Android如何使线程等到某个事件发生？

时间：2014-08-28 07:20:34

标签： android multithreading android-fragments android-asynctask

[我的情况]
现在在MainActivity中，我创建了一些片段，其中有一些TextView。并且MainActivity中有一个异步线程AsyncTask来从Web获取信息。在AsyncTask完成获取之后，它将通过回调方法更新上面提到的TextViews中的文本（Fragment实现OnInfoGotInterface并在接口中实现onInfoGot（）方法.onInfoGot（）将调用Fragment中定义的方法setTextView（）来更新信息）。

[我的问题]
执行程序时，我发现AsyncTask完成从web获取信息的时间点早于Fragment调用onCreateView（）的时间点。换句话说，当AsyncTask调用（OnInfoGotInterface）fragment.onInfoGot（）来设置TextViews时，TextViews还没有实现（TextViews是通过rootView.findViewById（）方法在CreateView（）上实现的）。结果，出现了NullPointerException。

[我需要一个决议]
现在我想这样做：当AsyncTask完成从Web获取信息并打算调用onInfoGot（）时，我们停止它，并让它等到Fragment完成onCreateView（）。之后我们唤醒了AsyncTask并允许它更新片段。

PS。有人建议我应该在Fragment的定义中的onCreateView（）之后调用新的AsyncTask.excute（）。但是这里的AsyncTask和Fragment都是在MainActivity中创建的。它们是MainActivity中的两个不同的任务线程，一个用于显示数据，另一个用于从Web获取数据。

有人可以提供一些建议吗？我真的很感激！ 这是代码：

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();
    }
}

3 个答案:

答案 0 :(得分：0)

您可以尝试ConditionVariable，它可用于保存AsyncTask 

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

答案 1 :(得分：0)

首先，您不应该使用带参数的Fragment构造函数。您应该将参数发送到bundle set中的片段到fragment参数。请参阅：https://stackoverflow.com/a/15392591/360211

我会让片段调用obtainCityInfo中的onCreate方法。OnCityInfoChanged。然后不再查找，我可以将onPostExecute传递给异步任务，以便在@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); } 上调用。

{{1}}

答案 2 :(得分：0)

我建议从MainActivity中删除AsyncTask AccessWebServiceTask并将其放入片段PageFragment中。然后在这个片段中，覆盖onActivityCreated，启动AsyncTask。

[编辑]

以下是您的代码的更新版本。测试它是否有效：

MainActivity 

    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);
     }
  }
}
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