Tasks and jobs

Tasks are bound to online or offline data or services, and provide methods to perform asynchronous operations using those resources. By using tasks you can:

  • Download, collect, and update geographic information using GeodatabaseSyncTask
  • Download and display tiled basemaps using ExportTileCacheTask
  • Locate addresses and find addresses from map locations using LocatorTask
  • Calculate point-to-point or multi-stop routes and get driving directions using RouteTask
  • Perform complex GIS analysis by executing geoprocessing models using GeoprocessingTask

Tasks either return their results directly from asynchronous methods on the task, or make use of jobs to provide status updates and results.

Tasks

Some operations return their results directly from asynchronous methods on the task, for example LocatorTask.GeocodeAsync and RouteTask.SolveRouteAsync. For more complex or longer running operations, tasks make use of jobs instead.

To use tasks that return results directly:

  1. Create the task by initializing it to use the required data or service.
  2. Define the task inputs.
    • Some operations require only simple value inputs (for example a simple geocode operation may only require an address string as input).
    • Others require parameters to be defined (for example, to limit a geocode operation to a specific country).
  3. Call the async operation method, passing in the inputs you defined.
  4. Use the results from the operation as required, for example to display geocode results on a map.

The following code creates a LocatorTask to geocode an address entered by the user. The best match is used to create a graphic to display on the map.

// Create a new LocatorTask from a geocode service endpoint
var locatorUri = new Uri("http://geocode.arcgis.com/arcgis/rest/services/World/GeocodeServer");
LocatorTask onlineLocator = new LocatorTask(locatorUri);
 // If the address text box is not empty, run the geocode task
if (!string.IsNullOrEmpty(FromAddressTextBox.Text.Trim()))
{
    // Pass the address to the GeocodeAsync method and get the results
    IReadOnlyList<GeocodeResult> matches = await onlineLocator.GeocodeAsync(FromAddressTextBox.Text);


    // Get the first match (one with the highest score)
    GeocodeResult bestMatch = matches[0];
         // Use geometry and attributes from the match (and a symbol created earlier) to create a new graphic
    Graphic fromGraphic = new Graphic(bestMatch.DisplayLocation, bestMatch.Attributes, fromPointSymbol);


    // Add the graphic to an existing graphics overlay in the map view
    MyMapView.GraphicsOverlays[0].Graphics.Add(fromGraphic);
}

Some Tasks are loadable, and will load themselves when you call an asynchronous method (like that shown above) that requires the task to be in a loaded state. Loadable objects are generally required to be loaded before their properties are filled out.

Define input parameters

Tasks offer numerous options that allow you to tailor the operation to your requirements. For example, when geocoding you can restrict your search to a specific area, country, category of place, and/or number of results. When an author publishes a service or packages a resource, they can choose default values for these options that suit the specific data or the most common use case for the service.

To find these default parameter values, tasks provide async methods that create parameters objects, initialized with these service-specific values. You can then make any changes to the parameter values, before using to execute an operation. Creating parameter objects in this way can be especially useful for operations with many options, such as solving a route.

The following example gets the default parameters for a route task, then explicitly sets values for some properties.

// get the default route parameters
var routeParams = await routeTask.CreateDefaultParametersAsync();


// explicitly set values for some parameters
routeParams.ReturnDirections = true;
routeParams.ReturnRoutes = true;
routeParams.OutputSpatialReference = MyMapView.SpatialReference;


// solve the route with these parameters
RouteResult routeTaskResult = await routeTask.SolveRouteAsync(routeParams);


// ... work with results ...

Alternatively, some parameters objects have constructors that you can use if you know the values of all the input parameters you want to use. This can be more efficient where parameter settings are simple.

For example, the following code creates a new GeocodeParameters that restricts the search country to France and limits the results (matches) to a maximum of 5.

GeocodeParameters geocodeParams = new GeocodeParameters();
geocodeParams.CountryCode = "France";
geocodeParams.MaxResults = 5;

Work online or offline

Many tasks can work either online by using services, or offline by using local data and resources. For example, you can geocode an address by using the default Esri geocoding service, your own geocoding service, a locator file (.loc) or a mobile map package (.mmpk).

