Basic Screen Logger
This tutorial is a community contribution by Christoph Ziegler working at “ARD Programmdirektion”, the programming directorate coordinating the joint TV programme “Das Erste” and the joint video-on-demand service “ARD Mediathek” of the regional public-service broadcasters in Germany.
HbbTV terminals have no console object to print messages for debugging. However, during development, you may want to be able to visualise state information of your HbbTV app; for example to see if everything works as expected or to find the cause for unexpected behaviour. This tutorial explains a basic logging utility contained in the GitHub repository that accompanies this series of tutorials. The utility prints log messages to the graphical user interface of your app. It achieves this by appending log messages to a <div> container, which it appends to the <body> element of the app’s document. There are plenty of other ways to implement screen loggers. This utility may help you debug your HbbTV app right away and may be a starting point and inspiration for you to develop your own more elaborate logging solution.
Full source code is available here: https://github.com/HbbTV-Association/Tutorials/tree/main/screen-logger
Create loggers and write log messages
All features you need to instantiate loggers, write log messages and to set global logging configurations are exposed by the global Logger element.
Example 1: Instantiate logger and write log message
Create instances of Logger with the new operator.
Example 2: Arguments of the log methods
Logging methods can handle arguments of type string or number. You can pass as many arguments as you like. Logger will concatenate them with a single space delimiter.
Example 3: Add a tag name to your logger
You can pass an optional tag name to the constructor that, if set, will prefix all log messages of that logger instance. You could, for example, create instances of Logger per class or per module, which may help keeping track of where in your code the log line has been invoked.
Example 4: Use multiple instances of Logger
All instances of logger print to the same log container in your app’s document.
Example 5: Make use of different log levels
Instances of Logger provide logging methods for different levels of importance of log messages. Log messages are prefixed with the respective log level. Following log levels are supported (sorted from low to high importance): VERBOSE, DEBUG, INFO, WARN, ERROR.
Example 6: Styling of log messages
For different log levels, logger adds different classes to elements in the document that represent log messages. By default, the font color of log and debug messages is black. Verbose messages appear in gray, info messages in green, warnings in orange and error messages in red. You can change the appearance of log messages by changing the default style definition in the logger.css stylesheet:
Example 7: Clear log
To clear the log run:
Visibility of the logs
By default, all log messages that reach the default verbosity level DEBUG will be appended to the container element for log messages. By default, this container will only be visible in the UI if a log message is written that reaches the default trigger level ERROR. Section Configuration explains how you can change the trigger and verbosity levels.
Logs written with logger.log() trigger the logger’s visibility at trigger level DEBUG. They are captured in the log container at verbosity level DEBUG or lower.
You can also programmatically show and hide the logger:
These methods could for example be invoked by key event handlers. This allows you, for example, to toggle the log container’s visibility with the press of a button on the remote.
The global Logger object exposes the Config object which you can use to configure the logger. Values in the below example are the default values: