Android 中的注解深入探究

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注解是我们经常接触的技术,Java有注解,Android也有注解,本文将试图介绍Android中的注解,以及ButterKnife和Otto这些基于注解的库的一些工作原理.

归纳而言,Android中的注解大概有以下好处

提高我们的开发效率 更早的发现程序的问题或者错误 更好的增加代码的描述能力 更加利于我们的一些规范约束 提供解决问题的更优解

准备工作

默认情况下,Android中的注解包并没有包括在framework中,它独立成一个单独的包,通常我们需要引入这个包.

dependencies {  compile 'com.android.support:support-annotations:22.2.0'}

但是如果我们已经引入了appcompat则没有必要再次引用support-annotations,因为appcompat默认包含了对其引用.

替代枚举

在最早的时候,当我们想要做一些值得限定实现枚举的效果,通常是

定义几个常量用于限定 从上面的常量选取值进行使用 一个比较描述上面问题的示例代码如下

public static final int COLOR_RED = 0;public static final int COLOR_GREEN = 1;public static final int COLOR_YELLOW = 2;public void setColor(int color) {  //some code here}//调用setColor(COLOR_RED)

然而上面的还是有不尽完美的地方

setColor(COLOR_RED)与setColor(0)效果一样,而后者可读性很差,但却可以正常运行

setColor方法可以接受枚举之外的值,比如setColor(3),这种情况下程序可能出问题

一个相对较优的解决方法就是使用Java中的Enum.使用枚举实现的效果如下

// ColorEnum.javapublic enum ColorEmun {  RED,  GREEN,  YELLOW}public void setColorEnum(ColorEmun colorEnum) {  //some code here}setColorEnum(ColorEmun.GREEN);

然而Enum也并非最佳,Enum因为其相比方案一的常量来说,占用内存相对大很多而受到曾经被Google列为不建议使用,为此Google特意引入了一些相关的注解来替代枚举.

Android中新引入的替代枚举的注解有IntDef和StringDef,这里以IntDef做例子说明一下.

public class Colors {  @IntDef({RED, GREEN, YELLOW})  @Retention(RetentionPolicy.SOURCE)  public @interface LightColors{}  public static final int RED = 0;  public static final int GREEN = 1;  public static final int YELLOW = 2;}

声明必要的int常量 声明一个注解为LightColors 使用@IntDef修饰LightColors,参数设置为待枚举的集合 使用@Retention(RetentionPolicy.SOURCE)指定注解仅存在与源码中,不加入到class文件中

Null相关的注解

和Null相关的注解有两个

@Nullable 注解的元素可以是Null@NonNull 注解的元素不能是Null

上面的两个可以修饰如下的元素

成员属性方法参数方法的返回值

@Nullableprivate String obtainReferrerFromIntent(@NonNull Intent intent) {  return intent.getStringExtra("apps_referrer");}

NonNull检测生效的条件

显式传入null在调用方法之前已经判断了参数为null时

setReferrer(null);//提示警告//不提示警告String referrer = getIntent().getStringExtra("apps_referrer");setReferrer(referrer);//提示警告String referrer = getIntent().getStringExtra("apps_referrer");if (referrer == null) {  setReferrer(referrer);}private void setReferrer(@NonNull String referrer) {  //some code here}

区间范围注解

Android中的IntRange和FloatRange是两个用来限定区间范围的注解,

float currentProgress;public void setCurrentProgress(@FloatRange(from=0.0f, to=1.0f) float progress) {  currentProgress = progress;}

如果我们传入非法的值,如下所示

setCurrentProgress(11);

就会得到这样的错误

Value must be >=0.0 and <= 1.0(was 11)

长度以及数组大小限制

限制字符串的长度

private void setKey(@Size(6) String key) {}

限定数组集合的大小

private void setData(@Size(max = 1) String[] data) {}setData(new String[]{"b", "a"});//error occurs

限定特殊的数组长度,比如3的倍数

private void setItemData(@Size(multiple = 3) String[] data) {}

权限相关

在Android中,有很多场景都需要使用权限,无论是Marshmallow之前还是之后的动态权限管理.都需要在manifest中进行声明,如果忘记了,则会导致程序崩溃. 好在有一个注解能辅助我们避免这个问题.使用RequiresPermission注解即可.

