LibGUI+HackStudio: Add an opt-in autocompletion interface to TextEditor

...and use that to implement autocomplete in HackStudio.

Now everyone can have autocomplete :^)

19 files changed, 211 insertions, 162 deletions

diff --git a/Base/res/icons/16x16/completion/cpp-identifier.png b/Base/res/icons/16x16/completion/cpp-identifier.png
new file mode 100644
index 0000000000..9805fb180e
--- /dev/null
+++ b/Base/res/icons/16x16/completion/cpp-identifier.png
diff --git a/Base/res/icons/16x16/completion/unspecified-identifier.png b/Base/res/icons/16x16/completion/unspecified-identifier.png
new file mode 100644
index 0000000000..9fc73d22e8
--- /dev/null
+++ b/Base/res/icons/16x16/completion/unspecified-identifier.png
diff --git a/Base/res/icons/16x16/completion/unspecified-unspecified.png b/Base/res/icons/16x16/completion/unspecified-unspecified.png
new file mode 100644
index 0000000000..74345c444a
--- /dev/null
+++ b/Base/res/icons/16x16/completion/unspecified-unspecified.png
diff --git a/DevTools/HackStudio/AutoCompleteResponse.h b/DevTools/HackStudio/AutoCompleteResponse.h
index e1fd298dec..70023bf7ec 100644
--- a/DevTools/HackStudio/AutoCompleteResponse.h
+++ b/DevTools/HackStudio/AutoCompleteResponse.h
@@ -28,45 +28,36 @@
 
 #include <AK/String.h>
 #include <AK/Types.h>
+#include <LibGUI/AutocompleteProvider.h>
 #include <LibIPC/Decoder.h>
 #include <LibIPC/Encoder.h>
 
-namespace HackStudio {
-
-enum class CompletionKind {
-    Identifier,
-};
-
-struct AutoCompleteResponse {
-    String completion;
-    size_t partial_input_length { 0 };
-    CompletionKind kind { CompletionKind::Identifier };
-};
-
-}
-
 namespace IPC {
 
 template<>
-inline bool encode(IPC::Encoder& encoder, const HackStudio::AutoCompleteResponse& response)
+inline bool encode(IPC::Encoder& encoder, const GUI::AutocompleteProvider::Entry& response)
 {
     encoder << response.completion;
     encoder << (u64)response.partial_input_length;
     encoder << (u32)response.kind;
+    encoder << (u32)response.language;
     return true;
 }
 
 template<>
-inline bool decode(IPC::Decoder& decoder, HackStudio::AutoCompleteResponse& response)
+inline bool decode(IPC::Decoder& decoder, GUI::AutocompleteProvider::Entry& response)
 {
     u32 kind = 0;
+    u32 language = 0;
     u64 partial_input_length = 0;
     bool ok = decoder.decode(response.completion)
         && decoder.decode(partial_input_length)
-        && decoder.decode(kind);
+        && decoder.decode(kind)
+        && decoder.decode(language);
 
     if (ok) {
-        response.kind = static_cast<HackStudio::CompletionKind>(kind);
+        response.kind = static_cast<GUI::AutocompleteProvider::CompletionKind>(kind);
+        response.language = static_cast<GUI::AutocompleteProvider::Language>(language);
         response.partial_input_length = partial_input_length;
     }
 
diff --git a/DevTools/HackStudio/CMakeLists.txt b/DevTools/HackStudio/CMakeLists.txt
index 6f78d59f91..59406cbcbc 100644
--- a/DevTools/HackStudio/CMakeLists.txt
+++ b/DevTools/HackStudio/CMakeLists.txt
@@ -2,7 +2,6 @@ add_subdirectory(LanguageServers)
 add_subdirectory(LanguageClients)
 
 set(SOURCES
-    AutoCompleteBox.cpp
     CodeDocument.cpp
     CursorTool.cpp
     Debugger/BacktraceModel.cpp
diff --git a/DevTools/HackStudio/Editor.cpp b/DevTools/HackStudio/Editor.cpp
index 79f59d403e..ecfbb17e3a 100644
--- a/DevTools/HackStudio/Editor.cpp
+++ b/DevTools/HackStudio/Editor.cpp
@@ -62,8 +62,6 @@ Editor::Editor()
     m_documentation_tooltip_window->set_rect(0, 0, 500, 400);
     m_documentation_tooltip_window->set_window_type(GUI::WindowType::Tooltip);
     m_documentation_page_view = m_documentation_tooltip_window->set_main_widget<Web::OutOfProcessWebView>();
-
-    m_autocomplete_box = make<AutoCompleteBox>(*this);
 }
 
