pat5i.htm

设计模式英文版 作者：Erich Gamma、Richard Helm、Ralph Johnson和John Vlissides 四人帮的书。 学设计模式的必读的书籍！经典中的经典

<A NAME="auto1056"></A>
<P>Consider the following implementation issues:</P>

<OL>

<A NAME="auto1057"></A>
<LI><EM>Defining the Strategy and Context interfaces.</EM>
The Strategy and Context interfaces must give a ConcreteStrategy
efficient access to any data it needs from a context, and vice versa.

<A NAME="auto1058"></A>
<P>One approach is to have Context pass data in parameters to Strategy
operations&#151;in other words, take the data to the strategy. This keeps
Strategy and Context decoupled. On the other hand, Context might
pass data the Strategy doesn't need.</P>

<A NAME="auto1059"></A>
<P>Another technique has a context pass <EM>itself</EM> as an argument, and
the strategy requests data from the context explicitly.
Alternatively, the strategy can store a reference to its context,
eliminating the need to pass anything at all.  Either way, the
strategy can request exactly what it needs.  But now Context must
define a more elaborate interface to its data, which couples Strategy
and Context more closely.</P>

<A NAME="auto1060"></A>
<P>The needs of the particular algorithm and its data requirements will
determine the best technique.</P>

</LI>

<A NAME="template-impl-strat"></A>
<LI><EM>Strategies as template parameters.</EM>
In C++ templates can be used to configure a class with a strategy.
This technique is only applicable if (1) the Strategy can be selected
at compile-time, and (2) it does not have to be changed at run-time.
In this case, the class to be configured (e.g., <CODE>Context</CODE>) is
defined as a template class that has a <CODE>Strategy</CODE> class as a
parameter:

<A NAME="auto1061"></A>
<PRE>
    template <class AStrategy>
    class Context {
        void Operation() { theStrategy.DoAlgorithm(); }
        // ...
    private:
        AStrategy theStrategy;
    };
</PRE>
The class is then configured with a <CODE>Strategy</CODE> class when it's
instantiated:
<A NAME="auto1062"></A>
<PRE>
    class MyStrategy {
    public:
        void DoAlgorithm();
    };
    
    <A NAME="auto1063"></A>
<P>Context<MyStrategy> aContext;
</PRE>
With templates, there's no need to define an abstract class that defines
the interface to the <CODE>Strategy.</CODE> Using <CODE>Strategy</CODE> as a
template parameter also lets you bind a <CODE>Strategy</CODE> to its
<CODE>Context</CODE> statically, which can increase efficiency.</P>

</LI>

<A NAME="strat-optional"></A>
<LI><EM>Making Strategy objects optional.</EM>
The Context class may be simplified if it's meaningful <EM>not</EM> to
have a Strategy object.  Context checks to see if it has a Strategy
object before accessing it.  If there is one, then Context uses it
normally.  If there isn't a strategy, then Context carries out default
behavior.  The benefit of this approach is that clients don't have to
deal with Strategy objects at all <EM>unless</EM> they don't like the
default behavior.</LI>

</OL>

<A NAME="samplecode"><A>
<H2><A HREF="#knownuses"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Sample Code</H2> 

<A NAME="auto1064"></A>
<P>We'll give the high-level code for the Motivation example, which is
based on the implementation of Composition and Compositor classes in
InterViews&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=InterViews3.1" TARGET="_mainDisplayFrame">LCI+92</A>].</P>

<A NAME="shrinkability"></A>
<A NAME="stretchability"></A>
<P>The <CODE>Composition</CODE> class maintains a collection of
<CODE>Component</CODE> instances, which represent text and graphical
elements in a document.  A composition arranges component objects into
lines using an instance of a <CODE>Compositor</CODE> subclass, which
encapsulates a linebreaking strategy.  Each component has an
associated natural size, stretchability, and shrinkability. The
stretchability defines how much the component can grow beyond its
natural size; shrinkability is how much it can shrink.  The
composition passes these values to a compositor, which uses them to
determine the best location for linebreaks.</P>

<A NAME="auto1065"></A>
<PRE>
    class Composition {
    public:
        Composition(Compositor*);
        void Repair();
    private:
        Compositor* _compositor;
        Component* _components;    // the list of components
        int _componentCount;       // the number of components
        int _lineWidth;            // the Composition's line width
        int* _lineBreaks;          // the position of linebreaks
                                   // in components
        int _lineCount;            // the number of lines
    };
</PRE>

