Paging ListBox with ReactiveUI and Caliburn.Micro - wpf

Question

I'm trying to implement a paging mechanism for a listbox using Caliburn.Micro.ReactiveUI with a call to EF using ".Skip(currentPage).Take(pageSize)". I'm new to ReactiveUI and Reactive in general. I'm sure this is supposed to be easy.
I've got a single "SearchParameters" class which I needs to be observed and the search function needs to execute when any of the properties on the SearchParameters object changes.
You can see from the commented-out code that I've tried to define the class as a ReactiveObject as well. The current implementation though is with CM's PropertyChangedBase. The individual properties are bound textboxes in my view using CM's conventions:
public class SearchParameters : PropertyChangedBase
{
private string _searchTerm;
public string SearchTerm
{
get { return _searchTerm; }
set
{
if (value == _searchTerm) return;
_searchTerm = value;
NotifyOfPropertyChange(() => SearchTerm);
}
}
private int _pageSize;
public int PageSize
{
get { return _pageSize; }
set
{
if (value == _pageSize) return;
_pageSize = value;
NotifyOfPropertyChange(() => PageSize);
}
}
private int _skipCount;
public int SkipCount
{
get { return _skipCount; }
set
{
if (value == _skipCount) return;
_skipCount = value;
NotifyOfPropertyChange(() => SkipCount);
}
}
//private string _searchTerm;
//public string SearchTerm
//{
// get { return _searchTerm; }
// set { this.RaiseAndSetIfChanged(ref _searchTerm, value); }
//}
//private int _pageSize;
//public int PageSize
//{
// get { return _pageSize; }
// set { this.RaiseAndSetIfChanged(ref _pageSize, value); }
//}
//private int _skipCount;
//public int SkipCount
//{
// get { return _skipCount; }
// set { this.RaiseAndSetIfChanged(ref _skipCount, value); }
//}
}
"SearchService" has the following method which needs to execute when any one of SearchParameter's values change:
public async Task<SearchResult> SearchAsync(SearchParameters searchParameters)
{
return await Task.Run(() =>
{
var query = (from m in _hrEntities.Departments select m);
if (!String.IsNullOrEmpty(searchParameters.SearchTerm))
{
searchParameters.SearchTerm = searchParameters.SearchTerm.Trim();
query = query.Where(
x => x.Employee.LastName.Contains(searchParameters.SearchTerm) || x.Employee.FirstName.Contains(searchParameters.SearchTerm)).Skip(searchParameters.SkipCount).Take(searchParameters.PageSize);
}
return new SearchResult
{
SearchTerm = searchParameters.SearchTerm,
Matches = new BindableCollection<DepartmentViewModel>(query.Select(x => new DepartmentViewModel{ Department = x }).Skip(searchParameters.SkipCount).Take(searchParameters.PageSize))
};
});
}
Here's how I've tried to wire all of this up in MainViewModel's ctor and where Rx gets hazy for me:
public class MainViewModel : ReactiveScreen
{
private SearchParameters _searchParameters;
public SearchParameters SearchParameters
{
get { return _searchParameters; }
set
{
if (value == _searchParameters) return;
_searchParameters = value;
NotifyOfPropertyChange(() => SearchParameters);
}
}
{
public void MainViewModel()
{
var searchService = new SearchService();
//default Skip and PageSize values
SearchParameters = new Services.SearchParameters { SkipCount = 0 , PageSize = 10};
var searchParameters = this.ObservableForProperty(x => x.SearchParameters)
.Value()
.Throttle(TimeSpan.FromSeconds(.3));
var searchResults = searchParameters.SelectMany(parameters => searchService.SearchAsync(parameters));
var latestMatches = searchParameters
.CombineLatest(searchResults,
(searchParameter, searchResult) =>
searchResult.SearchTerm != searchParameter.SearchTerm
? null
: searchResult.Matches)
.Where(matches => matches != null);
_departmentViewModels = latestMatches.ToProperty(this, x => x.DepartmentViewModels);
searchParameters.Subscribe(x => Debug.WriteLine(x));
}
}
In the above example the call to SearchAsync doesn't execute. It seems that changes to SearchParameter's properties aren't being observed.
Can anyone tell me what I'm doing wrong here?

Answer 1

Here's how I ended up doing this although I'd be interested in hearing other solutions if anyone has suggestions. I'm not sure if this is the best way but it works:
First, I defined a computed property in my SearchParameters class that returns a string and reevaluates anytime CurrentPage, SkipCount and PageSize are updated from the View:
public string ParameterString
{
get { return String.Format("SearchTerm={0}|SkipCount={1}|PageSize={2}", SearchTerm, SkipCount, PageSize); }
}
Next, in my MainViewModel ctor I simply observe the computed rather than attempting to react to SearchTerm, SkipCount and PageSize individually (which my original question was asking how to do):
var searchTerms = this
.ObservableForProperty(x => x.SearchParameters.ParameterString)
.Value()
.Throttle(TimeSpan.FromSeconds(.3));
var searchResults = searchTerms.SelectMany(parameters => SearchService.SearchAsync(parameters));
var latestMatches = searchTerms
.CombineLatest(searchResults,
(searchTerm, searchResult) =>
searchResult.SearchTerm != searchTerm
? null
: searchResult.Matches)
.Where(matches => matches != null);
Finally, in my SearchService I parse the parameter string to get the current values:
var parameters = searchParameters.Split('|');
var searchTerm = "";
var skipCount = 0;
var pageSize = 0;
foreach (var parameter in parameters)
{
if (parameter.Contains("SearchTerm="))
{searchTerm = parameter.Replace("SearchTerm=", "");}
else if (parameter.Contains("SkipCount="))
{ skipCount = Convert.ToInt32(parameter.Replace("SkipCount=", "")); }
else if (parameter.Contains("PageSize="))
{ pageSize = Convert.ToInt32(parameter.Replace("PageSize=", "")); }
}