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>章 19. 类与对象（PHP 5）</TD
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><A
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>Patterns</A
></H1
><P
> Patterns are ways to describe best practices and good designs.
They show a flexible solution to common programming problems.
</P
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><H2
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><A
NAME="language.oop5.patterns.factory"
>Factory</A
></H2
><P
> The Factory pattern allows for the instantiation of objects
at runtime. It is called a Factory Pattern since it is
responsible for "manufacturing" an object. A Parameterized Factory receives
the name of the class to instantiate as argument.
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><B
>例 19-25. Parameterized Factory Method</B
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><code><font color="#000000">
<font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">Example<br /></font><font color="#007700">{<br /> </font><font color="#FF8000">// The parameterized factory method<br /> </font><font color="#0000BB">public </font><font color="#007700">static function </font><font color="#0000BB">factory</font><font color="#007700">(</font><font color="#0000BB">$type</font><font color="#007700">)<br /> {<br /> if (include_once </font><font color="#DD0000">'Drivers/' </font><font color="#007700">. </font><font color="#0000BB">$type </font><font color="#007700">. </font><font color="#DD0000">'.php'</font><font color="#007700">) {<br /> </font><font color="#0000BB">$classname </font><font color="#007700">= </font><font color="#DD0000">'Driver_' </font><font color="#007700">. </font><font color="#0000BB">$type</font><font color="#007700">;<br /> return new </font><font color="#0000BB">$classname</font><font color="#007700">;<br /> } else {<br /> </font><font color="#0000BB">throw </font><font color="#007700">new </font><font color="#0000BB">Exception </font><font color="#007700">(</font><font color="#DD0000">'Driver not found'</font><font color="#007700">);<br /> }<br /> }<br />}<br /></font><font color="#0000BB">?></font>
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</code></TD
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> Defining this method in a class allows drivers to be loaded on the
fly. If the <TT
CLASS="literal"
>Example</TT
> class was a database
abstraction class, loading a <TT
CLASS="literal"
>MySQL</TT
> and
<TT
CLASS="literal"
>SQLite</TT
> driver could be done as follows:
</P
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><code><font color="#000000">
<font color="#0000BB"><?php<br /></font><font color="#FF8000">// Load a MySQL Driver<br /></font><font color="#0000BB">$mysql </font><font color="#007700">= </font><font color="#0000BB">Example</font><font color="#007700">::</font><font color="#0000BB">factory</font><font color="#007700">(</font><font color="#DD0000">'MySQL'</font><font color="#007700">);<br /><br /></font><font color="#FF8000">// Load a SQLite Driver<br /></font><font color="#0000BB">$sqlite </font><font color="#007700">= </font><font color="#0000BB">Example</font><font color="#007700">::</font><font color="#0000BB">factory</font><font color="#007700">(</font><font color="#DD0000">'SQLite'</font><font color="#007700">);<br /></font><font color="#0000BB">?></font>
</font>
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><H2
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><A
NAME="language.oop5.patterns.singleton"
>Singleton</A
></H2
><P
> The Singleton pattern applies to situations in which
there needs to be a single instance of a class.
The most common example of this is a database connection.
Implementing this pattern allows a programmer to make this
single instance easily accessible by many other objects.
</P
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><B
>例 19-26. Singleton Function</B
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><code><font color="#000000">
<font color="#0000BB"><?php<br /></font><font color="#007700">class </font><font color="#0000BB">Example<br /></font><font color="#007700">{<br /> </font><font color="#FF8000">// Hold an instance of the class<br /> </font><font color="#0000BB">private </font><font color="#007700">static </font><font color="#0000BB">$instance</font><font color="#007700">;<br /> <br /> </font><font color="#FF8000">// A private constructor; prevents direct creation of object<br /> </font><font color="#0000BB">private </font><font color="#007700">function </font><font color="#0000BB">__construct</font><font color="#007700">() <br /> {<br /> echo </font><font color="#DD0000">'I am constructed'</font><font color="#007700">;<br /> }<br /><br /> </font><font color="#FF8000">// The singleton method<br /> </font><font color="#0000BB">public </font><font color="#007700">static function </font><font color="#0000BB">singleton</font><font color="#007700">() <br /> {<br /> if (!isset(</font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">$instance</font><font color="#007700">)) {<br /> </font><font color="#0000BB">$c </font><font color="#007700">= </font><font color="#0000BB">__CLASS__</font><font color="#007700">;<br /> </font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">$instance </font><font color="#007700">= new </font><font color="#0000BB">$c</font><font color="#007700">;<br /> }<br /><br /> return </font><font color="#0000BB">self</font><font color="#007700">::</font><font color="#0000BB">$instance</font><font color="#007700">;<br /> }<br /> <br /> </font><font color="#FF8000">// Example method<br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">bark</font><font color="#007700">()<br /> {<br /> echo </font><font color="#DD0000">'Woof!'</font><font color="#007700">;<br /> }<br /><br /> </font><font color="#FF8000">// Prevent users to clone the instance<br /> </font><font color="#0000BB">public </font><font color="#007700">function </font><font color="#0000BB">__clone</font><font color="#007700">()<br /> {<br /> </font><font color="#0000BB">trigger_error</font><font color="#007700">(</font><font color="#DD0000">'Clone is not allowed.'</font><font color="#007700">, </font><font color="#0000BB">E_USER_ERROR</font><font color="#007700">);<br /> }<br /><br />}<br /><br /></font><font color="#0000BB">?></font>
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> This allows a single instance of the <TT
CLASS="literal"
>Example</TT
>
class to be retrieved.
</P
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<font color="#0000BB"><?php<br /></font><font color="#FF8000">// This would fail because the constructor is private<br /></font><font color="#0000BB">$test </font><font color="#007700">= new </font><font color="#0000BB">Example</font><font color="#007700">;<br /><br /></font><font color="#FF8000">// This will always retrieve a single instance of the class<br /></font><font color="#0000BB">$test </font><font color="#007700">= </font><font color="#0000BB">Example</font><font color="#007700">::</font><font color="#0000BB">singleton</font><font color="#007700">();<br /></font><font color="#0000BB">$test</font><font color="#007700">-></font><font color="#0000BB">bark</font><font color="#007700">();<br /><br /></font><font color="#FF8000">// This will issue an E_USER_ERROR.<br /></font><font color="#0000BB">$test_clone </font><font color="#007700">= </font><font color="#0000BB">clone $test</font><font color="#007700">;<br /><br /></font><font color="#0000BB">?></font>
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