让你的APP可定位（三）

显示位置信息

在前面的课程中，位置的更新以纬度和经度的形式获取。这种方式对计算距离或者在地图上显示图钉是很有用的，这种小数对终端用户又是毫无意义的。如果对用户显示位置，有很多更好的方式来代替。

Perform Reverse Geocoding

Reverse-geocoding是将经纬度坐标转化为可读地址的过程。Geocoder  API可以做到这一点。最后注意，这个API是基于一个web服务 。如果这个服务在设备上不允许，这个API将抛出"Service not Available exception" 的异常或者返回空的地址队列。在Android2.3(API level 9)之后一个很有用的方法isPresent()被加入，用来检查是否存在。

下面的代码片断用来展示 Geocoder API的 reverse-geocoding功能。因为getFromLocation() 是异步的，所以你不应该在UI线程中调用，因此AsyncTask 被使用在片断中。

private final LocationListener listener = new LocationListener() {

    public void onLocationChanged(Location location) {
        // Bypass reverse-geocoding if the Geocoder service is not available on the
        // device. The isPresent() convenient method is only available on Gingerbread or above.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.GINGERBREAD && Geocoder.isPresent()) {
            // Since the geocoding API is synchronous and may take a while.  You don't want to lock
            // up the UI thread.  Invoking reverse geocoding in an AsyncTask.
            (new ReverseGeocodingTask(this)).execute(new Location[] {location});
        }
    }
    ...
};

// AsyncTask encapsulating the reverse-geocoding API.  Since the geocoder API is blocked,
// we do not want to invoke it from the UI thread.
private class ReverseGeocodingTask extends AsyncTask<Location, Void, Void> {
    Context mContext;

    public ReverseGeocodingTask(Context context) {
        super();
        mContext = context;
    }

    @Override
    protected Void doInBackground(Location... params) {
        Geocoder geocoder = new Geocoder(mContext, Locale.getDefault());

        Location loc = params[0];
        List<Address> addresses = null;
        try {
            // Call the synchronous getFromLocation() method by passing in the lat/long values.
            addresses = geocoder.getFromLocation(loc.getLatitude(), loc.getLongitude(), 1);
        } catch (IOException e) {
            e.printStackTrace();
            // Update UI field with the exception.
            Message.obtain(mHandler, UPDATE_ADDRESS, e.toString()).sendToTarget();
        }
        if (addresses != null &s;&s; addresses.size() > 0) {
            Address address = addresses.get(0);
            // Format the first line of address (if available), city, and country name.
            String addressText = String.format("%s, %s, %s",
                    address.getMaxAddressLineIndex() > 0 ? address.getAddressLine(0) : "",
                    address.getLocality(),
                    address.getCountryName());
            // Update the UI via a message handler.
            Message.obtain(mHandler, UPDATE_ADDRESS, addressText).sendToTarget();
        }
        return null;
    }
}