一啸蜜、模式定義

建造者模式(Builder Pattern)：將一個復(fù)雜對象的構(gòu)建與它的表示分離，使得同樣的構(gòu)建過程可以創(chuàng)建不同的表示实檀。

建造者模式是一步一步創(chuàng)建一個復(fù)雜的對象惶洲，它允許用戶只通過指定復(fù)雜對象的類型和內(nèi)容就可以構(gòu)建它們按声，用戶不需要知道內(nèi)部的具體構(gòu)建細(xì)節(jié)。建造者模式屬于對象創(chuàng)建型模式恬吕。因為翻譯的不同签则，建造者模式又可以稱為生成器模式。

舉個栗子铐料，之前有講過ImageLoader的栗子渐裂，在ImageLoader中，提供了setImageCache钠惩、setLoadingImage柒凉、setLoadingFailed、setThreadCount篓跛、setPlaceHolder等方法膝捞，這些參數(shù)是ImageLoader類的成員變量，通過setter方法愧沟，讓調(diào)用者設(shè)置這些變量的值蔬咬，使這些參數(shù)都能個被用戶自由定制。

但是就會出現(xiàn)問題：1央渣、用戶可以再任意時間计盒，修改ImageLoader的配置渴频，如果已經(jīng)初始化過了線程池數(shù)量的情況下芽丹，再次調(diào)用setThreadCount 會出現(xiàn)什么結(jié)果？卜朗？什么結(jié)果不重要拔第，重要的是，這樣提高了用戶的使用成本场钉，里面暴露 函數(shù)過多蚊俺，需要用戶再使用的時候仔細(xì)選擇，會給使用的人造成困擾

二逛万、模式結(jié)構(gòu)

建造者模式包含如下角色（這些都是老邏輯了泳猬，新的實(shí)現(xiàn)方法，Builder是一個鏈?zhǔn)秸{(diào)用宇植，本次setter后都返回本身得封，Director可以被省略）：

• Builder：抽象建造者
• ConcreteBuilder：具體建造者
• Director：指揮者
• Product：產(chǎn)品角色

[圖片上傳失敗...(image-d3db8f-1554990544119)]

三、時序圖

[圖片上傳失敗...(image-6e86dd-1554990544119)]

四指郁、簡單實(shí)現(xiàn)

圖片加載的過程忙上，步驟也很多，需要配置url闲坎、占位圖疫粥、縮放類型茬斧、動畫、進(jìn)度條 等等梗逮，順序是不固定的项秉，下面就舉一個項目中正在使用的例子，解釋經(jīng)典的Builder模式
圖片加載時库糠，參數(shù)配置：

package cc.kaipao.dongjia.imageloader;

import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.support.annotation.ColorInt;
import android.support.annotation.DrawableRes;

import cc.kaipao.dongjia.imageloader.base.LoadListener;

/**
 * 圖片加載信息配置類
 *
 * Created by XiaoFeng on 2017/3/14.
 */

public class ImageConfig {

    private enum ScaleType {
        FIT_CENTER,
        CENTER_CROP,
        FIT_START
    }

    private String url;
    private @DrawableRes int placeholderResId;
    private @DrawableRes int failureResId;
    private ColorDrawable placeholderDrawable;
    private ColorDrawable failureDrawable;
    private ImageStyle style;
    private float roundRadius;
    private float borderWidth;
    private @ColorInt int borderColor;
    private boolean noFade;
    private int fadeDuration;
    private ScaleType scaleType;
    private boolean needProgress;
    private @DrawableRes int progressBgResId;
    private ColorDrawable progressBgDrawable;
    private LoadListener loadListener;
    private int width;
    private int height;

    private ImageConfig(Builder builder) {
        this.url = builder.url;
        this.placeholderResId = builder.placeholderResId;
        this.failureResId = builder.failureResId;
        this.placeholderDrawable = builder.placeholderDrawable;
        this.failureDrawable = builder.failureDrawable;
        this.style = builder.style;
        this.roundRadius = builder.roundRadius;
        this.borderWidth = builder.borderWidth;
        this.borderColor = builder.borderColor;
        this.noFade = builder.noFade;
        this.fadeDuration = builder.fadeDuration;
        this.scaleType = builder.scaleType;
        this.needProgress = builder.needProgress;
        this.progressBgResId = builder.progressBgResId;
        this.progressBgDrawable = builder.progressBgDrawable;
        this.loadListener = builder.loadListener;
        this.width = builder.width;
        this.height = builder.height;
    }
    public static Builder builder() {
        return new Builder();
    }

    public String getUrl() {
        return url;
    }

    public int getPlaceholderResId() {
        return placeholderResId;
    }

    public int getFailureResId() {
        return failureResId;
    }

    public ColorDrawable getPlaceholderDrawable() {
        return placeholderDrawable;
    }

    public ColorDrawable getFailureDrawable() {
        return failureDrawable;
    }

