建造者模式（構建者模式）— 創建型

簡介：

在面向對象的開發語言中，建造者模式和工廠模式一樣也是創建對象的；不同的是工廠模式是用來創建不同但是相關類型的對象（繼承同一父類或者接口的一組子類），由給定的參數來決定創建哪種類型的對象。建造者模式是用來創建一種類型的複雜對象，通過設置不同的可選參數，“定製化”地創建不同的對象。

應用場景

那些場景會應用到建造者模式呢，主要有以下需求是可以用使用：

1，一個類有很多屬性，必須創建的對象的初始化，如果利用構造函數初始化這些屬性，那構造函數回很長，太多參數也不利於使用，容易出錯，但是放到set方法中，又不能保證屬性的必須初始化；

2，如果類的屬性之間有一定的依賴關係或者約束條件，我們繼續使用構造函數配合 set() 方法的設計思路，那這些依賴關係或約束條件的校驗邏輯就無處安放了。

3，如果我們希望創建不可變對象，也就是說，對象在創建好之後，就不能再修改內部的屬性值，要實現這個功能，我們就不能在類中暴露 set() 方法。構造函數配合 set() 方法來設置屬性值的方式就不適用了。

建造者模式實現

來看一下Android源碼中的AlertDialog；從AlertDialog可以看出就是利用創建者模式實現的；

部分源碼：

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是構建者模式中創建對象的關鍵類，其內部一般都重複定義被構建者對象的成員變量，即：使用建造者模式來構建對象，代碼實際上是有點重複的，AlertDialog類中的成員變量(AlertController mAlert)，要在 Builder 類中(AlertController.AlertParams P 包含了mAlert成員變量)重新再定義一遍。關於P中包含mAlert成員變量可以從以下代碼看出

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

P.apply(dialog.mAlert);

構建者模式可以使用鏈式調用，來設置參數和使用構建者對象例如：

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


//或者，builder的show()方法會先調用create()方法
AlertDialog builder = new AlertDialog.Builder(this).setTitle("這是Title").setIcon(R.drawable.ic_launcher_background).setMessage("提示信息").show();

總結：通過構建者模式可以解決了以上三個開發需求或者說問題；