Java 如何在异步任务中从onpostexecute启动新活动
**我有三节课。我的Mainactivity、newactivity和扩展AsyncTask的活动,现在在扩展AsyncTask的活动中,我需要在AsyncTask中的OnPostExecute上启动新活动。我该怎么做?我已经完成了上下文,但我从mainactivity执行backgroundtask行中得到错误,但应用程序崩溃,请帮助** 这是活动课Java 如何在异步任务中从onpostexecute启动新活动,java,android,android-asynctask,Java,Android,Android Asynctask,**我有三节课。我的Mainactivity、newactivity和扩展AsyncTask的活动,现在在扩展AsyncTask的活动中,我需要在AsyncTask中的OnPostExecute上启动新活动。我该怎么做?我已经完成了上下文,但我从mainactivity执行backgroundtask行中得到错误,但应用程序崩溃,请帮助** 这是活动课 public class MainActivity extends AppCompatActivity { Button click;
public class MainActivity extends AppCompatActivity {
Button click;
//public static TextView data;
public static LineChart mchart;
List<Entry> x;
ArrayList<String> y;
private fetchdata list;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
click=(Button)findViewById(R.id.button);
mchart=(LineChart)findViewById(R.id.linechart);
//data=(TextView)findViewById(R.id.fetcheddata);
list=new fetchdata();
click.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
fetchdata process=new fetchdata();
process.execute();
}
});
}
}
public class fetchdata extends AsyncTask<Void,Void,Void> {
String data="";
String dataparsed="";
String singleparsed="";
List<Entry> x;
List<String> y;
boolean flag=false;
@Override
protected Void doInBackground(Void... voids) {
x = new ArrayList<Entry>();
y = new ArrayList<String>();
try {
URL url=new URL("https://io.adafruit.com/api/v2/Yarev/feeds/pir-sensor/data");
HttpURLConnection httpURLConnection=(HttpURLConnection) url.openConnection();
InputStream inputStream=httpURLConnection.getInputStream();
BufferedReader bufferedReader=new BufferedReader(new InputStreamReader(inputStream));
String line="";
while (line!=null)
{
line=bufferedReader.readLine();
data=data+line;
}
JSONArray JA=new JSONArray(data);
for(int i=0;i<JA.length();i++)
{
JSONObject JO= (JSONObject) JA.get(i);
singleparsed="Value:"+JO.get("value")+"\n"+
"Feed key:"+JO.get("feed_key")+"\n"+
"Created:"+JO.get("created_at")+"\n";
int value=JO.getInt("value");
float v1=value;
x.add(new Entry(i,v1));
dataparsed=dataparsed+singleparsed;
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
//start new activity?????
}
public List<Entry> getList() {
return x;
}
}
您必须使用上下文创建构造函数
public class fetchdata extends AsyncTask<Void,Void,Void> {
String data="";
String dataparsed="";
String singleparsed="";
List<Entry> x;
List<String> y;
boolean flag=false;
private WeakReference<Context> contextRef;
public fetchdata(Context context) {
contextRef = new WeakReference<>(context);
}
@Override
protected Void doInBackground(Void... voids) {
x = new ArrayList<Entry>();
y = new ArrayList<String>();
try {
URL url=new URL("https://io.adafruit.com/api/v2/Yarev/feeds/pir-sensor/data");
HttpURLConnection httpURLConnection=(HttpURLConnection) url.openConnection();
InputStream inputStream=httpURLConnection.getInputStream();
BufferedReader bufferedReader=new BufferedReader(new InputStreamReader(inputStream));
String line="";
while (line!=null)
{
line=bufferedReader.readLine();
data=data+line;
}
JSONArray JA=new JSONArray(data);
for(int i=0;i<JA.length();i++)
{
JSONObject JO= (JSONObject) JA.get(i);
singleparsed="Value:"+JO.get("value")+"\n"+
"Feed key:"+JO.get("feed_key")+"\n"+
"Created:"+JO.get("created_at")+"\n";
int value=JO.getInt("value");
float v1=value;
x.add(new Entry(i,v1));
dataparsed=dataparsed+singleparsed;
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
if (context != null) {
// start new activity with context
Intent intent=new Intent(context,NewActivity.class);
context.startActivity(intent);
}
}
public List<Entry> getList() {
return x;
}
}
你的活动背景在哪里?@pavan你需要校对和复制编辑你的问题,这很难理解,请像关注代码一样密切关注您的文本描述。您能否详细说明如何在postexecute中使用上下文和startactivity从mainactivity调用,我已经编辑了problem@pavansai我已经更新了答案
new fetchdata(this).execute();