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。