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Visitor 模式心得

最近讀到Visitor模式，還是一知半解的。偶然翻到Uncle Bob對(duì)該模式的推導(dǎo)過(guò)程，有所心得，和大家分享一下。 Uncle Bob 的鏈接是： http://butunclebob.com/ArticleS.UncleBob.VisitorVersusInstanceOf。個(gè)人覺(jué)得該模式用來(lái)操作復(fù)雜對(duì)象集合，特別適用于報(bào)表生成。因?yàn)閳?bào)表的來(lái)源相對(duì)穩(wěn)定(復(fù)雜數(shù)據(jù)集合)，但是表現(xiàn)形式卻是千變?nèi)f化。言歸正傳，我將該博客的內(nèi)容按照自己的理解分享出來(lái)，如果有什么不對(duì)的地方，請(qǐng)指正。

首先有一個(gè)如下簡(jiǎn)單的場(chǎng)景，Bob先生的公司提供計(jì)算機(jī)方面的培訓(xùn)服務(wù)，對(duì)社會(huì)人士開(kāi)設(shè)二門(mén)課程，一門(mén)是OOD, 一門(mén)是Java，java課程需要上機(jī)(結(jié)對(duì)編程，所以二個(gè)人用一臺(tái)機(jī)器)，所以需要根據(jù)報(bào)名的時(shí)間計(jì)算具體的機(jī)器數(shù)目：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
    }

    abstract public int getComputersInUse(GregorianCalendar date);
}

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public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    public int getComputersInUse(GregorianCalendar date) {
        return 0;
    }
}

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public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    public int getComputersInUse(GregorianCalendar date) {
        int resources = 0;
        if (!date.before(startDate) && !date.after(endDate)) {
            resources = Math.round(students/2);
        }
        return resources;
    }
}

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代碼實(shí)現(xiàn)了，B先生很happy。不久客戶(hù)說(shuō)需要生一份報(bào)表，B先生想了5分鐘，洋洋灑灑寫(xiě)下如下代碼：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReportLine() {
        StringBuffer line = new StringBuffer();
        line.append(dateFormat.format(startDate.getTime())).append(" ")
                .append(courseName()).append(" ")
                .append(computersInUse()).append(" Computers\n");
        return line.toString();
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    protected abstract String courseName();
    protected abstract String computersInUse();
}

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public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    protected String courseName() {
        return "AOOD";
    }

    @Override
    protected String computersInUse() {
        return "0";
    }
}

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public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    protected String courseName() {
        return "Java";
    }

    @Override
    protected String computersInUse() {
        return String.valueOf(Math.round(students/2));
    }
}

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public class CourseResourceTracker<T extends Course> {

    Set<T> courses = new HashSet<>();

    public String generateReport() {
        StringBuilder reports = new StringBuilder();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            T course = (T) i.next();
            reports.append(course.generateComputerReportLine()) ;
        }
        return reports.toString();
    }

    public void add(T course) {
        if (course == null) return;

        courses.add(course);
    }

    public void remove(T course) {
        if (course == null) return;

        courses.remove(course);
    }

    public static void main(String[] param) {
        CourseResourceTracker tracker = new CourseResourceTracker();
        tracker.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        tracker.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + tracker.generateReport());
    }
}

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報(bào)表很簡(jiǎn)單，打印結(jié)果如下，格式為：時(shí)間 + 名稱(chēng) + 數(shù)量

Report is : 04/20/2015 AOOD 0 Computers
04/20/2015 Java 5 Computers

B洋洋得意，客戶(hù)又來(lái)了新的需求，前面的報(bào)表太簡(jiǎn)單了，需要針對(duì)不同的課程打印不同的內(nèi)容：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract public int getComputersInUse(GregorianCalendar date);
}

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public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    public int getComputersInUse(GregorianCalendar date) {
        return 0;
    }
}

