Common notes for all descendants: most of them expose field or property "Value", this is (surprise, surprise!) the value of the field. Many of them also expose DefaultValue and DefaultValueExists fields/properties, these should be the default VRML value for this field. You can even change DefaultValue after the object is created.
Most of descendants include constructor that initializes both DefaultValue and Value to the same thing, as this is what you usually want.
Some notes about Assign method (inherited from TPersistent and overridied appropriately in TX3DField descendants):
There are some exceptions, but usually assignment is possible only when source and destination field classes are equal.
Assignment (by Assign, inherited from TPersistent) tries to copy everything: name (with alternative names), default value, IsClauseNames, ValueFromIsClause, Exposed, and of course current value.
Save method of SaveToStreamValue. May assume things that SaveToStreamValue may issume, for example: if this is used at all, then at least field value is not default (so there is a need to write this field) and such.
Call this inside overriden Assign methods. I don't want to place this inside TX3DField.Assign, since I want "inherited" in Assign methods to cause exception.
class function ExposedEventsFieldClass: TX3DFieldClass; virtual;
Class of the fields allowed in the exposed events of this field. This should usually be using ClassType of this object, and this is the default implementation of this method in TX3DField.
You can override this to return some ancestor (from which, and to which, you can assign) if your TX3DField descendant doesn't change how the Assign method works. E.g. TSFTextureUpdate class, that wants to be fully compatible with normal TSFString.
Handle exposed input event. In TX3DField class, this does everything usually needed — assigns value, sends an output event, notifies Changed.
You can override this for some special purposes. For special needs, you do not even need to call inherited in overriden versions. This is suitable e.g. for cases when TimeSensor.set_startTime or such must be ignored.
Descendants implementors notes: when implementing constructors in descendants, remember that Create in this class actually just calls CreateUndefined, and CreateUndefined is virtual. So when calling inherited Create, be aware that actually you may be calling your own overriden CreateUndefined.
In fact, in descendants you should focus on moving all the work to CreateUndefined constructor. The Create constructor should be just a comfortable extension of CreateUndefined, that does the same and addiionally gets parameters that specify default field value.
Later you can initialize such instance from string using it's Parse method.
Note that some exceptional fields simply cannot work when initialized by this constructor: these are SFEnum and SFBitMask fields. They simply need to know their TSFEnum.EnumNames, or TSFBitMask.FlagNames + TSFBitMask.NoneString + TSFBitMask.AllString before they can be parsed. I guess that's one of the reasons why these field types were entirely removed from VRML 2.0.
In this class, Parse only appends to IsClauseNames: if we stand on "IS" clause (see VRML 2.0 spec about "IS" clause) and IsClauseAllowed then we append specified identifier to IsClauseNames.
If "IS" clause not found, we call ParseValue which should actually parse field's value. Descendants should override ParseValue.
Parse field value from X3D XML encoded attribute using a Lexer. Attributes in X3D are generally encoded such that normal ParseValue(Lexer, nil) call is appropriate, so this is done in this class.
Parse field's value from XML Element children. This is used to read SFNode / MFNode field value inside <field> (for interface declaration default field value) and <fieldValue> inside <ProtoInstance>.
Save the field to the stream. Field name (if set, omitted if empty) and value are saved. Unless the current field value equals default value and FieldSaveWhenDefault is False (default), then nothing is saved.
IS clauses are not saved here (because they often have to be treated specially anyway, for XML encoding, for prototype declarations etc.).
Zwraca zawsze false w tej klasie. Mozesz to przedefiniowac w podklasach aby SaveToStream nie zapisywalo do strumienia pol o wartosci domyslnej.
function Equals(SecondValue: TX3DField; const EqualityEpsilon: Double): boolean; virtual; reintroduce;
True if the SecondValue object has exactly the same type and properties. For this class, this returns just (SecondValue.Name = Name).
All descendants (that add some property that should be compared) should override this like
Result := (inherited Equals(SecondValue, EqualityEpsilon)) and
(SecondValue is TMyType) and
(TMyType(SecondValue).MyProperty = MyProperty);
For varius floating-point fields in this unit: we compare each float using EqualityEpsilon, i.e. if the difference is < EqualityEpsilon then the floats are assumed equal. Pass EqualityEpsilon = 0.0 to perform *exact* comparison (this case will be optimized in implementation, by using routines like CompareMem instead of comparing float-by-float).
Note that this *doesn't* compare the default values of two fields instances. This compares only the current values of two fields instances, and eventually some other properties that affect parsing (like names for TSFEnum and TSFBitMask) or allowed future values (like TSFFloat.MustBeNonnegative).
function FastEqualsValue(SecondValue: TX3DField): boolean; virtual;
Compare value of this field, with other field, fast.
This compares only the values of the fields, not other properties (it doesn't care about names of the fields or such, or default values; only current values). In other words, it compares only the things copied by AssignValue.
