Lager
=====
Lager is a logging mixin.  It is designed to add class methods for logging, via `extend Lager`.  It provides a unified logging instance that you can use in both class and instance methods.  It is implemented with the familiar [Logger class](http://ruby-doc.org/stdlib-2.0/libdoc/logger/rdoc/Logger.html) from ruby's [stdlib](http://ruby-doc.org/stdlib/).  Only one Logger instance is used for the class.  Use #log_to to set the log destination and log level from inside or outside the class.

Usage
-----
```ruby
require 'lager'

class Foo
  extend Lager
  log_to $stdout, :debug  # sets up @lager at the class layer
  # ...
```

Now, within Foo, you can use the class instance variable @lager for logging.

```ruby
  # ...
  def self.bar(baz)
    unless baz.is_a?(String)
      @lager.debug { "baz #{baz} is a #{baz.class}, not a string" }
    end
  end
  # ...
```

What about instance methods, you ask?  Well, you will need to assign @lager yourself, within #initialize.

```ruby
  # ...
  def initialize
    # set the instance layer's @lager to the class layer's @lager
    @lager = self.class.lager
    # now both layers are using the same instance
  end

  def do_something_complicated
    @lager.debug { "about to do something complicated" }
    # ...
    @lager.debug { "whew! we made it!" }
  end
end
```

Everything under control
------------------------
Right now, Foo is spewing debug messages everywhere:

```ruby
Foo.bar(15)
f = Foo.new
f.do_something_complicated

[2013-07-05 15:14:52] DEBUG: baz 15 is a Fixnum, not a string
[2013-07-05 15:14:52] DEBUG: about to do something complicated
[2013-07-05 15:14:52] DEBUG: whew! we made it!
```

This is because we set the default logging to :debug level, above.  Let's calm things down a bit, shall we?

```ruby
Foo.log_level = :warn
Foo.new.do_something_complicated
```

We can tell Foo to log to a file:

```ruby
Foo.log_to '/tmp/foo.log'
```

Note that this will replace the class's Logger instance.  The old log level will be maintained unless you specify a new one.

Best practices
--------------
* Set default logging inside the class definition by calling log_to just after extend Lager
* Set the instance layer's @lager within #initialize
* Only call message methods (debug, info, warn, error, fatal) on @lager in your class and instance methods.
* Beyond the class default, let the log destination and log level be managed from the outside, by the users of your class.

For Logger, generally: use block invocation of message methods.

```ruby
@lager.debug { "hi" }
# rather than
@lager.debug "hi"
```

By using the first form, the block will not be evaluated unless you are logging at DEBUG level.  If using the second form, the message is evaluated no matter the current log level.  This can be significant when logging heavily processed  messages.

Artifacts
---------
* By mixing in Lager via extend, you introduce these class methods:
  * lager
  * log_to
  * log_level
  * log_level=
* By calling log_to, you introduce the *class instance variable* @lager
* By assigning @lager within initialize, you introduce the *instance variable* @lager

Now you have a unified interface for logging at both class and instance layers.

```ruby
  @lager.info { "So happy right now!" }
```

Use an existing Logger instance
-------------------------------
If your project already has an existing Logger, then you can set your class to use that Logger:

```ruby
class Foo
  extend Lager
  log_to $LOG # the global Logger instance
  # ...
end
```

Of course, $LOG will have to have already been defined at requiretime.  You can set it the same way at runtime:

```ruby
class Foo
  extend Lager
  log_to $stderr
  # ...
end

# now, in an irb session or another file
# where Project.log is your project's Logger:

Foo.log_to Project.log
```

Inheritance
-----------
```ruby
class Foo
  extend Lager
end

class Bar < Foo
end

# Bar will have Lager mixed in.  Its @lager is independent of Foo's.
# You can set Bar's to Foo's

Bar.log_to Foo.lager
```