@RequiresPermission(Manifest.permission.SET_WALLPAPER)  public void changeWallpaper(Bitmap bitmap) throws IOException {}

资源注解

在Android中几乎所有的资源都可以有对应的资源id.比如获取定义的字符串,我们可以通过下面的方法

public String getStringById(int stringResId) {  return getResources().getString(stringResId);}

使用这个方法,我们可以很容易的获取到定义的字符串,但是这样的写法也存在着风险.

getStringById(R.mipmap.ic_launcher)

如果我们在不知情或者疏忽情况下,传入这样的值,就会出现问题. 但是如果我们使用资源相关的注解修饰了参数,就能很大程度上避免错误的情况.

public String getStringById(@StringRes int stringResId) {  return getResources().getString(stringResId);}

在Android中资源注解如下所示

AnimRes AnimatorRes AnyRes ArrayRes AttrRes BoolRes ColorRes DimenRes DrawableRes FractionRes IdRes IntegerRes InterpolatorRes LayoutRes MenuRes PluralsRes RawRes StringRes StyleRes StyleableRes TransitionRes XmlRes

Color值限定

上面部分提到了ColorRes,用来限定颜色资源id,这里我们将使用ColorInt,一个用来限定Color值的注解. 在较早的TextView的setTextColor是这样实现的.

public void setTextColor(int color) {  mTextColor = ColorStateList.valueOf(color);  updateTextColors();}

然而上面的方法在调用时常常会出现这种情况

myTextView.setTextColor(R.color.colorAccent);

如上,如果传递过去的参数为color的资源id就会出现颜色取错误的问题,这个问题在过去还是比较严重的.好在ColorInt出现了,改变了这一问题.

public void setTextColor(@ColorInt int color) {  mTextColor = ColorStateList.valueOf(color);  updateTextColors();}

当我们再次传入Color资源值时,就会得到错误的提示.

CheckResult

这是一个关于返回结果的注解，用来注解方法，如果一个方法得到了结果，却没有使用这个结果，就会有错误出现，一旦出现这种错误，就说明你没有正确使用该方法。

@CheckResultpublic String trim(String s) {  return s.trim();}

线程相关

Android中提供了四个与线程相关的注解

@UiThread,通常可以等同于主线程,标注方法需要在UIThread执行,比如View类就使用这个注解 @MainThread 主线程,经常启动后创建的第一个线程 @WorkerThread 工作者线程,一般为一些后台的线程,比如AsyncTask里面的doInBackground就是这样的. @BinderThread 注解方法必须要在BinderThread线程中执行,一般使用较少.

一些示例

new AsyncTask<Void, Void, Void>() {    //doInBackground is already annotated with @WorkerThread    @Override    protected Void doInBackground(Void... params) {      return null;      updateViews();//error    }  };@UiThreadpublic void updateViews() {  Log.i(LOGTAG, "updateViews ThreadInfo=" + Thread.currentThread());}

注意,这种情况下不会出现错误提示

new Thread(){  @Override  public void run() {    super.run();    updateViews();  }}.start();

虽然updateViews会在一个新的工作者线程中执行,但是在compile时没有错误提示.

因为它的判断依据是,如果updateView的线程注解(这里为@UiThread)和run(没有线程注解)不一致才会错误提示.如果run方法没有线程注解,则不提示.

CallSuper

重写的方法必须要调用super方法

使用这个注解,我们可以强制方法在重写时必须调用父类的方法 比如Application的onCreate,onConfigurationChanged等.

Keep

在Android编译生成APK的环节,我们通常需要设置minifyEnabled为true实现下面的两个效果

混淆代码

删除没有用的代码

但是出于某一些目的,我们需要不混淆某部分代码或者不删除某处代码,除了配置复杂的Proguard文件之外,我们还可以使用@Keep注解 .

@Keeppublic static int getBitmapWidth(Bitmap bitmap) {  return bitmap.getWidth();}

ButterKnife

ButterKnife是一个用来绑定View,资源和回调的提高效率的工具.作者为Jake Wharton. ButterKnife的好处

使用BindView替代繁琐的findViewById和类型转换 使用OnClick注解方法来替换显式声明的匿名内部类 使用BindString,BindBool,BindDrawable等注解实现资源获取

一个摘自Github的示例

class ExampleActivity extends Activity { @BindView(R.id.user) EditText username; @BindView(R.id.pass) EditText password; @BindString(R.string.login_error) String loginErrorMessage; @OnClick(R.id.submit) void submit() {  // TODO call server... } @Override public void onCreate(Bundle savedInstanceState) {  super.onCreate(savedInstanceState);  setContentView(R.layout.simple_activity);  ButterKnife.bind(this);  // TODO Use fields... }}

ButterKnife工作原理

以BindView注解使用为例,示例代码为

public class MainActivity extends AppCompatActivity {  @BindView(R.id.myTextView)  TextView myTextView;  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    ButterKnife.bind(this);  }}

1.程序在compile时,会根据注解自动生成两个类,这里为MainActivity_ViewBinder.class和MainActivity_ViewBinding.class

2.当我们调用ButterKnife.bind(this);时,会查找当前类对应的ViewBinder类,并调用bind方法,这里会调用到MainActiivty_ViewBinder.bind方法.

