Android如何讓線程等到某個事件發生？

Question

[我的狀況]
現在在MainActivity中，我創建了一些片段，其中有一些TextView。 MainActivity中有一個異步線程AsyncTask，用於從Web獲取信息。 AsyncTask完成獲取后，它將通過回調方法更新上述TextViews中的文本（該Fragment實現一個OnInfoGotInterface並實例化接口中的onInfoGot（）方法。onInfoGot（）將調用Fragment中定義的setTextView（）方法以更新信息。 ）。

[我的問題]
執行程序時，我發現AsyncTask完成從Web獲取信息的時間點比Fragment調用onCreateView（）的時間點早。 換句話說，當AsyncTask調用（OnInfoGotInterface）fragment.onInfoGot（）設置TextView時，尚未實例化TextView（通過rootView.findViewById（）方法在CreateView（）上實例化TextView）。 結果，出現了NullPointerException。

[我需要解決方案]
現在我想這樣做：當AsyncTask完成從Web上獲取信息並要調用onInfoGot（）時，我們將其停止，並使其等待Fragment完成onCreateView（）。 之后，我們喚醒AsyncTask並允許它更新片段。

PS。 有人建議我在Fragment的定義中的onCreateView（）之后調用新的AsyncTask.excute（）。 但是這里AsyncTask和Fragment都是在MainActivity中創建的。 它們是MainActivity中兩個不同的任務線程，一個用於顯示數據，另一個用於從Web獲取數據。

任何人都可以提一些建議嗎？ 我真的很感激！ 這是代碼：

MainActivity.java：

public class MainActivity extends Activity{
private List<City> cityList;
private ViewPager mPager;  /*ViewPager for show the page of city info*/
private ScreenSlidePagerAdapter mPagerAdapter;  /* Adapter for the ViewPager, 
        * ScreenSlidePagerAdapter is a subclass of FragmentStatePagerAdapter*/
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_show_city);

    cityList = new ArrayList<City>();
    mPager = (ViewPager) findViewById(R.id.pager);
    mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
    mPager.setAdapter(mPagerAdapter);
    init();
}

private void init(){
    loadCities();  //Load cities from database into cityList
    initFragments();
    getCityInfo();
}

private void initFragments(){
    mPagerAdapter.removeAllFragments();
    for(City city: cityList){
        PageFragment fragment = new PageFragment(city.getName());
        mPagerAdapter.addFragment(fragment);
        mPagerAdapter.notifyDataSetChanged();
    }
}

private void getCityInfo(){
    for(City city: cityList){
        String cityName = city.getName(); 
        obtainCityInfo(cityName);  
    }
}

private obtainCityInfo(String cityName){
    String request = "http://example.abc.com/"
            + cityName + "&output=json";
    new AccessWebServiceTask().execute(request, cityName); 
}

private class AccessWebServiceTask extends AsyncTask<String, Void, CityInfo>{
    @Override
    protected CityInfo doInBackground(String... urls) {
        String result = getWebContent(urls[0]);  /*Access web through HTTP*/
        String cityName = urls[1];
        /*Transform the String result to a CityInfo object containing information of a city*/
        CityInfo cityInfo = encodeJason(result, cityName);  

        return cityInfo;
    }

    protected void onPostExecute(CityInfo cityInfo){
            OnCityGot(cityInfo); 
    }
}

public void onCityGot(CityInfo cityInfo){
    if(cityInfo != null){  
        String cityName = cityInfo.getCityName();
        /*Set the info field of a city object*/
        cityList.getByName(cityName).setCityInfo(cityInfo);
        mPagerAdapter.updateFragment(cityInfo);
    }
}

}

ScreenSlidePagerAdapter.java

public class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter{
    private List<PageFragment> fragmentsList;
    public ScreenSlidePagerAdapter(FragmentManager fm) {
        super(fm);
        fragmentsList = new ArrayList<PageFragment>();
    }

    public ScreenSlidePagerAdapter(FragmentManager fm, List<PageFragment> list){
        super(fm);
        fragmentsList = list;
    }