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public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    public int getComputersInUse(GregorianCalendar date) {
        int resources = 0;
        if (!date.before(startDate) && !date.after(endDate)) {
            resources = Math.round(students/2);
        }
        return resources;
    }

    public int getTotalComputers() {
        return students/2;
    }
}

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public class CourseComputerReport {
    protected SimpleDateFormat dateFormat;

    public CourseComputerReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        if (courses == null || courses.size() == 0) return null;

        StringBuffer report = new StringBuffer();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            if (course instanceof AOODCourse) {
                report.append("AOOD 0 Computers\n");
            } else if (course instanceof JavaCourse) {
                report.append("Java ");
                JavaCourse jc = (JavaCourse)course;
                report.append(String.valueOf(jc.getTotalComputers())).append(" Computers\n");
            }
        }
        return report.toString();
    }

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }
}

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從上面的UML 類(lèi)圖可以看出，CourseComputerReport 因?yàn)閷?duì)每門(mén)課程的format不一致，需要遍歷的時(shí)候針對(duì)不同的課程設(shè)置不同的格式從而導(dǎo)致CourseReport和Course之前出現(xiàn)強(qiáng)耦合，可以通過(guò)重構(gòu)方法 generateComputerReport 使之更加合理：

public abstract class CourseReport {
    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            if (course instanceof AOODCourse) {
                appendAOODLine((AOODCourse)course, report);
            } else if (course instanceof JavaCourse) {
                appendJavaLine((JavaCourse) course, report);
            }
        }
        return report.toString();
    }

    protected abstract void appendJavaLine(JavaCourse course, StringBuffer report);
    protected abstract void appendAOODLine(AOODCourse course, StringBuffer report);
}

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public class CourseComputerReport extends CourseReport{

    protected void appendJavaLine(JavaCourse course, StringBuffer report) {
        report.append("Java ");
        report.append(String.valueOf(course.getTotalComputers())).append(" Computers\n");
    }

    protected void appendAOODLine(AOODCourse course, StringBuffer report) {
        report.append("AOOD 0 Computers\n");
    }

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }
}

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老鳥(niǎo)L看了B的實(shí)現(xiàn)后，提出了自己的看法，為什么 CourseReport 需要依賴(lài)具體的Course，能否將Course告訴reporter 而不是Reporter來(lái)判斷具體的Course。并將Visitor的經(jīng)典類(lèi)圖隨手拋給小B：

小B恍然大悟，修改代碼如下：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract void accept(CourseVisitor courseVisitor);

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public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        courseVisitor.visit(this);
    }
}

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public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        courseVisitor.visit(this);
    }

    public int getTotalComputers() {
        return students/2;
    }
}

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public interface CourseVisitor {
    void visit(AOODCourse aoodCourse);
    void visit(JavaCourse javaCourse);
}

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public class ReportCourseVisitor implements CourseVisitor {

    protected StringBuffer report;

    public ReportCourseVisitor(StringBuffer report) {
        this.report = report;

    }

    @Override
    public void visit(AOODCourse aoodCourse) {
        report.append("AOOD 0 Computers\n");
    }

    @Override
    public void visit(JavaCourse javaCourse) {
        report.append("Java ");
        report.append(String.valueOf(javaCourse.getTotalComputers())).append(" Computers\n");
    }
}

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public abstract class CourseReport {

    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        CourseVisitor v = makeReportVisitor(report);
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            course.accept(v);
        }
        return report.toString();
    }

    protected abstract CourseVisitor makeReportVisitor(StringBuffer report);
}

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public class CourseComputerReport extends CourseReport{

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }

    @Override
    protected CourseVisitor makeReportVisitor(StringBuffer report) {
        return new ReportCourseVisitor(report);
    }
}

