Sam Lantinga f0a4fea89f Fixed bug 4608 - Android: not getting SDL_WINDOWEVENT_FOCUS_GAINED on start of our app
Dan Ginsburg

I've seen this on several devices including Moto Z running Android 7 and a Snapdragon 845 running Android 9.

What happens is as follows:

SDLActivity.onWindowFocusChanged(true) happens pretty early on, but it's before we've done SDL_CreateWindow and so Android_Window is 0x0 thus this message does not get sent:

JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeFocusChanged)(
                                    JNIEnv *env, jclass cls, jboolean hasFocus)
{
    SDL_LockMutex(Android_ActivityMutex);

    if (Android_Window) {
        __android_log_print(ANDROID_LOG_VERBOSE, "SDL", "nativeFocusChanged()");
        SDL_SendWindowEvent(Android_Window, (hasFocus ? SDL_WINDOWEVENT_FOCUS_GAINED : SDL_WINDOWEVENT_FOCUS_LOST), 0, 0);
    }

    SDL_UnlockMutex(Android_ActivityMutex);
}

When the window does get created, in Android_CreateWindow it does this:

 window->flags &= ~SDL_WINDOW_RESIZABLE;     /* window is NEVER resizeable */
    window->flags &= ~SDL_WINDOW_HIDDEN;
    window->flags |= SDL_WINDOW_SHOWN;          /* only one window on Android */
    window->flags |= SDL_WINDOW_INPUT_FOCUS;    /* always has input focus */

    /* One window, it always has focus */
    SDL_SetMouseFocus(window);
    SDL_SetKeyboardFocus(window);

The SDL_SetKeyboardFocus does send an SDL_WINDOWEVENT_FOCUS_GAINED message, but it gets eaten in SDL_SendWindowEvent because we've forced SDL_WINDOW_INPUT_FOCUS beforehand:

 case SDL_WINDOWEVENT_FOCUS_GAINED:
        if (window->flags & SDL_WINDOW_INPUT_FOCUS) {
            return 0;
        }
        window->flags |= SDL_WINDOW_INPUT_FOCUS;
        SDL_OnWindowFocusGained(window);
        break;

I can fix the problem if I comment out this line from Android_CreateWindow:

    window->flags |= SDL_WINDOW_INPUT_FOCUS;    /* always has input focus */

I would propose that as a fix unless there is a reason not to.
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                         Simple DirectMedia Layer

                                  (SDL)

                                Version 2.0

---
https://www.libsdl.org/

Simple DirectMedia Layer is a cross-platform development library designed
to provide low level access to audio, keyboard, mouse, joystick, and graphics
hardware via OpenGL and Direct3D. It is used by video playback software,
emulators, and popular games including Valve's award winning catalog
and many Humble Bundle games.

More extensive documentation is available in the docs directory, starting
with README.md

Enjoy!
	Sam Lantinga				(slouken@libsdl.org)
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Simple Directmedia Layer
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