feat(context): allow composing with zero input

let IsComposing = either input or composition is non-empty.

Author: lotem

src/rime/composition.cc

@@ -24,9 +24,6 @@ bool Composition::HasFinishedComposition() const {
 Preedit Composition::GetPreedit(const string& full_input, size_t caret_pos,
                                 const string& caret) const {
   Preedit preedit;
-  if (empty()) {
-    return preedit;
-  }
   preedit.caret_pos = string::npos;
   size_t start = 0;
   size_t end = 0;

src/rime/context.cc

@@ -32,7 +32,7 @@ string Context::GetScriptText() const {
   return composition_.GetScriptText();
 }
 
-static const string kCaretSymbol("\xe2\x80\xb8");
+static const string kCaretSymbol("\xe2\x80\xb8");  // U+2038 ‸ CARET
 
 string Context::GetSoftCursor() const {
   return get_option("soft_cursor") ? kCaretSymbol : string();
@@ -43,7 +43,7 @@ Preedit Context::GetPreedit() const {
 }
 
 bool Context::IsComposing() const {
-  return !input_.empty();
+  return !input_.empty() || !composition_.empty();
 }
 
 bool Context::HasMenu() const {

src/rime/switcher.cc

@@ -114,17 +114,17 @@ void Switcher::HighlightNextSchema() {
   ```yaml
   schema_list:
   - case: [mode/wubi, mode/wubi_pinyin]
-  schema: wubi_pinyin
+    schema: wubi_pinyin
   - case: [mode/wubi]
-  schema: wubi86
-  - case: [mode/default]
-  schema: pinyin
+    schema: wubi86
+    - case: [mode/default]
+    schema: pinyin
 
-  mode:
-  wubi: false
-  wubi_pinyin: false
-  default: true
-  ```
+    mode:
+    wubi: false
+    wubi_pinyin: false
+    default: true
+    ```
 */
 
 static an<ConfigValue> ParseSchemaListEntry(Config* config,
@@ -224,7 +224,8 @@ void Switcher::OnSelect(Context* ctx) {
 void Switcher::RefreshMenu() {
   Composition& comp = context_->composition();
   if (comp.empty()) {
-    context_->set_input(" ");  // make context_->IsComposing() == true
+    // no longer need this to make context_->IsComposing() == true
+    // context_->set_input(" ");
     Segment seg(0, 0);         // empty range
     seg.prompt = caption_;
     comp.AddSegment(seg);