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本文出自【前言】过年回家忙着干活，忙着给亲戚的孩子发红包，好累，忙里偷闲打开studio看了一下v4包，之前看过几个类，这次是每个类都看了一下，原来Android的v4包的源码也有一些是写的不是那么友好的，还有很多改善空间。
下面就拿其中的android.support.v4.text这个包里面的 TextUtilsCompat 和 TextUtilsCompatJellybeanMr1 这两个类来做一个具体讲解。
一、首先看一下Androidv4包下面的
TextUtilsCompat和TextUtilsCompatJellybeanMr1源码：
public final class TextUtilsCompat { private static class TextUtilsCompatImpl { TextUtilsCompatImpl() { } @NonNull public String htmlEncode(@NonNull String s) { StringBuilder sb = new StringBuilder(); char c; for (int i = 0; i < s.length(); i++) { c = s.charAt(i); switch (c) { case '<': sb.append("<"); //$NON-NLS-1$ break; case '>': sb.append(">"); //$NON-NLS-1$ break; case '&': sb.append("&"); //$NON-NLS-1$ break; case '\'': //http://www.w3.org/TR/xhtml1 // The named character reference ' (the apostrophe, U+0027) was // introduced in XML 1.0 but does not appear in HTML. Authors should // therefore use ' instead of ' to work as expected in HTML 4 // user agents. sb.append("'"); //$NON-NLS-1$ break; case '"': sb.append("""); //$NON-NLS-1$ break; default: sb.append(c); } } return sb.toString(); } public int getLayoutDirectionFromLocale(@Nullable Locale locale) { if (locale != null && !locale.equals(ROOT)) { final String scriptSubtag = ICUCompat.maximizeAndGetScript(locale); if (scriptSubtag == null) return getLayoutDirectionFromFirstChar(locale); // This is intentionally limited to Arabic and Hebrew scripts, since older // versions of Android platform only considered those scripts to be right-to-left. if (scriptSubtag.equalsIgnoreCase(ARAB_SCRIPT_SUBTAG) || scriptSubtag.equalsIgnoreCase(HEBR_SCRIPT_SUBTAG)) { return ViewCompat.LAYOUT_DIRECTION_RTL; } } return ViewCompat.LAYOUT_DIRECTION_LTR; } /** * Fallback algorithm to detect the locale direction. Rely on the first char of the * localized locale name. This will not work if the localized locale name is in English * (this is the case for ICU 4.4 and "Urdu" script) * * @param locale * @return the layout direction. This may be one of: * {@link ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ private static int getLayoutDirectionFromFirstChar(@NonNull Locale locale) { switch(Character.getDirectionality(locale.getDisplayName(locale).charAt(0))) { case Character.DIRECTIONALITY_RIGHT_TO_LEFT: case Character.DIRECTIONALITY_RIGHT_TO_LEFT_ARABIC: return ViewCompat.LAYOUT_DIRECTION_RTL; case Character.DIRECTIONALITY_LEFT_TO_RIGHT: default: return ViewCompat.LAYOUT_DIRECTION_LTR; } } } private static class TextUtilsCompatJellybeanMr1Impl extends TextUtilsCompatImpl { TextUtilsCompatJellybeanMr1Impl() { } @Override @NonNull public String htmlEncode(@NonNull String s) { return TextUtilsCompatJellybeanMr1.htmlEncode(s); } @Override public int getLayoutDirectionFromLocale(@Nullable Locale locale) { return TextUtilsCompatJellybeanMr1.getLayoutDirectionFromLocale(locale); } } private static final TextUtilsCompatImpl IMPL; static { final int version = Build.VERSION.SDK_INT; if (version >= 17) { // JellyBean MR1 IMPL = new TextUtilsCompatJellybeanMr1Impl(); } else { IMPL = new TextUtilsCompatImpl(); } } /** * Html-encode the string. * @param s the string to be encoded * @return the encoded string */ @NonNull public static String htmlEncode(@NonNull String s) { return IMPL.htmlEncode(s); } /** * Return the layout direction for