Android 后退按钮 - 从 Intent 中丢失信息

Question

刚接触android，所以我真的不知道这里发生了什么；

现在，在我的主要 Activity 中，我有一个下拉微调器；单击一个按钮会转到第二个 Activity ，其中有一个 ListView ，其中包含来自下拉微调器内容的信息。

当我单击另一个按钮并转到第三个 Activity ，然后按返回键时，下拉微调器内容中的信息不再存在；取而代之的是空值。

有人知道可能是什么问题吗？后退按钮是否发送 Intent ？

第二个 Activity :

public class classList extends AppCompatActivity {
    ListView listView;
    SimpleCursorAdapter mAdapter;
static private String[] classes = {"class1","class2"};

String course;
String number;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Intent intent = getIntent();
    course = intent.getStringExtra("com.naomi.classAlert.course");
    number = intent.getStringExtra("com.naomi.classAlert.number");

    Toast.makeText(getApplicationContext(), course+number,
            Toast.LENGTH_LONG).show();
    setContentView(R.layout.activity_class_list);

    listView = (ListView) findViewById(R.id.list);



    String[] values = new String[] {
            course+number+" 001", course+number+" 002", course+number+" 003"
    };

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_2, android.R.id.text1, values);
    listView.setAdapter(adapter);

    // ListView Item Click Listener
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

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

            // ListView Clicked item index
            int itemPosition     = position;

            // ListView Clicked item value
            String  itemValue    = (String) listView.getItemAtPosition(position);

            // Show Alert
            Toast.makeText(getApplicationContext(),
                    "Position :"+itemPosition+"  ListItem : " +itemValue , Toast.LENGTH_LONG)
                    .show();

            Intent intent1 = new Intent(classList.this, pickSection.class);
            intent1.putExtra("com.naomi.classAlert.classList.course",course);
            intent1.putExtra("com.naomi.classAlert.classList.number", number);
            intent1.putExtra("com.naomi.classAlert.classList.section",position+1);
            startActivity(intent1);

        }

    });
}

}

第三个 Activity :

public class pickSection extends AppCompatActivity {
    String course;
    String number;
    int section;
    //SharedPreferences faves = PreferenceManager
    //        .getDefaultSharedPreferences(this);
    SharedPreferences faves;
    SharedPreferences.Editor editor;

    Button button;
    TextView courseInfo;
    TextView capacity;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        faves = getSharedPreferences("preferencename",0);
        editor = faves.edit();
        setContentView(R.layout.activity_pick_section);

        button = (Button) findViewById(R.id.favouriteButton);
        courseInfo = (TextView) findViewById(R.id.courseinfo);
        capacity = (TextView) findViewById(R.id.capacity);
        Intent intent2 = getIntent();
        course = intent2.getStringExtra("com.naomi.classAlert.classList.course");
        number = intent2.getStringExtra("com.naomi.classAlert.classList.number");
        section = intent2.getIntExtra("com.naomi.classAlert.classList.section",0);

        courseInfo.setText(course+number+" "+section);
        capacity.setText("dummy text");


        String exists = faves.getString(course+number+section,"not found");
        if (!exists.equals("not found")){
            button.setText("Remove from favourites");
        }

    }

    public void addToFavourites(View v) {
        String exists = faves.getString(course+number+section,"not found");
        if (!exists.equals("not found")) { //it exists in favourites
            editor.remove(course + number + section);
            button.setText("Add to favourites");
        } else {
            editor.putString(course+number+section, "favourite");
            button.setText("Remove from favourites");
        }
        editor.commit();
    }
}

Answer 1

可以通过手机上的后退按钮关闭 Activity 。在这种情况下，执行 finish() 方法。如果 Activity 是通过调用 startActivity(Intent) 方法启动的，则调用者不需要来自现在已关闭的 Activity 的任何结果或反馈。

如果您使用 startActivityForResult() 方法调用启动 Activity ，您会期望来自子 Activity 的反馈。子 Activity 结束后，将调用子 Activity 的 onActivityResult() 方法，您可以根据结果执行操作。

 public void onItemClick (....){
     // start pickSection
      Intent intent1 = new Intent(classList.this, pickSection.class);
      intent1.putExtra("com.naomi.classAlert.classList.course",course);
      intent1.putExtra("com.naomi.classAlert.classList.number", number);
      intent1.putExtra("com.naomi.classAlert.classList.section",position+1);
      startActivity(intent1);
      startActivityForResult(intent1, 0);
    } 

当按下后退按钮时，pickSection 完成，它可以通过 Intent 将数据发送回其调用者。这是在 finish() 方法中完成的。 @覆盖

public void finish() {
  Intent intent1 = new Intent();
  intent1.putExtra("com.naomi.classAlert.classList.course",course);
  intent1.putExtra("com.naomi.classAlert.classList.number", number);
  intent1.putExtra("com.naomi.classAlert.classList.section",position+1);
  // Activity finished ok, return the data
  setResult(RESULT_OK, intent1);
  super.finish();
} 

一旦 pickSection 完成，就会调用 classList 中的 onActivityResult() 方法。

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  if (resultCode == RESULT_OK && requestCode == 0) {
    if (data.hasExtra("com.naomi.classAlert.classList.course")) {
      Toast.makeText(this, data.getExtras().getString("com.naomi.classAlert.classList.course"),
        Toast.LENGTH_SHORT).show();
    }
  }
}