Override keyboard layout correctly. Fixes #86.

Using numerous NSLog's led to the discovery that when McBopofomo lost
function (as described in #86), -setValue:forTag:client: was often called
not just on the context of the foreground app, but also on the contexts
of the background apps. This led to the theory that calling keyboard
layout override in that method (not a documented way of doing things
anyways) might corrupt the input method context. That we swapped out
language model and the builder when the method got called didn't help.

In this commit, we put back the keyboard layout override code to where
it belongs -- in -activateServer: -- and we now only swap the language
model and re-create the builder if the input method really changes (e.g.
from Bopomofo to Plain Bopomofo, or vice versa).

Similar defensive coding is also used in the function key handler in the
-handleEvent:client: method.

Source/InputMethodController.mm

@@ -251,6 +251,13 @@ public:
 {
     [[NSUserDefaults standardUserDefaults] synchronize];
 
+    // Override the keyboard layout. Use US if not set.
+    NSString *basisKeyboardLayoutID = [[NSUserDefaults standardUserDefaults] stringForKey:kBasisKeyboardLayoutPreferenceKey];
+    if (!basisKeyboardLayoutID) {
+        basisKeyboardLayoutID = @"com.apple.keylayout.US";
+    }
+    [client overrideKeyboardWithKeyboardNamed:basisKeyboardLayoutID];
+
     // reset the state
     _currentDeferredClient = nil;
     _currentCandidateClient = nil;
@@ -328,22 +335,32 @@ public:
 
 - (void)setValue:(id)value forTag:(long)tag client:(id)sender
 {
+    NSString *newInputMode;
+    Formosa::Gramambular::FastLM *newLanguageModel;
+
     if ([value isKindOfClass:[NSString class]] && [value isEqual:kPlainBopomofoModeIdentifier]) {
-        _inputMode = kPlainBopomofoModeIdentifier;
+        newInputMode = kPlainBopomofoModeIdentifier;
-        _languageModel = &gLanguageModelPlainBopomofo;
+        newLanguageModel = &gLanguageModelPlainBopomofo;
     }
     else {
-        _inputMode = kBopomofoModeIdentifier;
+        newInputMode = kBopomofoModeIdentifier;
-        _languageModel = &gLanguageModel;
+        newLanguageModel = &gLanguageModel;
     }
 
+    // Only apply the changes if the value is changed
+    if (![_inputMode isEqualToString:newInputMode]) {
+        [[NSUserDefaults standardUserDefaults] synchronize];
+
+        // Remember to override the keyboard layout again -- treat this as an activate eventy
         NSString *basisKeyboardLayoutID = [[NSUserDefaults standardUserDefaults] stringForKey:kBasisKeyboardLayoutPreferenceKey];
         if (!basisKeyboardLayoutID) {
             basisKeyboardLayoutID = @"com.apple.keylayout.US";
         }
-
         [sender overrideKeyboardWithKeyboardNamed:basisKeyboardLayoutID];
 
+        _inputMode = newInputMode;
+        _languageModel = newLanguageModel;
+
         if (!_bpmfReadingBuffer->isEmpty()) {
             _bpmfReadingBuffer->clear();
             [self updateClientComposingBuffer:sender];
@@ -358,6 +375,7 @@ public:
             _builder = new BlockReadingBuilder(_languageModel);
             _builder->setJoinSeparator("-");
         }
+    }
 }
 
 #pragma mark - IMKServerInput protocol methods
@@ -1110,24 +1128,30 @@ public:
 - (BOOL)handleEvent:(NSEvent *)event client:(id)client
 {
     if ([event type] == NSFlagsChanged) {
-        // function key pressed
-        BOOL includeShift = [[NSUserDefaults standardUserDefaults] boolForKey:kFunctionKeyKeyboardLayoutOverrideIncludeShiftKey];
-        if (([event modifierFlags] & ~NSShiftKeyMask) || (([event modifierFlags] & NSShiftKeyMask) && includeShift)) {
         NSString *functionKeyKeyboardLayoutID = [[NSUserDefaults standardUserDefaults] stringForKey:kFunctionKeyKeyboardLayoutPreferenceKey];
         if (!functionKeyKeyboardLayoutID) {
             functionKeyKeyboardLayoutID = @"com.apple.keylayout.US";
         }
 
-            [client overrideKeyboardWithKeyboardNamed:functionKeyKeyboardLayoutID];
-            return NO;
-        }
-
-        // reset when function key is released
         NSString *basisKeyboardLayoutID = [[NSUserDefaults standardUserDefaults] stringForKey:kBasisKeyboardLayoutPreferenceKey];
         if (!basisKeyboardLayoutID) {
             basisKeyboardLayoutID = @"com.apple.keylayout.US";
         }
 
+        // If no override is needed, just return NO.
+        if ([functionKeyKeyboardLayoutID isEqualToString:basisKeyboardLayoutID]) {
+            return NO;
+        }
+
+        // Function key pressed.
+        BOOL includeShift = [[NSUserDefaults standardUserDefaults] boolForKey:kFunctionKeyKeyboardLayoutOverrideIncludeShiftKey];
+        if (([event modifierFlags] & ~NSShiftKeyMask) || (([event modifierFlags] & NSShiftKeyMask) && includeShift)) {
+            // Override the keyboard layout and let the OS do its thing
+            [client overrideKeyboardWithKeyboardNamed:functionKeyKeyboardLayoutID];
+            return NO;
+        }
+
+        // Revert back to the basis layout when the function key is released
         [client overrideKeyboardWithKeyboardNamed:basisKeyboardLayoutID];
         return NO;
     }