a given Locale * * @param locale the Locale for which we want the layout direction. Can be null. * @return the layout direction. This may be one of: * {@link ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ public static int getLayoutDirectionFromLocale(@Nullable Locale locale) { return IMPL.getLayoutDirectionFromLocale(locale); } public static final Locale ROOT = new Locale("", ""); static String ARAB_SCRIPT_SUBTAG = "Arab"; static String HEBR_SCRIPT_SUBTAG = "Hebr"; private TextUtilsCompat() {}} /** * Jellybean MR1 - specific TextUtils API access. */@RequiresApi(17)@TargetApi(17)class TextUtilsCompatJellybeanMr1 { @NonNull public static String htmlEncode(@NonNull String s) { return TextUtils.htmlEncode(s); } public static int getLayoutDirectionFromLocale(@Nullable Locale locale) { return TextUtils.getLayoutDirectionFromLocale(locale); }} 二、分析一下以上源码的一些需要改进的问题（仅个人理解，如有不同意见，欢迎提出）：
通过以上源码来看，看起来确实有点不是很舒服。
/** * 格式化之后的TextUtilsCompat类 */public class TextUtilsCompat { private static final TextUtilsCompatImpl IMPL; public static final Locale ROOT = new Locale("", ""); static String ARAB_SCRIPT_SUBTAG = "Arab"; static String HEBR_SCRIPT_SUBTAG = "Hebr"; private TextUtilsCompat() {} static { final int version = Build.VERSION.SDK_INT; if (version >= 17) { // JellyBean MR1 IMPL = new TextUtilsCompatJellybeanMr1Impl(); } else { IMPL = new TextUtilsCompatImpl(); } } /** * Html-encode the string. * @param s the string to be encoded * @return the encoded string */ @NonNull public static String htmlEncode(@NonNull String s) { return IMPL.htmlEncode(s); } /** * Return the layout direction for a given Locale * * @param locale the Locale for which we want the layout direction. Can be null. * @return the layout direction. This may be one of: * {@link ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ public static int getLayoutDirectionFromLocale(@Nullable Locale locale) { return IMPL.getLayoutDirectionFromLocale(locale); } private static class TextUtilsCompatImpl { TextUtilsCompatImpl() {} @NonNull public String htmlEncode(@NonNull String s) { StringBuilder sb = new StringBuilder(); char c; for (int i = 0; i < s.length(); i++) { c = s.charAt(i); switch (c) { case '<': sb.append("<"); //$NON-NLS-1$ break; case '>': sb.append(">"); //$NON-NLS-1$ break; case '&': sb.append("&"); //$NON-NLS-1$ break; case '\'': //http://www.w3.org/TR/xhtml1 // The named character reference ' (the apostrophe, U+0027) was // introduced in XML 1.0 but does not appear in HTML. Authors should // therefore use ' instead of ' to work as expected in HTML 4 // user agents. sb.append("'"); //$NON-NLS-1$ break; case '"': sb.append("""); //$NON-NLS-1$ break; default: sb.append(c); } } return sb.toString(); } public int getLayoutDirectionFromLocale(@Nullable Locale locale) { if (locale != null && !locale.equals(ROOT)) { final String scriptSubtag = ICUCompat.maximizeAndGetScript(locale); if (scriptSubtag == null) { return getLayoutDirectionFromFirstChar(locale); } // This is intentionally limited to Arabic and Hebrew scripts, since older // versions of Android platform only considered those scripts to be right-to-left. if (scriptSubtag.equalsIgnoreCase(ARAB_SCRIPT_SUBTAG) || scriptSubtag.equalsIgnoreCase(HEBR_SCRIPT_SUBTAG)) { return ViewCompat.LAYOUT_DIRECTION_RTL; } } return ViewCompat.LAYOUT_DIRECTION_LTR; } /** * Fallback