    public boolean isNeedProgress() {
        return needProgress;
    }

    public int getProgressBgResId() {
        return progressBgResId;
    }

    public ColorDrawable getProgressBgDrawable() {
        return progressBgDrawable;
    }

    public ImageStyle getStyle() {
        return style;
    }

    public float getRoundRadius() {
        return roundRadius;
    }

    public float getBorderWidth() {
        return borderWidth;
    }

    public int getBorderColor() {
        return borderColor;
    }

    public boolean isNoFade() {
        return noFade;
    }

    public boolean isFitCenter() {
        return scaleType == ScaleType.FIT_CENTER;
    }

    public boolean isCenterCrop() {
        return scaleType == ScaleType.CENTER_CROP;
    }

    public int getFadeDuration() {
        return fadeDuration;
    }

    public LoadListener getLoadListener() {
        return loadListener;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }

    public static class Builder {
        // 圖片url
        private String url;
        // 占位圖
        private @DrawableRes int placeholderResId;
        // 失敗圖
        private @DrawableRes int failureResId;
        // 占位圖
        private ColorDrawable placeholderDrawable;
        // 失敗圖
        private ColorDrawable failureDrawable;
        // 圖片顯示風(fēng)格
        private ImageStyle style;
        // 圓角半徑
        private float roundRadius;
        // 邊框?qū)挾?        private float borderWidth;
        // 邊框顏色
        private @ColorInt int borderColor;
        // 是否是否淡入淡出
        private boolean noFade;
        // 淡入淡出時間間隔
        private int fadeDuration;
        // 縮放類型
        private ScaleType scaleType;
        // 是否顯示進(jìn)度條
        private boolean needProgress;
        // 進(jìn)度條背景圖
        private @DrawableRes int progressBgResId;
        // 進(jìn)度條背景圖
        private ColorDrawable progressBgDrawable;
        // 圖片加載成功或失敗的回調(diào)
        private LoadListener loadListener;

        /** 縮略圖尺寸 */
        private int width;
        /** 縮略圖尺寸 */
        private int height;

        public Builder() {
            this.style = ImageStyle.NORMAL;
            this.roundRadius = 0;
            this.borderWidth = 0;
            this.borderColor = Color.TRANSPARENT;
            this.noFade = false;
            this.fadeDuration = 0;
            this.scaleType = ScaleType.CENTER_CROP;
            this.needProgress = false;
            this.width =0;
            this.height = 0;
        }

        public Builder url(String url) {
            this.url = url;
            return this;
        }

        public Builder placeholderImage(@DrawableRes int placeholderResId) {
            this.placeholderResId = placeholderResId;
            return this;
        }

        public Builder failureImage(@DrawableRes int failureResId) {
            this.failureResId = failureResId;
            return this;
        }

        public Builder placeholderImage(ColorDrawable placeholderDrawable) {
            this.placeholderDrawable = placeholderDrawable;
            return this;
        }

        public Builder failureImage(ColorDrawable failureDrawable) {
            this.failureDrawable = failureDrawable;
            return this;
        }

        @Deprecated
        public Builder needProgress() {
            this.needProgress = true;
            return this;
        }

        public Builder progressBackground(@DrawableRes int progressBgResId) {
            this.progressBgResId = progressBgResId;
            return this;
        }

        public Builder progressBackground(ColorDrawable progressBgDrawable) {
            this.progressBgDrawable = progressBgDrawable;
            return this;
        }

        public Builder style(ImageStyle style) {
            this.style = style;
            return this;
        }

        public Builder roundRadius(float radius) {
            this.roundRadius = radius;
            return this;
        }

        public Builder borderWidth(float borderWidth) {
            this.borderWidth = borderWidth;
            return this;
        }

        public Builder borderColor(@ColorInt int borderColor) {
            this.borderColor = borderColor;
            return this;
        }

        public Builder noFade() {
            this.noFade = true;
            return this;
        }

        public Builder fade(int duration) {
            this.fadeDuration = duration;
            return this;
        }

        public Builder fitCenter() {
            this.scaleType = ScaleType.FIT_CENTER;
            return this;
        }
        public Builder fitStart() {
            this.scaleType = ScaleType.FIT_START;
            return this;
        }

        public Builder centerCrop() {
            this.scaleType = ScaleType.CENTER_CROP;
            return this;
        }

        public Builder width(int width) {
            this.width = width;
            return this;
        }


        public Builder height(int height) {
            this.height = height;
            return this;
        }

        public Builder loadListener(LoadListener loadListener) {
            this.loadListener = loadListener;
            return this;
        }

        public ImageConfig build() {
            return new ImageConfig(this);
        }


    }
}

在看看看使用的人是怎么使用的

ImageConfig config2 = ImageConfig.builder()
                    .url(UserAvatarUtil.getUrl(item.getZavatar()))
                    .placeholderImage(R.drawable.icon_set_avatar)
                    .failureImage(R.drawable.icon_set_avatar)
                    .style(ImageStyle.CIRCLE)
                    .build();
            ivAvatar.setImageWithConfig(config2);

