|
1 <?xml version="1.0" encoding="utf-8"?> |
|
2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
|
3 <!-- This component and the accompanying materials are made available under the terms of the License |
|
4 "Eclipse Public License v1.0" which accompanies this distribution, |
|
5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
|
6 <!-- Initial Contributors: |
|
7 Nokia Corporation - initial contribution. |
|
8 Contributors: |
|
9 --> |
|
10 <!DOCTYPE concept |
|
11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
|
12 <concept id="GUID-4957E6BB-C41D-52F1-94CD-68CF6D61C271" xml:lang="en"><title>Format |
|
13 layering</title><prolog><metadata><keywords/></metadata></prolog><conbody> |
|
14 <p>The Symbian platform uses the principle of format layering |
|
15 in which new formatting is added as a layer onto the previous layer of formatting. |
|
16 The following diagram shows a less efficient technique for applying format |
|
17 layering to text. The inefficiency arises because each format layer stores |
|
18 not only the new attribute which it introduces, but also every attribute which |
|
19 it inherits from the preceding format layer. </p> |
|
20 <fig id="GUID-DBA21807-9BC8-5FFB-962A-4CDCD9A72359"> |
|
21 <image href="GUID-CFD3CD78-D99A-57B8-80EB-8C67899E5D27_d0e427545_href.png" placement="inline"/> |
|
22 </fig> |
|
23 <p>The Symbian platform avoids this inefficiency by using the system of <keyword>based-on |
|
24 links</keyword>. Here, each new layer adds attributes in a natural way without |
|
25 restating all attributes previously set. A based-on link is a pointer to the |
|
26 format layer whose attributes are inherited by another layer. For example, |
|
27 when the italic attribute is applied to a region of bold text, the new layer |
|
28 stores only the italic attribute and a pointer to the previous attribute layer. |
|
29 Chains of format layers can thus be formed, with each layer based on the previous |
|
30 one. In order to determine a character or paragraph’s <keyword>effective format</keyword>, |
|
31 a chain of format layers must typically be examined. The final layer in the |
|
32 chain of based on links is the one whose based on link is NULL and is not |
|
33 therefore based upon any other layer.</p> |
|
34 <p>This method is demonstrated in the following two diagrams, in which each |
|
35 arrow represents a based-on link.</p> |
|
36 <fig id="GUID-3BE6B9A3-D8DB-543C-AA88-DD519B991B62"> |
|
37 <title>Format layering</title> |
|
38 <image href="GUID-52743192-B1E0-5B71-B85B-43487F18F412_d0e427565_href.png" placement="inline"/> |
|
39 </fig> |
|
40 <p>The following diagram demonstrates the system of based-on links when attributes |
|
41 are applied to different parts of text rather than added on top of existing |
|
42 attribute layers.</p> |
|
43 <fig id="GUID-1E0E17F0-BE93-5403-8474-ED612E118C8F"> |
|
44 <image href="GUID-F99938E9-9E06-53C0-B762-2CE51BC5F1E2_d0e427573_href.png" placement="inline"/> |
|
45 </fig> |
|
46 <p>In this case, several format layers are required, all of which are based |
|
47 on the normal layer.</p> |
|
48 </conbody></concept> |