algorithm to detect the locale direction. Rely on the first char of the * localized locale name. This will not work if the localized locale name is in English * (this is the case for ICU 4.4 and "Urdu" script) * * @param locale * @return the layout direction. This may be one of: * {@link ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ private static int getLayoutDirectionFromFirstChar(@NonNull Locale locale) { switch(Character.getDirectionality(locale.getDisplayName(locale).charAt(0))) { case Character.DIRECTIONALITY_RIGHT_TO_LEFT: case Character.DIRECTIONALITY_RIGHT_TO_LEFT_ARABIC: return ViewCompat.LAYOUT_DIRECTION_RTL; case Character.DIRECTIONALITY_LEFT_TO_RIGHT: default: return ViewCompat.LAYOUT_DIRECTION_LTR; } } } private static class TextUtilsCompatJellybeanMr1Impl extends TextUtilsCompatImpl { TextUtilsCompatJellybeanMr1Impl() { } @Override @NonNull public String htmlEncode(@NonNull String s) { return TextUtilsCompatJellybeanMr1.htmlEncode(s); } @Override public int getLayoutDirectionFromLocale(@Nullable Locale locale) { return TextUtilsCompatJellybeanMr1.getLayoutDirectionFromLocale(locale); } }} TextUtilsCompatImpl，一个是TextUtilsCompatJellybeanMr1Impl,TextUtilsCompatJellybeanMr1Impl是继承自TextUtilsCompatImpl的。htmlEncode 方法和 getLayoutDirectionFromLocale 方法。静态代码块里面通过TextUtilsCompatImpl IMPL这个常量来判断，当api大于17用TextUtilsCompatJellybeanMr1Impl，否则用TextUtilsCompatImpl，然后htmlEncode方法调用了对应内部类里面的htmlEncode方法，getLayoutDirectionFromLocale调用了对应内部类里面的getLayoutDirectionFromLocale方法。
htmlEncode 方法和 getLayoutDirectionFromLocale 方法，看看它们的区别。（1）TextUtilsCompatJellybeanMr1Impl这个内部类的方法解析：
htmlEncode(@NonNull String s) 方法 返回的是：TextUtilsCompatJellybeanMr1.htmlEncode(s); ==> 调用了TextUtils.htmlEncode(s);
getLayoutDirectionFromLocale(@Nullable Locale locale) 方法返回的是：TextUtilsCompatJellybeanMr1.getLayoutDirectionFromLocale(locale); ==> 调用了TextUtils.getLayoutDirectionFromLocale(locale);
（2）TextUtilsCompatImpl这个内部类的方法解析：
htmlEncode(@NonNull String s) 方法 返回的是：在这个方法内部写了一遍，跟TextUtils.htmlEncode(s);方法里面的一模一样。
getLayoutDirectionFromLocale(@Nullable Locale locale) 方法返回的是：重新写了一遍，这个方法是真正有所区别的地方。
三、根据我做过项目用到的MVP的开发模式，我把共同的htmlEncode方法和getLayoutDirectionFromLocale方法抽取出一个接口，然后分别用两个实现类去实现接口，然后用TextUtilsCompat这个类去判断调用哪个实现类的方法，这样看起来更直观一些。具体步骤如下：
/** * 抽取公用的接口 */public interface ITextUtilsCompat { /** * Html-encode the string. * @param s the string to be encoded * @return the encoded string */ public String htmlEncode(@NonNull String s); /** * Return the layout direction for a given Locale * * @param locale the Locale for which we want the layout direction. Can be null. * @return the layout direction. This may be one of: * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ public int getLayoutDirectionFromLocale(@Nullable Locale locale);} /** * 兼容Android 17+ 版本 * Jellybean MR1 - specific TextUtils API access. */@RequiresApi(17)@TargetApi(17)class TextUtilsCompatJellybeanMr1 implements ITextUtilsCompat{ @Override @NonNull public String htmlEncode(@NonNull String s) { return TextUtils.htmlEncode(s); } @Override public int getLayoutDirectionFromLocale(@Nullable Locale locale) { return TextUtils.getLayoutDirectionFromLocale(locale); }} /** * Android 17以下版本使用这个类 */class TextUtilsCompatImpl implements ITextUtilsCompat{ public static final Locale ROOT = new Locale("", ""); static String ARAB_SCRIPT_SUBTAG = "Arab"; static String