五伙狐、Android源碼中模式實(shí)現(xiàn)

在Android源碼中，我們最常用到的Builder模式就是AlertDialog.Builder瞬欧， 使用該Builder來構(gòu)建復(fù)雜的AlertDialog對象贷屎。簡單示例如下 :

//顯示基本的AlertDialog  
private void showDialog(Context context) {  
    AlertDialog.Builder builder = new AlertDialog.Builder(context);  
    builder.setIcon(R.drawable.icon);  
    builder.setTitle("Title");  
    builder.setMessage("Message");  
    builder.setPositiveButton("Button1",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("點(diǎn)擊了對話框上的Button1");  
                }  
            });  
    builder.setNeutralButton("Button2",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("點(diǎn)擊了對話框上的Button2");  
                }  
            });  
    builder.setNegativeButton("Button3",  
            new DialogInterface.OnClickListener() {  
                public void onClick(DialogInterface dialog, int whichButton) {  
                    setTitle("點(diǎn)擊了對話框上的Button3");  
                }  
            });  
    builder.create().show();  // 構(gòu)建AlertDialog， 并且顯示
} 

結(jié)果 : [圖片上傳失敗...(image-928a13-1554990544119)]

下面我們看看AlertDialog的相關(guān)源碼 :

// AlertDialog
public class AlertDialog extends Dialog implements DialogInterface {
    // Controller, 接受Builder成員變量P中的各個參數(shù)
    private AlertController mAlert;

    // 構(gòu)造函數(shù)
    protected AlertDialog(Context context, int theme) {
        this(context, theme, true);
    }

    // 4 : 構(gòu)造AlertDialog
    AlertDialog(Context context, int theme, boolean createContextWrapper) {
        super(context, resolveDialogTheme(context, theme), createContextWrapper);
        mWindow.alwaysReadCloseOnTouchAttr();
        mAlert = new AlertController(getContext(), this, getWindow());
    }

    // 實(shí)際上調(diào)用的是mAlert的setTitle方法
    @Override
    public void setTitle(CharSequence title) {
        super.setTitle(title);
        mAlert.setTitle(title);
    }

    // 實(shí)際上調(diào)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其他的代碼省略
    
    // ************  Builder為AlertDialog的內(nèi)部類   *******************
    public static class Builder {
        // 1 : 存儲AlertDialog的各個參數(shù), 例如title, message, icon等.
        private final AlertController.AlertParams P;
        // 屬性省略
        
        /**
         * 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的其他代碼省略 ......

        // 2 : 設(shè)置各種參數(shù)
        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 : 構(gòu)建AlertDialog, 傳遞參數(shù)
        public AlertDialog create() {
            // 調(diào)用new AlertDialog構(gòu)造對象艘虎， 并且將參數(shù)傳遞個體AlertDialog 
            final AlertDialog dialog = new AlertDialog(P.mContext, mTheme, false);
            // 5 : 將P中的參數(shù)應(yīng)用的dialog中的mAlert對象中
            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;
        }
    }
    
}

可以看到唉侄，通過Builder來設(shè)置AlertDialog中的title, message, button等參數(shù)， 這些參數(shù)都存儲在類型為AlertController.AlertParams的成員變量P中野建，AlertController.AlertParams中包含了與之對應(yīng)的成員變量属划。在調(diào)用Builder類的create函數(shù)時才創(chuàng)建AlertDialog, 并且將Builder成員變量P中保存的參數(shù)應(yīng)用到AlertDialog的mAlert對象中，即P.apply(dialog.mAlert)代碼段候生。我們看看apply函數(shù)的實(shí)現(xiàn) :

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);
        }
    }
}

實(shí)際上就是把P中的參數(shù)挨個的設(shè)置到AlertController中同眯， 也就是AlertDialog中的mAlert對象。從AlertDialog的各個setter方法中我們也可以看到唯鸭，實(shí)際上也都是調(diào)用了mAlert對應(yīng)的setter方法须蜗。在這里，Builder同時扮演了上文中提到的builder目溉、ConcreteBuilder明肮、Director的角色，簡化了Builder模式的設(shè)計缭付。

六柿估、優(yōu)缺點(diǎn)

優(yōu)點(diǎn)

• 良好的封裝性， 使用建造者模式可以使客戶端不必知道產(chǎn)品內(nèi)部組成的細(xì)節(jié)陷猫；
• 建造者獨(dú)立秫舌，容易擴(kuò)展；
• 在對象創(chuàng)建過程中會使用到系統(tǒng)中的一些其它對象绣檬，這些對象在產(chǎn)品對象的創(chuàng)建過程中不易得到足陨。

缺點(diǎn)

• 會產(chǎn)生多余的Builder對象，消耗內(nèi)存河咽；
• 對象的構(gòu)建過程暴露钠右。