    @Override
    public PageFragment getItem(int position) {
        return fragmentsList.get(position);
    }

    @Override
    public int getCount() {
        return fragmentsList.size();
    }

    public void setFragmentsList(List<PageFragment> fragmentsList){
        this.fragmentsList = fragmentsList;
        notifyDataSetChanged();
    }

    public void addFragment(PageFragment f){
        fragmentsList.add(f);
        notifyDataSetChanged();
    }

    public void removeFragment(int position){
        fragmentsList.remove(position);
        notifyDataSetChanged();
    }

    public void removeAllFragments(){
        fragmentsList.clear();
        notifyDataSetChanged();
    }

    private PageFragment findFragmentByName(String cityName){
        for(PageFragment fragment: fragmentsList){
            if(fragment.getCityName().equals(cityName))
                return fragment;
        }
        return null;
    }
    public void updateFragment(CityInfo cityInfo){
        String cityName = cityInfo.getCityName();
        OnCityInfoChanged fragment = (OnCityInfoChanged)findFragmentByName(cityName);
        String population = cityInfo.getPopulation();
        fragment.onCityInfoChanged(population);
        notifyDataSetChanged();
    }
}

PageFragment.java：

public class PageFragment extends Fragment implements OnCityInfoChanged{
    private TextView cityNameText;
    private TextView populationText;
    String cityName;
    String population;

    public PageFragment(){}

    public PageFragment(String cityName, String population){
        this.cityName = cityName;
        this.population = population
    }

    public PageFragment(CityInfo cityInfo){
        this.cityName = cityInfo.getCityName();
        this.population = cityInfo.getPopulation();
    }

    public PageFragment(String cityName){
        this.cityName = cityName;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState){
         ViewGroup rootView = (ViewGroup) inflater.inflate(
                R.layout.fragment_city_page2, container, false);
         cityNameText = (TextView)rootView.findViewById(R.id.city_name);
         populationText = (TextView)rootView.findViewById(R.id.population);

         setCityName(cityName);
         setPopulation(population)

         return rootView;
    }

    public void setCityName(String name){
        cityNameText.setText(name);
    }

    public void setPopulation(String population){
        populationText.setText(population);
    }

    public String getCityName(){
        return cityName;
    }  

    @Override
    public void onCityInfoChanged(String population) {
        //setCityName(cityName);
        setPopulation();
    }
}

Answer 1

您可以嘗試ConditionVariable ，它可以用於保存AsyncTask

private ConditionVariable mCondition = new ConditionVariable(false);
mCondition.block(); //used to block
// your condition
mCondition.open(); // used to open

Answer 2

首先，您不應該將Fragment構造函數與參數一起使用。 您應該將捆綁包中的片段的參數發送給片段參數集。 參見： https : //stackoverflow.com/a/15392591/360211

我將在其onCreate obtainCityInfo其片段稱為obtainCityInfo方法。 然后，不再進行查找，我可以將OnCityInfoChanged傳遞給異步任務，以使其調用onPostExecute 。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState){
     ViewGroup rootView = (ViewGroup) inflater.inflate(
            R.layout.fragment_city_page2, container, false);
     cityNameText = (TextView)rootView.findViewById(R.id.city_name);
     populationText = (TextView)rootView.findViewById(R.id.population);

     setCityName(cityName);
     obtainCityInfo(cityName, this);

     return rootView;
}

private obtainCityInfo(String cityName, OnCityInfoChanged callback){
    String request = "http://example.abc.com/"
            + cityName + "&output=json";
    new AccessWebServiceTask(callback).execute(request, cityName); 
}

Answer 3

我會建議刪除的AsyncTask AccessWebServiceTask從在MainActivity並把它放在片段PageFragment 。 然后在此片段中，重寫onActivityCreated ，啟動AsyncTask。

[編輯]

