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+//===--- Tweak.h -------------------------------------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Tweaks are small refactoring-like actions that run over the AST and produce
+// the set of edits as a result. They are local, i.e. they should take the
+// current editor context, e.g. the cursor position and selection into account.
+// The actions are executed in two stages:
+//   - Stage 1 should check whether the action is available in a current
+//     context. It should be cheap and fast to compute as it is executed for all
+//     available actions on every client request, which happen quite frequently.
+//   - Stage 2 is performed after stage 1 and can be more expensive to compute.
+//     It is performed when the user actually chooses the action.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_REFACTOR_ACTIONS_TWEAK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_REFACTOR_ACTIONS_TWEAK_H
+
+#include "ClangdUnit.h"
+#include "Protocol.h"
+#include "Selection.h"
+#include "clang/Tooling/Core/Replacement.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringRef.h"
+namespace clang {
+namespace clangd {
+
+/// An interface base for small context-sensitive refactoring actions.
+/// To implement a new tweak use the following pattern in a .cpp file:
+///   class MyTweak : public Tweak {
+///   public:
+///     const char* id() const override final; // defined by REGISTER_TWEAK.
+///     // implement other methods here.
+///   };
+///   REGISTER_TWEAK(MyTweak);
+class Tweak {
+public:
+  /// Input to prepare and apply tweaks.
+  struct Selection {
+    Selection(ParsedAST &AST, unsigned RangeBegin, unsigned RangeEnd);
+    /// The text of the active document.
+    llvm::StringRef Code;
+    /// Parsed AST of the active file.
+    ParsedAST &AST;
+    /// A location of the cursor in the editor.
+    SourceLocation Cursor;
+    // The AST nodes that were selected.
+    SelectionTree ASTSelection;
+    // FIXME: provide a way to get sources and ASTs for other files.
+  };
+  virtual ~Tweak() = default;
+  /// A unique id of the action, it is always equal to the name of the class
+  /// defining the Tweak. Definition is provided automatically by
+  /// REGISTER_TWEAK.
+  virtual const char *id() const = 0;
+  /// Run the first stage of the action. Returns true indicating that the
+  /// action is available and should be shown to the user. Returns false if the
+  /// action is not available.
+  /// This function should be fast, if the action requires non-trivial work it
+  /// should be moved into 'apply'.
+  /// Returns true iff the action is available and apply() can be called on it.
+  virtual bool prepare(const Selection &Sel) = 0;
+  /// Run the second stage of the action that would produce the actual changes.
+  /// EXPECTS: prepare() was called and returned true.
+  virtual Expected<tooling::Replacements> apply(const Selection &Sel) = 0;
+  /// A one-line title of the action that should be shown to the users in the
+  /// UI.
+  /// EXPECTS: prepare() was called and returned true.
+  virtual std::string title() const = 0;
+};
+
+// All tweaks must be registered in the .cpp file next to their definition.
+#define REGISTER_TWEAK(Subclass)                                               \
+  ::llvm::Registry<::clang::clangd::Tweak>::Add<Subclass>                      \
+      TweakRegistrationFor##Subclass(#Subclass, /*Description=*/"");           \
+  const char *Subclass::id() const { return #Subclass; }
+
+/// Calls prepare() on all tweaks, returning those that can run on the
+/// selection.
+std::vector<std::unique_ptr<Tweak>> prepareTweaks(const Tweak::Selection &S);
+
+// Calls prepare() on the tweak with a given ID.
+// If prepare() returns false, returns an error.
+// If prepare() returns true, returns the corresponding tweak.
+llvm::Expected<std::unique_ptr<Tweak>> prepareTweak(StringRef TweakID,
+                                                    const Tweak::Selection &S);
+
+} // namespace clangd
+} // namespace clang
+
+#endif