HEBR_SCRIPT_SUBTAG = "Hebr"; @Override @NonNull public String htmlEncode(@NonNull String s) { StringBuilder sb = new StringBuilder(); char c; for (int i = 0; i < s.length(); i++) { c = s.charAt(i); switch (c) { case '<': sb.append("<"); //$NON-NLS-1$ break; case '>': sb.append(">"); //$NON-NLS-1$ break; case '&': sb.append("&"); //$NON-NLS-1$ break; case '\'': //http://www.w3.org/TR/xhtml1 // The named character reference ' (the apostrophe, U+0027) was // introduced in XML 1.0 but does not appear in HTML. Authors should // therefore use ' instead of ' to work as expected in HTML 4 // user agents. sb.append("'"); //$NON-NLS-1$ break; case '"': sb.append("""); //$NON-NLS-1$ break; default: sb.append(c); } } return sb.toString(); } @Override public int getLayoutDirectionFromLocale(@Nullable Locale locale) { if (locale != null && !locale.equals(ROOT)) { final String scriptSubtag = ICUCompat.maximizeAndGetScript(locale); if (scriptSubtag == null) { return getLayoutDirectionFromFirstChar(locale); } // This is intentionally limited to Arabic and Hebrew scripts, since older // versions of Android platform only considered those scripts to be right-to-left. if (scriptSubtag.equalsIgnoreCase(ARAB_SCRIPT_SUBTAG) || scriptSubtag.equalsIgnoreCase(HEBR_SCRIPT_SUBTAG)) { return ViewCompat.LAYOUT_DIRECTION_RTL; } } return ViewCompat.LAYOUT_DIRECTION_LTR; } /** * Fallback algorithm to detect the locale direction. Rely on the first char of the * localized locale name. This will not work if the localized locale name is in English * (this is the case for ICU 4.4 and "Urdu" script) * * @param locale * @return the layout direction. This may be one of: * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ private static int getLayoutDirectionFromFirstChar(@NonNull Locale locale) { switch(Character.getDirectionality(locale.getDisplayName(locale).charAt(0))) { case Character.DIRECTIONALITY_RIGHT_TO_LEFT: case Character.DIRECTIONALITY_RIGHT_TO_LEFT_ARABIC: return ViewCompat.LAYOUT_DIRECTION_RTL; case Character.DIRECTIONALITY_LEFT_TO_RIGHT: default: return ViewCompat.LAYOUT_DIRECTION_LTR; } }} /** * v4包下面的TextUtilsCompat的简单优化 * 这里使用的是策略模式，根据不同api版本调用不同的接口实现类 * 这样写更好维护。 */public final class TextUtilsCompat { private static final ITextUtilsCompat IMPL; private TextUtilsCompat() {} static { final int version = Build.VERSION.SDK_INT; // JellyBean MR1 大于等于17 if (version >= Build.VERSION_CODES.JELLY_BEAN_MR1) { IMPL = new TextUtilsCompatJellybeanMr1(); } else { IMPL = new TextUtilsCompatImpl(); } } /** * Html-encode the string. * @param s the string to be encoded * @return the encoded string */ @NonNull public static String htmlEncode(@NonNull String s) { return IMPL.htmlEncode(s); } /** * Return the layout direction for a given Locale * * @param locale the Locale for which we want the layout direction. Can be null. * @return the layout direction. This may be one of: * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_LTR} or * {@link android.support.v4.view.ViewCompat#LAYOUT_DIRECTION_RTL}. * * Be careful: this code will need to be updated when vertical scripts will be supported */ public static int getLayoutDirectionFromLocale(@Nullable Locale locale) { return IMPL.getLayoutDirectionFromLocale(locale); } } TextUtilsCompat 这个类就封装完了。看完之后是不是很清爽？其实还有很多类似的类都可以根据类似的方式做一下改进的。源码不是完美的，只要掌握以上示例代码的思想还是很容易的让代码更好理解，更简洁清晰的。