mainpage.dox

/**
\mainpage
\htmlinclude manifest.html

\b message_filters is a set of message "filters" which take messages in, usually through a callback from somewhere else,
and may or may not spit them out at some time in the future, depending on the conditions which need to be met for that
filter to do so.

\b message_filters also sets up a common pattern for these filters, allowing you to chain them together, even though they
have no explicit base class.


\section codeapi Code API

The filters currently implemented in this package are:
- message_filters::Subscriber - A source filter, which wraps a ROS subscription.  Most filter chains will begin with a Subscriber.
- message_filters::Cache - Caches messages which pass through it, allowing later lookup by time stamp.
- message_filters::TimeSynchronizer - Synchronizes multiple messages by their timestamps, only passing them through when all have arrived.
- message_filters::TimeSequencer - Tries to pass messages through ordered by their timestamps, even if some arrive out of order.

There is also a base-class provided for simple filters: message_filters::SimpleFilter.  This provides callback management and disconnection
for any filter that derives from it.  A simple filter is defined as one that outputs a single message.  message_filters::SimpleFilter provides
the registerCallback() method for any of its derived classes.  message_filters::Subscriber, message_filters::Cache and message_filters::TimeSequencer
are all derived from message_filters::SimpleFilter.

Here's a simple example of using a Subscriber with a Cache:
\verbatim
void myCallback(const robot_msgs::Pose::ConstPtr& pose)
{}

rclcpp::Node::SharedPtr node = std::make_shared<rclcpp::Node>("test_node");
message_filters::Subscriber<robot_msgs::Pose> sub(nh, "pose_topic");
message_filters::Cache<robot_msgs::Pose> cache(sub, 10);
cache.registerCallback(myCallback);
\endverbatim

The Subscriber here acts as the source of messages.  Each message is passed to the cache, which then passes it through to the
user's callback (myCallback)

\section connections CONNECTIONS

Every filter can have up to two types of connections, input and output.  Source filters (such as message_filters::Subscriber) only
have output connections, whereas most other filters have both input and output connections.

The two connection types do not have to be identical.  For example, message_filters::TimeSynchronizer's input connection takes one
parameter, but its output connection has somewhere between 2 and 9 parameters, depending on the number of connections being
synchronized.

Input connections are registered either in the filter's constructor, or by calling connectInput() on the filter. For example:
\verbatim
message_filters::Cache<robot_msgs::Pose> cache(10);
cache.connectInput(sub);
\endverbatim 

This connects cache's input to sub's output.

Output connections are registered through the registerCallback() function.

*/