 Editor::~Editor()
@@ -315,50 +313,6 @@ void Editor::mousedown_event(GUI::MouseEvent& event)
     GUI::TextEditor::mousedown_event(event);
 }
 
-void Editor::keydown_event(GUI::KeyEvent& event)
-{
-    if (m_autocomplete_in_focus) {
-        if (event.key() == Key_Escape) {
-            m_autocomplete_in_focus = false;
-            m_autocomplete_box->close();
-            return;
-        }
-        if (event.key() == Key_Down) {
-            m_autocomplete_box->next_suggestion();
-            return;
-        }
-        if (event.key() == Key_Up) {
-            m_autocomplete_box->previous_suggestion();
-            return;
-        }
-        if (event.key() == Key_Return || event.key() == Key_Tab) {
-            m_autocomplete_box->apply_suggestion();
-            close_autocomplete();
-            return;
-        }
-    }
-
-    auto autocomplete_action = [this]() {
-        auto data = get_autocomplete_request_data();
-        if (data.has_value()) {
-            update_autocomplete(data.value());
-            if (m_autocomplete_in_focus)
-                show_autocomplete(data.value());
-        } else {
-            close_autocomplete();
-        }
-    };
-
-    if (event.ctrl() && event.key() == Key_Space) {
-        autocomplete_action();
-    }
-    GUI::TextEditor::keydown_event(event);
-
-    if (m_autocomplete_in_focus) {
-        autocomplete_action();
-    }
-}
-
 void Editor::enter_event(Core::Event& event)
 {
     m_hovering_editor = true;
@@ -486,6 +440,7 @@ void Editor::set_document(GUI::TextDocument& doc)
     }
 
     if (m_language_client) {
+        set_autocomplete_provider(make<LanguageServerAidedAutocompleteProvider>(*m_language_client));
         dbgln("Opening {}", code_document.file_path());
         int fd = open(code_document.file_path().characters(), O_RDONLY | O_NOCTTY);
         if (fd < 0) {
@@ -505,39 +460,21 @@ Optional<Editor::AutoCompleteRequestData> Editor::get_autocomplete_request_data(
     return Editor::AutoCompleteRequestData { cursor() };
 }
 
-void Editor::update_autocomplete(const AutoCompleteRequestData& data)
+void Editor::LanguageServerAidedAutocompleteProvider::provide_completions(Function<void(Vector<Entry>)> callback)
 {
-    if (!m_language_client)
-        return;
-
-    m_language_client->on_autocomplete_suggestions = [=, this](auto suggestions) {
-        if (suggestions.is_empty()) {
-            close_autocomplete();
-            return;
-        }
+    auto& editor = static_cast<Editor&>(*m_editor).wrapper().editor();
+    auto data = editor.get_autocomplete_request_data();
+    if (!data.has_value())
+        callback({});
 
-        show_autocomplete(data);
-
-        m_autocomplete_box->update_suggestions(move(suggestions));
-        m_autocomplete_in_focus = true;
+    m_language_client.on_autocomplete_suggestions = [callback = move(callback)](auto suggestions) {
+        callback(suggestions);
     };
 
-    m_language_client->request_autocomplete(
-        code_document().file_path(),
-        data.position.line(),
-        data.position.column());
-}
-
-void Editor::show_autocomplete(const AutoCompleteRequestData& data)
-{
-    auto suggestion_box_location = content_rect_for_position(data.position).bottom_right().translated(screen_relative_rect().top_left().translated(ruler_width(), 0).translated(10, 5));
-    m_autocomplete_box->show(suggestion_box_location);
-}
-
-void Editor::close_autocomplete()
-{
-    m_autocomplete_box->close();
-    m_autocomplete_in_focus = false;
+    m_language_client.request_autocomplete(
+        editor.code_document().file_path(),
+        data.value().position.line(),
+        data.value().position.column());
 }
 
 void Editor::on_edit_action(const GUI::Command& command)
diff --git a/DevTools/HackStudio/Editor.h b/DevTools/HackStudio/Editor.h
index 3c5c78a667..10c83a59d0 100644
--- a/DevTools/HackStudio/Editor.h
+++ b/DevTools/HackStudio/Editor.h
@@ -26,7 +26,6 @@
 
