编辑“Scopes”（章节）

== System scopes ==
There are special system scopes that refer to relationships between scopes.  These are <code>THIS</code>, <code>PREV</code>, <code>ROOT</code>, and <code>FROM</code>.

* THIS – Refers to the current scope. It is useless as context switch, but sometimes is used as input for certain effects.  If you are in a <code>pop</code> scope, <code>this</code> would refer to that pop.
* PREV – Refers to the previous scope. If you are in a <code>pop</code> scope, and change to the <code>planet</code> scope, <code>prev</code> would refer back to the pop.  <code>pop = { planet = { habitability = { who = prev value > 0.6 } } }</code> would check that the habitability of the current pop has over 60% habitability on its current planet.  Sometimes you will want to refer back more than one scope step, in which case you can repeat prev up to four times, i.e. <code>prevprev</code> up to <code>prevprevprevprev</code>
* FROM – Refers to the scope from which the current script was called. For example, if an event executed a planet event, the planet event could refer back to the objects in the first event using the <code>from</code>scope. Just like PREV, up to four FROMs can be repeated to refer back multiple times.
* ROOT – Refers to the main scope of the script. For events, this will be the object the event is called in.  For example, in a <code>planet_event</code>, <code>root</code> will be the specific planet the event was called on.  ROOT is usually the default scope for script blocks in the event, but is shorter and more clear than PREV to refer to when you have switched to other scopes.  For example, in a pop event's <code>immediate</code> block, the pop is the default scope.  But if you switch to the planet scope of the pop, and possibly to even more chained scopes, <code>root</code> will always refer back to the pop the event was called on.  Note that in some scripts, such as scripted_effects and scripted_triggers, the default scope, <code>this</code>, is not necessarily the same as <code>root</code>.

In some contexts, these relationships aren't intuitive.  For instance, <code>on_action</code>s will often override some of these system scopes to hold the objects the action refers to.  For example, in events called from <code>on_ship_disabled</code>, <code>this</code> will refer to the disabled ship, and <code>from</code> will refer to the ship that disabled it.  The vanilla <i>on_actions.txt</i> file has comments describing most of these, while others you will have to look at code to determine to which objects they refer.