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老鳥(niǎo)L看了以后表示滿意，同時(shí)指出如果添加新的Course，由于所有的course都依賴(lài)CourseVisitor，而courseVisitor需要添加新的接口，所有的course需要重新打包和編譯。是否可以新建一個(gè)空接口CourseVisitor 使Course依賴(lài)這個(gè)接口從而達(dá)到隔離變化的目的。需要修改的是需要將courseVisitor Cast到具體的實(shí)現(xiàn)類(lèi)，主要的代碼實(shí)現(xiàn)是：

 @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AOODCourseVisitor) {
            ((AOODCourseVisitor) courseVisitor).visit(this);
        }
    }

小B擼擼袖子，大筆一揮，修改代碼如下：

public abstract class Course {

    protected GregorianCalendar startDate;
    protected int students;
    protected SimpleDateFormat dateFormat;

    public Course(GregorianCalendar startDate, int students) {
        this.startDate = (GregorianCalendar) startDate.clone();
        this.students = students;
        this.dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public GregorianCalendar getStartDate() {
        return startDate;
    }

    abstract void accept(CourseVisitor courseVisitor);
}

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public class AOODCourse extends Course {

    public AOODCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AOODCourseVisitor) {
            ((AOODCourseVisitor) courseVisitor).visit(this);
        }
    }
}

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public class JavaCourse extends Course {

    private GregorianCalendar endDate;

    public JavaCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof JavaCourseVisitor) {
            ((JavaCourseVisitor) courseVisitor).visit(this);
        }
    }

    public int getTotalComputers() {
        return students/2;
    }
}

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public class AndroidCourse extends Course {

    private GregorianCalendar endDate;

    public AndroidCourse(GregorianCalendar startDate, int students) {
        super(startDate, students);
        endDate = (GregorianCalendar) startDate.clone();
        endDate.add(Calendar.DAY_OF_WEEK, 4);
    }

    @Override
    void accept(CourseVisitor courseVisitor) {
        if (courseVisitor instanceof AndroidCourseVisitor) {
            ((AndroidCourseVisitor) courseVisitor).visit(this);
        }
    }

    public int getTotalComputers() {
        return students/2;
    }
}

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public interface CourseVisitor {
}

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public interface AndroidCourseVisitor {
    void visit(AndroidCourse androidCourse);
}

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public interface AOODCourseVisitor {
    void visit(AOODCourse aoodCourse);
}

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public interface JavaCourseVisitor {
    void visit(JavaCourse javaCourse);
}

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public abstract class CourseReport {

    protected SimpleDateFormat dateFormat;

    public CourseReport() {
        dateFormat = new SimpleDateFormat("MM/dd/yyyy");
    }

    public String generateComputerReport(List courses) {
        StringBuffer report = new StringBuffer();
        CourseVisitor v = makeCourseVisitor(report);
        for (Iterator i = courses.iterator(); i.hasNext();) {
            Course course = (Course) i.next();
            report.append(dateFormat.format(course.getStartDate().getTime())).append(" ");
            course.accept(v);
        }
        return report.toString();
    }

    protected abstract CourseVisitor makeCourseVisitor(StringBuffer report);
}

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public class CourseComputerReport extends CourseReport {

    public static void main(String[] param) {
        CourseComputerReport report = new CourseComputerReport();
        List<Course> courses = new ArrayList<>();
        courses.add(new AOODCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new JavaCourse(new GregorianCalendar(2015, 3, 20), 10));
        courses.add(new AndroidCourse(new GregorianCalendar(2015, 3, 20), 10));
        System.out.println("Report is : " + report.generateComputerReport(courses));
    }

    @Override
    protected CourseVisitor makeCourseVisitor(StringBuffer report) {
        return new ReportCourseVisitor(report);
    }
}

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順便說(shuō)一下，上面使用的模式名稱(chēng)是Acyclic visitor，是為了解決添加新的item 導(dǎo)致所有的item需要重新編譯和打包的問(wèn)題。

posted on 2017-05-31 13:11 布兜兜閱讀(489) 評(píng)論(0) 收藏舉報(bào)

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