在 Android四大组件之service（一） 文中我们讲到了 service 的 基本概念 和 startService 启动方式、stopService。

不过这种方式是有个缺点，我们无法调用  FirstService 类里面的方法。

这个时候有人会说 new FirstService() 不就好了吗？其实不然，作为四大组件，你是不能直接new的，必须由系统创建，并服务他们context（即上下文）。下面，我们就来讲一下另外一种启动服务的方式。bindService() / unBindService()

第一步：继承 Binder 编写 InnerBinder 类

public class InnerBinder extends Binder {
        public void callServiceInnerMethod() {
            sayHello();
        }
}

第二步：在重写的 onBind() 方法中返回 InnerBinder 实例

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return new InnerBinder();
    }

第三步：编写 ServiceConnection 

    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Log.d(TAG, "onServiceConnected....");
            mRemoteBinder = (FirstService.InnerBinder) service;
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            Log.d(TAG, "onServiceDisconnected....");
            mRemoteBinder = null;
        }
    };

 第四步：绑定服务

    public void bindServiceClick(View view) {
        Intent intent = new Intent();
        intent.setClass(this, FirstService.class);
        isServiceBinded = bindService(intent, mConnection, BIND_AUTO_CREATE);
    }

第五步：解绑服务

    public void unBindServiceClick(View view) {
        if (mConnection != null && isServiceBinded) {
            unbindService(mConnection);
            isServiceBinded = false;
        }
    }

完整代码

MainActivity.java

package com.wust.testdemo;

import android.app.Activity;
import android.content.ComponentName;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.util.Log;
import android.view.View;

import services.FirstService;

public class MainActivity extends Activity {

    private static final String TAG = MainActivity.class.getSimpleName();
    private boolean isServiceBinded;
    private FirstService.InnerBinder mRemoteBinder;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Log.d(TAG, "onCreate");
    }

    public void startServiceClick(View view) {
        Intent intent = new Intent();
        intent.setClass(this, FirstService.class);
        startService(intent);
    }

    public void stopServiceClick(View view) {
        Intent intent = new Intent();
        intent.setClass(this, FirstService.class);
        stopService(intent);
    }

    public void callServiceMethod(View view) {
        Log.d(TAG, "call service inner method");
        mRemoteBinder.callServiceInnerMethod();
    }

    public void bindServiceClick(View view) {
        Intent intent = new Intent();
        intent.setClass(this, FirstService.class);
        isServiceBinded = bindService(intent, mConnection, BIND_AUTO_CREATE);
    }

    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Log.d(TAG, "onServiceConnected....");
            mRemoteBinder = (FirstService.InnerBinder) service;
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            Log.d(TAG, "onServiceDisconnected....");
            mRemoteBinder = null;
        }
    };

    public void unBindServiceClick(View view) {
        if (mConnection != null && isServiceBinded) {
            unbindService(mConnection);
            isServiceBinded = false;
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy");
    }
}

FirstService.java

package services;

import android.app.Service;
import android.content.Intent;
import android.os.Binder;
import android.os.IBinder;
import android.util.Log;
import android.widget.Toast;

import androidx.annotation.Nullable;

/**
 * ClassName: FirstService <br/>
 * Description: <br/>
 * date: 2021/10/17 21:44<br/>
 *
 * @author yiqi<br />
 * @QQ 1820762465
 * @微信 yiqiideallife
 * @技术交流QQ群 928023749
 */
public class FirstService extends Service {

    private static final String TAG = FirstService.class.getClass().getSimpleName();

    public class InnerBinder extends Binder {
        public void callServiceInnerMethod() {
            sayHello();
        }
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        Log.d(TAG, "onBind....");
        return new InnerBinder();
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Log.d(TAG, "onCreate...");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.d(TAG, "onStartCommand...");
        return super.onStartCommand(intent, flags, startId);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy...");
    }

    private void sayHello() {
        Toast.makeText(this, "Hello!", Toast.LENGTH_SHORT).show();
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:onClick="startServiceClick"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="开启服务"/>

    <Button
        android:onClick="stopServiceClick"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="停止服务"/>

    <Button
        android:onClick="bindServiceClick"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="绑定服务"/>

    <Button
        android:onClick="unBindServiceClick"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="解绑服务"/>

    <Button
        android:onClick="callServiceMethod"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="调用服务内部方法"/>

</LinearLayout>

效果展示

两种启动方式的优缺点

1. startService的方法可以长期在后台运行，而 bindService 的方法则不可以长期在后台运行。
2. bindService启动服务，可以跟服务进行通讯，但是startService启动服务不可以跟服务进行通讯。
3. bindService启动服务，在软件退出时一定要记住 unBindService() 服务，要不然会报错