建造者模式（构建者模式）— 创建型

简介：

在面向对象的开发语言中，建造者模式和工厂模式一样也是创建对象的；不同的是工厂模式是用来创建不同但是相关类型的对象（继承同一父类或者接口的一组子类），由给定的参数来决定创建哪种类型的对象。建造者模式是用来创建一种类型的复杂对象，通过设置不同的可选参数，“定制化”地创建不同的对象。

应用场景

那些场景会应用到建造者模式呢，主要有以下需求是可以用使用：

1，一个类有很多属性，必须创建的对象的初始化，如果利用构造函数初始化这些属性，那构造函数回很长，太多参数也不利于使用，容易出错，但是放到set方法中，又不能保证属性的必须初始化；

2，如果类的属性之间有一定的依赖关系或者约束条件，我们继续使用构造函数配合 set() 方法的设计思路，那这些依赖关系或约束条件的校验逻辑就无处安放了。

3，如果我们希望创建不可变对象，也就是说，对象在创建好之后，就不能再修改内部的属性值，要实现这个功能，我们就不能在类中暴露 set() 方法。构造函数配合 set() 方法来设置属性值的方式就不适用了。

建造者模式实现

来看一下Android源码中的AlertDialog；从AlertDialog可以看出就是利用创建者模式实现的；

部分源码：

public class AlertDialog extends Dialog implements DialogInterface {
    private AlertController mAlert;

  
    
    protected AlertDialog(Context context) {
        this(context, 0);
    }

    protected AlertDialog(Context context, boolean cancelable, OnCancelListener cancelListener) {
        this(context, 0);

        setCancelable(cancelable);
        setOnCancelListener(cancelListener);
    }

    
    protected AlertDialog(Context context, @StyleRes int themeResId) {
        this(context, themeResId, true);
    }

    AlertDialog(Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        super(context, createContextThemeWrapper ? resolveDialogTheme(context, themeResId) : 0, createContextThemeWrapper);

        mWindow.alwaysReadCloseOnTouchAttr();
        mAlert = AlertController.create(getContext(), this, getWindow());
    }

    @Override
    public void setTitle(CharSequence title) {
        super.setTitle(title);
        mAlert.setTitle(title);
    }

    public void setCustomTitle(View customTitleView) {
        mAlert.setCustomTitle(customTitleView);
    }

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

   
    public void setMessageMovementMethod(MovementMethod movementMethod) {
        mAlert.setMessageMovementMethod(movementMethod);
    }

    /** @hide */
    public void setMessageHyphenationFrequency(
            @Layout.HyphenationFrequency int hyphenationFrequency) {
        mAlert.setMessageHyphenationFrequency(hyphenationFrequency);
    }

    /**
     * Set the view to display in that dialog.
     */
    public void setView(View view) {
        mAlert.setView(view);
    }

    
    public void setView(View view, int viewSpacingLeft, int viewSpacingTop, int viewSpacingRight, int viewSpacingBottom) {
        mAlert.setView(view, viewSpacingLeft, viewSpacingTop, viewSpacingRight, viewSpacingBottom);
    }
    
    public void setIcon(@DrawableRes int resId) {
        mAlert.setIcon(resId);
    }

    public void setIcon(Drawable icon) {
        mAlert.setIcon(icon);
    }

    
    public void setIconAttribute(@AttrRes int attrId) {
        TypedValue out = new TypedValue();
        mContext.getTheme().resolveAttribute(attrId, out, true);
        mAlert.setIcon(out.resourceId);
    }

    public void setInverseBackgroundForced(boolean forceInverseBackground) {
        mAlert.setInverseBackgroundForced(forceInverseBackground);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mAlert.installContent();
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (mAlert.onKeyDown(keyCode, event)) return true;
        return super.onKeyDown(keyCode, event);
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (mAlert.onKeyUp(keyCode, event)) return true;
        return super.onKeyUp(keyCode, event);
    }

    public static class Builder {
        private final AlertController.AlertParams P;

        public Builder(Context context) {
            this(context, resolveDialogTheme(context, ResourceId.ID_NULL));
        }

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

        
        public Context getContext() {
            return P.mContext;
        }

        public Builder setTitle(@StringRes int titleId) {
            P.mTitle = P.mContext.getText(titleId);
            return this;
        }

        
        public Builder setTitle(CharSequence title) {
            P.mTitle = title;
            return this;
        }

       
        public Builder setMessage(@StringRes int messageId) {
            P.mMessage = P.mContext.getText(messageId);
            return this;
        }

