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More progress on our glorious Presents documentation.

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git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3314 542714f4-19e9-0310-aa3c-eee0fc999fb1
This commit is contained in:
Michael Bayne
2005-01-27 04:16:45 +00:00
parent c11e8c4d3c
commit ae5105185c
2 changed files with 304 additions and 19 deletions
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font-family: Courier New, monospace
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code {
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/* Headings */
h1 { font-size: 145% }
src/java/com/threerings/presents/package.html
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@@ -14,6 +14,14 @@
server. All information transfer takes place through the server using
the distributed object system documented below.
<ul>
<li><a href="#distributed_objects">Distributed Objects</a>
<li><a href="#event_listeners">Event Listeners</a>
<li><a href="#distributed_collections">Distributed Collections</a>
<li><a href="#invocation_services">Invocation Services</a>
<li><a href="#ant_tasks">Ant Tasks</a>
</ul>
<p> <em>A note to the reader:</em> the Presents system is a complex
one and though a great deal of code is provided in explaining the
services it provides, it is not the intent that one should start from
@@ -23,7 +31,7 @@
some working sample code which is provided in the <code>tests</code>
directory of this distribution.
<h3>Distributed Objects</h3>
<h3><a name="distributed_objects">Distributed Objects</a></h3>
The Presents services allow applications to access and update shared
information through a mechanism known as distributed objects.
@@ -61,8 +69,16 @@
public String owner;
}</pre>
This class definition is then run through a post-processor which turns
it into the following:
Note that all distributed fields, or attributes (fields in a
distributed object are frequently referred to as <i>attributes</i> in
this documentation and elsewhere in the system), are public fields in
our distributed object. Non-public fields will be ignored by the
system and not transmitted when a proxy object is delivered over the
network to a subscriber. Further, fields marked <code>transient</code>
will also be ignored by the system.
<p> We then run our class definition through a post-processor which
turns it into the following:
<pre class="example">
public class CageObject extends DObject
@@ -117,13 +133,29 @@
}</pre>
The contents of the methods are not too important, the main things to
note are that setter methods for the two attributes (fields in a
distributed object are referred to as <i>attributes</i> in this
documentation and elsewhere in the system) were generated and
note are that setter methods for the two attributes were generated and
constants were defined that will be used to identify which attribute
changed if we choose to inspect an event notifying us of such a
change.
<p> One may also notice that attribute change requests result in the
new value of the attribute being immediately written to the local copy
of the object. This is a convention that was decided upon after
repeatedly running into trouble when users of the system would set a
value in an object and immediately assume it held the new value rather
than realizing that an event would have to propagate back from the
server before the value was in fact updated. By setting the value
immediately, these problems are avoided and the opposite assumption is
almost never made. This is further justified by the fact that, in
general, attribute changes never originate on a client but instead
originate on the server after processing a request from the client
(via the below documented <a href="#invocation_services">invocation
services</a>) to do something application-specific that results in one
or more attribute changes taking place.
<p> See the section on <a href="#ant_tasks">Ant Tasks</a> for
information on how to configure and run this post-processor.
<h4>Creating an object</h4>
Generally, some entity on the server will choose to create a new
@@ -158,7 +190,7 @@
subscribed to the newly created object in the same manner as is
described below for all additional subscribers to the object. It is
possible to instruct an object to automatically destroy itself when
all subscribers have unsubscribed. (See {@link
all subscribers have unsubscribed. (See the not very terse {@link
com.threerings.presents.dobj.DObject}.setDestroyOnLastSubscriberRemoved()).
<h4>Subscribing to an object</h4>
@@ -205,8 +237,8 @@
asynchronous distributed environment. There is no guarantee that your
object subscription request will be completed before the client
decides to call shutdown() on its <code>ObjectUser</code>. Thus, in
the previous code, we would get a null pointer exception, and even
worse, we would remain subscribed to the object even though we didn't
the previous code, we could get a null pointer exception, and even
worse, we could remain subscribed to the object even though we didn't
want to be. To avoid these sorts of problems, the {@link
com.threerings.presents.util.SafeSubscriber} class is provided:
@@ -220,7 +252,7 @@
// inherited from interface Subscriber
public void objectAvailable (DObject object) {
// yay! we got our object
_object = object;
_object = (CageObject)object;
}
// inherited from interface Subscriber
@@ -234,7 +266,7 @@
}
<b>protected SafeSubscriber _safesub;</b>
protected DObject _object;
protected CageObject _object;
}</pre>
The safe subscriber will pass the object availability on to your
@@ -245,7 +277,7 @@
<code>unsubscribe()</code> even if the original subscription request
failed.
