1

我有一点 saveInstanceState 问题。

一个活动通过意图将一个字符串值发送到另一个活动，效果非常好。现在在第二个活动中，我选择一个行项目以进入第三个活动，这也有效。但是，如果我使用操作栏 BACK 按钮（到第二个活动），我会得到一个 NullPointerException。

原因当然是无法加载第一个活动的意图字符串。因此，在第二个活动中，我尝试将此意图字符串值保存为实例状态，并再次在 oncreate 方法中获取此值。我googlet了很多，我尝试了很多。但是 Nullpointer 不会消失。

有一点提示会很棒。

我的第二个活动代码：

    package de.cityknight.app;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Gravity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;


public class RouteChooseActivity extends ActionBarActivity implements OnItemClickListener {

    String citySave;
    String[] titelRoute;
    String[] descRoute;
    Integer[] bildRoute;

    ListView listView;
    List<RowItem> rowItems;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_route_choose);
        Bundle b = getIntent().getExtras();
        if (savedInstanceState != null) {
                savedInstanceState.getString("stadt");
         }
         else {
            b.getString("stadt");
         }
            TextView headingRoute = (TextView) findViewById(R.id.routeTitel);
            headingRoute.setText(String.format(getResources().getString(R.string.route_text)) + " " + b.getString("stadt") + " aus:");
            if (b.getString("stadt").equals("Passau")) {
                titelRoute = new String[]{"Die Altstadt", "Universität", "Essen und Trinken"};
                descRoute = new String[]{"Die Altstadt Passaus bietet viele Attraktion: Rathaus, Dom und die vielen kleinen Gassen.",
                        "Deutschlands schönster Campus.", "Kulinarische Köstlichkeiten in Passau"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else if (b.getString("stadt").equals("Augsburg")) {
                titelRoute = new String[]{"Augsburg1", "Augsburg2", "Augsburg3"};
                descRoute = new String[]{"Bla bla", "Mehr Bla", "Blubb"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else if (b.getString("stadt").equals("Bamberg")) {
                titelRoute = new String[]{"Bamberg", "Bamberg2", "Bamberg3"};
                descRoute = new String[]{"Bla bla", "Mehr Bla", "Blubb"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else {
                try {
                    TextView errorMessage = (TextView) findViewById(R.id.routeTitel);
                    errorMessage.setText(R.string.errorroute);
                } catch (NullPointerException e) {
                    Context context = getApplicationContext();
                    CharSequence text = getResources().getString(R.string.errorroute);
                    int duration = Toast.LENGTH_LONG;
                    Toast toast = Toast.makeText(context, text, duration);
                    toast.setGravity(Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, 0);
                    toast.show();
                }

            }

            try {
                rowItems = new ArrayList<RowItem>();
                for (int i = 0; i < titelRoute.length; i++) {
                    RowItem item = new RowItem(bildRoute[i], titelRoute[i], descRoute[i]);
                    rowItems.add(item);
                }
                Comparator<RowItem> sortRoutes = new Comparator<RowItem>() {
                    @Override
                    public int compare(RowItem rowItem1, RowItem rowItem2) {
                        return rowItem1.getTitle().compareTo(rowItem2.getTitle());
                    }
                };
                Collections.sort(rowItems, sortRoutes);
                listView = (ListView) findViewById(R.id.listRoute);
                CityListViewAdapter adapter = new CityListViewAdapter(this, R.layout.row_items, rowItems);
                listView.setAdapter(adapter);
                listView.setOnItemClickListener(this);
            } catch (ArrayIndexOutOfBoundsException rowItems) {
                Context context = getApplicationContext();
                CharSequence text = getResources().getString(R.string.ressourcefailed);
                int duration = Toast.LENGTH_LONG;
                Toast toast = Toast.makeText(context, text, duration);
                toast.setGravity(Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, 0);
                toast.show();
            }

    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        Intent intent = new Intent();
        intent.setClass(RouteChooseActivity.this, RouteView.class);
        RowItem item = (RowItem) parent.getItemAtPosition(position);
        Bundle b = new Bundle();
        b.putString("route", item.getTitle().toString());
        intent.putExtras(b);
        startActivity(intent);
    }

    protected void onSaveInstanceState(Bundle savedInstanceState) {
        Bundle b = getIntent().getExtras();
        b.getString("stadt");
        savedInstanceState.putString("stadt", citySave);
        super.onSaveInstanceState(savedInstanceState);
    }