這是您代碼的更新版本。 測試它是否有效：

主要活動

    public class MainActivity extends Activity{
    private List<City> cityList;
    private ViewPager mPager;  /*ViewPager for show the page of city info*/
    private ScreenSlidePagerAdapter mPagerAdapter;  /* Adapter for the ViewPager, 
            * ScreenSlidePagerAdapter is a subclass of FragmentStatePagerAdapter*/

     protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_show_city);

        cityList = new ArrayList<City>();
        mPager = (ViewPager) findViewById(R.id.pager);
        mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
        mPager.setAdapter(mPagerAdapter);
        init();
    }

    private void init(){
        loadCities();  //Load cities from database into cityList
        initFragments();

    }

    private void initFragments(){
        mPagerAdapter.removeAllFragments();
        for(City city: cityList){
            PageFragment fragment = PageFragment.newFragment(city.getName());
            mPagerAdapter.addFragment(fragment);       
        }
    }
}

ScreenSlidePagerAdapter：

    public class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter{
    private List<PageFragment> fragmentsList;
    public ScreenSlidePagerAdapter(FragmentManager fm) {
        super(fm);
        fragmentsList = new ArrayList<PageFragment>();
    }

    public ScreenSlidePagerAdapter(FragmentManager fm, List<PageFragment> list){
        super(fm);
        fragmentsList = list;
    }

    @Override
    public PageFragment getItem(int position) {
        return fragmentsList.get(position);
    }

    @Override
    public int getCount() {
        return fragmentsList.size();
    }

    public void setFragmentsList(List<PageFragment> fragmentsList){
        this.fragmentsList = fragmentsList;
        notifyDataSetChanged();
    }

    public void addFragment(PageFragment f){
        fragmentsList.add(f);
        notifyDataSetChanged();
    }

    public void removeFragment(int position){
        fragmentsList.remove(position);
        notifyDataSetChanged();
    }

    public void removeAllFragments(){
        fragmentsList.clear();
        notifyDataSetChanged();
    }    
}

PageFragment：

    public class PageFragment extends Fragment {
    private TextView cityNameText;
    private TextView populationText;
    private String cityName;
    private String population;

    public static final String CITY_NAME_KEY = "cityname"; 

    public PageFragment(){}


    public static PageFragment newFragment(String cityName){
      PageFragment fragment = new PageFragment();
      Bundle args = new Bundle();
      args.putString(CITY_NAME_KEY, cityName);
      fragment.setArguments(args);
      return fragment;
    }

    @Override
    public void onCreate (Bundle savedInstanceState){
       super.onCreate();
       if(savedInstanceState != null){
         this.cityName = savedInstanceState.getString(CITY_NAME_KEY);
       }       
    }   

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState){
         ViewGroup rootView = (ViewGroup) inflater.inflate(
                R.layout.fragment_city_page2, container, false);
         cityNameText = (TextView)rootView.findViewById(R.id.city_name);
         populationText = (TextView)rootView.findViewById(R.id.population);

         return rootView;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState){
       if(this.cityName != null){
         String request = "http://example.abc.com/" + cityName + "&output=json";
         new AccessWebServiceTask().execute(request, cityName); 
       }
    }

    public void setCityName(String name){
        cityNameText.setText(name);
    }

    public void setPopulation(String population){
        populationText.setText(population);
    }

    public String getCityName(){
        return cityName;
    }  


    private class AccessWebServiceTask extends AsyncTask<String, Void, CityInfo>{
    @Override
    protected CityInfo doInBackground(String... urls) {
        String result = getWebContent(urls[0]);  /*Access web through HTTP*/
        String cityName = urls[1];
        /*Transform the String result to a CityInfo object containing information of a city*/
        CityInfo cityInfo = encodeJason(result, cityName);  

        return cityInfo;
    }

    protected void onPostExecute(CityInfo cityInfo){
            OnCityGot(cityInfo); 
     }
   }

  public void onCityGot(CityInfo cityInfo){
     if(cityInfo != null){  
        String population = cityInfo.getPopulation();
        setPopulation(population);
     }
  }
}