<h3>Listeners</h3>
<h3><a name="event_listeners">Event Listeners</a></h3>
Once a client has subscribed to a distributed object, all events
pertaining to that object will be delivered to the client. Frequently,
@@ -258,8 +290,8 @@
<p> The basic listener is the {@link
com.threerings.presents.dobj.AttributeChangeListener} which is
informed of all simple attribute changes (setting a primitive field to
a new value is called an attribute change in this distributed object
system). We return to our trusty example:
a new value is called an attribute change). We return to our trusty
example:
<pre class="example">
public class ObjectUser
@@ -273,7 +305,7 @@
// inherited from interface Subscriber
public void objectAvailable (DObject object) {
// yay! we got our object
_object = object;
_object = (CageObject)object;
<b>_object.addListener(this);</b>
}
@@ -302,10 +334,22 @@
}
protected SafeSubscriber _safesub;
protected DObject _object;
protected CageObject _object;
}</pre>
It is useful to note that listeners are notified of a changed
The <code>attributeChanged()</code> method of our registered listener
will be called whenever an event is received as a result of one of the
setter methods being called on the <code>CageObject</code> by
<em>any</em> participant in the distributed system. The setter creates
an event which is sent to the server, the server dispatches the event
to all subscribers of the object and the Presents system dispatches
the event notification to all registered listeners when the event is
received on the client. Note that listeners are also used on the
server as entities on the server also frequently need to respond to
attribute changes. They are notified immediately after the server has
dispatched the event (over the network) to all subscribed clients.
<p> It is useful to note that listeners are notified of a changed
attribute <b>after</b> the change has been applied to the object. The
previous value of the attribute is available through the {@link
com.threerings.presents.dobj.AttributeChangedEvent#getOldValue}
@@ -313,9 +357,246 @@
in a variety of circumstances, we have rarely found that we cared to
know the previous value.
<h3>Distributed collections</h3>
<h3><a name="distributed_collections">Distributed collections</a></h3>
<h3>Invocation Services</h3>
One soon discovers that primitive object fields do not make for a very
useful information distribution mechanism and that more complex data
structures are necessary. Two collection types, sets and arrays, are
supported, and a mechanism is provided for allowing whole objects to
be passed around in toto as if they were a primitive field.
<p><b>Distributed Arrays</b><br>
Arrays of primitive types can be used in a distributed object and the
system will detect their use and provide a mechanism for updating the
entire array and an additional mechanism for updating a single element
at a time:
<pre class="example">
public class ChessObject extends DObject
{
// AUTO-GENERATED: FIELDS START
/** The field name of the <code>state</code> field. */
public static final String STATE = "state";
// AUTO-GENERATED: FIELDS END
/** Used to track our board state. */
public int[] state;
// AUTO-GENERATED: METHODS START
/**
* Requests that the <code>state</code> field be set to the
* specified value. The local value will be updated immediately and an
* event will be propagated through the system to notify all listeners
* that the attribute did change. Proxied copies of this object (on
* clients) will apply the value change when they received the
* attribute changed notification.
*/
public void setState (int[] value)
{
int[] ovalue = this.state;
requestAttributeChange(
STATE, value, ovalue);
this.state = (value == null) ? null : (int[])value.clone();
}
/**
* Requests that the <code>index</code>th element of
* <code>state</code> field be set to the specified value.
* The local value will be updated immediately and an event will be
* propagated through the system to notify all listeners that the
* attribute did change. Proxied copies of this object (on clients)
* will apply the value change when they received the attribute
* changed notification.
*/
public void setStateAt (int value, int index)
{
int ovalue = this.state[index];
requestElementUpdate(
STATE, index, new Integer(value), new Integer(ovalue));
this.state[index] = value;
}
// AUTO-GENERATED: METHODS END
}</pre>
To correspond with what is called an "element update" (the
modification of a single element in an array), there is the {@link
com.threerings.presents.dobj.ElementUpdateListener}. When an element
is updated, listeners implementing that interface will be notified.
Remember that if the whole array is changed using
<code>setState()</code>, the normal {@link
com.threerings.presents.dobj.AttributeChangeListener} is the interface
one uses to hear about it.
<p> <em>Note</em> that distributed arrays are <em>not</em>
automatically resized. If a request is made to update the element at
index 9 of an array, the array must be of at least size 10 or an array
index out of bounds exception will be thrown (as should be evident
from inspecting the code above). For more dynamic collections of
objects, see the documentation below about distributed sets.
<p> This mechanism is not actually limited to arrays of primitive
types. It also works for arrays of objects that implement the {@link
com.threerings.io.Streamable} interface which is documented next.
