在Android中探秘建造者模式

前言

在Android开发过程中，我发现很多安卓源代码里应用了设计模式，比较常用的有适配器模式(各种adapter)，建造者模式(Alert Dialog的构建)等等。虽然我们对大多数设计模式都有所了解，但是在应用设计模式的这个方面，感觉很多人在这方面有所不足。所以，我们可以看看Android源代码里面是怎样实现各种设计模式的，从而可以加深我们对设计模式的理解，从而方便我们以后对它的应用。今天，我们就来谈谈设计者模式(Builder Pattern)。

定义

建造者模式(Builder Pattern)也叫做生成器模式,其定义如下:

Separate the construction of a complex object from its representation so that the same construction process can create different representions.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不同的表示。

它的意思就是将一个对象和怎么构建这个对象分离开来，如果你想构建一个对象，你把这个消息告诉构建者，并且将自己对这个对象的各种要求告诉建造者，然后建造者根据这些要求进行捣鼓，然后，你所需要的一个对象就出来了。

Android源代码AlertDialog的实现

我们先来看下AlertDialog的源代码(只列出关键代码):

public class AlertDialog extends Dialog implements DialogInterface {    ...    protected AlertDialog(Context context, int theme) {    this(context, theme, true);    }    AlertDialog(Context context, int theme, boolean createThemeContextWrapper) {        super(context, resolveDialogTheme(context, theme), createThemeContextWrapper);        mWindow.alwaysReadCloseOnTouchAttr();        mAlert = new AlertController(getContext(), this, getWindow());    }    protected AlertDialog(Context context, boolean cancelable, OnCancelListener cancelListener) {        super(context, resolveDialogTheme(context, 0));        mWindow.alwaysReadCloseOnTouchAttr();        setCancelable(cancelable);        setOnCancelListener(cancelListener);        mAlert = new AlertController(context, this, getWindow());    }    public static class Builder {        private final AlertController.AlertParams P;        private int mTheme;        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;        }        public Builder setMessage(CharSequence message) {            P.mMessage = message;            return this;        }        public AlertDialog create() {            final AlertDialog dialog = new AlertDialog(P.mContext, mTheme, 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;        }        ...    }}

从上面我们可以清楚地看到:AlertDialog的Build是一个静态内部类(在有些情况下，我们会单独定义一个Build抽象类，然后用具体类来继承它，实现具体的功能)。我们对AlertDialog设置的属性会暂时保存在Build类的成员变量P（AlertController.AlertParams）中。同时，我们注意到我们设置的属性，它都回返回本身的AlertBuild对象，这样我们就可以不停地调用它设置的方法。

如果我们想获得这个AlertDialog。我们就需要调用建造者的create()方法，在create()方法里面它就会构造出一个Dialog实例，并且将我们刚才设置的属性全部赋给AlertDialog，最后返回AlertDialog的实例。

UniversalImageLoader上面建造者的实现

看完了AlertDialog，我们这边再看一个实例，那就是Android开源框架UniversalImageLoader的DisplayImageOptions类，下面是它的一些核心代码:

public final class DisplayImageOptions {private final int imageResOnLoading;private final int imageResForEmptyUri;private final int imageResOnFail;private final Drawable imageOnLoading;private final Drawable imageForEmptyUri;private final Drawable imageOnFail;private final boolean resetViewBeforeLoading;private final boolean cacheInMemory;private final boolean cacheOnDisk;private final ImageScaleType imageScaleType;private final Options decodingOptions;private final int delayBeforeLoading;private final boolean considerExifParams;private final Object extraForDownloader;private final BitmapProcessor preProcessor;private final BitmapProcessor postProcessor;private final BitmapDisplayer displayer;private final Handler handler;private final boolean isSyncLoading;private DisplayImageOptions(Builder builder) {    imageResOnLoading = builder.imageResOnLoading;    imageResForEmptyUri = builder.imageResForEmptyUri;    imageResOnFail = builder.imageResOnFail;    imageOnLoading = builder.imageOnLoading;    imageForEmptyUri = builder.imageForEmptyUri;    imageOnFail = builder.imageOnFail;    resetViewBeforeLoading = builder.resetViewBeforeLoading;    cacheInMemory = builder.cacheInMemory;    cacheOnDisk = builder.cacheOnDisk;    imageScaleType = builder.imageScaleType;    decodingOptions = builder.decodingOptions;    delayBeforeLoading = builder.delayBeforeLoading;    considerExifParams = builder.considerExifParams;    extraForDownloader = builder.extraForDownloader;    preProcessor = builder.preProcessor;    postProcessor = builder.postProcessor;    displayer = builder.displayer;    handler = builder.handler;    isSyncLoading = builder.isSyncLoading;}...public static class Builder {    private int imageResOnLoading = 0;    private int imageResForEmptyUri = 0;    private int imageResOnFail = 0;    private Drawable imageOnLoading = null;    private Drawable imageForEmptyUri = null;    private Drawable imageOnFail = null;    private boolean resetViewBeforeLoading = false;    private boolean cacheInMemory = false;    private boolean cacheOnDisk = false;    private ImageScaleType imageScaleType = ImageScaleType.IN_SAMPLE_POWER_OF_2;    private Options decodingOptions = new Options();    private int delayBeforeLoading = 0;    private boolean considerExifParams = false;    private Object extraForDownloader = null;    private BitmapProcessor preProcessor = null;    private BitmapProcessor postProcessor = null;    private BitmapDisplayer displayer = DefaultConfigurationFactory.createBitmapDisplayer();    private Handler handler = null;    private boolean isSyncLoading = false;public Builder() {        decodingOptions.inPurgeable = true;        decodingOptions.inInputShareable = true;    }    /**     * Stub image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware     * image aware view} during image loading     *     * @param imageRes Stub image resource     * @deprecated Use {@link #showImageOnLoading(int)} instead     */    @Deprecated    public Builder showStubImage(int imageRes) {        imageResOnLoading = imageRes;        return this;    }    /**     * Incoming image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware     * image aware view} during image loading     *     * @param imageRes Image resource     */    public Builder showImageOnLoading(int imageRes) {        imageResOnLoading = imageRes;        return this;    }    /** Builds configured {@link DisplayImageOptions} object */    public DisplayImageOptions build() {        return new DisplayImageOptions(this);    }    ...    }}

它比AlertDialog实现更为简单一点，直接将属性定义在Builder的成员变量中，然后将Builder对象返回给DisplayImageOptions中直接进行赋值，这样做的话，将初始化对象的复杂逻辑全部交给了Builder类，而使我们需要的DisplayImageOptions显得非常的干净,功能也相当的简洁明了