设计模式-建造者模式

1.建造者模式的定义及使用场景

Builder模式是一步一步创建一个复制对象的创建型模式，他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。改模式是为了将构造复杂对象的过程和它的部件解耦，使得构建过程和部件的表示隔离开来。

1.1定义

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同步的表示

1.2使用场景

• 相同的方法，不同的执行顺序，产生不同的事件结果时
• 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
• 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作用，这个时候使用建造者模式非常合适
• 当初始化一个对象特别复杂，如参数多，而且很多参数都具有默认值时

2.建造者模式的优缺点

2.1优点

1）封装性
2）建造者独立，容易扩展
2）便于控制细节风险

2.2缺点

会产生多余的Builder对象以及Director对象

3.建造者模式的实现方式

Product：

public class Product {    private  int id;    private  String  name;    public void doSomeThing(){        System.out.println("Product doSomeThing id:"+id+" name:"+name);    }    public void setId(int id) {        this.id = id;    }    public void setName(String name) {        this.name = name;    }    @Override    public String toString() {        return "Product{" +                "id=" + id +                ", name='" + name + '\'' +                '}';    }}

AbstactBuilder:

public abstract class AbstactBulider {    abstract void setId(int id);    abstract void setName(String name);    abstract Product build();}

Builder:

public class Builder extends AbstactBulider {    Product product = new Product();    @Override    void setId(int id) {        product.setId(id);    }    @Override    void setName(String name) {        product.setName(name);    }    @Override    Product build() {        return product;    }}

Director:

public class Director {    private AbstactBulider aBuilder, bBuilder;    public Product getAProduct() {        aBuilder = new Builder();        aBuilder.setId(1);        aBuilder.setName("allen");        return aBuilder.build();    }    public Product getBProduct() {        bBuilder = new Builder();        bBuilder.setId(2);        bBuilder.setName("joy");        return bBuilder.build();    }}

4.建造者模式在Android中的实际应用

在Android源码中，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。在开发过程中，我们经常用到AlertDialog，具体示例如下：
new AlertDialog.Builder(self)
  .setTitle("列表框")
  .setItems(new String[] {"列表项1","列表项2","列表项3"}, null)
  .setNegativeButton("确定", null)
  .show();
从类名就可以看出这是一个Builder模式，通过Builder对象来组装Dialog的各个部分，如title、buttons、items等。分析AlertDialog源码如下：

 public static class Builder {  private final AlertController.AlertParams P;  public Builder(Context context) {  this(context, resolveDialogTheme(context, 0));  }  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 setCustomTitle(View customTitleView) {  P.mCustomTitleView = customTitleView;  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 setIconAttribute(@AttrRes int attrId) {  TypedValue out = new TypedValue();  P.mContext.getTheme().resolveAttribute(attrId, out, true);  P.mIconId = out.resourceId;  return this;  }  public Builder setPositiveButton(@StringRes int textId, final OnClickListener listener) {  P.mPositiveButtonText = P.mContext.getText(textId);  P.mPositiveButtonListener = listener;  return this;  }  public Builder setPositiveButton(CharSequence text, final OnClickListener listener) {  P.mPositiveButtonText = text;  P.mPositiveButtonListener = listener;  return this;  }  public Builder setNegativeButton(@StringRes int textId, final OnClickListener listener) {  P.mNegativeButtonText = P.mContext.getText(textId);  P.mNegativeButtonListener = listener;  return this;  }  public Builder setNegativeButton(CharSequence text, final OnClickListener listener) {  P.mNegativeButtonText = text;  P.mNegativeButtonListener = listener;  return this;  }  public Builder setNeutralButton(@StringRes int textId, final OnClickListener listener) {  P.mNeutralButtonText = P.mContext.getText(textId);  P.mNeutralButtonListener = listener;  return this;  }  public Builder setNeutralButton(CharSequence text, final OnClickListener listener) {  P.mNeutralButtonText = text;  P.mNeutralButtonListener = listener;  return this;  }  public Builder setCancelable(boolean cancelable) {  P.mCancelable = cancelable;  return this;  }  public Builder setOnCancelListener(OnCancelListener onCancelListener) {  P.mOnCancelListener = onCancelListener;  return this;  }  public Builder setOnDismissListener(OnDismissListener onDismissListener) {  P.mOnDismissListener = onDismissListener;  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 setMultiChoiceItems(CharSequence[] items, boolean[] checkedItems,  final OnMultiChoiceClickListener listener) {  P.mItems = items;  P.mOnCheckboxClickListener = listener;  P.mCheckedItems = checkedItems;  P.mIsMultiChoice = true;  return this;  }  public Builder setMultiChoiceItems(Cursor cursor, String isCheckedColumn, String labelColumn,  final OnMultiChoiceClickListener listener) {  P.mCursor = cursor;  P.mOnCheckboxClickListener = listener;  P.mIsCheckedColumn = isCheckedColumn;  P.mLabelColumn = labelColumn;  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 setSingleChoiceItems(CharSequence[] items, int checkedItem, final OnClickListener listener) {  P.mItems = items;  P.mOnClickListener = listener;  P.mCheckedItem = checkedItem;  P.mIsSingleChoice = true;  return this;  }  public Builder setSingleChoiceItems(ListAdapter adapter, int checkedItem, final OnClickListener listener) {  P.mAdapter = adapter;  P.mOnClickListener = listener;  P.mCheckedItem = checkedItem;  P.mIsSingleChoice = true;  return this;  }  public Builder setOnItemSelectedListener(final AdapterView.OnItemSelectedListener listener) {  P.mOnItemSelectedListener = listener;  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;  }  /**  * @hide  */  public Builder setRecycleOnMeasureEnabled(boolean enabled) {  P.mRecycleOnMeasure = enabled;  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;  }  }

在AlertDialog的Builder模式中并没有看到Director角色出现，其实在很多场景中，Android并没有完全按照经典模式实现来做，而是做了一些修改，使得这个模式更易于使用。这里的AlertDialog.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。当模块比较稳定，不存在一些变化时，可以在经典模式实现的基础上做一些精简，而不是生搬硬套，使程序失去架构之美。正是由于灵活地运用设计模式，Android的源码很值得我们深入学习。