3.MainActiivty_ViewBinder.bind方法实际上是调用了findViewById然后在进行类型转换,赋值给MainActivity的myTextView属性

ButterKnife的bind方法

public static Unbinder bind(@NonNull Activity target) {  return getViewBinder(target).bind(Finder.ACTIVITY, target, target);}

ButterKnife的getViewBinder和findViewBinderForClass

@NonNull @CheckResult @UiThread static ViewBinder<Object> getViewBinder(@NonNull Object target) {  Class<?> targetClass = target.getClass();  if (debug) Log.d(TAG, "Looking up view binder for " + targetClass.getName());  return findViewBinderForClass(targetClass); } @NonNull @CheckResult @UiThread private static ViewBinder<Object> findViewBinderForClass(Class<?> cls) {  //如果内存集合BINDERS中包含,则不再查找  ViewBinder<Object> viewBinder = BINDERS.get(cls);  if (viewBinder != null) {   if (debug) Log.d(TAG, "HIT: Cached in view binder map.");   return viewBinder;  }  String clsName = cls.getName();  if (clsName.startsWith("android.") || clsName.startsWith("java.")) {   if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");   return NOP_VIEW_BINDER;  }  //noinspection TryWithIdenticalCatches Resolves to API 19+ only type.  try {   //使用反射创建实例   Class<?> viewBindingClass = Class.forName(clsName + "_ViewBinder");   //noinspection unchecked   viewBinder = (ViewBinder<Object>) viewBindingClass.newInstance();   if (debug) Log.d(TAG, "HIT: Loaded view binder class.");  } catch (ClassNotFoundException e) {    //如果没有找到,对父类进行查找   if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());   viewBinder = findViewBinderForClass(cls.getSuperclass());  } catch (InstantiationException e) {   throw new RuntimeException("Unable to create view binder for " + clsName, e);  } catch (IllegalAccessException e) {   throw new RuntimeException("Unable to create view binder for " + clsName, e);  }  //加入内存集合,便于后续的查找  BINDERS.put(cls, viewBinder);  return viewBinder; }

MainActivity_ViewBinder的反编译源码

➜ androidannotationsample javap -c MainActivity_ViewBinderWarning: Binary file MainActivity_ViewBinder contains com.example.admin.androidannotationsample.MainActivity_ViewBinderCompiled from "MainActivity_ViewBinder.java"public final class com.example.admin.androidannotationsample.MainActivity_ViewBinder implements butterknife.internal.ViewBinder<com.example.admin.androidannotationsample.MainActivity> { public com.example.admin.androidannotationsample.MainActivity_ViewBinder();  Code:    0: aload_0    1: invokespecial #1         // Method java/lang/Object."<init>":()V    4: return public butterknife.Unbinder bind(butterknife.internal.Finder, com.example.admin.androidannotationsample.MainActivity, java.lang.Object);  Code:    0: new      #2         // class com/example/admin/androidannotationsample/MainActivity_ViewBinding    3: dup    4: aload_2    5: aload_1    6: aload_3              // 创建ViewBinding实例    7: invokespecial #3         // Method com/example/admin/androidannotationsample/MainActivity_ViewBinding."<init>":(Lcom/example/admin/androidannotationsample/MainActivity;Lbutterknife/internal/Finder;Ljava/lang/Object;)V   10: areturn public butterknife.Unbinder bind(butterknife.internal.Finder, java.lang.Object, java.lang.Object);  Code:    0: aload_0    1: aload_1    2: aload_2    3: checkcast   #4         // class com/example/admin/androidannotationsample/MainActivity    6: aload_3              //调用上面的重载方法    7: invokevirtual #5         // Method bind:(Lbutterknife/internal/Finder;Lcom/example/admin/androidannotationsample/MainActivity;Ljava/lang/Object;)Lbutterknife/Unbinder;   10: areturn}MainActivity_ViewBinding的反编译源码➜ androidannotationsample javap -c MainActivity_ViewBindingWarning: Binary file MainActivity_ViewBinding contains com.example.admin.androidannotationsample.MainActivity_ViewBindingCompiled from "MainActivity_ViewBinding.java"public class com.example.admin.androidannotationsample.MainActivity_ViewBinding<T extends com.example.admin.androidannotationsample.MainActivity> implements butterknife.Unbinder { protected T target; public com.example.admin.androidannotationsample.MainActivity_ViewBinding(T, butterknife.internal.Finder, java.lang.Object);  Code:    0: aload_0    1: invokespecial #1         // Method java/lang/Object."<init>":()V    4: aload_0    5: aload_1    6: putfield   #2         // Field target:Lcom/example/admin/androidannotationsample/MainActivity;    9: aload_1   10: aload_2   11: aload_3              //调用Finder.findRequireViewAsType找到View,并进行类型转换,并复制给MainActivity中对一个的变量   12: ldc      #4         // int 2131427412   14: ldc      #5         // String field 'myTextView'   16: ldc      #6         // class android/widget/TextView                      // 内部实际调用了findViewById   18: invokevirtual #7         // Method butterknife/internal/Finder.findRequiredViewAsType:(Ljava/lang/Object;ILjava/lang/String;Ljava/lang/Class;)Ljava/lang/Object;   21: checkcast   #6         // class android/widget/TextView   24: putfield   #8         // Field com/example/admin/androidannotationsample/MainActivity.myTextView:Landroid/widget/TextView;   27: return public void unbind();  Code:    0: aload_0    1: getfield   #2         // Field target:Lcom/example/admin/androidannotationsample/MainActivity;    4: astore_1    5: aload_1    6: ifnonnull   19    9: new      #9         // class java/lang/IllegalStateException   12: dup   13: ldc      #10         // String Bindings already cleared.   15: invokespecial #11         // Method java/lang/IllegalStateException."<init>":(Ljava/lang/String;)V   18: athrow   19: aload_1   20: aconst_null            // 解除绑定,设置对应的变量为null   21: putfield   #8         // Field com/example/admin/androidannotationsample/MainActivity.myTextView:Landroid/widget/TextView;   24: aload_0   25: aconst_null   26: putfield   #2         // Field target:Lcom/example/admin/androidannotationsample/MainActivity;   29: return}

