mDNS 與 DNS-SD 實作
===

本實作主要參考 [nsdchat](https://android.googlesource.com/platform/development/+/master/samples/training/NsdChat/src/com/example/android/nsdchat)，出處應來自 Android Developers 官方，但據下方文章作者提及官方好像已經移除且也找不到了，這是舊的連結，不過官方依然有 sample code 只是比較簡化就是。

本人參考後無法正常運行，且範例是沒有區別 server 與 client ，在 trace 過程中有點難理解，故自行簡化重新修改成兩種版本：`dns_server`與`dns_client`，可用兩台不同裝置分別安裝來測試看看：

兩支不同專案實作時，記得在 `AndroidManifest.xml` 加上
`<uses-permission android:name="android.permission.INTERNET"/>`

### dns_server

* `activity_main.xml`

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_state"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btn_registger"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:textAllCaps="false"
        android:text="Register Service"/>

    <Button
        android:id="@+id/btn_unregistger"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toBottomOf="@id/btn_registger"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:text="Unregister Service"
        android:textAllCaps="false"/>

</androidx.constraintlayout.widget.ConstraintLayout>

* `MainActivity`

package com.example.nsdchat_server_test;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.net.nsd.NsdManager;
import android.net.nsd.NsdServiceInfo;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    //region Properties
    public static final String TAG = "MainActivity";

    public static final String SERVICE_NAME = "NsdChat_server"; // 要註冊的 service 名稱
    public static final String SERVICE_TYPE = "_http._tcp.";    // 要註冊的 service 類型
    public static final int SERVICE_PORT = 2222; // 要註冊的 service port

    private TextView tv_state;
    private Button btn_registger;
    private Button btn_unregistger;

    private NsdManager mNsdManager;
    private static NsdServiceInfo mServiceInfo;
    private NsdManager.RegistrationListener mRegistrationListener;
    //endregion

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mNsdManager = (NsdManager) getSystemService(Context.NSD_SERVICE);

        initView();
        initListener();
    }

    @Override
    protected void onStop() {
        tearDown();
        super.onStop();
    }

    private void initView() {
        tv_state = (TextView) findViewById(R.id.tv_state);
        btn_registger = (Button) findViewById(R.id.btn_registger);
        btn_unregistger = (Button) findViewById(R.id.btn_unregistger);
    }
    
    private void initListener() {

        btn_registger.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (mServiceInfo != null) {
                    updateRegisterStateUI("您已註冊過 service: " + mServiceInfo.getServiceName());
                    return;
                }

                NsdServiceInfo serviceInfo = createServiceInfo();
                registerService(serviceInfo);
            }
        });

        btn_unregistger.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                tearDown();
            }
        });
    }

    private void registerService(NsdServiceInfo serviceInfo) {
        mRegistrationListener = new NsdManager.RegistrationListener() {
            @Override
            public void onServiceRegistered(NsdServiceInfo nsdServiceInfo) {
                String serviceName = nsdServiceInfo.getServiceName();
                Log.d(TAG, "Service registered: " + serviceName);
                mServiceInfo = nsdServiceInfo;
                updateRegisterStateUI("註冊 service 成功: " + serviceName);
            }

            @Override
            public void onRegistrationFailed(NsdServiceInfo nsdServiceInfo, int i) {
                Log.d(TAG, "Service registration failed: " + i);
                updateRegisterStateUI("Service registration failed");
            }

            @Override
            public void onServiceUnregistered(NsdServiceInfo nsdServiceInfo) {
                Log.d(TAG, "Service unregistered: " + nsdServiceInfo.getServiceName());
                updateRegisterStateUI("您已註銷 service");
            }

            @Override
            public void onUnregistrationFailed(NsdServiceInfo nsdServiceInfo, int i) {
                Log.d(TAG, "Service unregistration failed: " + i);
                updateRegisterStateUI("Service unregistration failed");
            }
        };

        mNsdManager.registerService(serviceInfo, NsdManager.PROTOCOL_DNS_SD, mRegistrationListener);
    }

    private NsdServiceInfo createServiceInfo() {
        NsdServiceInfo serviceInfo  = new NsdServiceInfo();
        serviceInfo.setServiceName(SERVICE_NAME);
        serviceInfo.setServiceType(SERVICE_TYPE);
        serviceInfo.setPort(SERVICE_PORT);
        return serviceInfo;
    }

    private void tearDown() {
        if (mRegistrationListener != null) {
            try {
                mNsdManager.unregisterService(mRegistrationListener);
            } finally {
            }
            mRegistrationListener = null;
            mServiceInfo = null;
        }
    }
    
    private void updateRegisterStateUI(String msg) {
        new Thread(new Runnable() {
            @Override
            public void run() {

                tv_state.post(new Runnable() {
                    @Override
                    public void run() {
                        tv_state.setText(msg);
                    }
                });

