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bitbake: bitbake-user-manual-metadata.xml: Edits to flexible inheritance section.

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Fixes [YOCTO #5472]

Applied review edits from Paul Eggleton to this section.
Minor edits and some re-writing.

(Bitbake rev: 7259d9d40aad8254751f7674653cd362a9023054)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark
2014-04-11 11:47:57 -07:00
committed by Richard Purdie
parent 392de9d568
commit 9889a91a5b
1 changed files with 37 additions and 26 deletions
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63
bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
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@@ -793,36 +793,35 @@
</para>
</section>
<section id='automatically-mapping-functions-within-the-context-of-a-class'>
<title>Automatically Mapping Functions Within the Context of a Class</title>
<section id='flexible-inheritance-for-class-functions'>
<title>Flexible Inheritance for Class Functions</title>
<para>
Through coding techniques and the use of
<filename>EXPORT_FUNCTIONS</filename>, BitBake supports
automatic mapping for functions within the context of
a class.
exporting a function from a class such that the
class function appears as the default implementation
of the function, but can still be called if a recipe
inheriting the class needs to define its own version of
the function.
</para>
<para>
To understand the benefits of this feature, consider the basic scenario
where a class defines a function and your recipe inherits the class.
In this basic scenario, your recipe has access to the function in the
class by way of inheritance and can freely call and use the function
as defined in the class.
However, if you need to have a modified version of that function
in your recipe you are limited to using either your modified version
of the function or using "prepend_" or "_append" operators to add
code to be executed before or after the original function in the
class.
Your recipe cannot use both versions of the fucntion.
To understand the benefits of this feature, consider
the basic scenario where a class defines a task function
and your recipe inherits the class.
In this basic scenario, your recipe inherits the task
function as defined in the class.
If desired, your recipe can add to the start and end of the
function by using the "_prepend" or "_append" operations
respectively, or it can redefine the function completely.
However, if it redefines the function, there is
no means for it to call the class version of the function.
</para>
<para>
Function mapping allows you to access both your custom function
function that is defined in the recipe and the original function that
is defined in the class.
You have this access all from within your recipe.
To accomplish this, you need some things in place:
To make use of this technique, you need the following
things in place:
<itemizedlist>
<listitem><para>
The class needs to define the function as follows:
@@ -853,12 +852,24 @@
<listitem><para>
You need to call the function appropriately from within your
recipe.
Continuing with the same example,
your recipe would call the <filename>do_foo</filename> function
from the recipe by referring to it as
<filename>bar_do_foo</filename>.
To call your modified version of the function as defined in your
recipe, call it as <filename>do_foo</filename>.
Continuing with the same example, if your recipe
needs to call the class version of the function,
it should call <filename>bar_do_foo</filename>.
Assuming <filename>do_foo</filename> was a shell function
and <filename>EXPORT_FUNCTIONS</filename> was used as above,
the recipe's function could conditionally call the
class version of the function as follows:
<literallayout class='monospaced'>
do_foo() {
if [ somecondition ] ; then
bar_do_foo
else
# Do something else
fi
}
</literallayout>
To call your modified version of the function as defined
in your recipe, call it as <filename>do_foo</filename>.
</para></listitem>
</itemizedlist>
With these conditions met, your single recipe