如何使异步任务每隔一段时间执行一次,就像定时器一样...实际上,我正在开发一个应用程序,它将自动下载来自服务器的所有最新未读问候,并且为此我必须在一些固定的时间间隔后从服务器检查更新....我知道可以很容易地通过计时器实现,但我想使用异步任务,因为我认为它对Android应用程序更有效。
如何使异步任务每隔一段时间执行一次,就像定时器一样...实际上,我正在开发一个应用程序,它将自动下载来自服务器的所有最新未读问候,并且为此我必须在一些固定的时间间隔后从服务器检查更新....我知道可以很容易地通过计时器实现,但我想使用异步任务,因为我认为它对Android应用程序更有效。
public void callAsynchronousTask() {
final Handler handler = new Handler();
Timer timer = new Timer();
TimerTask doAsynchronousTask = new TimerTask() {
@Override
public void run() {
handler.post(new Runnable() {
public void run() {
try {
PerformBackgroundTask performBackgroundTask = new PerformBackgroundTask();
// PerformBackgroundTask this class is the class that extends AsynchTask
performBackgroundTask.execute();
} catch (Exception e) {
// TODO Auto-generated catch block
}
}
});
}
};
timer.schedule(doAsynchronousTask, 0, 50000); //execute in every 50000 ms
}
//Every 10000 ms
private void doSomethingRepeatedly() {
Timer timer = new Timer();
timer.scheduleAtFixedRate( new TimerTask() {
public void run() {
try{
new SendToServer().execute();
}
catch (Exception e) {
// TODO: handle exception
}
}
}, 0, 10000);
}
private int m_interval = 5000; // 5 seconds by default, can be changed later
private Handle m_handler;
@Override
protected void onCreate(Bundle bundle)
{
...
m_handler = new Handler();
}
Runnable m_statusChecker = new Runnable()
{
@Override
public void run() {
updateStatus(); //this function can change value of m_interval.
m_handler.postDelayed(m_statusChecker, m_interval);
}
}
void startRepeatingTask()
{
m_statusChecker.run();
}
void stopRepeatingTask()
{
m_handler.removeCallback(m_statusChecker);
}
但我建议您检查这个框架:http://code.google.com/intl/de-DE/android/c2dm/。这是一种不同的方法:当有东西准备好时,服务器会通知手机(从而节省带宽和性能:))。
TimerTask mDoAsynchronousTask;
@Override
public void onStop(){
super.onStop();
mDoAsynchronousTask.cancel();
mHandler.removeCallbacks(null);
...
}
public void callAsynchronousTask(final boolean stopTimer) {
Timer timer = new Timer();
mDoAsynchronousTask = new TimerTask() {
@Override
public void run() {
mHandler.post(new Runnable() {
...
尽量避免使用异步任务,如果必须使用,则使用调度程序服务来运行异步任务。或者使用应用程序类,例如这个好主意: https://fattybeagle.com/2011/02/15/android-asynctasks-during-a-screen-rotation-part-ii/
或者使用简单的处理程序(不使用定时器,只使用postDelayed),并且在onStop()中调用cancel异步任务是一个好习惯。这段代码使用postDelayed很好地工作:
public class MainActivity extends AppCompatActivity {
MyAsync myAsync = new MyAsync();
private final Handler mSendSSLMessageHandler = new Handler();
private final Runnable mSendSSLRunnable = new Runnable(){
..
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ConnectivityManager connMgr = (ConnectivityManager)
getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
if (networkInfo != null && networkInfo.isConnected()) {
mSendSSLMessageHandler.post(mSendSSLRunnable);
}else
..
@Override
public void onStop(){
super.onStop();
if ( progressDialog!=null && progressDialog.isShowing() ){
progressDialog.dismiss();
}
mSendSSLMessageHandler.removeCallbacks(mSendSSLRunnable);
myAsync.cancel(false);
}
private final Runnable mSendSSLRunnable = new Runnable(){
@Override
public void run(){
try {
myAsync = new MyAsync();
myAsync.execute();
} catch (Exception e) {
// TODO Auto-generated catch block
}
mSendSSLMessageHandler.postDelayed(mSendSSLRunnable, 5000);
}
};
class MyAsync extends AsyncTask<Void, Void, String> {
boolean running = true;
@Override
protected void onPreExecute() {
super.onPreExecute();
progressDialog = ProgressDialog.show
(MainActivity.this, "downloading", "please wait");
}
@Override
protected String doInBackground(Void... voids) {
if (!running) {
return null;
}
String result = null;
try{
URL url = new URL("http://192...");
HttpURLConnection urlConnection = (HttpURLConnection)
url.openConnection();
InputStream in = new BufferedInputStream (urlConnection.getInputStream());
result = inputStreamToString(in);
}catch(Exception e){
e.printStackTrace();
}
return result;
}
@Override
protected void onCancelled() {
boolean running = false;
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
progressDialog.dismiss();
try {
..
} catch (JSONException e) {
textView.append("json is invalid");
e.printStackTrace();
}
}
}