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

// AlertDialogpublic 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。