Author: Duncan Parsons
Some properties of a component can 'drop-down' to reveal other properties (such as
a Font property reveals various properties within itself). These are objects within
the component, and a simple demonstration of how to add you own new 'drop-down'
properties is given here. (You can also get a Static Analogue Clock Component
too!!!)
Answer:
Please Note: I will only include here pertinent aspects of what is being explained.
I will not flesh out all the examples for the sake of clarity.
To include an object within a component is a fairly simple matter, simply declare a
field, and make a property public.
1 interface
2
3 type
4 TMyComp = class(TComponent)
5 private
6 fFont: TFont;
7 public
8 property Font: TFont read fFont write fFont;
9 constructor create(AOwner: TComponent); override;
10 destructor destroy; override;
11 end;
12
13 implementation
14
15 constructor TMyComp.create(AOwner: TComponent);
16 begin
17 inherited create(AOwner);
18 fFont := TFont.Create; //create the Object into the field reference,
19 // so it will not raise an exception
20 end;
21
22 destructor TMyComp.destroy;
23 begin
24 fFont.free; //free the field to avoid memory leaks, etc.
25 inherited;
26 end;
NB: When creating Objects, ALWAYS remember to free them, unless a help file tells
you overwise (happens very rarely, eg exception handlers). Notice that what is
created in the constructor is explicitly freed in the destructor.
This creates a fairly useless component admittedly, but it is an example after all!
When accessing the Font property, it can be referenced in code using:
27 begin
28 with MyComp1.Font do
29 begin
30 Color := clBlue;
31 Size := 10;
32 end;
33 end;
This is all well and good, but what about the Object Inspector?
If we move the property from public to published, the Font property is now
available, with the plus sign to 'drop-down' as required.
This is a step in the right direction.
However, this is not the whole story. What if we were devising a component which
could logically take completely new objects as properties. For instance an analogue
clock face - three similar objects would be obvious.. the hour, minute and second
hands! Each is the same, save for customisable features, such as colour, thickness,
etc.
So - let us construst our AnalogueHand object:
34 type
35 TAnalogueHand = class
36 Colour: TColor;
37 Thickness: integer;
38 end;
Here is an object, descended from TObject, which has the properties we require.
Let us put it into a Clock face component:
39 type
40 TAnalogueClock = class(TGraphicControl)
41 private
42 fHourHand, fMinuteHand, FSecHand: TAnalogueHand;
43 protected
44 procedure SetHand(index: integer; value: TAnalogueHand);
45 public
46 constructor create(AOwner: TComponent); override;
47 destructor destroy; override;
48 published
49 property HourHand: TAnalogueHand index 0 read fHourHand write SetHand;
50 property MinuteHand: TAnalogueHand index 1 read fMinuteHand write SetHand;
51 property SecHand: TAnalogueHand index 2 read fSecHand write SetHand;
52 end;
53
54 in the constructor, each field must be created separately, and freed on
55 destruction:
56
57 constructor TAnalogueClock.create(AOwner: TComponent);
58 begin
59 inherited create(AOwner);
60 //Set up the Hand Objects
61 fHourHand := TAnalogueHand.create;
62 with fHourHand do
63 begin
64 colour := clBlue;
65 Thickness := 2;
66 end;
67 fMinuteHand := TAnalogueHand.create;
68 with fMinuteHand do
69 begin
70 colour := clRed;
71 Thickness := 2;
72 end;
73 fSecHand := TAnalogueHand.create;
74 with fSecHand do
75 begin
76 colour := clRed;
77 Thickness := 1;
78 end;
79 end;
80
81 destructor TAnalogueClock.destroy;
82 begin
83 fSecHand.free;
84 fMinuteHand.free;
85 fHourHand.free;
86 inherited;
87 end;
88
89 procedure TAnalogueClock.SetHand(index: integer; value: TAnalogueHand);
90 begin
91 case index of
92 0: fHourHand := Value;
93 1: fMinuteHand := Value;
94 2: fSecHand := Value;
95 end;
96 invalidate;
97 end;
Notice that the Hands are written to all using the same procedure, SetHand, each
with a different index to refer to it.
If we install this, we end up with our object, but the object inspector gives an
Access Violation if we try to view the properties - not what we wanted!
The reason being that to descend our Hand Object from TObject is the wrong
ancestor.. For objects which are of a temporary nature, this is fine, but to allow
properties to exist abit longer, to have their properties stored in a persistent
fashion (put very simply!) - we must descend from TPersistent.
So, our new hand declaration looks like:
98 type
99 TAnalogueHand = class(TPersistent)
100 Colour: TColor;
101 Thickness: integer;
102 end;
Rebuild, and the Access Violation has gone - hooray!! But, there are no
subproperties!! An inspection of the Hand object could provide a clue.. With a
standard component, for a property to appear in the object inspector, it must be
published:
103 type
104 TAnalogueHand = class(TPersistent)
105 private
106 fColour: TColor;
107 fThickness: integer;
108 published
109 property Colour: TColor read fColour write fColour;
110 property Thickness: integer read fThickness write fThickness;
111 end;
Rebuild again - and we have subproperties within properties, droppong down without
Access Violations, etc.
At runtime the new subproperties can be accessed by:
112 with AnalagueClock1.HourHand do
113 begin
114 Colour := clOlive;
115 Thickness := 4;
116 end;
AnalagueClock1.SecHand.Colour := clFuchsia;
This has been a quick and simple overview to providing subproperties in a
component. More complicated user defined objects can be created, which may have
further subproperties (try publishing a TCanvas Object, and see how many layers you
get..).
In summary:
descend your new object from TPersistant (if it is COMPLETELY new - as in the
example);
ensure that any methods declared in the object are written (such as constructors,
setting procedures, functions, etc). - I've forgotten this a few times!!
use the standard of fields and published properties (and any public as required).
The published properties will appear as subproperties.
ensure that when the new object is contained within a component that it is
explicitly created and freed at the appropriate times.
This worked example appears in an expanded form in the component attached to this
article. I had a requirement for a Clock face, but I needed it to be static - for
inputting. All the Clock faces I found were very nice, but the darn things moved!!
So I created my own static analogue clock face.
I make no apology for using British English within the component! Light
diffractions have a 'U' (coloUr), and the free floating state contrary to digital
has a 'UE' suffix (analogUE). If you don't like it - you have the source!!
Component Download: http://www.baltsoft.com/files/dkb/attachment/AnalogueClock.zip
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