Finder的源码

package butterknife.internal;import android.app.Activity;import android.app.Dialog;import android.content.Context;import android.support.annotation.IdRes;import android.view.View;@SuppressWarnings("UnusedDeclaration") // Used by generated code.public enum Finder { VIEW {  @Override public View findOptionalView(Object source, @IdRes int id) {   return ((View) source).findViewById(id);  }  @Override public Context getContext(Object source) {   return ((View) source).getContext();  }  @Override protected String getResourceEntryName(Object source, @IdRes int id) {   final View view = (View) source;   // In edit mode, getResourceEntryName() is unsupported due to use of BridgeResources   if (view.isInEditMode()) {    return "<unavailable while editing>";   }   return super.getResourceEntryName(source, id);  } }, ACTIVITY {  @Override public View findOptionalView(Object source, @IdRes int id) {   return ((Activity) source).findViewById(id);  }  @Override public Context getContext(Object source) {   return (Activity) source;  } }, DIALOG {  @Override public View findOptionalView(Object source, @IdRes int id) {   return ((Dialog) source).findViewById(id);  }  @Override public Context getContext(Object source) {   return ((Dialog) source).getContext();  } }; //查找对应的Finder,如上面的ACTIVITY, DIALOG, VIEW public abstract View findOptionalView(Object source, @IdRes int id); public final <T> T findOptionalViewAsType(Object source, @IdRes int id, String who,   Class<T> cls) {  View view = findOptionalView(source, id);  return castView(view, id, who, cls); } public final View findRequiredView(Object source, @IdRes int id, String who) {  View view = findOptionalView(source, id);  if (view != null) {   return view;  }  String name = getResourceEntryName(source, id);  throw new IllegalStateException("Required view '"    + name    + "' with ID "    + id    + " for "    + who    + " was not found. If this view is optional add '@Nullable' (fields) or '@Optional'"    + " (methods) annotation."); } //来自ViewBinding的调用 public final <T> T findRequiredViewAsType(Object source, @IdRes int id, String who,   Class<T> cls) {  View view = findRequiredView(source, id, who);  return castView(view, id, who, cls); } public final <T> T castView(View view, @IdRes int id, String who, Class<T> cls) {  try {   return cls.cast(view);  } catch (ClassCastException e) {   String name = getResourceEntryName(view, id);   throw new IllegalStateException("View '"     + name     + "' with ID "     + id     + " for "     + who     + " was of the wrong type. See cause for more info.", e);  } } @SuppressWarnings("unchecked") // That's the point. public final <T> T castParam(Object value, String from, int fromPos, String to, int toPos) {  try {   return (T) value;  } catch (ClassCastException e) {   throw new IllegalStateException("Parameter #"     + (fromPos + 1)     + " of method '"     + from     + "' was of the wrong type for parameter #"     + (toPos + 1)     + " of method '"     + to     + "'. See cause for more info.", e);  } } protected String getResourceEntryName(Object source, @IdRes int id) {  return getContext(source).getResources().getResourceEntryName(id); } public abstract Context getContext(Object source);}