 #pragma once
 
-#include "AutoCompleteBox.h"
 #include "CodeDocument.h"
 #include "Debugger/BreakpointCallback.h"
 #include "LanguageClient.h"
@@ -72,7 +71,6 @@ private:
     virtual void paint_event(GUI::PaintEvent&) override;
     virtual void mousemove_event(GUI::MouseEvent&) override;
     virtual void mousedown_event(GUI::MouseEvent&) override;
-    virtual void keydown_event(GUI::KeyEvent&) override;
     virtual void enter_event(Core::Event&) override;
     virtual void leave_event(Core::Event&) override;
 
@@ -87,18 +85,26 @@ private:
         GUI::TextPosition position;
     };
 
-    Optional<AutoCompleteRequestData> get_autocomplete_request_data();
+    class LanguageServerAidedAutocompleteProvider final : virtual public GUI::AutocompleteProvider {
+    public:
+        LanguageServerAidedAutocompleteProvider(LanguageClient& client)
+            : m_language_client(client)
+        {
+        }
+        virtual ~LanguageServerAidedAutocompleteProvider() override { }
+
+    private:
+        virtual void provide_completions(Function<void(Vector<Entry>)> callback) override;
+        LanguageClient& m_language_client;
+    };
 
-    void update_autocomplete(const AutoCompleteRequestData&);
-    void show_autocomplete(const AutoCompleteRequestData&);
-    void close_autocomplete();
+    Optional<AutoCompleteRequestData> get_autocomplete_request_data();
 
     void flush_file_content_to_langauge_server();
 
     explicit Editor();
 
     RefPtr<GUI::Window> m_documentation_tooltip_window;
-    OwnPtr<AutoCompleteBox> m_autocomplete_box;
     RefPtr<Web::OutOfProcessWebView> m_documentation_page_view;
     String m_last_parsed_token;
     GUI::TextPosition m_previous_text_position { 0, 0 };
diff --git a/DevTools/HackStudio/LanguageClient.cpp b/DevTools/HackStudio/LanguageClient.cpp
index 462f5b9791..a10df6d4b6 100644
--- a/DevTools/HackStudio/LanguageClient.cpp
+++ b/DevTools/HackStudio/LanguageClient.cpp
@@ -61,7 +61,7 @@ void LanguageClient::request_autocomplete(const String& path, size_t cursor_line
     m_connection.post_message(Messages::LanguageServer::AutoCompleteSuggestions(path, cursor_line, cursor_column));
 }
 
-void LanguageClient::provide_autocomplete_suggestions(const Vector<AutoCompleteResponse>& suggestions)
+void LanguageClient::provide_autocomplete_suggestions(const Vector<GUI::AutocompleteProvider::Entry>& suggestions)
 {
     if (on_autocomplete_suggestions)
         on_autocomplete_suggestions(suggestions);
diff --git a/DevTools/HackStudio/LanguageClient.h b/DevTools/HackStudio/LanguageClient.h
index bd3298d7a8..bf294a7ea6 100644
--- a/DevTools/HackStudio/LanguageClient.h
+++ b/DevTools/HackStudio/LanguageClient.h
@@ -104,9 +104,9 @@ public:
     virtual void remove_text(const String& path, size_t from_line, size_t from_column, size_t to_line, size_t to_column);
     virtual void request_autocomplete(const String& path, size_t cursor_line, size_t cursor_column);
 
-    void provide_autocomplete_suggestions(const Vector<AutoCompleteResponse>&);
+    void provide_autocomplete_suggestions(const Vector<GUI::AutocompleteProvider::Entry>&);
 
-    Function<void(Vector<AutoCompleteResponse>)> on_autocomplete_suggestions;
+    Function<void(Vector<GUI::AutocompleteProvider::Entry>)> on_autocomplete_suggestions;
 
 private:
     ServerConnection& m_connection;
diff --git a/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.cpp b/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.cpp
index 90d48756d6..a6fae1c425 100644
--- a/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.cpp
+++ b/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.cpp
@@ -32,10 +32,10 @@
 
 namespace LanguageServers::Cpp {
 
-Vector<AutoCompleteResponse> AutoComplete::get_suggestions(const String& code, const GUI::TextPosition& autocomplete_position)
+Vector<GUI::AutocompleteProvider::Entry> AutoComplete::get_suggestions(const String& code, const GUI::TextPosition& autocomplete_position)
 {
     auto lines = code.split('\n', true);
-    Lexer lexer(code);
+    Cpp::Lexer lexer(code);
     auto tokens = lexer.lex();
 
     auto index_of_target_token = token_in_position(tokens, autocomplete_position);
@@ -75,10 +75,10 @@ Optional<size_t> AutoComplete::token_in_position(const Vector<Cpp::Token>& token
     return {};
 }
 
