如何在WPF中对聚合函数（如sum）进行数据绑定？

Question

Imagine I have the following class that implements INotifyPropertyChanged . (For brevity, I'm not showing the code that raises the PropertyChanged events, but imagine that each property does.)

public class Employee : INotifyPropertyChanged
{
    public string Title { get; set; }
    public string Department { get; set; }
    public double Salary { get; set; }
}

Now, let's say I have four employees:

     Title         Department    Salary
     -----------------------------------
(1)  Professor     Math           90,000
(2)  Professor     CompSci       120,000
(3)  Instructor    Math           50,000
(4)  Instructor    CompSci        60,000

I'd like to create a table using WPF like the following:

Title         Math      CompSci    Total
------------------------------------------
Professor     90,000    120,000    210,000
Instructor    50,000     60,000    110,000

It re-arranges the employees by title, showing their salaries for each department in separate columns. The last column shows the total salary per title.

Furthermore, I'd like the user to be able to interact with the table, such as being able to add or remove an employee, or change anyone's salary (and therefore automatically update the total).

I've been able to do this by re-creating the entire table anytime a change happens. But I'd like to be able to do this more efficiently (for example, only update a total when a salary is changed).

I've tried to do this, but the code becomes messy really quickly. My view model needs to listen to collection changes in Employees, and whenever one is added, it must listen to its Salary in order to update the Total. It's hard to follow what's going on, and I imagine it will be a maintenance nightmare later on.

I wish there was a way to bind the total and have it automatically keep track of the necessary changes. Here's some made-up code to show what I mean:

<TextBlock Text="{Binding Employees, Aggregate=Sum, Property=Salary}" />

I know this doesn't exist, but is there some easy way to implement what I'm trying to do that's understandable and maintainable? It doesn't have to be in XAML. I'm fine with a programmatic solution as long as it's clear what's going on.

Answer 1

I'm guessing somewhere you have a master class that has a list of employees like:

public ObservableCollection<Employee> Employees { get; private set; }

that you are adding your data to.

Now you can create a new output class from it like this:

public class Summary : ViewModelBase
{
    private string _Title;
    public string Title
    {
        get { return _Title; }
        set { Set(ref _Title, value); }
    }

    private decimal _MathSalary;
    public decimal MathSalary
    {
        get { return _MathSalary; }
        set { Set(ref _MathSalary, value); }
    }

    private decimal _CompSciSalary;
    public decimal CompSciSalary
    {
        get { return _CompSciSalary; }
        set { Set(ref _CompSciSalary, value); }
    }

    private decimal _Total;
    public decimal Total
    {
        get { return _Total; }
        set { Set(ref _Total, value); }
    }
}

and then you can create a total class property that is a get only using this linq query:

public List<Summary> EmpSum
    {
        get
        {
            return this.Employees.GroupBy(x => x.Title).Select(y => new Summary { Title = y.First().Title, MathSalary = y.Where(s => s.Department == "Math").Sum(s => s.Salary), CompSciSalary = y.Where(s => s.Department == "CompSci").Sum(s => s.Salary), Total = y.Sum(s => s.Salary) }).ToList();
        }
    }

I'm using MVVMLight (ViewModelBase) but you can do your own notification code. Just need to raisepropertychanged("EmpSum") when you add/remove.change the main list.