一、模式定义

造者模式(Builder Pattern):将一个复杂对象的构建与它的表示分离,使得同样的构建过程可以创建不同的表示。

建造者模式是一步一步创建一个复杂的对象,它允许用户只通过指定复杂对象的类型和内容就可以构建它们,用户不需要知道内部的具体构建细节。建造者模式属于对象创建型模式。根据中文翻译的不同,建造者模式又可以称为生成器模式。

二、模式结构

建造者模式包含如下角色:

  • Builder:抽象建造者
  • ConcreteBuilder:具体建造者
  • Director:指挥者
  • Product:产品角色

三、时序图

四、简单实现

电脑的组装过程较为复杂,步骤繁多,但是顺序却是不固定的。下面我们以组装电脑为例来演示一下简单且经典的builder模式

package com.dp.example.builder;

/**
 * Computer产品抽象类, 为了例子简单, 只列出这几个属性
 * 
 * @author mrsimple
 *
 */
public abstract class Computer {

    protected int mCpuCore = 1;
    protected int mRamSize = 0;
    protected String mOs = "Dos";

    protected Computer() {

    }

    // 设置CPU核心数
    public abstract void setCPU(int core);

    // 设置内存
    public abstract void setRAM(int gb);

    // 设置操作系统
    public abstract void setOs(String os);

    @Override
    public String toString() {
        return "Computer [mCpuCore=" + mCpuCore + ", mRamSize=" + mRamSize
                + ", mOs=" + mOs + "]";
    }

}

package com.dp.example.builder;

/**
 * Apple电脑
 */
public class AppleComputer extends Computer {

    protected AppleComputer() {

    }

    @Override
    public void setCPU(int core) {
        mCpuCore = core;
    }

    @Override
    public void setRAM(int gb) {
        mRamSize = gb;
    }

    @Override
    public void setOs(String os) {
        mOs = os;
    }

}

package com.dp.example.builder;


package com.dp.example.builder;

/**
 * builder抽象类
 *
 */
public abstract class Builder {
    // 设置CPU核心数
    public abstract void buildCPU(int core);

    // 设置内存
    public abstract void buildRAM(int gb);

    // 设置操作系统
    public abstract void buildOs(String os);

    // 创建Computer
    public abstract Computer create();

}

package com.dp.example.builder;

public class ApplePCBuilder extends Builder {
    private Computer mApplePc = new AppleComputer();

    @Override
    public void buildCPU(int core) {
        mApplePc.setCPU(core);
    }

    @Override
    public void buildRAM(int gb) {
        mApplePc.setRAM(gb);
    }

    @Override
    public void buildOs(String os) {
        mApplePc.setOs(os);
    }

    @Override
    public Computer create() {
        return mApplePc;
    }

}

package com.dp.example.builder;

public class Director {
    Builder mBuilder = null;

    /**
     * 
     * @param builder
     */
    public Director(Builder builder) {
        mBuilder = builder;
    }

    /**
     * 构建对象
     * 
     * @param cpu
     * @param ram
     * @param os
     */
    public void construct(int cpu, int ram, String os) {
        mBuilder.buildCPU(cpu);
        mBuilder.buildRAM(ram);
        mBuilder.buildOs(os);
    }
}

/**
 * 经典实现较为繁琐
 * 
 * @author mrsimple
 *
 */
public class Test {
    public static void main(String[] args) {
        // 构建器
        Builder builder = new ApplePCBuilder();
        // Director
        Director pcDirector = new Director(builder);
        // 封装构建过程, 4核, 内存2GB, Mac系统
        pcDirector.construct(4, 2, "Mac OS X 10.9.1");
        // 构建电脑, 输出相关信息
        System.out.println("Computer Info : " + builder.create().toString());
    }
}

五、Android源码中模式实现

在Android源码中,我们最常用到的Builder模式就是AlertDialog.Builder, 使用该Builder来构建复杂的AlertDialog对象。简单示例如下 :

//显示基本的AlertDialog  
private void showDialog(Context context) {  
    AlertDialog.Builder builder = new AlertDialog.Builder(context);  
    builder.setIcon(R.drawable.icon);  
    builder.setTitle("Title");  
    builder.setMessage("Message");  
    builder.setPositiveButton("Button1",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("点击了对话框上的Button1");  
                }  
            });  
    builder.setNeutralButton("Button2",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("点击了对话框上的Button2");  
                }  
            });  
    builder.setNegativeButton("Button3",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("点击了对话框上的Button3");  
                }  
            });  
    builder.create().show();  // 构建AlertDialog, 并且显示
}

结果 : result

下面我们看看AlertDialog的相关源码 :

// AlertDialog
public class AlertDialog extends Dialog implements DialogInterface {
    // Controller, 接受Builder成员变量P中的各个参数
    private AlertController mAlert;

    // 构造函数
    protected AlertDialog(Context context, int theme) {
        this(context, theme, true);
    }

    // 4 : 构造AlertDialog
    AlertDialog(Context context, int theme, boolean createContextWrapper) {
        super(context, resolveDialogTheme(context, theme), createContextWrapper);
        mWindow.alwaysReadCloseOnTouchAttr();
        mAlert = new AlertController(getContext(), this, getWindow());
    }

    // 实际上调用的是mAlert的setTitle方法
    @Override
    public void setTitle(CharSequence title) {
        super.setTitle(title);
        mAlert.setTitle(title);
    }

    // 实际上调用的是mAlert的setCustomTitle方法
    public void setCustomTitle(View customTitleView) {
        mAlert.setCustomTitle(customTitleView);
    }

    public void setMessage(CharSequence message) {
        mAlert.setMessage(message);
    }

    // AlertDialog其他的代码省略

    // ************  Builder为AlertDialog的内部类   *******************
    public static class Builder {
        // 1 : 存储AlertDialog的各个参数, 例如title, message, icon等.
        private final AlertController.AlertParams P;
        // 属性省略

        /**
         * Constructor using a context for this builder and the {@link AlertDialog} it creates.
         */
        public Builder(Context context) {
            this(context, resolveDialogTheme(context, 0));
        }


        public Builder(Context context, int theme) {
            P = new AlertController.AlertParams(new ContextThemeWrapper(
                    context, resolveDialogTheme(context, theme)));
            mTheme = theme;
        }

        // Builder的其他代码省略 ......

        // 2 : 设置各种参数
        public Builder setTitle(CharSequence title) {
            P.mTitle = title;
            return this;
        }


        public Builder setMessage(CharSequence message) {
            P.mMessage = message;
            return this;
        }

        public Builder setIcon(int iconId) {
            P.mIconId = iconId;
            return this;
        }

        public Builder setPositiveButton(CharSequence text, final OnClickListener listener) {
            P.mPositiveButtonText = text;
            P.mPositiveButtonListener = listener;
            return this;
        }


        public Builder setView(View view) {
            P.mView = view;
            P.mViewSpacingSpecified = false;
            return this;
        }

        // 3 : 构建AlertDialog, 传递参数
        public AlertDialog create() {
            // 调用new AlertDialog构造对象, 并且将参数传递个体AlertDialog 
            final AlertDialog dialog = new AlertDialog(P.mContext, mTheme, false);
            // 5 : 将P中的参数应用的dialog中的mAlert对象中
            P.apply(dialog.mAlert);
            dialog.setCancelable(P.mCancelable);
            if (P.mCancelable) {
                dialog.setCanceledOnTouchOutside(true);
            }
            dialog.setOnCancelListener(P.mOnCancelListener);
            if (P.mOnKeyListener != null) {
                dialog.setOnKeyListener(P.mOnKeyListener);
            }
            return dialog;
        }
    }

}

可以看到,通过Builder来设置AlertDialog中的title, message, button等参数, 这些参数都存储在类型为AlertController.AlertParams的成员变量P中,AlertController.AlertParams中包含了与之对应的成员变量。在调用Builder类的create函数时才创建AlertDialog, 并且将Builder成员变量P中保存的参数应用到AlertDialog的mAlert对象中,即P.apply(dialog.mAlert)代码段。我们看看apply函数的实现 :

public void apply(AlertController dialog) {
    if (mCustomTitleView != null) {
        dialog.setCustomTitle(mCustomTitleView);
    } else {
        if (mTitle != null) {
            dialog.setTitle(mTitle);
        }
        if (mIcon != null) {
            dialog.setIcon(mIcon);
        }
        if (mIconId >= 0) {
            dialog.setIcon(mIconId);
        }
        if (mIconAttrId > 0) {
            dialog.setIcon(dialog.getIconAttributeResId(mIconAttrId));
        }
    }
    if (mMessage != null) {
        dialog.setMessage(mMessage);
    }
    if (mPositiveButtonText != null) {
        dialog.setButton(DialogInterface.BUTTON_POSITIVE, mPositiveButtonText,
                mPositiveButtonListener, null);
    }
    if (mNegativeButtonText != null) {
        dialog.setButton(DialogInterface.BUTTON_NEGATIVE, mNegativeButtonText,
                mNegativeButtonListener, null);
    }
    if (mNeutralButtonText != null) {
        dialog.setButton(DialogInterface.BUTTON_NEUTRAL, mNeutralButtonText,
                mNeutralButtonListener, null);
    }
    if (mForceInverseBackground) {
        dialog.setInverseBackgroundForced(true);
    }
    // For a list, the client can either supply an array of items or an
    // adapter or a cursor
    if ((mItems != null) || (mCursor != null) || (mAdapter != null)) {
        createListView(dialog);
    }
    if (mView != null) {
        if (mViewSpacingSpecified) {
            dialog.setView(mView, mViewSpacingLeft, mViewSpacingTop, mViewSpacingRight,
                    mViewSpacingBottom);
        } else {
            dialog.setView(mView);
        }
    }
}

实际上就是把P中的参数挨个的设置到AlertController中, 也就是AlertDialog中的mAlert对象。从AlertDialog的各个setter方法中我们也可以看到,实际上也都是调用了mAlert对应的setter方法。在这里,Builder同时扮演了上文中提到的builder、ConcreteBuilder、Director的角色,简化了Builder模式的设计。

六、优缺点

优点

  • 良好的封装性, 使用建造者模式可以使客户端不必知道产品内部组成的细节;
  • 建造者独立,容易扩展;
  • 在对象创建过程中会使用到系统中的一些其它对象,这些对象在产品对象的创建过程中不易得到。

缺点

  • 会产生多余的Builder对象以及Director对象,消耗内存;
  • 对象的构建过程暴露。
Copyright © ruheng.com 2017 all right reserved,powered by Gitbook该文件修订时间: 2018-05-12 01:48:13

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