Xamarin 表單觸發器示例

Triggers 是一種為應用程式新增一些 UX 響應的簡單方法。一個簡單的方法是新增一個 Trigger，根據其相關的 Entry 是否輸入了文字來更改 Label 的 TextColor。

使用 Trigger 可以使 Label.TextColor 從灰色（沒有輸入文字時）變為黑色（一旦使用者輸入文字）：

轉換器（每個轉換器都有一個 Instance 變數，用於繫結，因此每次使用時都不會建立類的新例項）：

/// <summary>
/// Used in a XAML trigger to return <c>true</c> or <c>false</c> based on the length of <c>value</c>.
/// </summary>
public class LengthTriggerConverter : Xamarin.Forms.IValueConverter {

    /// <summary>
    /// Used so that a new instance is not created every time this converter is used in the XAML code.
    /// </summary>
    public static LengthTriggerConverter Instance = new LengthTriggerConverter();

    /// <summary>
    /// If a `ConverterParameter` is passed in, a check to see if <c>value</c> is greater than <c>parameter</c> is made. Otherwise, a check to see if <c>value</c> is over 0 is made.
    /// </summary>
    /// <param name="value">The length of the text from an Entry/Label/etc.</param>
    /// <param name="targetType">The Type of object/control that the text/value is coming from.</param>
    /// <param name="parameter">Optional, specify what length to test against (example: for 3 Letter Name, we would choose 2, since the 3 Letter Name Entry needs to be over 2 characters), if not specified, defaults to 0.</param>
    /// <param name="culture">The current culture set in the device.</param>
    /// <returns><c>object</c>, which is a <c>bool</c> (<c>true</c> if <c>value</c> is greater than 0 (or is greater than the parameter), <c>false</c> if not).</returns>
    public object Convert(object value, System.Type targetType, object parameter, CultureInfo culture)     { return DoWork(value, parameter); }
    public object ConvertBack(object value, System.Type targetType, object parameter, CultureInfo culture) { return DoWork(value, parameter); }

    private static object DoWork(object value, object parameter) {
        int parameterInt = 0;

        if(parameter != null) { //If param was specified, convert and use it, otherwise, 0 is used

            string parameterString = (string)parameter;

            if(!string.IsNullOrEmpty(parameterString)) { int.TryParse(parameterString, out parameterInt); }
        }

        return (int)value > parameterInt;
    }
}

XAML（XAML 程式碼使用 Entry 的 x:Name 來計算 Entry.Text 屬性超過 3 個字元。）：

<StackLayout>
    <Label Text="3 Letter Name">
        <Label.Triggers>
            <DataTrigger TargetType="Label"
                         Binding="{Binding Source={x:Reference NameEntry},
                                           Path=Text.Length,
                                           Converter={x:Static helpers:LengthTriggerConverter.Instance},
                                           ConverterParameter=2}"
                         Value="False">
                <Setter Property="TextColor"
                        Value="Gray"/>
            </DataTrigger>
        </Label.Triggers>
    </Label>
    <Entry x:Name="NameEntry"
           Text="{Binding MealAmount}"
           HorizontalOptions="StartAndExpand"/>
</StackLayout>