<A NAME="auto1066"></A>
<P>When a new layout is required, the composition asks its compositor to
determine where to place linebreaks. The composition passes the
compositor three arrays that define natural sizes, stretchabilities,
and shrinkabilities of the components.  It also passes the number of
components, how wide the line is, and an array that the compositor
fills with the position of each linebreak.  The compositor returns the
number of calculated breaks.</P>

<A NAME="auto1067"></A>
<P>The <CODE>Compositor</CODE> interface lets the composition pass the
compositor all the information it needs.  This is an example of
"taking the data to the strategy":</P>

<A NAME="auto1068"></A>
<PRE>
    class Compositor {
    public:
        virtual int Compose(
            Coord natural[], Coord stretch[], Coord shrink[],
            int componentCount, int lineWidth, int breaks[]
        ) = 0;
    protected:
        Compositor();
    };
</PRE>

<A NAME="auto1069"></A>
<P>Note that <CODE>Compositor</CODE> is an abstract class.  Concrete
subclasses define specific linebreaking strategies.</P>

<A NAME="auto1070"></A>
<P>The composition calls its compositor in its <CODE>Repair</CODE>
operation.  <CODE>Repair</CODE> first initializes arrays with the natural
size, stretchability, and shrinkability of each component (the details
of which we omit for brevity).  Then it calls on the compositor to
obtain the linebreaks and finally lays out the components according to
the breaks (also omitted):</P>

<A NAME="auto1071"></A>
<PRE>
    void Composition::Repair () {
        Coord* natural;
        Coord* stretchability;
        Coord* shrinkability;
        int componentCount;
        int* breaks;
    
        // prepare the arrays with the desired component sizes
        // ...
    
        // determine where the breaks are:
        int breakCount;
        breakCount = _compositor->Compose(
            natural, stretchability, shrinkability,
            componentCount, _lineWidth, breaks
        );
    
        // lay out components according to breaks
        // ...
    }
</PRE>

<A NAME="simplecompositor2"></A>
<P>Now let's look at the <CODE>Compositor</CODE> subclasses.
<CODE>SimpleCompositor</CODE> examines components a line at a time to
determine where breaks should go:</P>

<A NAME="auto1072"></A>
<PRE>
    class SimpleCompositor : public Compositor {
    public:
        SimpleCompositor();
    
        virtual int Compose(
            Coord natural[], Coord stretch[], Coord shrink[],
            int componentCount, int lineWidth, int breaks[]
        );
        // ...
    };
</PRE>

<A NAME="doc-color"></A>
<A NAME="tex-comp2"></A>
<P><CODE>TeXCompositor</CODE> uses a more global strategy.  It examines a
<EM>paragraph</EM> at a time, taking into account the components' size
and stretchability.  It also tries to give an even "color" to the
paragraph by minimizing the whitespace between components.</P>

<A NAME="auto1073"></A>
<PRE>
    class TeXCompositor : public Compositor {
    public:
        TeXCompositor();
    
        virtual int Compose(
            Coord natural[], Coord stretch[], Coord shrink[],
            int componentCount, int lineWidth, int breaks[]
        );
        // ...
    };
</PRE>

<A NAME="breaks"></A>
<P><CODE>ArrayCompositor</CODE> breaks the components into lines at regular
intervals.</P>

<A NAME="auto1074"></A>
<PRE>
    class ArrayCompositor : public Compositor {
    public:
        ArrayCompositor(int interval);
    
        virtual int Compose(
            Coord natural[], Coord stretch[], Coord shrink[],
            int componentCount, int lineWidth, int breaks[]
        );
        // ...
    };
</PRE>

<A NAME="auto1075"></A>
<P>These classes don't use all the information passed in
<CODE>Compose</CODE>.  <CODE>SimpleCompositor</CODE> ignores the stretchability
of the components, taking only their natural widths into account.
<CODE>TeXCompositor</CODE> uses all the information passed to it, whereas
<CODE>ArrayCompositor</CODE> ignores everything.</P>

<A NAME="auto1076"></A>
<P>To instantiate <CODE>Composition</CODE>, you pass it the compositor
you want to use:</P>

<A NAME="auto1077"></A>
<PRE>
    Composition* quick = new Composition(new SimpleCompositor);
    Composition* slick = new Composition(new TeXCompositor);
    Composition* iconic = new Composition(new ArrayCompositor(100));
</PRE>

<A NAME="auto1078"></A>
<P><CODE>Compositor</CODE>'s interface is carefully designed to support all
layout algorithms that subclasses might implement.  You don't want to
have to change this interface with every new subclass, because that will
require changing existing subclasses.  In general, the Strategy and
Context interfaces determine how well the pattern achieves its intent.</P>

