java - 使用 BatteryManager 接收广播 Intent 时出错

Question

我在接收广播 Intent 时遇到错误，我正在使用 BatteryManager。我不知道为什么会收到此错误。

请有人帮忙。提前致谢

如果您需要更多代码，请告诉我。

错误如下:

> Error receiving broadcast Intent {
> act=android.intent.action.BATTERY_CHANGED flg=0x60000010 (has extras)
> } in com.example.android.login.MainActivity$1@260e12dc
>             at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:933)
>             at android.os.Handler.handleCallback(Handler.java:739)
>             at android.os.Handler.dispatchMessage(Handler.java:95)
>             at android.os.Looper.loop(Looper.java:145)
>             at android.app.ActivityThread.main(ActivityThread.java:5972)
>             at java.lang.reflect.Method.invoke(Native Method)
>             at java.lang.reflect.Method.invoke(Method.java:372)
>             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
>             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)
>      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void
> android.widget.TextView.setText(java.lang.CharSequence)' on a null
> object reference
>             at com.example.android.login.MainActivity$1.onReceive(MainActivity.java:37)
>             at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:923)
>             at android.os.Handler.handleCallback(Handler.java:739)
>             at android.os.Handler.dispatchMessage(Handler.java:95)
>             at android.os.Looper.loop(Looper.java:145)
>             at android.app.ActivityThread.main(ActivityThread.java:5972)
>             at java.lang.reflect.Method.invoke(Native Method)
>             at java.lang.reflect.Method.invoke(Method.java:372)

Activity 类别

package com.example.android.login;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.res.Resources;
import android.os.BatteryManager;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import com.parse.LogOutCallback;
import com.parse.Parse;
import com.parse.ParseObject;
import com.parse.ParseUser;

import com.parse.ParseException;


public class MainActivity extends ActionBarActivity {
    private TextView batteryTxt;
    private BroadcastReceiver mBatInfoReceiver = new BroadcastReceiver(){
        @Override
        public void onReceive(Context ctxt, Intent intent) {
            int level = intent.getIntExtra(BatteryManager.EXTRA_LEVEL, 0);
            batteryTxt.setText(String.valueOf(level) + "%");
        }
    };
    private Toolbar toolbar;
    public Button logoutButton;
    public int level;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        batteryTxt = (TextView) this.findViewById(R.id.percent);
        this.registerReceiver(this.mBatInfoReceiver, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_appbar);
        toolbar = (Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
        drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawerLayout), toolbar);

        logoutButton = (Button) findViewById(R.id.logoutButton);
        logoutButton.setOnClickListener(new View.OnClickListener() {
            @Override
             public void onClick(View v) {
                // Set up a progress dialog
                final ProgressDialog logout = new ProgressDialog(MainActivity.this);
                logout.setTitle("Please wait.");
                logout.setMessage("Logging out.  Please wait.");
                logout.show();
                ParseUser.logOutInBackground(new LogOutCallback() {

                    public void done(ParseException e) {
                        logout.dismiss();
                        if (e == null) {
                            Intent logoutDone = new Intent(MainActivity.this, DispatchActivity.class);
                            startActivity(logoutDone);
                        } else {
                            Toast.makeText(MainActivity.this, "Logout Unsuccessful", Toast.LENGTH_LONG).show();
                        }
                    }
                });
            }
        });


    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, 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.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

}

Answer 1

根据您的堆栈跟踪，看起来它正在抛出 NullPointerException，因为 batteryTxt收到 Intent 时字段为空。正如 Daniel Nugent 指出的，这是因为接收者在调用 setContentView() 之前就已注册。 。但是，看起来您也永远不会取消注册接收器，因此我建议将用于注册接收器的代码移动到 onResume()方法，并在 onPause() 中添加取消注册调用方法来防止在您的 Activity 停止时发生相同的 NullPointerException。

@Override
protected void onResume() {
    super.onResume();
    registerReceiver(mBatInfoReceiver, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
}

@Override
protected void onPause() {
    unregisterReceiver(mBatInfoReceiver);
    super.onPause();
}