// Create a new LocatorTask from a geocode service endpoint
var locatorUri = new Uri("http://geocode.arcgis.com/arcgis/rest/services/World/GeocodeServer");
LocatorTask onlineLocator = new LocatorTask(locatorUri);


// Create an offline locator from a local .loc file - coverage will depend on the packaged locator dataset...
LocatorTask offlineLocator = new LocatorTask(localLocatorUri);


// Mobile map packages can also contain locators - use this to get a reference to an offline locator...
MobileMapPackage mmpk = await MobileMapPackage.OpenAsync(localMmpkPath);
LocatorTask mmpkLocator = mmpk.LocatorTask;

Tasks and jobs

Some tasks expose operations that have multiple stages (like preparing and downloading a geodatabase), and can generate multiple progress messages (such as percentage complete). These types of tasks are always bound to ArcGIS Server or Local Server. An example is GenerateGeodatabase on GeodatabaseSyncTask.

Instead of returning results directly, these tasks make use of jobs to monitor status, return progress updates, and return their results. Each Job represents a specific operation of a task. Jobs are useful for longer-running operations, because they can also be paused, resumed, and cancelled. Your app can support a user action or host OS terminating a running job object, and then re-create and resume the job later.

To use operations like these:

  1. Create the task by initializing it to use the required data or service.
  2. Define the input parameters for the task.
  3. Call the async operation method to get a job, passing in the input parameters you defined.
  4. Start the job.
  5. Optionally, listen for changes to the job status and check the job messages, for example to update a UI and report progress to the user.
  6. Listen for the job completion and get the results from the operation. Check for errors in the job, and if successful, use the results.

The following example creates a task to synchronize local edits to a service. The JobChanged event is handled to report success or failure of the job.

// Function to submit a geodatabase synchronization job 
// URL for the feature service and the path to the local geodatabase are passed in
public async Task SyncronizeEditsAsync(string serviceUrl, string geodatabasePath)
{
    // create a sync task with the URL of the feature service to sync
    var syncTask = await GeodatabaseSyncTask.CreateAsync(new Uri(serviceUrl));


    // create sync parameters
    var taskParameters = new SyncGeodatabaseParameters()
    {
        RollbackOnFailure = true,
        GeodatabaseSyncDirection = SyncDirection.Bidirectional
    };
    // open the local geodatabase
    var gdb = await Esri.ArcGISRuntime.Data.Geodatabase.OpenAsync(geodatabasePath);


    // create a synchronize geodatabase job, pass in the parameters and the geodatabase
    SyncGeodatabaseJob job = syncTask.SyncGeodatabase(taskParameters, gdb);


    // handle the JobChanged event for the job
    job.JobChanged += (s, e) =>
    {
        // report changes in the job status
        if (job.Status == Esri.ArcGISRuntime.Tasks.JobStatus.Succeeded)
        {
            // report success ...
            statusMessage = "Synchronization is complete!";
        }
        else if (job.Status == Esri.ArcGISRuntime.Tasks.JobStatus.Failed)
        {
            // report failure ...                
            statusMessage = job.Error.Message;
        }
        else
        {
            statusMessage = "Sync in progress ...";
        }
    };


    // await the completion of the job
    var result = await job.GetResultAsync();
}

Calling Job.Status retrieves the current point in the job's workflow. Started jobs periodically receive job changed events; this happens with decreasingly frequency as a job progresses; more than one JobMessage may have been added to the job for each call. The job done listener is called as soon as the job is complete, for both successes and failures. Completed jobs, whether they have succeeded or failed, cannot be restarted.

Report job progress

A job represents an asynchronously running operation that might take some time to finish. As described previously, you can monitor changes to job status for notification when a job has completed, failed, or been cancelled, but what about the time in-between? Users may become frustrated waiting for a long job to complete without getting feedback on it's progress. Fortunately, jobs provide a mechanism for reporting the current progress (percentage complete) for the running operation they represent.

As the job runs it raises a ProgressChanged event. You can get the current progress of the job from the job's Progress property, an integer representing the percentage of the operation that has been completed. This allows your app to provide more specific information about the status of a running job using UI elements like progress bars, for example.

The following example updates a label in the UI to show the percentage complete for the job.