Otto

Otto Bus 是一个专为Android改装的Event Bus,在很多项目中都有应用.由Square开源共享.public class EventBusTest {  private static final String LOGTAG = "EventBusTest";  Bus mBus = new Bus();  public void test() {    mBus.register(this);  }  class NetworkChangedEvent {  }  @Produce  public NetworkChangedEvent sendNetworkChangedEvent() {    return new NetworkChangedEvent();  }  @Subscribe  public void onNetworkChanged(NetworkChangedEvent event) {    Log.i(LOGTAG, "onNetworkChanged event=" + event);  }}

Otto 的工作原理

使用@Produce和@Subscribe标记方法 当调用bus.register方法,去检索注册对象的标记方法,并cache映射关系 当post事件时,将事件与handler方法对应加入事件队列 抽取事件队列,然后调用handler处理

如下为对Otto如何利用注解的分析

register的源码

public void register(Object object) {  if (object == null) {   throw new NullPointerException("Object to register must not be null.");  }  enforcer.enforce(this);  //查找object中的Subscriber  Map<Class<?>, Set<EventHandler>> foundHandlersMap = handlerFinder.findAllSubscribers(object);  for (Class<?> type : foundHandlersMap.keySet()) {   Set<EventHandler> handlers = handlersByType.get(type);   if (handlers == null) {    //concurrent put if absent    Set<EventHandler> handlersCreation = new CopyOnWriteArraySet<EventHandler>();    handlers = handlersByType.putIfAbsent(type, handlersCreation);    if (handlers == null) {      handlers = handlersCreation;    }   }   final Set<EventHandler> foundHandlers = foundHandlersMap.get(type);   if (!handlers.addAll(foundHandlers)) {    throw new IllegalArgumentException("Object already registered.");   }  }  for (Map.Entry<Class<?>, Set<EventHandler>> entry : foundHandlersMap.entrySet()) {   Class<?> type = entry.getKey();   EventProducer producer = producersByType.get(type);   if (producer != null && producer.isValid()) {    Set<EventHandler> foundHandlers = entry.getValue();    for (EventHandler foundHandler : foundHandlers) {     if (!producer.isValid()) {      break;     }     if (foundHandler.isValid()) {      dispatchProducerResultToHandler(foundHandler, producer);     }    }   }  } }

HandlerFinder源码

interface HandlerFinder { Map<Class<?>, EventProducer> findAllProducers(Object listener); Map<Class<?>, Set<EventHandler>> findAllSubscribers(Object listener); //Otto注解查找器 HandlerFinder ANNOTATED = new HandlerFinder() {  @Override  public Map<Class<?>, EventProducer> findAllProducers(Object listener) {   return AnnotatedHandlerFinder.findAllProducers(listener);  }  @Override  public Map<Class<?>, Set<EventHandler>> findAllSubscribers(Object listener) {   return AnnotatedHandlerFinder.findAllSubscribers(listener);  } };

具体查找实现

/** This implementation finds all methods marked with a {@link Subscribe} annotation. */ static Map<Class<?>, Set<EventHandler>> findAllSubscribers(Object listener) {  Class<?> listenerClass = listener.getClass();  Map<Class<?>, Set<EventHandler>> handlersInMethod = new HashMap<Class<?>, Set<EventHandler>>();  Map<Class<?>, Set<Method>> methods = SUBSCRIBERS_CACHE.get(listenerClass);  if (null == methods) {   methods = new HashMap<Class<?>, Set<Method>>();   loadAnnotatedSubscriberMethods(listenerClass, methods);  }  if (!methods.isEmpty()) {   for (Map.Entry<Class<?>, Set<Method>> e : methods.entrySet()) {    Set<EventHandler> handlers = new HashSet<EventHandler>();    for (Method m : e.getValue()) {     handlers.add(new EventHandler(listener, m));    }    handlersInMethod.put(e.getKey(), handlers);   }  }  return handlersInMethod; }

以上就是关于Android中注解的一些总结,文章部分内容参考自 Support Annotations ,希望能帮助大家对注解有基础的认识,并运用到实际的日常开发之中。

通过此文希望能帮助你彻底了解Android 注解机制,谢谢大家对本站的支持！

如果有可能，我带你去远行。