MVP 패턴

using System;
using System.Collections.Generic;
using System.Linq;

var view = new ConsoleSearchView();
var presenter = new SearchPresenter(view, new InMemoryCatalog());

view.Type("ke");
view.ClickSearch();
view.Type("k");
view.ClickSearch();

abstract record SearchResult;
sealed record Found(IReadOnlyList<string> Items) : SearchResult;
sealed record Failed(string Reason) : SearchResult;

// 모델: 화면을 모르는 검색 기능이다.
interface ICatalog { SearchResult Search(string query); }

sealed class InMemoryCatalog : ICatalog
{
    private readonly string[] products = ["keyboard", "mouse", "monitor", "keycap"];
    public SearchResult Search(string query) =>
        new Found(products.Where(p => p.Contains(query, StringComparison.OrdinalIgnoreCase)).ToList());
}

// 뷰 인터페이스: 프레젠터가 화면에 대해 아는 것은 이것뿐이다.
interface ISearchView
{
    string Query { get; }
    event Action? SearchRequested;
    void ShowResults(IReadOnlyList<string> items);
    void ShowError(string message);
}

// 프레젠터: 입력을 검사하고 모델을 호출한 뒤, 뷰에 보여 줄 내용을 정한다.
sealed class SearchPresenter
{
    private readonly ISearchView view;
    private readonly ICatalog catalog;

    public SearchPresenter(ISearchView view, ICatalog catalog)
    {
        this.view = view;
        this.catalog = catalog;
        view.SearchRequested += OnSearchRequested;
    }

    private void OnSearchRequested()
    {
        var query = view.Query.Trim();
        if (query.Length < 2)
        {
            view.ShowError("Enter at least 2 characters.");
            return;
        }
        switch (catalog.Search(query))
        {
            case Found found: view.ShowResults(found.Items); break;
            case Failed failed: view.ShowError("Search failed: " + failed.Reason); break;
            default: view.ShowError("Unexpected search result."); break;
        }
    }
}

// 콘솔 뷰: 입력을 받아 이벤트로 넘기고, 받은 내용을 그대로 출력만 한다.
sealed class ConsoleSearchView : ISearchView
{
    public string Query { get; private set; } = "";
    public event Action? SearchRequested;

    public void Type(string text)
    {
        Query = text;
        Console.WriteLine($"> search \"{text}\"");
    }
    public void ClickSearch() => SearchRequested?.Invoke();
    public void ShowResults(IReadOnlyList<string> items) => Console.WriteLine("results: [" + string.Join(", ", items) + "]");
    public void ShowError(string message) => Console.WriteLine("error: " + message);
}

> search "ke"
results: [keyboard, keycap]
> search "k"
error: Enter at least 2 characters.

using System;
using System.Collections.Generic;
using System.Linq;

var passed = 0;
var failed = 0;

// 1) 입력이 짧으면 모델을 부르지 않고, 이전 결과를 비운 뒤 검증 오류를 보여 준다.
{
    var view = new FakeSearchView();
    var catalog = new ScriptedCatalog();
    _ = new SearchPresenter(view, catalog);
    view.Query = "k";
    view.ClickSearch();
    Check("short query", view, catalog,
        ["ShowResults()", "ShowError(Enter at least 2 characters.)"], []);
}

// 2) 모델이 실패하면 이전 결과를 비우고 오류를 보여 준다.
{
    var view = new FakeSearchView();
    var catalog = new ScriptedCatalog(new Found(["keyboard"]), new Failed("catalog timeout"));
    _ = new SearchPresenter(view, catalog);
    view.Query = "key";
    view.ClickSearch();
    view.ClickSearch();
    Check("failure clears old results", view, catalog,
        ["ShowResults(keyboard)", "ShowResults()", "ShowError(Search failed: catalog timeout)"],
        ["key", "key"]);
}

// 3) 앞뒤 공백은 프레젠터가 잘라서 모델에 넘긴다.
{
    var view = new FakeSearchView();
    var catalog = new ScriptedCatalog(new Found([]));
    _ = new SearchPresenter(view, catalog);
    view.Query = "  zz  ";
    view.ClickSearch();
    Check("trim and no match", view, catalog,
        ["ShowResults()", "ShowError(No products match \"zz\".)"], ["zz"]);
}

Console.WriteLine($"{passed} passed, {failed} failed");

void Check(string name, FakeSearchView view, ScriptedCatalog catalog, string[] expectedCalls, string[] expectedQueries)
{
    var ok = view.Calls.SequenceEqual(expectedCalls) && catalog.Queries.SequenceEqual(expectedQueries);
    if (ok) passed++; else failed++;
    Console.WriteLine($"{(ok ? "PASS" : "FAIL")} {name}");
    Console.WriteLine("  view   : " + string.Join(" | ", view.Calls));
    Console.WriteLine("  catalog: " + (catalog.Queries.Count == 0 ? "(not called)" : string.Join(", ", catalog.Queries)));
}

abstract record SearchResult;
sealed record Found(IReadOnlyList<string> Items) : SearchResult;
sealed record Failed(string Reason) : SearchResult;

interface ICatalog { SearchResult Search(string query); }

// 가짜 모델: 정해 둔 결과를 순서대로 돌려주고, 받은 검색어를 기록한다.
sealed class ScriptedCatalog(params SearchResult[] script) : ICatalog
{
    private int next;
    public List<string> Queries { get; } = [];
    public SearchResult Search(string query)
    {
        Queries.Add(query);
        return script[next++];
    }
}