            }
        }).start();
    }
}

### dns_client

* `activity_main.xml`

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@+id/cl_serviceName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="16dp">

        <TextView
            android:id="@+id/tv_serviceName_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            android:textAllCaps="false"
            android:text="Search Service Name: "
            android:gravity="start"/>

        <TextView
            android:id="@+id/tv_serviceName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintStart_toEndOf="@id/tv_serviceName_title"
            app:layout_constraintEnd_toEndOf="parent"
            android:textAllCaps="false"
            android:textStyle="bold"
            android:text="SERVICE NAME"/>

    </androidx.constraintlayout.widget.ConstraintLayout>


    <Button
        android:id="@+id/btn_discovery"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toBottomOf="@id/cl_serviceName"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:layout_marginTop="16dp"
        android:textAllCaps="false"
        android:text="Discovery"/>

    <Button
        android:id="@+id/btn_connectService"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toBottomOf="@id/btn_discovery"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:textAllCaps="false"
        android:lines="3"
        android:text="Connect service\n(or Bind Service)\n(or Resolve Service)"/>

    <Button
        android:id="@+id/btn_disconnect"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toBottomOf="@id/btn_connectService"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:textAllCaps="false"
        android:text="Disconnect"/>

    <ScrollView
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_margin="16dp"
        app:layout_constraintTop_toBottomOf="@id/btn_disconnect"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintBottom_toTopOf="@id/cl_connectedState"
        android:background="#B5B5B5">

        <TextView
            android:id="@+id/tv_state"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

    </ScrollView>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:id="@+id/cl_connectedState"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintBottom_toBottomOf="parent">

        <TextView
            android:id="@+id/tv_connectedState_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Connect state:"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintStart_toStartOf="parent"/>

        <TextView
            android:id="@+id/tv_connectedState"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_marginTop="16dp"
            app:layout_constraintTop_toBottomOf="@id/tv_connectedState_title"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            android:textColor="#FF0000"
            android:lines="5"/>

    </androidx.constraintlayout.widget.ConstraintLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

* `MainActivity`

package com.example.nsdchat_client_test;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.net.nsd.NsdManager;
import android.net.nsd.NsdServiceInfo;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    //region Properties
    public static final String TAG = "MainActivity";

    public static final String FIND_SERVICE_NAME = "NsdChat_server"; // 要尋找的 service 名稱
    public static final String FIND_SERVICE_TYPE = "_http._tcp."; // 要尋找的 service 類型

    private TextView tv_serviceName;
    private TextView tv_state;
    private Button btn_discovery;
    private Button btn_connectService;
    private Button btn_disconnect;
    private TextView tv_connectedState;

    private NsdManager mNsdManager;
    private NsdManager.ResolveListener mResolveListener;
    private NsdManager.DiscoveryListener mDiscoveryListener;
    private static NsdServiceInfo mTryConnServiceInfo; // 符合 service 名稱可以嘗試 connect 的 service
    private static NsdServiceInfo mConnServiceInfo;    // 已 connect 的 service
    
    //endregion

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mNsdManager = (NsdManager) getSystemService(Context.NSD_SERVICE);
        
        initView();
        initListener();

        tv_serviceName.setText(FIND_SERVICE_NAME);
    }

    private void initView() {
        tv_serviceName = (TextView) findViewById(R.id.tv_serviceName);
        tv_state = (TextView) findViewById(R.id.tv_state); 
        btn_discovery = (Button) findViewById(R.id.btn_discovery);
        btn_connectService = (Button) findViewById(R.id.btn_connectService);
        btn_disconnect = (Button) findViewById(R.id.btn_disconnect);
        tv_connectedState = (TextView) findViewById(R.id.tv_connectedState);
    }

    private void initListener() {
        //region Discovery 按鈕
        btn_discovery.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                stopDiscovery();
                tv_state.setText("");
                tv_connectedState.setText("");
                startDiscovery();
            }
        });
        //endregion

        //region Connect Service 按鈕
        btn_connectService.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                // 要找的 service name 不存在
                if (mTryConnServiceInfo == null) {
                    tv_connectedState.setText("service name not exist.");
                    return;
                }
                // 已存在 connected service
                else if (mConnServiceInfo != null) {
                    String msg = getConnectedServiceInfo(mConnServiceInfo);
                    tv_connectedState.setText("已存在 connected service: " + "\n" + msg);
                    return;
                }

                startConnectService(mTryConnServiceInfo);

            }
        });
        //endregion

        //region Disconnect 按鈕
        btn_disconnect.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                tv_connectedState.setText("");
                mTryConnServiceInfo = null;
                mConnServiceInfo = null;

                // 刷新列表
                reloadDiscovery();
            }
        });
        //endregion
    }

    private void startDiscovery() {
        mDiscoveryListener = new NsdManager.DiscoveryListener() {

