Java 如何在Android中通过反射实例化侦听器

我必须为安卓1.6（API 4）开发一个应用程序，它应该能够在安卓2.2或更高版本的手机上使用OnAudioFocusChangeListener（自安卓2.2-API 8开始提供）

谁能告诉我如何通过反射实例化侦听器？ 我已经通过反射运行了静态和非静态方法，但我不知道如何处理侦听器

这是要反映的侦听器：

AudioManager  audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);

OnAudioFocusChangeListener audioListener = new OnAudioFocusChangeListener() {
    @Override
    public void onAudioFocusChange(int focusChange) {
    // code to execute
    }
};

public void getAudioFocus() {
    audioManager.requestAudioFocus(audioListener, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
}

public void releaseAudioFocus() {
    audioManager.abandonAudioFocus(audioListener);
}

这是一个代码示例，其中包含我通过反射运行的方法：

Class BluetoothAdapter = Class.forName("android.bluetooth.BluetoothAdapter");
Method methodGetDefaultAdapter = BluetoothAdapter.getMethod("getDefaultAdapter"); // static method from the BluetoothAdapter class returning a BluetoothAdapter object
Object bluetooth = methodGetDefaultAdapter.invoke(null);
Method methodGetState = bluetooth.getClass().getMethod("getState"); // non-static method executed from the BluetoothAdapter object (which I called "bluetooth") returning an int
int bluetoothState = (Integer) methodGetState.invoke(bluetooth);

---

IMHO反射会降低类的可读性。此外，反射比正常的字段或类访问要慢一些

作为替代方案，请参见此处描述的包装类方法：

---

创建接口及其两个实现，一个用于API 8+，另一个用于早期版本。在API8类中，您可以使用API8类，包括

OnAudioFocusChangeListener

。然后根据操作系统的版本实例化版本，您可以通过查看。

最后，我使用代理类解决了这个问题。这是密码

private AudioManager theAudioManager;
private Object myOnAudioFocusChangeListener = null;

private static final int AUDIOMANAGER_AUDIOFOCUS_GAIN = 1;
private static final int AUDIOMANAGER_AUDIOFOCUS_LOSS = -1;

theAudioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);

// instantiating the OnAudioFocusChangeListener by reflection (as it only exists from Android 2.2 onwards)
// we use a Proxy class for implementing the listener
public void setOnAudioFocusChangeListener() {
    Log.i(this, "setOnAudioFocusChangeListener()");
    Class<?>[] innerClasses = theAudioManager.getClass().getDeclaredClasses();
    for (Class<?> interfaze : innerClasses) {
        if (interfaze.getSimpleName().equalsIgnoreCase("OnAudioFocusChangeListener")) {
            Class<?>[] classArray = new Class<?>[1];
            classArray[0] = interfaze;
            myOnAudioFocusChangeListener = Proxy.newProxyInstance(interfaze.getClassLoader(), classArray, new ProxyOnAudioFocusChangeListener());
        }
    }
}

// called by onResume
public void getAudioFocus() {
    if (myOnAudioFocusChangeListener != null) {
        Log.i(this, "getAudioFocus()");
        try {
            Method[] methods = theAudioManager.getClass().getDeclaredMethods();
            for (Method method : methods) {
                if (method.getName().equalsIgnoreCase("requestAudioFocus")) {
                    method.invoke(theAudioManager, myOnAudioFocusChangeListener, AudioManager.STREAM_MUSIC, AUDIOMANAGER_AUDIOFOCUS_GAIN);
                    Log.i(this, "requestAudioFocus");
                }
            }
        } catch (Exception e) {
            Log.e(this, e.getMessage());
        }
    }
}

// called by onPause
public void releaseAudioFocus() {
    if (myOnAudioFocusChangeListener != null) {
        Log.i(this, "releaseAudioFocus()");
        try {
            Method[] methods = theAudioManager.getClass().getDeclaredMethods();
            for (Method method : methods) {
                if (method.getName().equalsIgnoreCase("abandonAudioFocus"))
                    method.invoke(theAudioManager, myOnAudioFocusChangeListener);
            }
        } catch (Exception e) {
            Log.e(this, e.getMessage());
        }
    }
}

---

这是一个很好的例子，我不完全确定是否可以使用包装器，因为我需要将

OnAudioFocusChangeListener

对象作为参数传递给

请求AudioFocus

和

放弃AudioFocus

对象的

方法。我想，如果传递一个AudioFocusChangeListener包装，它就不起作用了。您可以将它作为包装类中的一个内部类。这会有用的。我也会忽略反射——这是代码的味道——特别是当有更好的方法解决它的时候。使用wrapper方法更好。wrapper方法的缺点是需要将目标sdk版本设置为更高的版本。这意味着您需要小心，高级API中可用的所有类都不会被具有较低API的设备执行。由于eclipse（据我所知）无法突出显示哪些代码仅在较高的API上可用，并且需要“阻止”到较低的API，因此很容易忘记为较低的API重新编码。因此，我的建议是使用反射，以防在更高的API中只需要使用一个或两个可用的类。但您仍然可以制作包含所有版本相关代码的包装器，每个版本（或一组版本）一个包装器，然后在Eclipse中简单地切换目标版本，就可以在一个地方看到错误。这将使捕获与版本相关的错误变得更容易。
private class ProxyOnAudioFocusChangeListener implements InvocationHandler {

    // implements the method onAudioFocusChange from the OnAudioFocusChangeListener
    public void onAudioFocusChange(int focusChange) {
        Log.e(this, "onAudioFocusChange() focusChange = " + focusChange);
        if (focusChange == AUDIOMANAGER_AUDIOFOCUS_LOSS) {
            Log.i(this, "AUDIOMANAGER_AUDIOFOCUS_LOSS");
            Message msg = mHandler.obtainMessage(ControllerHandler.SET_ON_PAUSE);
            mHandler.sendMessage(msg);
        } else if (focusChange == AUDIOMANAGER_AUDIOFOCUS_GAIN) {
            Log.i(this, "AUDIOMANAGER_AUDIOFOCUS_GAIN");
            // no action is taken
        }
    }

    // implements the method invoke from the InvocationHandler interface
    // it intercepts the calls to the listener methods
    // in this case it redirects the onAudioFocusChange listener method to the OnAudioFocusChange proxy method
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = null;
        try {
            if (args != null) {
                if (method.getName().equals("onAudioFocusChange") && args[0] instanceof Integer) {
                    onAudioFocusChange((Integer) args[0]);
                }
            }
        } catch (Exception e) {
            throw new RuntimeException("unexpected invocation exception: " + e.getMessage());
        }
        return result;
    }   
}