<p><b>Streamable and its good friend SimpleStreamableObject</b><br>
The {@link com.threerings.io.Streamable} interface is used to mark
objects that can be sent over the network by using them in distributed
object fields by using arrays of such objects as a field. This
interface functions in much the same way that {@link
java.io.Serializable} does in that it simply marks the class and an
underlying mechanism uses reflection to actually marshall and
unmarshall the object on the network. In fact, all
non-<code>transient</code> fields of a streamable object are included
during the marhsalling process. Here's an example:
<pre class="example">
public class Player implements Streamable
{
/** This player's name. */
public String name;
/** This player's rating. */
public int rating;
}
public class ChessObject extends DObject
{
/** A record for each player in the game. */
public Player[] players;
}</pre>
The generated methods are ommitted for the sake of brevity, but as you
would expect, both a <code>setPlayers(Player[] value)</code> and a
<code>setPlayersAt(Player value, int index)</code> method will be
generated and do just what you expect.
<p> It should be pointed out that streamable objects sent over the
network are sent in their entirety. No mechanism is provided for
updating just a single field in a streamable instance both because
that would increase the complexity of the system tremendously and
because it is generally not very useful. If conservation of bandwidth
is of extreme importance, special {@link
com.threerings.presents.dobj.DEvent} derived classes can be created to
transmit precisely what is desired and nothing more. Doing so is
beyond the scope of this introduction, but will hopefully be covered
in an additional tutorial.
<p> The {@link com.threerings.io.SimpleStreamableObject} class is a
convenient way to create a simple record like the <code>Player</code>
record above that implements <code>Streamable</code> and provides a
default implementation of <code>toString()</code> that uses reflection
to print out the actual values of the fields in the object (a boon
when logging and debugging).
<p><b>Distributed Sets</b><br>
In developing a distributed system, one frequently encounters
situations where one wants distributed collection of objects where
order is generally not important but the ability to fluidly add and
remove elements is. For such occasions we provide the distributed set
or {@link com.threerings.presents.dobj.DSet}.
<p> A <code>DSet</code> contains entries (called entries rather than
elements to avoid confusion with array "element updating") which must
implement the {@link com.threerings.presents.dobj.DSet.Entry}
interface. This automatically makes them {@link
com.threerings.io.Streamable} and requires that they provide a {@link
java.lang.Comparable} key which is used to distinguish them from other
entries in the set (and look them up via an efficient binary search).
<p> When using a <code>DSet</code> one is provided with three new
operations: <code>addEntry()</code>, <code>updateEntry()</code> and
<code>removeEntry()</code>. Once again an example is in order:
<pre class="example">
public class Monkey implements DSet.Entry
{
/** The monkey's name. */
public String name;
/** The monkey's age. */
public int age;
// documentation inherited from interface DSet.Entry
public Comparable getKey ()
{
return name;
}
}
public class CageObject extends DObject
{
// AUTO-GENERATED: FIELDS START
/** The field name of the <code>monkeys</code> field. */
public static final String MONKEYS = "monkeys";
// AUTO-GENERATED: FIELDS END
/** A collection of monkeys. */
public DSet monkeys;
// AUTO-GENERATED: METHODS START
/**
* Requests that the specified entry be added to the
* <code>monkeys</code> set. The set will not change until the event is
* actually propagated through the system.
*/
public void addToMonkeys (DSet.Entry elem)
{
requestEntryAdd(MONKEYS, monkeys, elem);
}
/**
* Requests that the entry matching the supplied key be removed from
* the <code>monkeys</code> set. The set will not change until the
* event is actually propagated through the system.
*/
public void removeFromMonkeys (Comparable key)
{
requestEntryRemove(MONKEYS, monkeys, key);
}
/**
* Requests that the specified entry be updated in the
* <code>monkeys</code> set. The set will not change until the event is
* actually propagated through the system.
*/
public void updateMonkeys (DSet.Entry elem)
{
requestEntryUpdate(MONKEYS, monkeys, elem);
}
/**
* Requests that the <code>monkeys</code> field be set to the
* specified value. Generally one only adds, updates and removes
* entries of a distributed set, but certain situations call for a
* complete replacement of the set value. The local value will be
* updated immediately and an event will be propagated through the
* system to notify all listeners that the attribute did
* change. Proxied copies of this object (on clients) will apply the
* value change when they received the attribute changed notification.
*/
public void setMonkeys (DSet value)
{
requestAttributeChange(MONKEYS, value, this.monkeys);
this.monkeys = (value == null) ? null : (DSet)value.clone();
}
// AUTO-GENERATED: METHODS END
}</pre>
It is possible to set the entire set (which is necessary to establish
its original value even if one decides to set it to the empty set),
but more commonly one will simply add entries to the set, update those
entries and remove them using the provided methods.
<h3><a name="invocation_services">Invocation Services</a></h3>
TBD
<h3><a name="ant_tasks">Ant Tasks</a></h3>
TBD
</body>
</html>