Facilitate typing `C-SPC' on Android

* doc/emacs/android.texi (Android Windowing): Mention C-SPC
interception and how it may be disabled.

* java/org/gnu/emacs/EmacsNative.java (shouldForwardCtrlSpace):
New function.

* java/org/gnu/emacs/EmacsView.java (onKeyPreIme): New function.
If the provided key code is SPC and the event's modifier key
mask contains ControlMask, relay it directly to onKeyDown.

* java/org/gnu/emacs/EmacsWindow.java (eventModifiers): Export
and make static.

* src/android.c (shouldForwardCtrlSpace): New function.

* src/androidfns.c (syms_of_androidfns)
<android_intercept_control_space>: New defvar.

2 files changed, 19 insertions, 0 deletions

diff --git a/src/android.c b/src/android.c
index ed304baf0e6..1ccb724247d 100644
--- a/src/android.c
+++ b/src/android.c
@@ -2267,6 +2267,12 @@ NATIVE_NAME (shouldForwardMultimediaButtons) (JNIEnv *env,
   return !android_pass_multimedia_buttons_to_system;
 }
 
+JNIEXPORT jboolean JNICALL
+NATIVE_NAME (shouldForwardCtrlSpace) (JNIEnv *env, jobject object)
+{
+  return !android_intercept_control_space;
+}
+
 JNIEXPORT void JNICALL
 NATIVE_NAME (blitRect) (JNIEnv *env, jobject object,
                         jobject src, jobject dest,
diff --git a/src/androidfns.c b/src/androidfns.c
index 9e8372f524b..51421f0a68a 100644
--- a/src/androidfns.c
+++ b/src/androidfns.c
@@ -3205,6 +3205,19 @@ Note that if you set this, you will no longer be able to quit Emacs
 using the volume down button.  */);
   android_pass_multimedia_buttons_to_system = false;
 
+  DEFVAR_BOOL ("android-intercept-control-space",
+               android_intercept_control_space,
+    doc: /* Whether Emacs should intercept C-SPC.
+When this variable is set, Emacs intercepts C-SPC events as they are
+delivered to a frame before they are registered and filtered by the
+input method.
+
+For no apparent purpose, Android input methods customarily discard SPC
+events with the Ctrl modifier set without delivering them to Emacs
+afterwards, which is an impediment to typing key sequences
+incorporating such keys.  */);
+  android_intercept_control_space = true;
+
   DEFVAR_BOOL ("android-use-exec-loader", android_use_exec_loader,
     doc: /* Whether or not to bypass system restrictions on program execution.