java - 如何设置 doInbackground 在继续 mainActivity 之前完成其任务？

Question

这些是我的MainActivity :

    database_connector wp_terms = new database_connector("SELECT * FROM  `dse120071750`.`wp_terms` ",progressDialog,this);
    wp_terms.execute();
    wp_terms.onPreExecute();
    try {

        for (int i=0; i<wp_terms.getJsonArray().length(); i++){
            JSONObject obj = wp_terms.getJsonArray().getJSONObject(i);
            this.wp_terms.put(obj.getString("term_id"), obj.getString("name"));
        }
    } catch (JSONException e) {
        e.printStackTrace();
    }

数据库连接器:

package hk.hoome.www.mobilehoome;

public class database_connector extends AsyncTask<Void,Void, Void> {

//String mode;
HttpResponse response;
String sql;
JSONArray jsonArray;
searchPage searchPage;

public database_connector(String sql, searchPage searchPage){
    //this.mode = mode;
    this.sql = sql;
    this.searchPage = searchPage;
    jsonArray = new JSONArray();
}

@Override
protected Void doInBackground(Void... params) {
    connect();
    publishProgress();
    return null;
}



@Override
protected void onProgressUpdate(Void... values) {

}

public void connect() {
    List<NameValuePair> nameValuePair = new ArrayList<NameValuePair>(1);
    nameValuePair.add(new BasicNameValuePair("sql", sql));
    //nameValuePair.add(new BasicNameValuePair("mode", mode));
    try {
        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost("http://www.hoome.hk/hoomeMobileApps/connectDB.php");
        httpPost.setEntity(new UrlEncodedFormEntity(nameValuePair, "UTF-8"));
        response = httpClient.execute(httpPost);
        HttpEntity httpEntity  = response.getEntity();
        String entityResponse  = EntityUtils.toString(httpEntity);
        Log.e("Entity Response ", entityResponse.substring(2));
        jsonArray = new JSONArray(entityResponse.substring(2));

    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }
}



public JSONArray getJsonArray(){
    return jsonArray;
}

}

当我运行此代码时，for (int i=0; i<wp_terms.getJsonArray().length(); i++){这导致 nullPointerException 。

我相信这是因为doInbackground尚未完成其过程，但 mainActivity继续运行。我该如何设置doInbackground在继续运行 mainActivity 之前必须完成吗？

解决方案？

   try {
        while (wp_posts.getJsonArray().equals(null))
            Thread.sleep(1000);                 
    } catch(InterruptedException ex) {
        Thread.currentThread().interrupt();
    }

这是一个好的解决方案吗？

Answer 1

在数据库连接器类中使用回调并传递完成回调。为了简单起见，我使用 Runnable 接口(interface)。一般来说，尝试拥有自己的接口(interface)，并通过自定义接口(interface)在后台线程和主线程之间传递参数。

package hk.hoome.www.mobilehoome;

public class database_connector extends AsyncTask<Void,Void, Void> {

//String mode;
HttpResponse response;
String sql;
JSONArray jsonArray;
searchPage searchPage;
private Runnable activityCallback;

public void setCallback(Runnable callback) {
    this.activityCallback = callback;
}

public database_connector(String sql, searchPage searchPage){
    //this.mode = mode;
    this.sql = sql;
    this.searchPage = searchPage;
    jsonArray = new JSONArray();
}

@Override
protected Void doInBackground(Void... params) {
    connect();
    publishProgress();
    return null;
}

protected void onPostExecute(Void result) {

    if(activityCallback != null) {
        activityCallback.run();
    }
}


@Override
protected void onProgressUpdate(Void... values) {

}

public void connect() {
    List<NameValuePair> nameValuePair = new ArrayList<NameValuePair>(1);
    nameValuePair.add(new BasicNameValuePair("sql", sql));
    //nameValuePair.add(new BasicNameValuePair("mode", mode));
    try {
        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost("http://www.hoome.hk/hoomeMobileApps/connectDB.php");
        httpPost.setEntity(new UrlEncodedFormEntity(nameValuePair, "UTF-8"));
        response = httpClient.execute(httpPost);
        HttpEntity httpEntity  = response.getEntity();
        String entityResponse  = EntityUtils.toString(httpEntity);
        Log.e("Entity Response ", entityResponse.substring(2));
        jsonArray = new JSONArray(entityResponse.substring(2));

    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }
}



public JSONArray getJsonArray(){
    return jsonArray;
}
}

在你的MainActivity中

database_connector wp_terms = new database_connector("SELECT * FROM  `dse120071750`.`wp_terms` ",progressDialog,this);
wp_terms.setCallback(new Runnable() {

        public void run() {

        try {

                for (int i=0; i<wp_terms.getJsonArray().length(); i++){
                    JSONObject obj = wp_terms.getJsonArray().getJSONObject(i);
                    this.wp_terms.put(obj.getString("term_id"),      obj.getString("name"));
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    });
    wp_terms.execute();