Xamarin Forms - calling rest service from viewmodel of pcl - xamarin

Question

I created a class that connected to the API to retrieve the required data using httpclient. That file was called in the code behind file of the view and worked perfectly. Than I decided to implement the MVVM approach. As a result, I moved the code that initialized the rest service class to the view-model.
After doing that, i stopped getting the data. To investigate, I stated the the debugging session with the breakpoint placed at the line where i initialize the rest service class. Than i executed that line. By doing that, I found out that a huge android mono exception is thrown and the debugging session if stopped. The app exits the debugging session.
This has happened for the first time since i stated developing my app in Xamarin Forms. I have no idea about why it is breaking like that. Your help will be greatly appreciated.
This is the code that was working properly.
In the view code behind file
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class SubtaskPage : ContentPage
{
protected override void OnAppearing()
{
base.OnAppearing();
PopulateSubtaskData();
}
private async void PopulateSubtaskData()
{
lstSubtasks.IsRefreshing = true;
try
{
RestService rs = new RestService();
SResponse = await rs.GetSubtasksAsync(Convert.ToInt32(Application.Current.Properties["UserId"]));
if (SResponse.Status == 1)
{
lstSubtasks.ItemsSource = SResponse.Subtasks;
}
else
{
await DisplayAlert("Error", SResponse.Message, "Ok");
}
}
catch (Exception E)
{
Debug.WriteLine(#"GetSubtasksAsync -> ERROR {0}", E.Message);
}
lstSubtasks.IsRefreshing = false;
}
}
The rest service class is as follows
This class is in a separate folder named "Services". ip and url have been changed for security reason.
class RestService
{
HttpClient client;
public List<Ticket> Tickets { get; private set; }
string Server1 = "server ip";
string Server2 = "server ip";
public RestService()
{
client = new HttpClient();
client.MaxResponseContentBufferSize = 256000;
}
public async Task<SubtasksResponse> GetSubtasksAsync(int UserId)
{
SubtasksResponse SubtaskResponse = new SubtasksResponse();
string ApiUrl = "URL";
string Url = "";
HttpResponseMessage response;
if (CrossConnectivity.Current.IsConnected)
{
Url = await GetActiveServerAsync();
if (Url != "")
{
var uri = string.Format(Url + ApiUrl, UserId);
try
{
response = await client.GetAsync(uri);
if (response.IsSuccessStatusCode)
{
var content = await response.Content.ReadAsStringAsync();
SubtaskResponse.Subtasks = JsonConvert.DeserializeObject<List<Ticket>>(content);
SubtaskResponse.Status = 1;
}
else
{
SubtaskResponse.Subtasks = null;
SubtaskResponse.Status = 0;
SubtaskResponse.Message = "Attempt to fetch data from server was unsuccessful. Please try again";
}
}
catch (Exception E)
{
SubtaskResponse.Subtasks = null;
SubtaskResponse.Status = 0;
SubtaskResponse.Message = "Error occured while fetching data from the server. Please try again";
}
}
else
{
SubtaskResponse.Subtasks = null;
SubtaskResponse.Status = 0;
SubtaskResponse.Message = "Remote Server Not Responding! Please try again later";
}
}
else
{
SubtaskResponse.Subtasks = null;
SubtaskResponse.Status = 0;
SubtaskResponse.Message = "No Network Connection Found! Please connect to a network and try again";
}
return SubtaskResponse;
}
}
}
This was working fine until I added the view model into the mix.
This is how I am calling the function in the view model.
async Task<SubtasksResponse> PopulateSubtaskList()
{
RestService rs = new RestService();
IsBusy = true;
_subtaskList = await rs.GetSubtasksAsync(Convert.ToInt32(Application.Current.Properties["UserId"]));
IsBusy = false;
return _subtaskList;
}
"RestService rs = new RestService();" this is the line where the code breaks.
Here is the image of the exception that occurs when the code breaks.
Hope you get the clear picture of the situation. Please let me know if additional information is required.
Thanks

Answer 1

Don't do this. If you want to call rest from a mvvm Xamarin Forms app I can advice Refit. All the difficult work is already done for you and abstracted away. With a few lines of code you are up and running.

Answer 2

BTW the error message you are showing probably has nothing to do with your code but is a bug in a recent Xamarin version. See here: https://bugzilla.xamarin.com/show_bug.cgi?id=56787

Answer 3

Found the answer on this page (https://releases.xamarin.com/common-issues-in-the-xamarin-15-2-2-release-being-tracked-by-the-xamarin-team/).
The solution is as follows
Download the missing Mono.Posix file and unzip the archive.
Right-click the Mono.Posix.dll file in Explorer and select Properties.
Check the Digital Signatures tab to ensure the file shows a valid Xamarin Inc. signature.
At the bottom of the General tab, if an Unblock checkbox appears, enable it and select OK. (This checkbox appears depending on how the file was downloaded.)
For Visual Studio 2017, copy the Mono.Posix.dll file into the “Xamarin.VisualStudio” extension directory. For example, for a default installation of the Enterprise edition, copy the file into:
C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Extensions\Xamarin.VisualStudio
For Visual Studio 2015, copy the file into the “Xamarin\Xamarin” extension directory:
C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\Extensions\Xamarin\Xamarin\
Quit and restart Visual Studio.
For more detail, visit the link given above.