Message handler functions in ATL

Message handler functions of ATL

In the active template library (ATL), the message handler function is three types:

Message handler

In the message map, the second parameter of the MESSAGE_HANDLER macro defines the MessageHandler function.

Syntax

LRESULT MessageHandler(
    UINT uMsg,
    WPARAM wParam,
    LPARAM lParam,
    BOOL& bHandled);

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Parameters

Example

class MyBaseWindow: public CWindowImpl<CMyBaseWindow>
{
public:
   BEGIN_MSG_MAP(MyBaseWindow)
      MESSAGE_HANDLER(WM_CREATE, OnCreate)
   END_MSG_MAP()

   // When a MyBaseWindow object will receive a WM_CREATE message, the message
   // will be directed to MyBaseWindow::OnCreate for accurate processing.
   LRESULT OnCreate(UINT /*nMsg*/, WPARAM /*wParam*/, LPARAM /*lParam*/, 
      BOOL& /*bHandled*/)
   {
      return 0;   
   }
};

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CommandHandler

In the message map, the third parameter of the COMMAND_HANDLER macro defines the CommandHandler function.

Syntax

LRESULT CommandHandler(
    WORD wNotifyCode,
    WORD wID,
    HWND hWndCtl,
    BOOL& bHandled);

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Parameters

Example

class ATL_NO_VTABLE CPolyProp :
   public CComObjectRootEx<CComSingleThreadModel>,
   public CComCoClass<CPolyProp, &CLSID_PolyProp>,
   public IPropertyPageImpl<CPolyProp>,
   public CDialogImpl<CPolyProp>
{
public:
BEGIN_COM_MAP(CPolyProp)
   COM_INTERFACE_ENTRY(IPropertyPage)
END_COM_MAP()

BEGIN_MSG_MAP(CPolyProp)
   COMMAND_HANDLER(IDC_SIDES, EN_CHANGE, OnEnChangeSides)
   CHAIN_MSG_MAP(IPropertyPageImpl<CPolyProp>)
END_MSG_MAP()

   // When a CPolyProp object will receive a WM_COMMAND message identified 
   // by IDC_SIDES and EN_CHANGE, the message will be directed to 
   // CPolyProp::OnEnChangeSides for accurate processing.
   LRESULT OnEnChangeSides(WORD /*wNotifyCode*/, WORD /*wID*/, HWND /*hWndCtl*/, 
      BOOL& /*bHandled*/);

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Notify handler

In message map, the third parameter of the NOTIFY_HANDLER macro defines the NotifyHandler function.

Syntax

LRESULT NotifyHandler(
    int idCtrl,
    LPNMHDR pnmh,
    BOOL& bHandled);

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Parameters

Example

class MyDialog: public CDialogImpl<MyDialog>
{
public:
   enum { IDD = IDD_MYDLG };

   BEGIN_MSG_MAP(CMyDialog2)
      NOTIFY_HANDLER(IDC_TREE1, NM_CLICK, OnNMClickTree1)
   END_MSG_MAP()

public:
   // When a MyDialog object will receive a WM_NOTIFY message 
   // identified by IDC_TREE1 and NM_CLICK, the message will be 
   // directed to MyDialog::OnNMClickTree1 for accurate processing
   LRESULT OnNMClickTree1(int /*idCtrl*/, LPNMHDR pNMHDR, BOOL& /*bHandled*/);
};

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Frequently Asked Questions

What is the result of successful message processing?

The result of successful message processing is 0.

What is the parameter of Notify handler in the NOTIFY_HANDLER macro?

In the message map, the third parameter of the NOTIFY_HANDLER macro defines the NotifyHandler function.

What happens to bHandler in Message handler?

Before calling CommandHandler, the message map sets bHandler to TRUE. To signal that the message requires more processing, CommandHandler should set bHandled to FALSE if it cannot fully handle the news.

Why do we use classes?

We use classes to implement and create particular objects having multiple data types. We also use classes for encryption and encapsulation.

Conclusion

We hope this blog successfully helped you understand the message handler function of ATL.

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