UAProf
Profile Resolution
Now
you know how to find the URL of a UAProf profile, the next step is to
download it and retrieve some attribute values from it. Retrieving a
UAProf attribute value is not as simple as retrieving the value of an
HTTP header like Accept or
User-Agent. The
reason is that UAProf is a complex specification that has taken many
things into consideration. This makes UAProf powerful but also makes
it contains a lot of rules. For example:
The
UAProf specification allows more than one UAProf profile for a
mobile device, which means you may find more than one URL in the
x-wap-profile header.
The
UAProf profiles may contain a default description block, which is
used to specify default values to attributes. The default values may
be specified in the document locally or in an external profile.
Two
headers, x-wap-profile-diff
and Profile-diff, can
be used to specify changes to the original user agent profiles.
To
find the final value of an attribute, we have to perform a process
called profile resolution:
Read
the default attribute values.
Read
the attribute values specified in the UAProf profiles.
Read
the changes specified in the x-wap-profile-diff
headers or the Profile-diff
headers.
Merge
all these values by following a certain order and a set of
rules.
When a default value merges a non-default value, the default value is always
overriden by the non-default value. When a non-default value merges another
non-default value, the UAProf parser follows the resolution rule of the
attribute. The UAProf specification has defined three resolution rules
(Locked, Override and Append) that are used to tell the
UAProf parser how to merge non-default values. Every attribute are associated
with one of the resolution rules. The meanings of the three resolution rules
are provided in the table below:
|
Resolution
rule
|
Meaning
|
|
Locked
|
The
attribute's final value is the first non-default value.
For
example, the resolution rule of the ScreenSize
attribute is Locked.
|
|
Override
|
The
attribute's final value is the last non-default value.
For
example, the resolution rule of the ColorCapable
attribute is Override.
|
|
Append
|
The
attribute's final value is the list of all non-default values.
For
example, the resolution rule of the CcppAccept
attribute is Append.
(The CcppAccept attribute
tells us the MIME media types acceptable by a mobile device.)
Let's
say the first non-default occurrence of the CcppAccept
attribute contains the items:
image/bmp
image/gif
The
second non-default occurrence contains the items:
image/jpeg
image/png
After
profile resolution, the CcppAccept
attribute contains the items:
image/bmp
image/gif
image/jpeg
image/png
|
For
more details about profile resolution, please refer to the UAProf
specification.
Free Library / Tool for
Retrieving UAProf Attribute Values
As
you can see above, writing a UAProf parser to retrieve the values of
attributes according to the UAProf specification requires a lot of
work. Fortunately there is an open source library called DELI that
can be used to query UAProf or CC/PP profiles in Java Servlets. It
can be downloaded free of charge from the website
http://delicon.sourceforge.net/.
A Simple Way for
Retrieving UAProf Attribute Values
As
mentioned earlier, determining the final value of an attribute
involves a lot of steps according to the UAProf specification.
However, in real life, mobile device manufacturers and mobile
operators keep things simple. In most situations:
There
is only one UAProf profile for a mobile device. So, you will not
find more than one URL in HTTP headers such as x-wap-profile,
Profile, etc.
The
UAProf profile of a mobile device does not specify any default
description blocks.
No
x-wap-profile-diff or
Profile-diff headers
are used to specify additional changes to the UAProf profile.
Hence,
to get the final value of an attribute, no more profile merging is
necessary. All you need to do is to obtain the URL of the user agent
profile, download the user agent profile and then retrieve the
attribute values you needed from the document using any method you
like, such as simple text matching and XML parsing.
The
advantage of using this way is that it is very simple and
straightforward and it works most of the time. The disadvantage is
that the attribute values retrieved may be incorrect if the above
three points are not true.
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