Return true if the left mouse button is currently down, and false on all other occasions. This is regardless of if it was pressed on this frame, or a previous one.
You can also attach a block onto 'isLeftDown', and the current mouse x and y location will be passed into the block if the left mouse button is currently down.
/* Moves the player to the current mouse location,
* when the mouse button is down. */
getControls().isLeftDown() do |x, y|
player.setLocation( x, y )