-Vector<AutoCompleteResponse> AutoComplete::identifier_prefixes(const Vector<String>& lines, const Vector<Cpp::Token>& tokens, size_t target_token_index)
+Vector<GUI::AutocompleteProvider::Entry> AutoComplete::identifier_prefixes(const Vector<String>& lines, const Vector<Cpp::Token>& tokens, size_t target_token_index)
 {
     auto partial_input = text_of_token(lines, tokens[target_token_index]);
-    Vector<AutoCompleteResponse> suggestions;
+    Vector<GUI::AutocompleteProvider::Entry> suggestions;
 
     HashTable<String> suggestions_lookup; // To avoid duplicate results
 
@@ -88,7 +88,7 @@ Vector<AutoCompleteResponse> AutoComplete::identifier_prefixes(const Vector<Stri
             continue;
         auto text = text_of_token(lines, token);
         if (text.starts_with(partial_input) && suggestions_lookup.set(text) == AK::HashSetResult::InsertedNewEntry) {
-            suggestions.append({ text, partial_input.length(), HackStudio::CompletionKind::Identifier });
+            suggestions.append({ text, partial_input.length(), GUI::AutocompleteProvider::CompletionKind::Identifier });
         }
     }
     return suggestions;
diff --git a/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.h b/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.h
index 650afc36a2..b4096bd793 100644
--- a/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.h
+++ b/DevTools/HackStudio/LanguageServers/Cpp/AutoComplete.h
@@ -35,18 +35,17 @@
 namespace LanguageServers::Cpp {
 
 using namespace ::Cpp;
-using ::HackStudio::AutoCompleteResponse;
 
 class AutoComplete {
 public:
     AutoComplete() = delete;
 
-    static Vector<AutoCompleteResponse> get_suggestions(const String& code, const GUI::TextPosition& autocomplete_position);
+    static Vector<GUI::AutocompleteProvider::Entry> get_suggestions(const String& code, const GUI::TextPosition& autocomplete_position);
 
 private:
     static Optional<size_t> token_in_position(const Vector<Cpp::Token>&, const GUI::TextPosition&);
     static StringView text_of_token(const Vector<String>& lines, const Cpp::Token&);
-    static Vector<AutoCompleteResponse> identifier_prefixes(const Vector<String>& lines, const Vector<Cpp::Token>&, size_t target_token_index);
+    static Vector<GUI::AutocompleteProvider::Entry> identifier_prefixes(const Vector<String>& lines, const Vector<Cpp::Token>&, size_t target_token_index);
 };
 
 }
diff --git a/DevTools/HackStudio/LanguageServers/LanguageClient.ipc b/DevTools/HackStudio/LanguageServers/LanguageClient.ipc
index 66a63d53fb..31b5d5b2c5 100644
--- a/DevTools/HackStudio/LanguageServers/LanguageClient.ipc
+++ b/DevTools/HackStudio/LanguageServers/LanguageClient.ipc
@@ -1,4 +1,4 @@
 endpoint LanguageClient = 8002
 {
-    AutoCompleteSuggestions(Vector<HackStudio::AutoCompleteResponse> suggestions) =|
+    AutoCompleteSuggestions(Vector<GUI::AutocompleteProvider::Entry> suggestions) =|
 }
diff --git a/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.cpp b/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.cpp
index 25e5334aae..89df78365a 100644
--- a/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.cpp
+++ b/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.cpp
@@ -35,7 +35,7 @@
 
 namespace LanguageServers::Shell {
 
-Vector<AutoCompleteResponse> AutoComplete::get_suggestions(const String& code, size_t offset)
+Vector<GUI::AutocompleteProvider::Entry> AutoComplete::get_suggestions(const String& code, size_t offset)
 {
     // FIXME: No need to reparse this every time!
     auto ast = ::Shell::Parser { code }.parse();
@@ -49,7 +49,7 @@ Vector<AutoCompleteResponse> AutoComplete::get_suggestions(const String& code, s
 #endif
 
