老司机带你重构Android的v4包的部分源码
2018-02-23 本文已影响1095人
AWeiLoveAndroid
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【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。
本文源码同步发布在github，详情请点击 https://github.com/AweiLoveAndroid/refactor-android-support-v4
一、首先看一下Androidv4包下面的
TextUtilsCompat和TextUtilsCompatJellybeanMr1源码：
（一）TextUtilsCompat 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() {}
}
（二）TextUtilsCompatJellybeanMr1 源码：
/**
* 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。
（一）排列顺序有点乱，我格式化了一下，如下，看的稍微清楚了一些：
/**
* 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);
}
}
}
（二）TextUtilsCompat 这个类里面有两个内部类，一个是TextUtilsCompatImpl，一个是TextUtilsCompatJellybeanMr1Impl,TextUtilsCompatJellybeanMr1Impl是继承自TextUtilsCompatImpl的。
（三）从静态代码块看出，api 大于17 使用 new TextUtilsCompatJellybeanMr1Impl(); api小于17 使用TextUtilsCompatImpl。TextUtilsCompat，TextUtilsCompatImpl和TextUtilsCompatJellybeanMr1Impl里面都有 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。
（四）TextUtilsCompatImpl和TextUtilsCompatJellybeanMr1Impl里面都有 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：
（一）抽取公共接口ITextUtilsCompat
/**
* 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);
}
（二）写一个TextUtilsCompatJellybeanMr1实现ITextUtilsCompat 接口，然后把之前TextUtilsCompatJellybeanMr1类里面的复制过来，具体如下：
/**
* 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);
}
}
（三）写一个类TextUtilsCompatImpl实现ITextUtilsCompat 接口，然后把之前TextUtilsCompat类里面的有关代码复制过来，具体如下：
/**
* 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;
}
}
}
（四）封装TextUtilsCompat给调用者使用，具体如下：
/**
* 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);
}
}