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

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

       
        public Builder setIcon(Drawable icon) {
            P.mIcon = icon;
            return this;
        } 

        
        public Builder setOnKeyListener(OnKeyListener onKeyListener) {
            P.mOnKeyListener = onKeyListener;
            return this;
        }

       
        public Builder setItems(@ArrayRes int itemsId, final OnClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnClickListener = listener;
            return this;
        }

        
        public Builder setItems(CharSequence[] items, final OnClickListener listener) {
            P.mItems = items;
            P.mOnClickListener = listener;
            return this;
        }

        
        public Builder setAdapter(final ListAdapter adapter, final OnClickListener listener) {
            P.mAdapter = adapter;
            P.mOnClickListener = listener;
            return this;
        }

        
        public Builder setCursor(final Cursor cursor, final OnClickListener listener,
                String labelColumn) {
            P.mCursor = cursor;
            P.mLabelColumn = labelColumn;
            P.mOnClickListener = listener;
            return this;
        }

       
        public Builder setMultiChoiceItems(@ArrayRes int itemsId, boolean[] checkedItems,
                final OnMultiChoiceClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnCheckboxClickListener = listener;
            P.mCheckedItems = checkedItems;
            P.mIsMultiChoice = true;
            return this;
        }
       
        public Builder setSingleChoiceItems(@ArrayRes int itemsId, int checkedItem,
                final OnClickListener listener) {
            P.mItems = P.mContext.getResources().getTextArray(itemsId);
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mIsSingleChoice = true;
            return this;
        }

        
        public Builder setSingleChoiceItems(Cursor cursor, int checkedItem, String labelColumn,
                final OnClickListener listener) {
            P.mCursor = cursor;
            P.mOnClickListener = listener;
            P.mCheckedItem = checkedItem;
            P.mLabelColumn = labelColumn;
            P.mIsSingleChoice = true;
            return this;
        }
        
        public Builder setView(int layoutResId) {
            P.mView = null;
            P.mViewLayoutResId = layoutResId;
            P.mViewSpacingSpecified = false;
            return this;
        }

        
        public Builder setView(View view) {
            P.mView = view;
            P.mViewLayoutResId = 0;
            P.mViewSpacingSpecified = false;
            return this;
        }

       
        @Deprecated
        public Builder setView(View view, int viewSpacingLeft, int viewSpacingTop,
                int viewSpacingRight, int viewSpacingBottom) {
            P.mView = view;
            P.mViewLayoutResId = 0;
            P.mViewSpacingSpecified = true;
            P.mViewSpacingLeft = viewSpacingLeft;
            P.mViewSpacingTop = viewSpacingTop;
            P.mViewSpacingRight = viewSpacingRight;
            P.mViewSpacingBottom = viewSpacingBottom;
            return this;
        }

       
        @Deprecated
        public Builder setInverseBackgroundForced(boolean useInverseBackground) {
            P.mForceInverseBackground = useInverseBackground;
            return this;
        }


        public AlertDialog create() {
            // Context has already been wrapped with the appropriate theme.
            final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);
            P.apply(dialog.mAlert);
            dialog.setCancelable(P.mCancelable);
            if (P.mCancelable) {
                dialog.setCanceledOnTouchOutside(true);
            }
            dialog.setOnCancelListener(P.mOnCancelListener);
            dialog.setOnDismissListener(P.mOnDismissListener);
            if (P.mOnKeyListener != null) {
                dialog.setOnKeyListener(P.mOnKeyListener);
            }
            return dialog;
        }

        
        public AlertDialog show() {
            final AlertDialog dialog = create();
            dialog.show();
            return dialog;
        }
    }

}

Builder是构建者模式中创建对象的关键类，其内部一般都重复定义被构建者对象的成员变量，即：使用建造者模式来构建对象，代码实际上是有点重复的，AlertDialog类中的成员变量(AlertController mAlert)，要在 Builder 类中(AlertController.AlertParams P 包含了mAlert成员变量)重新再定义一遍。关于P中包含mAlert成员变量可以从以下代码看出

//在Builer类的create方法中有如下代码
final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);            

P.apply(dialog.mAlert);

构建者模式可以使用链式调用，来设置参数和使用构建者对象例如：

 AlertDialog builder = new AlertDialog.Builder(this).setTitle("这是Title").setIcon(R.drawable.ic_launcher_background).setMessage("提示信息").create();
 builder.show();


//或者，builder的show()方法会先调用create()方法
AlertDialog builder = new AlertDialog.Builder(this).setTitle("这是Title").setIcon(R.drawable.ic_launcher_background).setMessage("提示信息").show();

总结：通过构建者模式可以解决了以上三个开发需求或者说问题；