returned from the DM_DRAGOVER messages.

Misc page

This page contains miscellaneous options. First, you can choose between letting the containers accept all kinds of drops on them and only allowing drops with a mechanism of DRM_OS2FILE. DRM_OS2FILE is the most common type of Rendering Mechanism.

Also on this page are choices on how the Debug windows will function. In default mode, the Debug windows will display all structures from all Drag/Drop messages. Also, if you drag more than one icon, all structures related to all items will be displayed. This can be very useful if you need to know everything that's going on. It can also be quite verbose and slow down the drag. So if you don't need to know all the information, there are some choices you can make on this notebook page that affect the Debug window operation. You can choose to only display the message names rather than all the structures involved and you can choose to only display the output from the first Drag Item rather than all of them on a multi-item drag.

All Settings Notebook options take affect immediately, i.e. the next time you do a Drag/Drop operation they will be used. You don't need to close the notebook. In fact it is convenient to keep it open to experiment with changing the options and then trying out the new changes and doing it again, etc.

NOTE: The options on the settings notebook are stored in drgdrop.ini when the program is shut down. So your settings are persistent between program invocations.

A little about Drag N' Drop

This program uses containers to implement Drag/Drop. The reason for this is that the container control is very conducive to direct manipulation and makes it easy on a program to implement it. The concepts illustrated in this program are directly transferable to Drag/Drop in a regular PM window and any other PM control.

The CN_ messages that are used in this program are WM_CONTROL messages sent by the container to its owner when it gets the normal PM messages. The reason for these messages is so that the container can provide its owner with more information than it would normally get with the regular PM messages. The mapping goes like this:

PM message (container gets this) Message sent by container to owner

The ValueSet control also provides this type of interface to its owner. Obviously if your program just uses a normal client window you would use the messages on the left-hand side.

THE MOST IMPORTANT THING that you need to know about Drag/Drop is that the information passed between the source and target is in shared memory so it has to be freed by both sides when they are done with it. Since you don't allocate the shared memory (PMDRAG.DLL does), it is more difficult to think in terms of freeing it. The memory that has to be freed is the DRAGINFO structure and all the string handles in it and its attached DRAGITEM structures, and any DRAGTRANSFER structures that were used if rendering takes place (this program does not do any rendering - my DRGRENDR and DRGTHRND samples do if you need some sample code for that).

You use DrgFreeDraginfo() to free the DRAGINFO structure and DrgFreeDragtransfer() to free the DRAGTRANSFER structures. The DRAGINFO structure is allocated by the source when it starts the drag (using the DrgAllocDraginfo() API), and PM takes care of disbursing this memory to processes that need it when they call DrgAccessDraginfo(). The DRAGTRANSFER structures are allocated by the target (using the DrgAllocDragtransfer() API) when drop processing starts. Whenever it communicates with the source, it uses DrgSend/PostTransferMsg() API's that cause PM to give their process access to the DRAGTRANSFER structure.

Here are the documented times for freeing these structures when no rendering is involved. Since this program doesn't do any rendering, I won't go into that aspect. When rendering is involved, things change a little:


If DrgDrag() returns NULLHANDLE (which it will if the user hits F1 or Esc during the drop so that there is no target window), it should make the following calls:


DrgDeleteDraginfoStrHandles() deletes all the string handles in all the DRAGITEMs associated with the DRAGINFO structure. DrgFreeDraginfo() frees the DRAGINFO structure.

If DrgDrag() doesn't return NULLHANDLE, that means a drop occurred and the return value is the target's window handle. In this case, it is the target's responsibility to delete the string handles but the source needs to free the DRAGINFO structure. Here's what it should do:

 comes thru, the source needs to call DrgFreeDraginfo(). Keep in mind that
 this means a global counter must be used (or store it in a window word like
 this program does) so it can keep track of when the last one comes thru.


The target needs to make the following calls under the CN_DROP (or DM_DROP if not a container owner) message:


DRGDROP frees these structures when the documentation recommends freeing them (as illustrated above). Unfortunately the documentation isn't real clear on this and you have to search to find it. REMEMBER THAT IF YOUR APPLICATION DOES NOT FREE THESE STRUCTURES WHEN IT IS SUPPOSED TO THAT YOUR APP COULD SCREW UP OTHER APPS IF IT ALLOWS DIRECT MANIPULATION TO OCCUR WITH OTHER APPS. This is an important thing to note. If direct manipulation is not done properly it could lead to a PM resource leak because shared memory might never be freed that should be freed.

NOTE: If rendering takes place after the drop, this scenario changes a bit. So as not to confuse things, I'm leaving this little bit of trivia out of this sample. For that info, find my DRGRENDR or DRGTHRND sample programs. DRGRENDR does rendering on the main thread. DRGTHRND does rendering on secondary threads.

DRGDROP.EXE is built from 10 source modules:

DRGDROP.C  - Main source module that starts things off.
DEBUGWIN.C - Debug window code that creates/handles Drag/Drop output windows.
NOTEBOOK.C - Settings Notebook code (exclusive of the dialogs).
DLGDRAGI.C - DragInfo dialog within the Settings Notebook.
DLGMISC.C  - Miscellaneous Options dialog within the Settings Notebook.
DLGREPLY.C - Reply dialog within the Settings Notebook.
DLGRMF.C   - RMF dialog within the Settings Notebook.
DRAG.C     - All Drag/Drop code.
MENU.C     - Handles the context menu.
SHOW.C     - Displays Drag/Drop data structures in the debug windows.

I know that this little document doesn't go nearly into enough detail about Drag/Drop. The problem is that there is enough information on direct manipulation to fill a book. For more information, look at the information in the Programming Guide that comes with the IBM toolkit technical library. There is a sample program called DRAGDROP that comes with the toolkit that is a very good sample.

Hope this sample program helps someone.