<A NAME="knownuses"><A>
<H2><A HREF="#relatedpatterns"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Known Uses</H2> 

<A NAME="auto1079"></A>
<P>Both ET++&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=et++" TARGET="_mainDisplayFrame">WGM88</A>] and InterViews use strategies to encapsulate
different linebreaking algorithms as we've described.</P>

<A NAME="rtlsmall-use-strat"></A>
<P>In the RTL System for compiler code optimization&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=RTLSystem92" TARGET="_mainDisplayFrame">JML92</A>],
strategies define different register allocation schemes
(RegisterAllocator) and instruction set scheduling policies
(RISCscheduler, CISCscheduler). This provides flexibility in targeting the
optimizer for different machine architectures.</P>

<A NAME="et-swapman"></A>
<A NAME="future-cashflow"></A>
<A NAME="swaps"></A>
<P>The ET++SwapsManager calculation engine framework computes prices for
different financial instruments&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=egg-gamma_swaps" TARGET="_mainDisplayFrame">EG92</A>]. Its key
abstractions are Instrument and YieldCurve. Different instruments are
implemented as subclasses of Instrument. YieldCurve calculates
discount factors, which determine the present value of future cash
flows. Both of these classes delegate some behavior to Strategy
objects. The framework provides a family of ConcreteStrategy classes
for generating cash flows, valuing swaps, and calculating discount
factors. You can create new calculation engines by configuring
Instrument and YieldCurve with the different ConcreteStrategy objects.
This approach supports mixing and matching existing Strategy
implementations as well as defining new ones.</P>

<A NAME="template-impl-strat2"></A>
<P>The Booch components&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=booch_components" TARGET="_mainDisplayFrame">BV90</A>] use strategies as template
arguments. The Booch collection classes support three different kinds of
memory allocation strategies: managed (allocation out of a pool),
controlled (allocations/deallocations are protected by locks), and
unmanaged (the normal memory allocator). These strategies are passed as
template arguments to a collection class when it's instantiated. For
example, an UnboundedCollection that uses the unmanaged strategy is
instantiated as <CODE>UnboundedCollection<MyItemType*, Unmanaged></CODE>.</P>

<A NAME="rapp-use-strategy"></A>
<P>RApp is a system for integrated circuit layout&nbsp;.
RApp must lay out and route wires that connect subsystems on the
circuit. Routing algorithms in RApp are defined as
subclasses of an abstract Router class.  Router is a Strategy class.</P>

<A NAME="auto1080"></A>
<P>Borland's ObjectWindows&nbsp;[<A HREF="vfs.htm?doc=bib-0.htm&fid=bb&hid=objectwindows" TARGET="_mainDisplayFrame">Bor94</A>] uses strategies in dialogs
boxes to ensure that the user enters valid data. For example, numbers might
have to be in a certain range, and a numeric entry field should accept
only digits. Validating that a string is correct can require a
table look-up.</P>

<A NAME="validator"></A>
<P>ObjectWindows uses Validator objects to encapsulate validation
strategies.  Validators are examples of Strategy objects.  Data entry
fields delegate the validation strategy to an optional Validator
object. The client attaches a validator to a field if validation is
required (an example of an optional strategy).  When the dialog is
closed, the entry fields ask their validators to validate the data.
The class library provides validators for common cases, such as a
RangeValidator for numbers.  New client-specific validation strategies
can be defined easily by subclassing the Validator class.</P>

<A NAME="relatedpatterns"></A>
<H2><A HREF="#last"><IMG SRC="gifsb/down3.gif" BORDER=0></A> Related Patterns</H2> 

<A NAME="auto1081"></A>
<P><A HREF="pat4ffs.htm" TARGET="_mainDisplayFrame">Flyweight&nbsp;(195)</A>:
Strategy objects often make good flyweights.</P>

<A NAME="last"></A>
<P><A HREF="#intent"><IMG SRC="gifsb/up3.gif" BORDER=0></A><BR>
<A HREF="pat5jfs.htm" TARGET="_mainDisplayFrame"><IMG SRC="gifsb/rightar3.gif"
        ALIGN=TOP BORDER=0></A> <A HREF="pat5jfs.htm"
        TARGET="_mainDisplayFrame">Template Method</A><BR>
<A HREF="pat5hfs.htm" TARGET="_mainDisplayFrame"><IMG SRC="gifsb/leftarr3.gif"
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</P>

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