// Execute a task to return the Job
GenerateOfflineMapJob job = task.GenerateOfflineMap(parameters, packagePath);

// Hook into the ProgressChanged event to track the current progress
job.ProgressChanged += (s, e) =>
{
    // Note: The event might be raised in a background thread.
    //       Make sure updates to UI elements are pushed to the UI thread.
    Dispatcher.Invoke(() =>
    {
        // Update a UI element to show the updated progress value (percent complete)
        PercentageLabel.Text = "Operation is %" + job.Progress.ToString() + " complete ...";
    });
};

// Run the job and await the results asynchronously 
GenerateOfflineMapResult results = await job.GetResultAsync();

Pause, resume, or cancel job

Jobs are designed to effectively handle a user exiting an app while the job is running, and to handle the app being terminated by the host operating system. Jobs also deal with explicit pausing and cancellation of the operation.

Cancel a job

Sometimes the results of a job are no longer required. For example, a user could change their mind about the area of a tile cache they want to download and want to cancel the job and start over.

Calling Job.Cancel on a job immediately changes its status to JobStatus.Failed and adds additional information to the job error object. The job completion listener will be called. The error object indicates the error domain and code, which allows you to identify when cancellation has occurred.

The following example shows code you would add to the JobChanged handler to check for a System.OperationCanceledException when the job fails. Canceling the job might require additional cleanup of incomplete data or other code on the client.

if (job.Status == Esri.ArcGISRuntime.Tasks.JobStatus.Failed)
{
    // get the exception from the job
    Exception jobException = job.Error;
                    
    // check for cancellation
    if (jobException is OperationCanceledException)
    {
        statusMessage = "Job was canceled.";
    }
    else
    {
        // report the exception message
        statusMessage = "Job failed: " + jobException.Message;
    }

    // ... code here to clean up (delete incomplete files, for example) ...
}

Cancelled jobs cannot be restarted, although you can start a new job by re-using the same parameters. Cancelling a job does not necessarily stop any server-side processes, because not all services support cancellation on the server side.

Pause and resume a job

Jobs can be long running operations, so there is no guarantee that they will be completed while the app is running. You can pause a job explicitly using Job.Pause. For example when an app is backgrounded and does not have permissions for background operation. Pausing may also be useful if a user wishes to temporarily stop network access for any reason.

Job changed messages will not be received for a paused job. Pausing a job does not stop any server-side processes from executing. While a job is paused, outstanding requests can complete, and therefore it's possible that a resuming a job will result in it having a different state to when it was paused.

You can serialize a job to JSON to persist it if your app is backgrounded, or the process is otherwise terminated. When you deserialize it again, the job will be in the JobStatus.Paused state regardless of its state when serialized, and should be restarted to resume listening for completion. The job changed listener can be a good place to update the job JSON for storage by your app.

The following example shows code to save the state of a job in its JobChanged event handler. Additional code could be used to save this text (JSON) as a local file for deserialization later.

// handle the JobChanged event for the job
job.JobChanged += (s, e) =>
{
    // when the job changes, save its current state to JSON
    _jobJson = job.ToJson();


    // ... code to check job status ...
};

The Job can be deserialized from a JSON string and resumed if necessary. You must re-wire event handlers to monitor the job status (JobChanged, for example), as shown in the following example.

private void DeserializeJob(string jobJson)
{
    // recreate the SyncGeodatabaseJob from the JSON representation
    SyncGeodatabaseJob job = SyncGeodatabaseJob.FromJson(jobJson);
         // handle the JobChanged event for the job
    job.JobChanged += (s, e) =>
    {
        // ... code to check job status, and so on ...
    };


    // start (resume) the job
    job.Start();
}

Loss of network connection

Additionally, jobs using services are designed to handle situations where network connectivity is temporarily lost without needing to be immediately paused. A started job will ignore errors such as network errors for a period of up to 10 minutes. If errors continue for longer, the job will fail and the message will indicate the loss of network connection.

To deal with partially-connected workflows, you can serialize and pause a job when your app detects that network connectivity has been lost for a few minutes to avoid job failure purely due to this lack of network connectivity (as failed jobs cannot be restarted). The job can then be deserialized and resumed when connectivity is reinstated.