     auto result = ast->complete_for_editor(m_shell, offset);
-    Vector<AutoCompleteResponse> completions;
+    Vector<GUI::AutocompleteProvider::Entry> completions;
     for (auto& entry : result) {
 #ifdef DEBUG_AUTOCOMPLETE
         dbgln("Suggestion: '{}' starting at {}", entry.text_string, entry.input_offset);
diff --git a/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.h b/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.h
index 924864bcf4..fece37dcb7 100644
--- a/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.h
+++ b/DevTools/HackStudio/LanguageServers/Shell/AutoComplete.h
@@ -34,8 +34,6 @@
 
 namespace LanguageServers::Shell {
 
-using namespace ::HackStudio;
-
 class AutoComplete {
 public:
     AutoComplete()
@@ -43,7 +41,7 @@ public:
     {
     }
 
-    Vector<AutoCompleteResponse> get_suggestions(const String& code, size_t autocomplete_position);
+    Vector<GUI::AutocompleteProvider::Entry> get_suggestions(const String& code, size_t autocomplete_position);
 
 private:
     NonnullRefPtr<::Shell::Shell> m_shell;
diff --git a/DevTools/HackStudio/AutoCompleteBox.cpp b/Libraries/LibGUI/AutocompleteProvider.cpp
index d5950c125c..9ca6b14372 100644
--- a/DevTools/HackStudio/AutoCompleteBox.cpp
+++ b/Libraries/LibGUI/AutocompleteProvider.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2020, the SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -24,20 +24,21 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include "AutoCompleteBox.h"
-#include "Editor.h"
+#include <LibGUI/AutocompleteProvider.h>
 #include <LibGUI/Model.h>
 #include <LibGUI/TableView.h>
+#include <LibGUI/TextEditor.h>
 #include <LibGUI/Window.h>
 #include <LibGfx/Bitmap.h>
 
-namespace HackStudio {
+static RefPtr<Gfx::Bitmap> s_cpp_identifier_icon;
+static RefPtr<Gfx::Bitmap> s_unspecified_identifier_icon;
 
-static RefPtr<Gfx::Bitmap> s_cplusplus_icon;
+namespace GUI {
 
-class AutoCompleteSuggestionModel final : public GUI::Model {
+class AutocompleteSuggestionModel final : public GUI::Model {
 public:
-    explicit AutoCompleteSuggestionModel(Vector<AutoCompleteResponse>&& suggestions)
+    explicit AutocompleteSuggestionModel(Vector<AutocompleteProvider::Entry>&& suggestions)
         : m_suggestions(move(suggestions))
     {
     }
@@ -64,11 +65,20 @@ public:
                 return suggestion.completion;
             }
             if (index.column() == Column::Icon) {
-                // TODO: Have separate icons for fields, functions, methods etc
-                // FIXME: Probably should have different icons for the different kinds, rather than for "c++".
-                if (suggestion.kind == CompletionKind::Identifier)
-                    return *s_cplusplus_icon;
-                return *s_cplusplus_icon;
+                // TODO
+                if (suggestion.language == GUI::AutocompleteProvider::Language::Cpp) {
+                    if (!s_cpp_identifier_icon) {
+                        s_cpp_identifier_icon = Gfx::Bitmap::load_from_file("/res/icons/16x16/completion/cpp-identifier.png");
+                    }
+                    return *s_cpp_identifier_icon;
+                }
+                if (suggestion.language == GUI::AutocompleteProvider::Language::Unspecified) {
+                    if (!s_unspecified_identifier_icon) {
+                        s_unspecified_identifier_icon = Gfx::Bitmap::load_from_file("/res/icons/16x16/completion/unspecified-identifier.png");
+                    }
+                    return *s_unspecified_identifier_icon;
+                }
+                return {};
             }
         }
 
@@ -83,18 +93,14 @@ public:
     virtual void update() override {};
 
 private:
-    Vector<AutoCompleteResponse> m_suggestions;
+    Vector<AutocompleteProvider::Entry> m_suggestions;
 };
 
-AutoCompleteBox::~AutoCompleteBox() { }
+AutocompleteBox::~AutocompleteBox() { }
 