            /** 開始搜尋 service */
            @Override
            public void onDiscoveryStarted(String regType) {
                Log.d(TAG, "Service discovery started");
            }

            /** 搜尋 service 中 */
            @Override
            public void onServiceFound(NsdServiceInfo service) {
                Log.d(TAG, "Service discovery success: " + service);

                if (!service.getServiceType().equals(FIND_SERVICE_TYPE)) {
                    Log.d(TAG, "Unknown Service Type: " + service.getServiceType());
                }

                String list_msg;

                // 若已存在 connected service, 且 service 名字相同 (目前使用名稱來判斷 service 是否相同)
                if (mConnServiceInfo != null && service.getServiceName().equals(FIND_SERVICE_NAME)) {
                    list_msg = String.valueOf(service) + " (已連接)" + "\n\n";
                } else {
                    list_msg = String.valueOf(service) + "\n\n";
                }

                updateDiscoveryStateUI(list_msg);

                if (service.getServiceName().equals(FIND_SERVICE_NAME)) {
                    mTryConnServiceInfo = service;
                }
            }

            /** service 斷線 */
            @Override
            public void onServiceLost(NsdServiceInfo service) {
                Log.e(TAG, "service lost: " + service);

                if (mConnServiceInfo != null) {
                    tv_connectedState.setText("service lost");
                    mTryConnServiceInfo = null;
                    mConnServiceInfo = null;
                }
                
                // 刷新列表
                reloadDiscovery();
            }

            /** 停止搜尋 service */
            @Override
            public void onDiscoveryStopped(String serviceType) {
                Log.i(TAG, "Discovery stopped: " + serviceType);
            }

            /** 開始搜尋 service fialed */
            @Override
            public void onStartDiscoveryFailed(String serviceType, int errorCode) {
                Log.e(TAG, "Discovery failed: Error code:" + errorCode);
            }

            /** 停止搜尋 service failed */
            @Override
            public void onStopDiscoveryFailed(String serviceType, int errorCode) {
                Log.e(TAG, "Discovery failed: Error code:" + errorCode);
            }
        };

        mNsdManager.discoverServices(FIND_SERVICE_TYPE, NsdManager.PROTOCOL_DNS_SD, mDiscoveryListener);
    }

    private void startConnectService(NsdServiceInfo service) {

        mResolveListener = new NsdManager.ResolveListener() {

            /** connect service 失敗 */
            @Override
            public void onResolveFailed(NsdServiceInfo serviceInfo, int errorCode) {
                Log.e(TAG, "Resolve failed" + errorCode);
                tv_connectedState.setText("Resolve failed: " + serviceInfo.getServiceName());
            }

            /** connect service 成功 */
            @Override
            public void onServiceResolved(NsdServiceInfo serviceInfo) {
                Log.e(TAG, "Resolve Succeeded. " + serviceInfo);

                mConnServiceInfo = serviceInfo;

                // 刷新 Connect state
                String msg = getConnectedServiceInfo(serviceInfo);
                tv_connectedState.setText("Connect Succeeded" + "\n" + msg);

                // 刷新列表
                reloadDiscovery();
            }
        };

        mNsdManager.resolveService(service, mResolveListener);
    }

    private void stopDiscovery() {
        if (mDiscoveryListener != null) {
            try {
                mNsdManager.stopServiceDiscovery(mDiscoveryListener);
            } finally {
            }
            mDiscoveryListener = null;
        }
    }
    
    private void updateDiscoveryStateUI(String msg) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                tv_state.post(new Runnable() {
                    @Override
                    public void run() {
                        tv_state.append(msg);
                    }
                });
            }
        }).start();
    }

    /**
     * 重新刷新搜尋 service 列表
     */
    private void reloadDiscovery() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                btn_discovery.post(new Runnable() {
                    @Override
                    public void run() {
                        stopDiscovery();
                        tv_state.setText("");
                        startDiscovery();
                    }
                });
            }
        }).start();
    }

    private String getConnectedServiceInfo(NsdServiceInfo serviceInfo) {
        return
            "service name: " + serviceInfo.getServiceName() + "\n" +
            "host_from_server: " + serviceInfo.getHost() + "\n" +
            "port from server: " + serviceInfo.getPort();
    }
}

* 在測試時注意 **dns_client** 的 `FIND_SERVICE_NAME` 須與 **dns_server** 的 `SERVICE_NAME` 一致，connect 才能 work

* **dns_server** 的 `SERVICE_PORT` 隨機即可。

## Ref.
[Listener already in use (Service Discovery)](https://stackoverflow.com/a/35108713)

[nsdchat](https://android.googlesource.com/platform/development/+/master/samples/training/NsdChat/src/com/example/android/nsdchat)

[android NSD服务详解](https://blog.csdn.net/wenzhi20102321/article/details/79575836)

[Use network service discovery](https://developer.android.com/training/connect-devices-wirelessly/nsd#java) (官方)

###### tags: `實作相關`