PropertyChangeEvent监听javabean的变化

相信大家会有这样的需求，一个javabean内有多个变量，每次改变都需要获取到他们的状态，一般我们想到的可能是用接口回调的方法去监听变量的改变，一个javabean还好处理，那如果有很多呢，会使代码显得很臃肿且不好扩展，所以这里有个很好方法来处理以上需求。

在JavaBean的设计中，按照属性的不同作用又细分为四类：单值属性、索引属性、关联属性、限制属性。关联属性，也称之为绑定属性。绑定属性会在属性值发生变化时，通知所有相关的监听器。

先介绍几个api吧

类PropertyChangeSupport

1. addPropertyChangeListener(PropertyChangeListener listener)
   顾名思义，添加对bean的监听。
2. removePropertyChangeListener(PropertyChangeListener listener)
   移除监听。
3. firePropertyChange(String propertyName, int oldValue, int newValue)
   添加对bean内某个变量的监听，第一个参数最好是变量名，第二个是变量改变前的值，第二个是变量改变后的值

类PropertyChangeEvent

https://docs.oracle.com/javase/7/docs/api/java/beans/PropertyChangeEvent.html

1. getPropertyName()
   获取发生改变的变量名。
2. getSource()
   获取改变的bean对象
3. getOldValue()
   获取发生改变的变量的旧值。
4. getNewValue()
   获取发生改变的变量的新值。

ok，不多说，上代码

javabean类

public class DeviceInfo {

private static DeviceInfo deviceInfo;

public static DeviceInfo getInstance() {
    if (deviceInfo == null) {
        deviceInfo = new DeviceInfo();
    }
    return deviceInfo;
}

private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);

public String getDeviceName() {
    return mDeviceName;
}

public void setDeviceName(String deviceName) {
    String oldValue = mDeviceName;
    this.mDeviceName = deviceName;
    changeSupport.firePropertyChange("deviceName", oldValue, deviceName);
}

public String getDeviceStatus() {
    return mDeviceStatus;
}

public void setDeviceStatus(String deviceStatus) {
    String oldValue = mDeviceStatus;
    this.mDeviceStatus = deviceStatus;
    changeSupport.firePropertyChange("deviceStatus", oldValue, deviceStatus);
}

private String mDeviceName;
private String mDeviceStatus;

public void addPropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.addPropertyChangeListener(listener);
}

public void removePropertyChangeListener(PropertyChangeListener listener) {
    changeSupport.removePropertyChangeListener(listener);
}
}

监听类

public class MainActivity extends AppCompatActivity implements PropertyChangeListener{

private static final String TAG = "MainActivity";
private EditText deviceName;
private EditText deviceStatus;
private Button change;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    deviceName = (EditText) findViewById(R.id.et_deviceName);
    deviceStatus = (EditText) findViewById(R.id.et_deviceStatus);
    change = (Button) findViewById(R.id.bt_change);
    change.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            String device_name = deviceName.getText().toString().trim();
            String device_status = deviceStatus.getText().toString().trim();
            DeviceInfo.getInstance().setDeviceName(device_name);
            DeviceInfo.getInstance().setDeviceStatus(device_status);
        }
    });

    DeviceInfo.getInstance().addPropertyChangeListener(this);
}

@Override
public void propertyChange(PropertyChangeEvent propertyChangeEvent) {
    Log.d(TAG,"Source:"+propertyChangeEvent.getSource());
    Log.d(TAG,"PropertyName:"+propertyChangeEvent.getPropertyName());
    Log.d(TAG,"OldValue:"+propertyChangeEvent.getOldValue());
    Log.d(TAG,"NewValue:"+propertyChangeEvent.getNewValue().toString());
    Log.d(TAG,"---------------------------------------------------");
}
}

好的，操作后的打印值：
初次赋值时oldvalue是null，记得判空。

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恩，好啦，特别是当bean很多的时候特别好用，用propertyChangeEvent.getSource()就能区分是哪个bean了。