-AutoCompleteBox::AutoCompleteBox(WeakPtr<Editor> editor)
-    : m_editor(move(editor))
+AutocompleteBox::AutocompleteBox(TextEditor& editor)
+    : m_editor(editor)
 {
-    if (!s_cplusplus_icon) {
-        s_cplusplus_icon = Gfx::Bitmap::load_from_file("/res/icons/16x16/filetype-cplusplus.png");
-    }
-
     m_popup_window = GUI::Window::construct();
     m_popup_window->set_window_type(GUI::WindowType::Tooltip);
     m_popup_window->set_rect(0, 0, 200, 100);
@@ -103,13 +109,13 @@ AutoCompleteBox::AutoCompleteBox(WeakPtr<Editor> editor)
     m_suggestion_view->set_column_headers_visible(false);
 }
 
-void AutoCompleteBox::update_suggestions(Vector<AutoCompleteResponse>&& suggestions)
+void AutocompleteBox::update_suggestions(Vector<AutocompleteProvider::Entry>&& suggestions)
 {
     if (suggestions.is_empty())
         return;
 
     bool has_suggestions = !suggestions.is_empty();
-    m_suggestion_view->set_model(adopt(*new AutoCompleteSuggestionModel(move(suggestions))));
+    m_suggestion_view->set_model(adopt(*new AutocompleteSuggestionModel(move(suggestions))));
 
     if (!has_suggestions)
         m_suggestion_view->selection().clear();
@@ -117,18 +123,23 @@ void AutoCompleteBox::update_suggestions(Vector<AutoCompleteResponse>&& suggesti
         m_suggestion_view->selection().set(m_suggestion_view->model()->index(0));
 }
 
-void AutoCompleteBox::show(Gfx::IntPoint suggstion_box_location)
+bool AutocompleteBox::is_visible() const
+{
+    return m_popup_window->is_visible();
+}
+
+void AutocompleteBox::show(Gfx::IntPoint suggstion_box_location)
 {
     m_popup_window->move_to(suggstion_box_location);
     m_popup_window->show();
 }
 
-void AutoCompleteBox::close()
+void AutocompleteBox::close()
 {
     m_popup_window->hide();
 }
 
-void AutoCompleteBox::next_suggestion()
+void AutocompleteBox::next_suggestion()
 {
     GUI::ModelIndex new_index = m_suggestion_view->selection().first();
     if (new_index.is_valid())
@@ -142,7 +153,7 @@ void AutoCompleteBox::next_suggestion()
     }
 }
 
-void AutoCompleteBox::previous_suggestion()
+void AutocompleteBox::previous_suggestion()
 {
     GUI::ModelIndex new_index = m_suggestion_view->selection().first();
     if (new_index.is_valid())
@@ -156,22 +167,25 @@ void AutoCompleteBox::previous_suggestion()
     }
 }
 
-void AutoCompleteBox::apply_suggestion()
+void AutocompleteBox::apply_suggestion()
 {
     if (m_editor.is_null())
         return;
 
+    if (!m_editor->is_editable())
+        return;
+
     auto selected_index = m_suggestion_view->selection().first();
     if (!selected_index.is_valid())
         return;
 
-    auto suggestion_index = m_suggestion_view->model()->index(selected_index.row(), AutoCompleteSuggestionModel::Column::Name);
+    auto suggestion_index = m_suggestion_view->model()->index(selected_index.row(), AutocompleteSuggestionModel::Column::Name);
     auto suggestion = suggestion_index.data().to_string();
-    size_t partial_length = suggestion_index.data((GUI::ModelRole)AutoCompleteSuggestionModel::InternalRole::PartialInputLength).to_i64();
+    size_t partial_length = suggestion_index.data((GUI::ModelRole)AutocompleteSuggestionModel::InternalRole::PartialInputLength).to_i64();
 
     ASSERT(suggestion.length() >= partial_length);
     auto completion = suggestion.substring_view(partial_length, suggestion.length() - partial_length);
     m_editor->insert_at_cursor_or_replace_selection(completion);
 }
 
-};
+}
diff --git a/DevTools/HackStudio/AutoCompleteBox.h b/Libraries/LibGUI/AutocompleteProvider.h
index ef4bdcf4a5..6987f3b0ca 100644
--- a/DevTools/HackStudio/AutoCompleteBox.h
+++ b/Libraries/LibGUI/AutocompleteProvider.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2020, the SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -26,20 +26,57 @@
 
 #pragma once
 
-#include "AutoCompleteResponse.h"
-#include <AK/WeakPtr.h>
-#include <LibGUI/Widget.h>
+#include <LibGUI/Forward.h>
+#include <LibGUI/TextEditor.h>
+#include <LibGUI/Window.h>
 
