Hello, Task!¶

This guide will show how to use the task service to create a new task that will run asynchronously and display a simple toast when it has completed.

Generate a task¶

We will start by creating a new plugin named "Sleepy" that will sleep for a period of time and return a message when it is done. We will need to generate the following:

A translation module
A command to put into the action menu in the table view
An endpoint that creates the task
The task itself

lime-project new package
cd sleepy
lime-project generate translation-module
lime-project generate command sleep
lime-project generate tasks

The command¶

The command will take a number as argument that represents the number of seconds that the task will be asleep for. The timeout will be configurable from the config when it is added to the action menu.

Add the timeout property to the SleepCommand class.

// frontend/src/commands/sleep/sleep.command.ts

@Command({
    id: 'sleepy.sleep'
})
export class SleepCommand {
    public timeout: number;
}

In the command handler, we are going to use the TaskService to create the task.

// frontend/src/commands/sleep/sleep.handler.ts

export class SleepHandler implements CommandHandler {
    constructor(private tasks: TaskService) {}

    public async handle(command: SleepCommand) {
        const url = 'sleepy/task/sleep/';

        const id = await this.tasks.create(url, command);
        console.log(`Created task with id ${id}`);
    }
}

We also need to register our handler with our command in the Loader component

// frontend/src/components/lwc-sleepy-loader/lwc-sleepy-loader.tsx

export class Loader implements LimePluginLoader {
    public componentWillLoad() {
        const commandBus = this.platform.get(PlatformServiceName.CommandBus);
        const taskService = this.platform.get(PlatformServiceName.TaskState);
        commandBus.register(SleepCommand, new SleepHandler(taskService));
    }
}

All frontend coding is done now so we can go ahead and build the project.

bash cd frontend npm run build

The endpoint¶

The endpoint that was generated is responsible for creating the task. The file sleepy/endpoints/task_endpoint.py contains some autogenerated code that we can replace with our own.

sleep_args = {
    "timeout": fields.Number(required=True)
}

class SleepyTaskHandler(webserver.LimeResource):
    @use_args(sleep_args)
    def post(self, args):
        return lime_task.send_task(
                'sleepy.tasks.tasks.sleep',
                self.application,
                args['timeout']).to_dict()

api.add_resource(SleepyTaskHandler, '/task/sleep/')

The task¶

The task itself is really simple, it will just sleep for the given amount of time and then return a message. We can remove all of the autogenerated code in sleepy/tasks/tasks.py and add our own.

from lime_task import task
import time

@task
def sleep(app, timeout):
    time.sleep(timeout)

    return "Slept for " + str(timeout) + " seconds"

Installation and configuration¶

All coding should be done now and we can go ahead and install our plugin. Please note that the plugin needs to be installed both on the appserver and on the taskhandler. Once installed, both services have to be restarted.

To add our command to the action menu in the table view, we need to configure the table view for the one of the limetypes in lime-admin. The following snippet will add our action and configure our task to sleep for 15 seconds.

{
  "actions": [{
    "id": "sleepy.sleep",
    "params": {
      "timeout": 15
    }
  }]
}

Testing our plugin¶

We can now go ahead and test our plugin. Navigate to the table that has the action added to the menu and click it. A notification should be displayed indicating that the task is about to be created. Unless it is canceled, a new notification should be displayed telling us that it was created successfully. Depending on how much work is being done on the taskhandler, the task might have been queued up so there is no way of knowing how long it will take until it returns a result. After a while though it should return and a new message should be displayed with the result of the operation.