    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        // Restore UI state from the savedInstanceState.
        // This bundle has also been passed to onCreate.
        savedInstanceState.getString(citySave);
        Log.i("debug", "saved data: " + citySave);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.route, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        switch (item.getItemId()) {
            case R.id.action_stadt:
                setContentView(R.layout.activity_stadt);
                Intent stadt = new Intent(RouteChooseActivity.this, StadtActivity.class);
                startActivity(stadt);
                return true;
            case R.id.action_help:
                setContentView(R.layout.activity_help);
                Intent help = new Intent(RouteChooseActivity.this, Help.class);
                startActivity(help);
                return true;
            case R.id.action_exit:
                moveTaskToBack(true);
                System.exit(0);
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
    }
4

3 回答 3

2

  1. 这将引发 NullPointerException ：

    savedInstanceState.getString(citySave);
Log.i("debug", "saved data: " + citySave);

    它应该是 ：

    citySave = savedInstanceState.getString("stadt");
Log.i("debug", "saved data: " + citySave);

  2. 在onSaveInstanceState您应该super.onSaveInstanceState(savedInstanceState);在添加字符串之前调用savedInstanceState（否则它将被清除......）

  3. 整体savedInstanceState并不适合这个目的。当您进行第 3 个活动时，Android可以将其删除！（这取决于系统是否认为它需要内存并因此完全破坏第二个活动及其 savedInstanceState）。

    做事的好方法是覆盖onBackPressed()您的第 3 个活动的方法，以触发一个意图以启动您的第 2 个活动，并将值放入 Intent 的“Extra”包中。

    使用这种方法，您的问题应该得到解决。

于 2014-07-18T09:02:26.217 回答
1

如我所见，您有两个错误：

首先，您实际上并没有使用 savedInstanceState。是的，您正在调用该函数，但您实际上并没有使用它返回的值。

String myNewString;
Bundle b = getIntent().getExtras();
if (savedInstanceState != null) {
    myNewString = savedInstanceState.getString("stadt");
} else {
    myNewString = b.getString("stadt");
}

现在使用myNewString而不是b.getString("stadt").

其次，尝试将代码放入onResume函数中。如果您按下后退按钮返回第二个活动，onCreate() 函数将不会运行。

于 2014-07-18T08:56:19.827 回答
0

我想通了。

您需要三件事来获得良好的生命周期 - 如果您的活动被销毁，则为 savedInstanceState， - 如果您的活动被另一个应用程序或活动屏幕暂停，例如如果您选择一个启动新活动的 rowItem。

对于我的问题，做了以下事情： 第二个活动：

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_route_choose);
        Bundle b = getIntent().getExtras();
        citySave = b.getString("stadt");

@Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        Intent intent = new Intent();
        intent.setClass(RouteChooseActivity.this, RouteView.class);
        RowItem item = (RowItem) parent.getItemAtPosition(position);
        Bundle b = new Bundle();
        b.putString("route", item.getTitle().toString());
        b.putString("stadt", citySave);
        intent.putExtras(b);
        startActivity(intent);
    }

    protected void onSaveInstanceState(Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);
        Bundle b = getIntent().getExtras();
        citySave = b.getString("stadt");
        savedInstanceState.putString("stadt", citySave);
    }

    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        // Restore UI state from the savedInstanceState.
        // This bundle has also been passed to onCreate.
        citySave = savedInstanceState.getString("stadt");
        Log.i("debug", "saved data: " + citySave);
    }

    public void onResume() {
        super.onResume();

            Bundle b = getIntent().getExtras();
            citySave = b.getString("stadt");

    }

第三项活动：

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_route_view);
        Bundle b = getIntent().getExtras();
        b.getString("route");
        citySave = b.getString("stadt");

    public void onBackPressed() {
        super.onBackPressed();
        Intent intent = new Intent();
        intent.setClass(RouteView.this, RouteChooseActivity.class);
        Bundle b = new Bundle();
        b.putString("stadt", citySave);
        intent.putExtras(b);
        startActivity(intent);
        finish();
    }

    public void onPause() {
        super.onPause();
        Intent intent = new Intent();
        intent.setClass(RouteView.this, RouteChooseActivity.class);
        Bundle b = new Bundle();
        b.putString("stadt", citySave);
        intent.putExtras(b);
        startActivity(intent);
        finish();
    }
于 2014-07-19T11:54:02.940 回答