-namespace HackStudio {
+namespace GUI {
 
-class Editor;
+class AutocompleteProvider {
+    AK_MAKE_NONCOPYABLE(AutocompleteProvider);
+    AK_MAKE_NONMOVABLE(AutocompleteProvider);
 
-class AutoCompleteBox final {
 public:
-    explicit AutoCompleteBox(WeakPtr<Editor>);
-    ~AutoCompleteBox();
+    virtual ~AutocompleteProvider() { }
 
-    void update_suggestions(Vector<AutoCompleteResponse>&& suggestions);
+    enum class CompletionKind {
+        Identifier,
+    };
+
+    enum class Language {
+        Unspecified,
+        Cpp,
+    };
+
+    struct Entry {
+        String completion;
+        size_t partial_input_length { 0 };
+        CompletionKind kind { CompletionKind::Identifier };
+        Language language { Language::Unspecified };
+    };
+
+    virtual void provide_completions(Function<void(Vector<Entry>)>) = 0;
+
+    void attach(TextEditor& editor)
+    {
+        ASSERT(!m_editor);
+        m_editor = editor;
+    }
+    void detach() { m_editor.clear(); }
+
+protected:
+    AutocompleteProvider() { }
+
+    WeakPtr<TextEditor> m_editor;
+};
+
+class AutocompleteBox final {
+public:
+    explicit AutocompleteBox(TextEditor&);
+    ~AutocompleteBox();
+
+    void update_suggestions(Vector<AutocompleteProvider::Entry>&& suggestions);
+    bool is_visible() const;
     void show(Gfx::IntPoint suggstion_box_location);
     void close();
 
@@ -48,10 +85,9 @@ public:
     void apply_suggestion();
 
 private:
-    void complete_suggestion(const GUI::ModelIndex&);
-
-    WeakPtr<Editor> m_editor;
+    WeakPtr<TextEditor> m_editor;
     RefPtr<GUI::Window> m_popup_window;
     RefPtr<GUI::TableView> m_suggestion_view;
 };
+
 }
diff --git a/Libraries/LibGUI/CMakeLists.txt b/Libraries/LibGUI/CMakeLists.txt
index a4514e8f1e..3aefaaab94 100644
--- a/Libraries/LibGUI/CMakeLists.txt
+++ b/Libraries/LibGUI/CMakeLists.txt
@@ -6,9 +6,10 @@ set(SOURCES
     Action.cpp
     ActionGroup.cpp
     Application.cpp
+    AutocompleteProvider.cpp
     BoxLayout.cpp
-    Button.cpp
     BreadcrumbBar.cpp
+    Button.cpp
     Calendar.cpp
     CheckBox.cpp
     Clipboard.cpp
diff --git a/Libraries/LibGUI/TextEditor.cpp b/Libraries/LibGUI/TextEditor.cpp
index 133db5c567..ecbd0913c8 100644
--- a/Libraries/LibGUI/TextEditor.cpp
+++ b/Libraries/LibGUI/TextEditor.cpp
@@ -25,10 +25,12 @@
  */
 
 #include <AK/QuickSort.h>
+#include <AK/ScopeGuard.h>
 #include <AK/StringBuilder.h>
 #include <AK/TemporaryChange.h>
 #include <LibCore/Timer.h>
 #include <LibGUI/Action.h>
+#include <LibGUI/AutocompleteProvider.h>
 #include <LibGUI/Clipboard.h>
 #include <LibGUI/InputBox.h>
 #include <LibGUI/Menu.h>
@@ -711,6 +713,27 @@ void TextEditor::sort_selected_lines()
 
 void TextEditor::keydown_event(KeyEvent& event)
 {
+    if (m_autocomplete_box && m_autocomplete_box->is_visible() && (event.key() == KeyCode::Key_Return || event.key() == KeyCode::Key_Tab)) {
+        m_autocomplete_box->apply_suggestion();
+        m_autocomplete_box->close();
+        return;
+    }
+
+    if (m_autocomplete_box && m_autocomplete_box->is_visible() && event.key() == KeyCode::Key_Escape) {
+        m_autocomplete_box->close();
+        return;
+    }
+
+    if (m_autocomplete_box && m_autocomplete_box->is_visible() && event.key() == KeyCode::Key_Up) {
+        m_autocomplete_box->previous_suggestion();
+        return;
+    }
+
+    if (m_autocomplete_box && m_autocomplete_box->is_visible() && event.key() == KeyCode::Key_Down) {
+        m_autocomplete_box->next_suggestion();
+        return;
+    }
+
     if (is_single_line() && event.key() == KeyCode::Key_Tab)
         return ScrollableWidget::keydown_event(event);
 
