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71 lines
3.9 KiB
Markdown
71 lines
3.9 KiB
Markdown
# Module Template for Full Applications
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An application can be written to run as either a _task_ (a module with its own stack and process priority) or as a _work queue task_ (a module that runs on a work queue thread, sharing the stack and thread priority with other tasks on the work queue).
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In most cases a work queue task can be used, as this minimizes resource usage.
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::: info
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[Architectural Overview > Runtime Environment](../concept/architecture.md#runtime-environment) provides more information about tasks and work queue tasks.
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:::
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::: info
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All the things learned in the [First Application Tutorial](../modules/hello_sky.md) are relevant for writing a full application.
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:::
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## Work Queue Task
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PX4-Autopilot contains a template for writing a new application (module) that runs as a _work queue task_:
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[src/examples/work_item](https://github.com/PX4/PX4-Autopilot/tree/main/src/examples/work_item).
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A work queue task application is just the same as an ordinary (task) application, except that it needs to specify that it is a work queue task, and schedule itself to run during initialisation.
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The example shows how.
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In summary:
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1. Specify the dependency on the work queue library in the cmake definition file ([CMakeLists.txt](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/work_item/CMakeLists.txt)):
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```txt
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...
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DEPENDS
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px4_work_queue
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```
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2. In addition to `ModuleBase`, the task should also derive from `ScheduledWorkItem` (included from [ScheduledWorkItem.hpp](https://github.com/PX4/PX4-Autopilot/blob/main/platforms/common/include/px4_platform_common/px4_work_queue/ScheduledWorkItem.hpp))
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3. Specify the queue to add the task to in the constructor initialisation.
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The [work_item](https://github.com/PX4/PX4-Autopilot/blob/main/src/examples/work_item/WorkItemExample.cpp#L42) example adds itself to the `wq_configurations::test1` work queue as shown below:
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```cpp
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WorkItemExample::WorkItemExample() :
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ModuleParams(nullptr),
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ScheduledWorkItem(MODULE_NAME, px4::wq_configurations::test1)
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{
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}
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```
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::: info
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The available work queues (`wq_configurations`) are listed in [WorkQueueManager.hpp](https://github.com/PX4/PX4-Autopilot/blob/main/platforms/common/include/px4_platform_common/px4_work_queue/WorkQueueManager.hpp#L49).
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:::
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4. Implement the `ScheduledWorkItem::Run()` method to perform "work".
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5. Implement the `task_spawn` method, specifying that the task is a work queue (using the `task_id_is_work_queue` id).
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6. Schedule the work queue task using one of the scheduling methods.
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In the example, `init()` calls `registerCallback()` on a uORB subscription so that `Run()` is triggered whenever a new `sensor_accel` message is published.
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`ScheduleOnInterval` is an alternative for fixed-rate scheduling.
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## Tasks
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PX4/PX4-Autopilot contains a template for writing a new application (module) that runs as a task on its own stack:
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[src/templates/template_module](https://github.com/PX4/PX4-Autopilot/tree/main/src/templates/template_module).
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The template demonstrates the following additional features/aspects that are required or are useful for a full application:
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- Accessing parameters and reacting to parameter updates.
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- uORB subscriptions and waiting for topic updates.
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- Controlling the task that runs in the background via `start`/`stop`/`status`.
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The `module start [<arguments>]` command can then be directly added to the
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[startup script](../concept/system_startup.md).
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- Command-line argument parsing.
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- Documentation: the `PRINT_MODULE_*` methods serve two purposes (the API is
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documented [in the source code](https://github.com/PX4/PX4-Autopilot/blob/v1.17/platforms/common/include/px4_platform_common/module.h)):
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- They are used to print the command-line usage when entering `module help` on the console.
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- They are automatically extracted via script to generate the [Modules & Commands Reference](../modules/modules_main.md) page.
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