CODEWITHSUNDEEP
windowswinformswinapifocus
Visions
Visions

Reputation: 947

Understanding SystemParametersInfo SPI_SETFOREGROUNDLOCKTIMEOUT

My App needs to get the focus when it get's called by an external tool (via an API), i know that the default is, that it should just flash in the Taskbar, but in this case this is absolutely not the behaviour that i want. In this case i try to get the focus by "this.Activate()" (C#).

This is where the ForeGroundLockTimeOut comes into play. However, I got a little problem understanding the SystemParameterInfo SPI_SETFOREGROUNDLOCKTIMEOUT.

I know that it's used to set the ForeGroundLockTimeOut which defines how long your app has to wait until it gets the focus it requested.

(for further information the variable "val" is an IntPtr that is set to 0)

SystemParametersInfo(SPI_SETFOREGROUNDLOCKTIMEOUT,0,val,SPIF_SENDWININICHANGE + SPIF_UPDATEINIFILE);

this one will change the Registry key that that handles the timeout (HKEY_CURRENT_USER\Control Panel\Desktop\ForeGroundLockTimout) Since this will change the behaviour of all apps, it's really the last resort to use.

Now i thought what if i don't update the registry key. So i tried this:

SystemParametersInfo(SPI_SETFOREGROUNDLOCKTIMEOUT, 0, val, 0);

However it doesn't change the behavior of my app in any way, but 

SystemParametersInfo(SPI_SETFOREGROUNDLOCKTIMEOUT,0,val,SPIF_SENDWININICHANGE);

does.

What i do not understand is why this only works for my app, which is absolutely what i want, but i do not understandit .Why do i have to broadcast a change, that only works for my app, when there has been no change made to any registry key or whatsoever, and why does this work only for my app.

Note: If you want to test this behavior, test it when Visual Studio is not running, while it's running (even if this Solution is not loaded) it changes the behavior of the App into getting focus in any case.

Upvotes: 1

Views: 4882

Answers (2)

shaoheng
shaoheng

Reputation: 11

SPIF_UPDATEINIFILE--Writes the new system-wide parameter setting to the user profile. SPIF_SENDCHANGE--Broadcasts the WM_SETTINGCHANGE message after updating the user profile.

Upvotes: 0

Cody Gray
Cody Gray

Reputation: 244913

This is not a per-app setting, it is a global system setting. There's no way to set it for just your application, so when you call SystemParametersInfo with 0 for the last parameter, nothing happens.

On the other hand, when you use SPIF_SENDWININICHANGE, the new setting gets broadcast in the form of a WM_SETTINGCHANGE message. (Which is why manually editing the registry is the wrong thing to do; always call the documented API.)

Aside from that, this code is wrong:

SPIF_SENDWININICHANGE + SPIF_UPDATEINIFILE

To concatenate two flags, you must use the boolean OR operator (|), not the addition operator (+). In this particular case, because 0x1 + 0x2 and 0x1 | 0x2 are both equal to 3, it seems to work, but that's just an accident.

The real solution to this problem needs not involve manipulating global settings (because you surely don't want to do that on client machines, even if you're OK with it on your own). You need to work with the system, rather than trying to work against it. In the system's model, the process that currently has the focus is the one with the privilege of setting/changing the focus. So there are basically three options:

  1. Just have the external tool call SetForegroundWindow itself and pass your application's window as the parameter. This altogether avoids your app having to activate itself.

  2. Use the AllowSetForegroundWindow function to delegate the external tool's foreground-window-setting privileges to your application. It should call this function, passing the ID of your app's process as the parameter. That way, when your app calls SetForegroundWindow, it will work as expected.

  3. Depending on how your tool and application are designed, you could just have the external tool launch the application. Like the documentation says, if a process is launched by the foreground process, it is allowed to set the foreground window.

Upvotes: 5

Related Questions