Question

我有一个包含100多个对象的数据库。每个对象都有一个特定的图像文件名，主题字符串，标题字符串和pdf文件名。

我有一个活动，它是对象的网格视图。

首次创建时，网格视图将加载要显示的对象数组。数组的确切内容可能会有所不同，具体取决于我在首次打开活动时运行的数据库查询。

默认设置是从数据库中检索并显示所有对象。但是我有时只检索和显示对象的一个子集（基于主题）。

单击网格中的按钮将打开一个活动，该活动显示该对象的pdf。很好

问题是，当我从PDF活动中“返回”时，将重新创建网格视图。始终使用“默认”所有对象。

因此，如果我有一个仅显示某个“主题”对象的网格视图，则如果我打开pdf活动，则返回时将重新创建网格视图并显示所有对象，而不显示pdf活动之前的原始网格视图开始。

问题： 1）如何在不重新创建网格视图的情况下返回网格视图？

代码：

public class SubjectListActivity extends AppCompatActivity {

String DB_NAME = "XXXXX.sqlite";
String TABLE_NAME = "XXXXX";
DataBaseHelper myDBHelper;


private GridView gridView;
private MyAdapter myAdapter;

private ArrayList<Subject> subjectsArrayList;
private Context context;
private String subjectAreas = ""; // this is sent to the activity from whichever activity opened it.  


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

    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        subjectAreas = extras.getString("subjectAreas");
    }

    gridView = findViewById(R.id.gv);


    // Database
    AssetDatabaseOpenHelper assetDatabaseOpenHelper = new AssetDatabaseOpenHelper(this, DB_NAME);
    assetDatabaseOpenHelper.saveDatabase();
    myDBHelper = new DataBaseHelper(this, DB_NAME);

    // populates subjectsArray
    subjectsArrayList = populateSubjects();

    loadGrid();

    // listener
    gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            String title = subjectsArrayList.get(position).getSubjectTitle();
            String iconStyle = subjectsArrayList.get(position).getSubjectIconStyle();
            String pdf = subjectsArrayList.get(position).getSubjectPDF();
            String ref = subjectsArrayList.get(position).getSubjectRefs();
            String linked = subjectsArrayList.get(position).getSubjectLinked();

            Intent intent =  new Intent(view.getContext(), SubjectActivity.class);

            intent.putExtra("title", title);
            intent.putExtra("icon", iconStyle);
            intent.putExtra("pdf", pdf);
            intent.putExtra("ref", ref);
            intent.putExtra("linked", linked);
            intent.putExtra("subjectAreas", subjectAreas);

            startActivityForResult(intent, 1);


        }
    });

}


private void loadGrid(){

    myAdapter = new MyAdapter(getApplicationContext(), subjectsArrayList);
    gridView.setAdapter(myAdapter);

}

// The SQL query that populates the subjectArray depends on the 'subjectAreas' string which is set when the activity first opens. The default is 'getAllSubjects'
private ArrayList<Subject>  populateSubjects() {

    Cursor res = myDBHelper.getAllSubjects(TABLE_NAME);

    if (subjectAreas.equals("emerg")){res = myDBHelper.getEmergSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("all")){res = myDBHelper.getAllSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("ax")){res = myDBHelper.getAxSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("mx")){res = myDBHelper.getMxSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("ref")){res = myDBHelper.getRefSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("cal")){res = myDBHelper.getCalSubjects(TABLE_NAME);}
    else if (subjectAreas.equals("search")){res = myDBHelper.getSearchSubjects(TABLE_NAME, keywords);}


        ArrayList<Subject> list = new ArrayList<>();

            if (res.getCount() == 0) {

            } else {
                while (res.moveToNext()) {
                    Subject subject = new Subject();
                    subject.setSubjectID(res.getString(0));
                    subject.setSubjectTitle(res.getString(1));
                    subject.setSubjectIconStyle(res.getString(2));
                    subject.setSubjectPDF(res.getString(3));
                    subject.setSubjectKeywords(res.getString(4));
                    subject.setSubjectLinked(res.getString(5));
                    subject.setSubjectRefs(res.getString(6));
                    list.add(subject);
                }

            }

            return list;
        }


// Tried sending 'back' subjectAreas from PDF activity and re-populating the array and gridview - this doesn't seem to be called  
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == 1) {
        if(resultCode == RESULT_OK) {
            subjectAreas = data.getStringExtra("subjectAreas");
            subjectsArrayList = populateSubjects();
            myAdapter = new MyAdapter(getApplicationContext(), subjectsArrayList);
            gridView.invalidateViews();
            gridView.setAdapter(myAdapter);
        }
    }
}

}

预先感谢

更新：使用savedInstanceState“保存”我的subjectAreas字符串，我使用了以下内容：

    private String subjectAreas = "";
private static final String subjectAreasSaved = "";

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

    Bundle extras = getIntent().getExtras();
    if (extras != null) {
        subjectAreas = extras.getString("subjectAreas");
        title = extras.getString("title");
        keywords = extras.getStringArrayList("keywords");
    }

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        subjectAreas = savedInstanceState.getString(subjectAreasSaved);
    }

并且：

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putString(subjectAreasSaved, subjectAreas);
    super.onSaveInstanceState(savedInstanceState);
}

当第二个活动是开放式活动时，onSaveINstanceState是自动调用的还是我需要调用它？目前似乎什么也没发生

Answer 1

娱乐取决于框架。如果它确定需要资源，则无论是否需要它都会销毁先前的副本。你无法阻止它。

您可以做的是实现onSaveInstanceState和onRestoreInstanceState来保存需要从该确切状态重新创建的所有数据。

Answer 2

首先，应该对帮助很大的作者们表示敬意。

我的代码问题得到了很好的解释，并通过以下线程解决了：Stackoverflow thread

基本上，操作栏中的“主页”按钮会导致重新创建上一个活动。因此，我的问题。当我尝试实现InstanceState时，我一直为saveInstanceState获取一个'null'...然后将我引向我上面链接的线程。

我真的希望这对其他人有帮助

Android-无需重新创建即可返回上一个活动

2 个答案: