Android设计模式学习之Builder模式

Android设计模式学习之观察者模式

建造者模式（Builder Pattern），是创造性模式之一，Builder 模式的目的则是为了将对象的构建与展示分离。Builder 模式是一步一步创建一个复杂对象的创建型模式，它允许用户在不知道内部构建细节的情况下，可以更精细地控制对象的构造流程。

模式的使用场景

1.相同的方法，不同的执行顺序，产生不同的事件结果时；
2.多个部件或零件，都可以装配到一个对象中，但是产生的运行结果又不相同时；
3.产品类非常复杂，或者产品类中的调用顺序不同产生了不同的效能，这个时候使用建造者模式非常合适。

UML类图

角色介绍

Product 产品类 : 产品的抽象类；
Builder : 抽象类， 规范产品的组建，一般是由子类实现具体的组件过程；
ConcreteBuilder : 具体的构建器；
Director : 统一组装过程(可省略)。

Builder模式简单实现

Builder模式最典型的例子，就是组装电脑的例子了
创建产品类

public class Computer {
    private String mCpu;
    private String mRam;

    public void setmCpu(String mCpu) {
        this.mCpu = mCpu;
    }


    public void setmRam(String mRam) {
        this.mRam = mRam;
    }
}

创建Builder类
组装电脑有一套组装方法的模版，就是一个抽象的Builder类,里面提供了安装CPU、内存的方法，以及组装成电脑的create方法：

public abstract class Builder {
    public abstract void buildCpu(String cpu);
    public abstract void buildRam(String ram);
    public abstract Computer create();
}

实现了抽象的Builder类，ComputerBuilder类用于组装电脑：

public class ComputerBuilder extends Builder { 
   
    private Computer mComputer = new Computer();
    @Override
    public void buildCpu(String cpu) {
        mComputer.setmCpu(cpu);
    }



    @Override
    public void buildRam(String ram) {
        mComputer.setmRam(ram);
    }

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

用Dirextor指挥者类来统一组装过程

public class Direcror {
    Builder mBuild=null;
    public Direcror(Builder build){
        this.mBuild=build;
    }
    public Computer CreateComputer(String cpu,String mainboard,String ram){
        //规范建造流程
       this.mBuild.buildMainboard(mainboard);     
       this.mBuild.buildRam(ram);
       return mBuild.create();
    }
}

客户端调用指挥者类

最后商家用指挥者类组装电脑。我们只需要提供我们想要的CPU，内存就可以了，至于商家怎样组装的电脑我们无需知道。

public class CreatComputer {
    public static void main(String[]args){
        Builder mBuilder=new MoonComputerBuilder();
        Direcror mDirecror=new Direcror(mBuilder);
        //组装电脑
        mDirecror.CreateComputer("i5-3210","DDR4");
    }
}

Android源码中的Builder模式

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("头部");  
        builder.setMessage("内容");  
        builder.setPositiveButton("Button1",  
                new DialogInterface.OnClickListener() {  
                    public void onClick(DialogInterface dialog, int whichButton) {  
                        setTitle("点击了对话框上的Button1");  
                    }  
                })
　　　　　　　　.setNeutralButton("Button2",  
                new DialogInterface.OnClickListener() {  
                    public void onClick(DialogInterface dialog, int whichButton) {  
                        setTitle("点击了对话框上的Button2");  
                    }  
                });  
        builder.create().show();  // 构建AlertDialog， 并且显示
    }

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。

在实际项目中的应用

我们可以采用系统已经提供好的Builder设计模式构建整个应用的万能Dialog，代码可以参考系统的AlertDialog

public static class Builder { 
   

        private AlertController.AlertParams P;

        public Builder(Context context) {
            this(context, 0);
        }

        public Builder(Context context, int themeResId) {
            P = new AlertController.AlertParams();
            P.themeResId = themeResId;
            P.context = context;
        }

        public Builder setText(int viewId, CharSequence text) {
            P.textArray.put(viewId, text);
            return this;
        }

        public Builder setOnClickListener(int viewId, View.OnClickListener listener) {
            P.clickArray.put(viewId, listener);
            return this;
        }


        public Builder setContentView(int layoutId) {
            P.view = null;
            P.layoutId = layoutId;
            return this;
        }

        public Builder setContentView(View view) {
            P.layoutId = 0;
            P.view = view;
            return this;
        }

        /** * Sets whether the dialog is cancelable or not. Default is true. * * @return This Builder object to allow for chaining of calls to set methods */
        public Builder setCancelable(boolean cancelable) {
            P.cancelable = cancelable;
            return this;
        }


        /** * Sets the callback that will be called if the dialog is canceled. * <p> * <p>Even in a cancelable dialog, the dialog may be dismissed for reasons other than * being canceled or one of the supplied choices being selected. * If you are interested in listening for all cases where the dialog is dismissed * and not just when it is canceled, see * {@link #setOnDismissListener(OnDismissListener) setOnDismissListener}.</p> * * @return This Builder object to allow for chaining of calls to set methods * @see #setCancelable(boolean) * @see #setOnDismissListener(OnDismissListener) */
        public Builder setOnCancelListener(OnCancelListener onCancelListener) {
            P.onCancelListener = onCancelListener;
            return this;
        }

        /** * Sets the callback that will be called when the dialog is dismissed for any reason. * * @return This Builder object to allow for chaining of calls to set methods */
        public Builder setOnDismissListener(OnDismissListener onDismissListener) {
            P.onDismissListener = onDismissListener;
            return this;
        }

        /** * Sets the callback that will be called if a key is dispatched to the dialog. * * @return This Builder object to allow for chaining of calls to set methods */
        public Builder setOnKeyListener(OnKeyListener onKeyListener) {
            P.onKeyListener = onKeyListener;
            return this;
        }


        /** * Creates an {@link AlertDialog} with the arguments supplied to this * builder. * <p/> * Calling this method does not display the dialog. If no additional * processing is needed, {@link #show()} may be called instead to both * create and display the dialog. */
        public BaseDialog create() {
            // Context has already been wrapped with the appropriate theme.
            final BaseDialog dialog = new BaseDialog(P.context, P.themeResId);
            P.apply(dialog.mAlert);
            dialog.setCancelable(P.cancelable);
            if (P.cancelable) {
                dialog.setCanceledOnTouchOutside(true);
            }
            dialog.setOnCancelListener(P.onCancelListener);
            dialog.setOnDismissListener(P.onDismissListener);
            if (P.onKeyListener != null) {
                dialog.setOnKeyListener(P.onKeyListener);
            }
            return dialog;
        }

        /** * Creates an {@link AlertDialog} with the arguments supplied to this * builder and immediately displays the dialog. * <p/> * Calling this method is functionally identical to: * <pre> * AlertDialog dialog = builder.create(); * dialog.show(); * </pre> */
        public BaseDialog show() {
            final BaseDialog dialog = create();
            dialog.show();
            return dialog;
        }
    }

class AlertController {

    private DialogViewHelper mViewHelper;
    private BaseDialog mDialog;
    private Window mWindow;

    public AlertController(BaseDialog dialog, Window window) {
        mDialog = dialog;
        mWindow = window;
    }

    /** * 获取Dialog * @return */
    public BaseDialog getDialog() {
        return mDialog;
    }

    /** * 获取window * @return */
    public Window getWindow() {
        return mWindow;
    }

    public DialogViewHelper getViewHelper() {
        return mViewHelper;
    }

    /** * 设置View的辅助 * @param viewHelper */
    public void setDialogViewHelper(DialogViewHelper viewHelper) {
        this.mViewHelper = viewHelper;
    }

    /** * 设置文本 * @param viewId * @param text */
    public void setText(int viewId, CharSequence text) {
        mViewHelper.setText(viewId, text);
    }

    /** * 设置点击事件 * @param viewId * @param listener */
    public void setOnClickListener(int viewId, View.OnClickListener listener) {
        mViewHelper.setOnClickListener(viewId, listener);
    }

    /** * 通过id获取View * @param viewId * @param <T> * @return */
    public <T extends View> T getView(int viewId) {
        return mViewHelper.getView(viewId);
    }
}

代码调用

  @Override
    public void onClick(View v) {
        BaseDialog dialog = new BaseDialog.Builder(this)
                .setContentView(R.layout.detail_dialog).fullWith()
                .fromBottom(false)
                .show();
    }

最后总结一下Buider模式的优缺点：

Builder 模式的优点：
1.将一个复杂对象的创建过程封装起来，使得客户端不必知道产品内部组成的细节；
2.允许对象通过多个步骤来创建，并且可以改变过程和选择需要的过程；
3.产品的实现可以被替换，因为客户端只看到一个抽象的接口；
创建者独立，容易扩展。
Builder 模式缺点：
1.会产生多余的 Builder 对象以及 Director 对象，消耗内存；
2.与工厂模式相比，采用 Builder 模式创建对象的客户，需要具备更多的领域知识。