Add simple bag player examples

[sangteak601]

Fixes #1449.

Added simple bag player examples in cpp and Python. I also tried to read or write bag files without specifying topic types. However, it became too complicated and was no longer a "simple" example, so I decided not to push the changes.

[sangteak601]

Hi @MichaelOrlov, I think I screwed up with reviewers. Do you mind fixing it and reviewing this PR ?

[MichaelOrlov]

@sangteak601 @fujitatomoya I am curious how this PR overlaps with the existing tutorial and examples from https://docs.ros.org/en/rolling/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html ?

[fujitatomoya]

@MichaelOrlov

I am curious how this PR overlaps with the existing tutorial and examples from https://docs.ros.org/en/rolling/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html ?

that is what i found too.

i was thinking that https://docs.ros.org/en/rolling/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html can also be updated to point the source code from this example.

[MichaelOrlov]

i was thinking that https://docs.ros.org/en/rolling/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html can also be updated to point the source code from this example.

I would prefer to make them aligned as much as possible but in the tutorial preferable to have the full text with source code instead of just a link on it.

Also, I think that in the examples still unclear point that if the message always shall be deserialized or not before publishing.
To make it clear and show that it is optional I would suggest adding some class member variable or global variable with name like message_needs_to_be_edit_before_send and have an if-else statement showing two options for publishing. With deserialized and altering value before publishing and non-serialized publishing.

cc: @sangteak601

[sangteak601]

@MichaelOrlov Thanks for the review. I will make a modification.

Based on my knowledge(correct me if I'm wrong), we can also receive serialized messages directly. Therefore, perhaps we can modify the recorder example in a similar manner?

[sangteak601]

@fujitatomoya @MichaelOrlov I have addressed issues. Do you mind taking another look?

[fujitatomoya]

overall lgtm, but i have a few comments.

besides,

I would prefer to make them aligned as much as possible but in the tutorial preferable to have the full text with source code instead of just a link on it.

i do agree with this. we need to update https://github.com/ros2/ros2_documentation accordingly to align with these samples.

[fujitatomoya]

if i am not mistaken, this is intentionally enabled as sqlite3? but this bag file can be also red from simple_bag_player.cpp, because it reads the meta data from the bag file and use the appropriate storage plugin if available? just checking this because user does not meet the failure during the tutorial.

[sangteak601]

It was set as sqlite3 in simple_bag_recorder.py. I don't know the reason exactly.
It is possible to read sqlite3 files from simple_bag_player.cpp, but the opposite(reading mcap files from simple_bag_player.py) isn't.

Maybe we can delete storage_id?

[fujitatomoya]

No, i think that is okay with current code. this also includes an example how to specify the storage id, i think that is fine.

[sangteak601]

I would prefer to make them aligned as much as possible but in the tutorial preferable to have the full text with source code instead of just a link on it.

i do agree with this. we need to update https://github.com/ros2/ros2_documentation accordingly to align with these samples.

I will update the documentation once this is merged.

[fujitatomoya]

comment for the documentation. this needs to be described how to enable with command line parameter argument. so that user can understand we can publish either publishing serialized data as it is or editing data before publishing in the example.

[MichaelOrlov]

@sangteak601 As far as I recall we need another type of publisher aka GenericPublisher to be able to publish serialized data.