interface ISearchView
{
    string Query { get; }
    event Action? SearchRequested;
    void ShowResults(IReadOnlyList<string> items);
    void ShowError(string message);
}

sealed class SearchPresenter
{
    private readonly ISearchView view;
    private readonly ICatalog catalog;

    public SearchPresenter(ISearchView view, ICatalog catalog)
    {
        this.view = view;
        this.catalog = catalog;
        view.SearchRequested += OnSearchRequested;
    }

    private void OnSearchRequested()
    {
        var query = view.Query.Trim();
        if (query.Length < 2)
        {
            ShowFailure("Enter at least 2 characters.");
            return;
        }
        switch (catalog.Search(query))
        {
            case Found { Items.Count: 0 }:
                ShowFailure($"No products match \"{query}\".");
                break;
            case Found found:
                view.ShowResults(found.Items);
                break;
            case Failed failure:
                ShowFailure("Search failed: " + failure.Reason);
                break;
            default:
                ShowFailure("Unexpected search result.");
                break;
        }
    }

    // 오류와 함께 남은 이전 목록이 이번 검색의 결과처럼 보이지 않도록 먼저 비운다.
    private void ShowFailure(string message)
    {
        view.ShowResults([]);
        view.ShowError(message);
    }
}

// 가짜 뷰: 화면을 그리지 않고, 프레젠터가 보낸 호출을 문자열로 남긴다.
sealed class FakeSearchView : ISearchView
{
    public string Query { get; set; } = "";
    public event Action? SearchRequested;
    public List<string> Calls { get; } = [];

    public void ClickSearch() => SearchRequested?.Invoke();
    public void ShowResults(IReadOnlyList<string> items) => Calls.Add($"ShowResults({string.Join(", ", items)})");
    public void ShowError(string message) => Calls.Add($"ShowError({message})");
}

PASS short query
  view   : ShowResults() | ShowError(Enter at least 2 characters.)
  catalog: (not called)
PASS failure clears old results
  view   : ShowResults(keyboard) | ShowResults() | ShowError(Search failed: catalog timeout)
  catalog: key, key
PASS trim and no match
  view   : ShowResults() | ShowError(No products match "zz".)
  catalog: zz
3 passed, 0 failed

type SearchResult =
  | { kind: "found"; items: readonly string[] }
  | { kind: "failed"; reason: string };

// 모델: 화면을 모르는 검색 기능이다.
interface Catalog { search(query: string): SearchResult; }

class InMemoryCatalog implements Catalog {
  private readonly products = ["keyboard", "mouse", "monitor", "keycap"];
  search(query: string): SearchResult {
    const q = query.toLowerCase();
    return { kind: "found", items: this.products.filter((p) => p.includes(q)) };
  }
}

class DownCatalog implements Catalog {
  search(_query: string): SearchResult {
    return { kind: "failed", reason: "catalog timeout" };
  }
}

// 뷰 인터페이스: 프레젠터가 화면에 대해 아는 것은 이것뿐이다.
interface SearchView {
  readonly query: string;
  onSearch(handler: () => void): void;
  showResults(items: readonly string[]): void;
  showError(message: string): void;
}

// 프레젠터: 입력을 검사하고 모델을 호출한 뒤, 뷰에 보여 줄 내용을 정한다.
class SearchPresenter {
  constructor(private readonly view: SearchView, private readonly catalog: Catalog) {
    view.onSearch(() => this.search());
  }

  private search(): void {
    const query = this.view.query.trim();
    if (query.length < 2) {
      this.showFailure("Enter at least 2 characters.");
      return;
    }
    const result = this.catalog.search(query);
    if (result.kind === "failed") {
      this.showFailure("Search failed: " + result.reason);
      return;
    }
    if (result.items.length === 0) {
      this.showFailure(`No products match "${query}".`);
      return;
    }
    this.view.showResults(result.items);
  }

  // 오류와 함께 남은 이전 목록이 이번 검색의 결과처럼 보이지 않도록 먼저 비운다.
  private showFailure(message: string): void {
    this.view.showResults([]);
    this.view.showError(message);
  }
}

// 콘솔 뷰: 입력을 받아 이벤트로 넘기고, 받은 내용을 그대로 출력만 한다.
class ConsoleSearchView implements SearchView {
  query = "";
  private handler: () => void = () => {};
  onSearch(handler: () => void): void { this.handler = handler; }
  type(text: string): void {
    this.query = text;
    console.log(`> search "${text}"`);
  }
  clickSearch(): void { this.handler(); }
  showResults(items: readonly string[]): void { console.log("results: [" + items.join(", ") + "]"); }
  showError(message: string): void { console.log("error: " + message); }
}

const view = new ConsoleSearchView();
new SearchPresenter(view, new InMemoryCatalog());
view.type("k");
view.clickSearch();
view.type(" ke ");
view.clickSearch();
view.type("zz");
view.clickSearch();

const offline = new ConsoleSearchView();
new SearchPresenter(offline, new DownCatalog());
offline.type("ke");
offline.clickSearch();

> search "k"
results: []
error: Enter at least 2 characters.
> search " ke "
results: [keyboard, keycap]
> search "zz"
results: []
error: No products match "zz".
> search "ke"
results: []
error: Search failed: catalog timeout