Pass the composition to TextInputContext via a parameter (#630)

We previously used an internal state for this, which is unclear from the API.
Furthermore, this was wrong and outdated after all the changes made to
TextInputContext throughout the time; a method called "SetText" should not be
used to set any composition, as it may be used for purposes other than just IME.

Backends/RmlUi_Platform_Win32.cpp

@@ -698,7 +698,7 @@ void TextInputMethodEditor_Win32::ConfirmComposition(Rml::StringView composition
 	if (input_context != nullptr)
 	{
 		input_context->SetCompositionRange(composition_range_start, composition_range_end);
-		input_context->CommitComposition();
+		input_context->CommitComposition(composition);
 	}
 
 	// Move the cursor to the end of the string.

Include/RmlUi/Core/TextInputContext.h

@@ -74,15 +74,18 @@ class RMLUICORE_API TextInputContext {
 	/// @param[in] text The string to replace the character range with.
 	/// @param[in] start The first character to be replaced.
 	/// @param[in] end The first character *after* the range.
+	/// @note This method does not respect internal restrictions, such as the maximum length.
 	virtual void SetText(StringView text, int start, int end) = 0;
 
 	/// Update the range of the text being composed (for IME).
 	/// @param[in] start The first character in the range.
 	/// @param[in] end The first character *after* the range.
 	virtual void SetCompositionRange(int start, int end) = 0;
 
-	/// Commit the current IME composition.
-	virtual void CommitComposition() = 0;
+	/// Commit an composition string (from IME), and respect internal restrictions (e.g., the maximum length).
+	/// @param[in] composition The string to replace the composition range with.
+	/// @note If the composition range equals to [0, 0], it takes no action.
+	virtual void CommitComposition(StringView composition) = 0;
 };
 
 } // namespace Rml

Source/Core/Elements/WidgetTextInput.cpp

@@ -90,13 +90,12 @@ class WidgetTextInputContext final : public TextInputContext {
 	void SetCursorPosition(int position) override;
 	void SetText(StringView text, int start, int end) override;
 	void SetCompositionRange(int start, int end) override;
-	void CommitComposition() override;
+	void CommitComposition(StringView composition) override;
 
 private:
 	TextInputHandler* handler;
 	WidgetTextInput* owner;
 	ElementFormControl* element;
-	String composition;
 };
 
 WidgetTextInputContext::WidgetTextInputContext(TextInputHandler* handler, WidgetTextInput* owner, ElementFormControl* element) :
@@ -139,16 +138,14 @@ void WidgetTextInputContext::SetText(StringView text, int start, int end)
 	value.replace(start, end - start, text.begin(), text.size());
 
 	element->SetValue(value);
-
-	composition = String(text);
 }
 
 void WidgetTextInputContext::SetCompositionRange(int start, int end)
 {
 	owner->SetCompositionRange(start, end);
 }
 
-void WidgetTextInputContext::CommitComposition()
+void WidgetTextInputContext::CommitComposition(StringView composition)
 {
 	int start_byte, end_byte;
 	owner->GetCompositionRange(start_byte, end_byte);
@@ -172,12 +169,13 @@ void WidgetTextInputContext::CommitComposition()
 		if (value_length + composition_length - (start - end) > owner->GetMaxLength())
 		{
 			int new_length = owner->GetMaxLength() - (value_length - composition_length);
-			composition.erase(StringUtilities::ConvertCharacterOffsetToByteOffset(composition, new_length));
+			int new_length_byte = StringUtilities::ConvertCharacterOffsetToByteOffset(composition, new_length);
+			composition = StringView(composition.begin(), composition.begin() + new_length_byte);
 		}
 	}
 
 	RMLUI_ASSERTMSG(end_byte >= start_byte, "Invalid end character offset.");
-	value.replace(start_byte, end_byte - start_byte, composition.data(), composition.size());
+	value.replace(start_byte, end_byte - start_byte, composition.begin(), composition.size());
 
 	element->SetValue(value);
 }