データバインディングとイベント処理

Blazor のバインディング

一方向データバインディング

<!-- One-way: C# -> DOM -->
<p>Count: @_count</p>
<p>Full name: @($"{_firstName} {_lastName}")</p>
<p>Price: @Price.ToString("C")</p>

@code {
    private int _count = 0;
    private string _firstName = "Alice";
    private string _lastName = "Smith";
    [Parameter] public decimal Price { get; set; }
}

@bind による双方向バインディング

<input @bind="_name" placeholder="Enter name" />
<input type="number" @bind="_quantity" />
<input type="date" @bind="_dueDate" />
<input type="checkbox" @bind="_isActive" />

<p>Name: @_name | Qty: @_quantity | Active: @_isActive</p>

@code {
    private string _name = "";
    private int _quantity = 1;
    private DateTime _dueDate = DateTime.Today;
    private bool _isActive = true;
}

カスタムイベントでの @bind

@* Updates on every keystroke instead of on focus-loss *@
<input @bind="_search"
       @bind:event="oninput"
       placeholder="Live search..." />

<ul>
    @foreach (var item in Filtered)
    {
        <li>@item</li>
    }
</ul>

@code {
    private string _search = "";
    private string[] Items = { "Apple", "Banana", "Cherry", "Avocado" };
    private IEnumerable<string> Filtered =>
        Items.Where(i => i.Contains(_search, StringComparison.OrdinalIgnoreCase));
}

@onclick によるイベント処理

<button @onclick="Increment">+</button>
<button @onclick="DecrementAsync">-</button>
<button @onclick="() => _count = 0">Reset</button>
<p>Count: @_count</p>

@code {
    private int _count = 0;

    private void Increment() => _count++;

    private async Task DecrementAsync()
    {
        await Task.Delay(200); // simulate async work
        _count--;
    }
}

イベント引数

<input @onkeydown="OnKeyDown" @onfocus="OnFocus" />
<div @onmousemove="OnMouseMove">Move here</div>
<p>@_info</p>

@code {
    private string _info = "";

    private void OnKeyDown(KeyboardEventArgs e)
        => _info = $"Key: {e.Key} (Ctrl={e.CtrlKey})";

    private void OnFocus(FocusEventArgs e)
        => _info = "Input focused";

    private void OnMouseMove(MouseEventArgs e)
        => _info = $"Mouse: ({e.ClientX}, {e.ClientY})";
}

親子間通信のための EventCallback

@* Child: ProductCard.razor *@
<button @onclick="() => OnBuy.InvokeAsync(Product)">Buy</button>

@code {
    [Parameter] public Product Product { get; set; } = default!;
    [Parameter] public EventCallback<Product> OnBuy { get; set; }
}

@* Parent: *@
<ProductCard Product="p" OnBuy="HandleBuy" />

@code {
    private async Task HandleBuy(Product p)
    {
        await CartService.AddAsync(p);
        // Parent re-renders automatically
    }
}

デフォルト動作の防止と伝播の停止

@* Prevent form submission *@
<form @onsubmit="HandleSubmit" @onsubmit:preventDefault>
    <input @bind="_value" />
    <button type="submit">Save</button>
</form>

@* Stop click from bubbling to parent div *@
<div @onclick="ParentClicked">
    <button @onclick="ChildClicked"
            @onclick:stopPropagation>
        Click me
    </button>
</div>

カスタムコンポーネントでの @bind-Value による双方向バインディング

@* RatingInput.razor *@
@for (int i = 1; i <= 5; i++)
{
    int star = i;
    <span class="@(star <= Value ? "filled" : "")"
          @onclick="() => ValueChanged.InvokeAsync(star)">★</span>
}

@code {
    [Parameter] public int Value { get; set; }
    [Parameter] public EventCallback<int> ValueChanged { get; set; }
}

@* Usage: *@
<RatingInput @bind-Value="_rating" />

入力イベントのデバウンス

@code {
    private string _query = "";
    private CancellationTokenSource? _cts;

    private async Task OnSearchInput(ChangeEventArgs e)
    {
        _query = e.Value?.ToString() ?? "";
        _cts?.Cancel();
        _cts = new CancellationTokenSource();

        try
        {
            await Task.Delay(300, _cts.Token); // wait 300ms
            Results = await SearchService.SearchAsync(_query);
        }
        catch (TaskCanceledException) { /* user still typing */ }
    }

    private List<string> Results = new();
}

確認問題

まとめ：データバインディングとイベント処理