Consolidate input completion code in preparation for advanced completion strategies (#4639)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/controllers/completion/sources/UserSource.hpp

@@ -0,0 +1,53 @@
+#pragma once
+
+#include "common/Channel.hpp"
+#include "controllers/completion/sources/Source.hpp"
+#include "controllers/completion/strategies/Strategy.hpp"
+
+#include <QString>
+
+#include <functional>
+#include <memory>
+#include <utility>
+#include <vector>
+
+namespace chatterino::completion {
+
+using UserItem = std::pair<QString, QString>;
+
+class UserSource : public Source
+{
+public:
+    using ActionCallback = std::function<void(const QString &)>;
+    using UserStrategy = Strategy<UserItem>;
+
+    /// @brief Initializes a source for UserItems from the given channel.
+    /// @param channel Channel to initialize users from. Must be a TwitchChannel
+    /// or completion is a no-op.
+    /// @param strategy Strategy to apply
+    /// @param callback ActionCallback to invoke upon InputCompletionItem selection.
+    /// See InputCompletionItem::action(). Can be nullptr.
+    /// @param prependAt Whether to prepend @ to string completion suggestions.
+    UserSource(const Channel *channel, std::unique_ptr<UserStrategy> strategy,
+               ActionCallback callback = nullptr, bool prependAt = true);
+
+    void update(const QString &query) override;
+    void addToListModel(GenericListModel &model,
+                        size_t maxCount = 0) const override;
+    void addToStringList(QStringList &list, size_t maxCount = 0,
+                         bool isFirstWord = false) const override;
+
+    const std::vector<UserItem> &output() const;
+
+private:
+    void initializeFromChannel(const Channel *channel);
+
+    std::unique_ptr<UserStrategy> strategy_;
+    ActionCallback callback_;
+    bool prependAt_;
+
+    std::vector<UserItem> items_{};
+    std::vector<UserItem> output_{};
+};
+
+}  // namespace chatterino::completion