Why pass SPIF_ Sendchange flag systemparametersinfo will hang?

If you deliver SPIF_SENDCHANGE Mark for SystemParametersInfo function , Then it will send to the system in the form of broadcast WM_SETTINGCHANGE news , And send the message to wParam The parameter is set to the flag you pass to the function . for example , If you call ：
SystemParametersInfo(SPI_SETDOUBLECLICKTIME,500, 0, SPIF_UPDATEINIFILE | SPIF_SENDCHANGE);

The system will broadcast the following messages ：
SendMessage(HWND_BROADCAST, WM_SETTINGCHANGE, SPI_SETDOUBLECLICKTIME, 0);

If a window in the system does not respond to this summer limit , The broadcast will be suspended until the window responds to the message or the window is destroyed .

If you want to avoid unresponsive behavior in other windows , There are some ways , But it can also have an unexpected impact on your program .

You can execute in a background thread SystemParametersInfo call , Then even if it is suspended , Only the background thread is suspended , The main interface still works normally .

With this message , The background thread can notify the main thread when the broadcast is finally completed , At this point your program now knows that all windows have received their notifications and are using the new settings .

You can still not have SPIF_SENDCHANGE In case of a sign SystemParametersInfo call , Then change by manually broadcasting , as follows ：
DWORD dwResult;
SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE,SPI_SETDOUBLECLICKTIME, 0,SMTO_ABORTIFHUNG | SMTO_NOTIMEOUTIFNOTHUNG, 5000, &dwResult);

This does mean that unresponsive windows will not be notified that system parameters have changed . If you think your settings have changed very little , So that the lack of notification procedures is no big deal , This is acceptable . let me put it another way , When the unresponsive program finally wakes up , It doesn't know that the settings have changed , Because it missed the notice .

You can combine the above two methods ： Use a background thread and send a timeout message .

Perhaps the best technique is to use SendNotifyMessage function . As mentioned earlier ,SendNotifyMessage Function and SendMessage similar , It just doesn't wait for a response . This will get your program back to work , It won't mess up the procedures that are temporarily unresponsive when you decide to broadcast a notification .

SendNotifyMessage(HWND_BROADCAST, WM_SETTINGCHANGE, SPI_SETDOUBLECLICKTIME, 0);

The disadvantage is that , You don't know when all windows finally receive and process notifications . All you know is , One day , They will eventually discover . Usually you don't care about this aspect of broadcasting , So this lack of information is not an obstacle .

summary

Independent , Independent , Independent .
Where possible , Try to make your program and other components less dependent ： Do not call their functions , Do not import their libraries , Do not reference their header files . such , In the world of programs , You are free .
Be careful , The premise of the above practice is ： Where possible .

Last

Raymond Chen Of 《The Old New Thing》 It's one of my favorite blogs , There's a lot about Windows Little knowledge , For the vast Windows For platform developers , It's really helpful .
This article is from ：《Why does SystemParametersInfo hang when I pass the SPIF_SENDCHANGE flag?》