Discussing the nuts and bolts of software development

Monday, June 09, 2008

 

Detecting the phone buttons on WM5/6

When working on a project that involved creating a custom dialer for Windows Mobile devices, one of the more important tasks is enabling the detection of the Send (green, off-hook) and Hang-up (red, on-hook) button presses at all times. The difficulty with this feat is that the Windows Mobile OS has specific needs for these buttons so you have to be careful with how you go about using them.

For example the Send button is used for launching the phone application (cprog.exe by default), and also perform phone related tasks like making and answering calls or calling a contact. The Hang-up button is used to minimize dialogs and hang up active calls.

On some devices the Hang-up button can have other behaviors such as acting as a power button or enabling keyboard lock. It’s important to know these facts, because depending on what you intend to do with the Send or Hang-up buttons, you may cause existing features of the OS to not function.

There are several methods to detect the Send and Hang-up buttons, but only a few methods will let you safely co-exist with the existing features of the Windows Mobile OS. I’ll first talk about obtaining the key presses through use of keyboard hook (SetWindowsHook) and why you shouldn’t use it. This is the first method I used for the project but quickly learned about the various limitations. If you do an Internet search for “Windows Mobile SetWindowsHook” you will obtain several pages explaining how to use SetWindowsHook on the device to detect each key press. Using this method to detect the Send or Hang-up keys you may discover several set backs.
  1. You are not just intercepting the Send or Hang-up keys but every key press so you’ll have to be extra careful about handling them.
  2. If an existing application already has the keyboard hook, you may not be able to use it. On a device like the Palm Treo 700/750 you may find that the Key-guard feature no longer works. This is because the Palm Key-guard also relies on the keyboard hook and can no longer use it if another application has control of it.
  3. You may find that certain features that rely on Send/Hang-up no long work. For example you can no longer dial a number from Contacts with the Send button.

There are ways around these limitations of course, but it would require a lot of manual handling from your application.

Another method of key detection is the use of hotkey registration. Microsoft provides us with the “RegisterHotKey” function which essentially allows you to register any button with your application window as a hotkey using the virtual key codes (VK_TTALK for Send, VK_TEND for Hang-up). What this does is, when the registered key is pressed; it will send you a WM_HOTKEY message, even if your window is not in the foreground, allowing you to handle it.

However there are a couple limitations when dealing with the Send and Hang-up keys. You may find that attempting to register either buttons as a hot key will fail. The reason is that the TaskBar (PPC) or Tray (Smartphone) already has Send button registered as a hotkey and the phone application has the Hang-up button.

You can forcibly “steal” the hot key using either the “UnregisterHotKey” function or the hidden “UnregisterFunc1” function; however taking the hot key away from their respective owners would render some features useless as with the cases when using “SetWindowsHook”.

After some experimentation I have found that there is a unique way in which you may register the Send and Hang-up buttons (and possibly other buttons) as hot keys. I’ve discovered that it is possible to register a button using 0x2001 as a modifier (some others will work as well such 0x2002, etc). What this does is it will generate a WM_HOTKEY message for you when pressing the Send button without interfering with the existing hot key registration held by the TaskBar or Tray or phone application. The limitation here, however, is that you will only get “one shot”; that is, only one WM_HOTKEY is generated and successive key presses will have no notifications. The solution is simply to register the key again. The code would look as follows (note that error and results checking are left out for simplicity)


// setup the window to receive the hot key message
ON_MESSAGE(WM_HOTKEY, &MyappDlg::OnHotkey)

...

// define some of the values to use for registering hot keys
#define ONE_SHOT_MOD 0x2001
#define VK_TTALK_ID 0x07
#define VK_TEND_ID 0x08

...

// register the Send and Hang-up buttons with the unique modifier
::RegisterHotKey(m_hWnd, VK_TTALK_ID, ONE_SHOT_MOD, VK_TTALK);
::RegisterHotKey(m_hWnd, VK_TEND_ID, ONE_SHOT_MOD, VK_TEND);

...

// wait for and handle the hot key messages
afx_msg LRESULT MyappDlg::OnHotkey( WPARAM wParam, LPARAM lParam )
{
switch (lParam >> 16)
{
case VK_TTALK:
// reregister VK_TTALK to get the next key press
::RegisterHotKey(m_hWnd, VK_TTALK_ID, ONE_SHOT_MOD, VK_TTALK);
// handle the keypress...
break;
case VK_TEND:
// reregister VK_TEND to get the next key press
::RegisterHotKey(m_hWnd, VK_TEND_ID, ONE_SHOT_MOD, VK_TEND);
// handle the keypress...
break;

...

So there you have it. Each time the Send and Hang-up buttons are pressed you get a notification and you don’t have to worry about interfering with the normal operations of each button.

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Comments:
In your experiments when pressing the talk key, did the phone dial app still pop up when you had your hook in place.
 
It should so long as the Tray or HHTask window receives VK_TTALK as a WM_HOTKEY when the talk key is pressed. If your goal is to prevent the phone dialer from popping up, then you'd have to prevent this message.
 
Hi, for me it works only for VK_TTALK! When a press the red button the phone dialer pop up!
 
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