当另一个类中的先决属性更改时，为依赖属性提高 PropertyChanged？

Question

我有这个Bank 类：

public class Bank : INotifyPropertyChanged
{
    public Bank(Account account1, Account account2)
    {
        Account1 = account1;
        Account2 = account2;
    }

    public Account Account1 { get; }
    public Account Account2 { get; }

    public int Total => Account1.Balance + Account2.Balance;

    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    public void RaisePropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

Bank 依赖于其他类，并有一个属性Total，它是根据这些其他类的属性计算得出的。每当这些 Account.Balance 属性中的任何一个发生更改时，都会为 Account.Balance 引发 PropertyChanged：

public class Account : INotifyPropertyChanged
{
    private int _balance;

    public int Balance
    {
        get { return _balance; }
        set
        {
            _balance = value;
            RaisePropertyChanged();
        }
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    public void RaisePropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

每当任何必备属性发生更改时，我都想为Total 提高PropertyChanged。我怎样才能以易于测试的方式做到这一点？

TL;DR当另一个类中的先决属性发生更改时，如何为依赖属性引发 PropertyChanged？

Answer 1

您可以通过多种不同的方式做到这一点。我见过许多不同的解决方案，包括自定义属性或在单个属性setter 中引发多个PropertyChanged 事件。我认为这些灵魂提法大部分都是反模式，不易测试。

我和一位同事 (Robert Jørgensgaard Engdahl) 想出的最好方法是这个静态类：

public static class PropertyChangedPropagator
{
    public static PropertyChangedEventHandler Create(string sourcePropertyName, string dependantPropertyName, Action<string> raisePropertyChanged)
    {
        var infiniteRecursionDetected = false;
        return (sender, args) =>
        {
            try
            {
                if (args.PropertyName != sourcePropertyName) return;
                if (infiniteRecursionDetected)
                {
                    throw new InvalidOperationException("Infinite recursion detected");
                }
                infiniteRecursionDetected = true;
                raisePropertyChanged(dependantPropertyName);
            }
            finally
            {
                infiniteRecursionDetected = false;
            }
        };
    }
}

它会创建一个PropertyChangedEventHandler，您可以将其设置为侦听其他类的 PropertyChanged。它在抛出 StackOverflowException 之前使用 InvalidOperationException 处理循环依赖关系。

要在上面的示例中使用静态 PropertyChangedPropagator，您必须为每个必备属性添加一行代码：

public class Bank : INotifyPropertyChanged
{
    public Bank(Account account1, Account account2)
    {
        Account1 = account1;
        Account2 = account2;
        Account1.PropertyChanged += PropertyChangedPropagator.Create(nameof(Account.Balance), nameof(Total), RaisePropertyChanged);
        Account2.PropertyChanged += PropertyChangedPropagator.Create(nameof(Account.Balance), nameof(Total), RaisePropertyChanged);
    }

    public Account Account1 { get; }
    public Account Account2 { get; }

    public int Total => Account1.Balance + Account2.Balance;


    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    public void RaisePropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

这很容易测试（伪代码）：

[Test]
public void Total_PropertyChanged_Is_Raised_When_Account1_Balance_Is_Changed()
{
    var bank = new Bank(new Account(), new Account());

    bank.Account1.Balance += 10;

    Assert.PropertyChanged(bank, nameof(Bank.Total));
}