This tries to compare very fast, which means that for large (multi-valued) fields it may give up and answer False even when they are in fact equal. So this is usable only for optimization purposes: when it answers True, it is True. When it answers False, it actually doesn't know.
Default implementation in this class (TX3DField) just returns False.
Exposed events of this field. Nil if this field is not exposed. EventIn is always equivalent to ExposedEvents[true], EventOut is always equivalent to ExposedEvents[false].
Field type name in VRML/X3D. As for VRML/X3D interface declaration statements. In base TX3DField class, this teturns XFAny (name indicating any type, used by instantreality and us).
class function CreateEvent(const AParentNode: TX3DFileItem; const AName: string; const AInEvent: boolean): TX3DEvent; virtual;
Create TX3DEvent descendant suitable as exposed event for this field.
Copies the current field value. Contrary to TPersistent.Assign, this doesn't copy the rest of properties.
After setting, our ValueFromIsClause is always changed to False. You can manually change it to True, if this copy indeed was done following "IS" clause.
Descendants implementors notes:
In this class, implementation takes care of setting our ValueFromIsClause to False. In descendants, you should do like
if Source is <appropriate class> thenbegininherited;
Value := Source.value;
endelse
AssignValueRaiseInvalidClass(Source);
Raised in case of any field assignment problem. It's guaranteed that in case of such problem, our value will not be modified before raising the exception.
Note that for now this doesn't guarantee that every possible field's value can be stored as default value. In case of trouble, it will silently record "no default is known" information, so e.g. EqualsDefaultValue will always return False. Our default value mechanisms are sometimes limited, not every value can be a default value. For example, for multiple-valued nodes, we usually cannot save arrays longer than one as default value. This is not a problem, since X3D specification doesn't specify too long default values. But it may be a problem for prototypes, since then user can assign any value as default value. May be corrected in the future.
Assigns value to this node calculated from linear interpolation between two given nodes Value1, Value2. Just like other lerp functions in our units (like CastleVectors.Lerp).
First of all, AssignLerp is defined only for fields where CanAssignLerp returns True, so always check CanAssignLerp first. All float-based VRML fields should have this implemented.
Use this only if Value1 and Value2 are equal or descendant of target (Self) class.
For multiple-value fields, counts of Value1 and Value2 must be equal, or EX3DMultFieldDifferentCount will be raised.
What happens when the value of this field changes. This is called, exactly once, by TCastleSceneCore.ChangedField to determine what must be done when we know that value of this field changed.
In overridden descendants, this can also do something immediately. Overriding this is similar to registering your callback in OnReceive list, with two benefits:
This method may be not called (although no guarantees) when the actual field value did not change. In contrast, the OnReceive event is always fired, even when you send the same value to an exposed field, because VRML/X3D events and routes must be fired anyway.
This is useful also for fields that are not exposed, and can be changed only by ObjectPascal code.
So overridding this is closer to "do something when field value changes" than registering callback in OnReceive list.
Set the value of the field, notifying the scenes and events engine. This sets the value of this field in the nicest possible way for any possible TCastleSceneCore (with events on or off) containing the node with this field.
Precise specification:
If this is an exposed field and we have events engine working:
We will send this value through it's input event. In this case, this is equivalent to doing EventIn.Send(Value, Scene.Time). The scenes (including events engine) will be notified correctly by exposed events handler already.
Otherwise, we will just set the fields value. And then notify the scenes (including events engine).
Notifications when exposed field received new value through VRML/X3D event. Only for exposed fields (Nil for not exposed fields). This is simply a shortcut for EventOut.OnReceive, see TX3DEvent.OnReceive for details how does this work.
Note that this observes the "out" event (not the "in" event). This way you know inside the handler that the field value is already changed as appropriate. Inside "in" event handlers, you would not know this (it would depend on the order in which handlers are run, one "in" handler sets the field value).
Note that "out" event handlers are executed before Scene is notified about the field value change (before TCastleSceneCore.ChangedField is called). This is also usually exactly what you want — you can change the scene graph inside the event handler (for example, load something on Inline.load or Inline.url changes), and let the TX3DField.ChangesAlways cause appropriate action on this change.
Does current field value came from expanding "IS" clause. If yes, then saving this field to stream will only save it's "IS" clauses, never saving actual value.
When True (default) we will automatically handle exposed events behavior. This means that we will listen on EventIn, and when something will be received we will set current field's value and produce appropriate EventOut.
You almost certainly want to leave this as True in all typical situations, as it takes care of implementing required exposed events behavior.
That said, in special cases you may decide to break this.
What always happens when the value of this field changes.
This is included in the ExecuteChanges method result. So instead of using this property, you could always override ExecuteChanges method. But often it's easier to use the property.
By default this is an empty set. This is suitable for things that aren't *directly* an actual content (but only an intermediate value to change other stuff). This includes all metadata fields and nodes, all fields in event utilities, Script node, interpolators...