IsDialogMessageが処理する前にメッセージを傍受する

Last time, we looked at recognizing that the user clicked the Close button (or equivalent nonclient gestures) on a dialog. The other system-provided pathway to dismissing a dialog is pressing ESC, and we saw in our flow diagram that this is done by the Is­Dialog­Message

If your dialog box doesn’t have an IDCANCEL

Now, you might be concerned that additional pathways for system-provided dialog dismissal may be added later. (Who knows, maybe a new touch gesture will be invented.) But you can’t predict what pathway that future system-provide dismissal will take, so you have nothing to code to. All you can do is cover the pathways that you know and hope that any future dismissal mechanisms will follow one of them.

We saw from our diagram that the ESC pathway consists of the Is­Dialog­Message

One way to do this is to recognize the ESC key before calling IsDialogMessage

Your first try might go like this:

while (⟦ dialog still active ⟧ && GetMessage(&msg, NULL, 0, 0, 0)) { if (msg.message == WM_KEYDOWN && msg.wParam == VK_ESCAPE) { ⟦ do custom ESC key handling ⟧ } else if (!IsDialogMessage(hdlg, &msg)) { TranslateMessage(&msg); DispatchMessage(&msg); } }

However, this fails to honor controls that declare DLGC_WANT­ALL­KEYS

DLGC_WANT­MESSAGE

bool IsDialogESC(HDLG hdlg, MSG const* msg) { if (msg->message != WM_KEYDOWN || msg->wParam != VK_ESCAPE) { return false; } if (msg->hwnd != hdlg && !IsChild(hdlg, msg->hwnd)) { return false; } auto code = SendMessage(msg->hwnd, WM_GETDLGCODE, msg->wParam, (LPARAM)msg); if (code & (DLGC_WANTALLKEYS | DLGC_WANTMESSAGE)) { return false; } return true; }

In order for this to be a potential dialog-dismissing ESC, the message must be a WM_KEY­DOWN

I carefully ordered the tests so that we can early-out as quickly as possible. Checking for the ESC key can be done by inspecting the message. Checking that the target window is acceptable is a little more work, but not too bad. Checking whether the target window wants to handle the message is the most expensive test, so we do that last.

Now we can incorporate this helper function into our custom message loop.

while (⟦ dialog still active ⟧ && GetMessage(&msg, NULL, 0, 0, 0)) { if (IsDialogESC(hdlg, &msg)) { ⟦ do custom ESC key handling ⟧ } else if (!IsDialogMessage(hdlg, &msg)) { TranslateMessage(&msg); DispatchMessage(&msg); } }

This all sounds great, but you might not be able to make this change. For example, maybe the dialog box’s dialog procedure uses End­Dialog()

We’ll study this problem next time.