Tasks and jobs
Tasks are bound to online or offline data or services and provide methods to perform asynchronous operations using those resources.
With tasks you can:
- Download, collect, and update geographic information using
- Download and display tiled basemaps using
- Locate addresses on the map and interpolate addresses from map locations using
- Calculate point-to-point or multi-stop routes and get driving directions using
- Perform complex GIS analysis by executing geoprocessing models using
Tasks either return their results directly from asynchronous methods on the task, or make use of jobs to provide status updates and results.
Some operations return results directly from asynchronous methods on the task. For more complex or longer running operations, tasks make use of jobs instead.
To use tasks that return results directly:
- Create the task by initializing it to use the required data or service.
- Some operations can work both online and offline.
- 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 input parameters (for example, to limit a geocode operation to a specific country).
- Call the async operation method, passing in the inputs you defined.
- Use the results from the operation as required, for example to display geocode results on a map.
You can create a
LocatorTask using the Esri geocode service, and pass in an address to geocode. When the operation is complete, the result location is could displayed in a
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 use these default parameter values, tasks provide helper methods that create parameter objects initialized with service-specific values. You can then make any changes to the parameter values before passing them to an operation. Creating these default parameter objects is useful for operations with many options, such as tracing a utility network.
You could get the default parameters for a
RouteTask and ensure that results using these parameters will return both a route and directions. Also the output spatial reference could match that of the map view.
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 when parameter settings are simple.
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 (
.loz), or a mobile map package (
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 for platforms that support it). An example is
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 canceled. Your app can support a user action or host OS terminating a running job object, and then recreate and resume the job later.
To use operations like these:
- Create the task by initializing it to use the required data or service.
- Define the input parameters for the task.
- Call the async operation method to get a job, passing in the input parameters you defined.
- Start the job.
- 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.
- Listen for the job completion and get the results from the operation. Check for errors in the job, and if successful, use the results.
Job.status retrieves the current
Job.Status in the job's workflow. Jobs periodically fire a changed event as they are running, usually with decreasingly frequency as a job progresses. More than one
JobMessage may appear in a change event. The job complete listener is called as soon as the job finishes. Whether successful or not, jobs 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 canceled, but what about the time in-between? Users may become frustrated waiting for a long job to complete without getting feedback on its progress. Fortunately, jobs provide a mechanism for reporting the current progress (percentage complete) for the running operation they represent.
As the job runs it provides a
Job.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.
Add an observer to the ongoing job to monitor the
fraction. This code displays this
fraction value in the status completion block. Alternatively, you could pass the
observed to the
UIProgress and the value will be automatically updated in the UI.
Remember to remove the observer when the job has completed.
Pause, resume, or cancel a job
Jobs are designed to handle a user exiting an app while the job is running or having the app terminated by the host operating system. Jobs also provide a mechanism for explicitly pausing or canceling 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.
canceling, cancels the
Job, and waits for any asynchronous, server-side operations to be canceled. After all cancelation tasks complete (including any server-side tasks),
Job.Status changes to
Job.cancel() call back is fired . If one or more jobs cannot be canceled,
Job.cancel() fires an error .
GenerateOfflineMapJob is a server-side job that launches several more server-side jobs, depending on the layers in your map. Other examples of server-side jobs include
You should always cancel unneeded jobs (for example when exiting your app) to avoid placing unnecessary load on the server.
You could examine the
Job error once a job has failed. The error code
NSUser would indicate that the job has been terminated by the user.
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 the
pause method through the
NSProgress API (exposed via
NSProgress through the
Job.progress property). 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. Therefore, when resuming a job it may have a different state than 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.Status will be in the paused state regardless of its state when serialized and should be restarted to resume listening for completion. The job changed listener is a good place to update the job JSON for storage by your app.
You could serialize a job to JSON.
You could deserialize a job from JSON and then restart the job.
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 handle inconsistent connectivity, you can serialize and pause a job when your app loses connectivity for a few minutes to avoid job failure (as failed jobs cannot be restarted). The job can then be deserialized and resumed when connectivity returns.