@@ -720,6 +743,14 @@ void TextEditor::keydown_event(KeyEvent& event)
         return;
     }
 
+    ArmedScopeGuard update_autocomplete { [&] {
+        if (m_autocomplete_box && m_autocomplete_box->is_visible()) {
+            m_autocomplete_provider->provide_completions([&](auto completions) {
+                m_autocomplete_box->update_suggestions(move(completions));
+            });
+        }
+    } };
+
     if (event.key() == KeyCode::Key_Escape) {
         if (on_escape_pressed)
             on_escape_pressed();
@@ -997,6 +1028,18 @@ void TextEditor::keydown_event(KeyEvent& event)
         return;
     }
 
+    if (!event.shift() && !event.alt() && event.ctrl() && event.key() == KeyCode::Key_Space) {
+        if (m_autocomplete_provider) {
+            m_autocomplete_provider->provide_completions([&](auto completions) {
+                m_autocomplete_box->update_suggestions(move(completions));
+                auto position = content_rect_for_position(cursor()).bottom_right().translated(screen_relative_rect().top_left().translated(ruler_width(), 0).translated(10, 5));
+                m_autocomplete_box->show(position);
+            });
+            update_autocomplete.disarm();
+            return;
+        }
+    }
+
     if (is_editable() && !event.ctrl() && !event.alt() && event.code_point() != 0) {
         StringBuilder sb;
         sb.append_code_point(event.code_point());
@@ -1759,6 +1802,25 @@ void TextEditor::set_syntax_highlighter(OwnPtr<SyntaxHighlighter> highlighter)
         document().set_spans({});
 }
 
+const AutocompleteProvider* TextEditor::autocomplete_provider() const
+{
+    return m_autocomplete_provider.ptr();
+}
+
+void TextEditor::set_autocomplete_provider(OwnPtr<AutocompleteProvider>&& provider)
+{
+    if (m_autocomplete_provider)
+        m_autocomplete_provider->detach();
+    m_autocomplete_provider = move(provider);
+    if (m_autocomplete_provider) {
+        m_autocomplete_provider->attach(*this);
+        if (!m_autocomplete_box)
+            m_autocomplete_box = make<AutocompleteBox>(*this);
+    }
+    if (m_autocomplete_box)
+        m_autocomplete_box->close();
+}
+
 int TextEditor::line_height() const
 {
     return font().glyph_height() + m_line_spacing;
diff --git a/Libraries/LibGUI/TextEditor.h b/Libraries/LibGUI/TextEditor.h
index 6f55d39a44..f5b8b2284c 100644
--- a/Libraries/LibGUI/TextEditor.h
+++ b/Libraries/LibGUI/TextEditor.h
@@ -30,6 +30,7 @@
 #include <AK/NonnullOwnPtrVector.h>
 #include <AK/NonnullRefPtrVector.h>
 #include <LibCore/ElapsedTimer.h>
+#include <LibGUI/Forward.h>
 #include <LibGUI/ScrollableWidget.h>
 #include <LibGUI/TextDocument.h>
 #include <LibGUI/TextRange.h>
@@ -159,6 +160,9 @@ public:
     const SyntaxHighlighter* syntax_highlighter() const;
     void set_syntax_highlighter(OwnPtr<SyntaxHighlighter>);
 
+    const AutocompleteProvider* autocomplete_provider() const;
+    void set_autocomplete_provider(OwnPtr<AutocompleteProvider>&&);
+
     bool is_in_drag_select() const { return m_in_drag_select; }
 
 protected:
@@ -317,6 +321,8 @@ private:
     NonnullOwnPtrVector<LineVisualData> m_line_visual_data;
 
     OwnPtr<SyntaxHighlighter> m_highlighter;
+    OwnPtr<AutocompleteProvider> m_autocomplete_provider;
+    OwnPtr<AutocompleteBox> m_autocomplete_box;
 
     RefPtr<Core::Timer> m_automatic_selection_scroll_timer;
     Gfx::IntPoint m_last_mousemove_position;