Android系統匿名共享記憶體Ashmem（Anonymous Shared Memory）簡要介紹和學習計劃 (四)

 接著，我們就來看Client端的實現。Client端是一個Activity，實現在src/shy/luo/ashmem/Client.java檔案中：

package shy.luo.ashmem;

import java.io.FileDescriptor;
import java.io.IOException;

import shy.luo.ashmem.R;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.MemoryFile;
import android.os.ParcelFileDescriptor;
import android.os.ServiceManager;
import android.os.RemoteException;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Client extends Activity implements OnClickListener {
	private final static String LOG_TAG = "shy.luo.ashmem.Client";
	
	IMemoryService memoryService = null;
	MemoryFile memoryFile = null;
	
	private EditText valueText = null;
	private Button readButton = null;
	private Button writeButton = null;
	private Button clearButton = null;
	
    	@Override
    	public void onCreate(Bundle savedInstanceState) {
        	super.onCreate(savedInstanceState);
        	setContentView(R.layout.main);

		IMemoryService ms = getMemoryService();
		if(ms == null) {        
        		startService(new Intent("shy.luo.ashmem.server"));
		} else {
			Log.i(LOG_TAG, "Memory Service has started.");
		}

        	valueText = (EditText)findViewById(R.id.edit_value);
        	readButton = (Button)findViewById(R.id.button_read);
        	writeButton = (Button)findViewById(R.id.button_write);
        	clearButton = (Button)findViewById(R.id.button_clear);

		readButton.setOnClickListener(this);
        	writeButton.setOnClickListener(this);
        	clearButton.setOnClickListener(this);
        
        	Log.i(LOG_TAG, "Client Activity Created.");
    	}

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

		Log.i(LOG_TAG, "Client Activity Resumed.");
    	}

    	@Override
    	public void onPause() {
		super.onPause();

		Log.i(LOG_TAG, "Client Activity Paused.");
    	}
    
    	@Override
    	public void onClick(View v) {
    		if(v.equals(readButton)) {
    			int val = 0;
    		
    			MemoryFile mf = getMemoryFile();
    			if(mf != null) {
				try {
    					byte[] buffer = new byte[4];
    					mf.readBytes(buffer, 0, 0, 4);
    			
    					val = (buffer[0] << 24) | ((buffer[1] & 0xFF) << 16) | ((buffer[2] & 0xFF) << 8) | (buffer[3] & 0xFF);
				} catch(IOException ex) {
					Log.i(LOG_TAG, "Failed to read bytes from memory file.");
					ex.printStackTrace();
				}
    			}	
    		
    			String text = String.valueOf(val);
    			valueText.setText(text);
    		} else if(v.equals(writeButton)) {
    			String text = valueText.getText().toString();
    			int val = Integer.parseInt(text);
    		
    			IMemoryService ms = getMemoryService();
    			if(ms != null) {
				try {
    					ms.setValue(val);
				} catch(RemoteException ex) {
					Log.i(LOG_TAG, "Failed to set value to memory service.");
					ex.printStackTrace();
				}
    			}
    		} else if(v.equals(clearButton)) {
    			String text = "";
    			valueText.setText(text);
    		}
    	}
    
    	private IMemoryService getMemoryService() {
    		if(memoryService != null) {
    			return memoryService;
    		}
    	
    		memoryService = IMemoryService.Stub.asInterface(
                			ServiceManager.getService("AnonymousSharedMemory"));

		Log.i(LOG_TAG, memoryService != null ? "Succeed to get memeory service." : "Failed to get memory service.");
    	
    		return memoryService;
    	}
    
    	private MemoryFile getMemoryFile() {
    		if(memoryFile != null) {
    			return memoryFile;
    		}
    		
    		IMemoryService ms = getMemoryService();
    		if(ms != null) {
			try {
    				ParcelFileDescriptor pfd = ms.getFileDescriptor();
				if(pfd == null) {
					Log.i(LOG_TAG, "Failed to get memory file descriptor.");
					return null;
				}

				try {
					FileDescriptor fd = pfd.getFileDescriptor();
					if(fd == null) {
						Log.i(LOG_TAG, "Failed to get memeory file descriptor.");
						return null;                      
					}	

    					memoryFile = new MemoryFile(fd, 4, "r");
				} catch(IOException ex) {
					Log.i(LOG_TAG, "Failed to create memory file.");
					ex.printStackTrace();
				}
    			} catch(RemoteException ex) {
				Log.i(LOG_TAG, "Failed to get file descriptor from memory service.");
				ex.printStackTrace();
			}
		}
    	
    		return memoryFile;
    	}
}

        Client端的介面主要包含了三個按鈕Read、Write和Clear，以及一個用於顯示內容的文字框。

        這個Activity在onCreate時，會透過startService介面來啟動我們前面定義的Server程式。呼叫startService時，需要指定要啟動的服務的名稱，這裡就是"shy.luo.ashmem.server"了，後面我們會在程式的描述檔案AndroidManifest.xml看到前面的Server類是如何和名稱"shy.luo.ashmem.server"關聯起來的。關於呼叫startService函式來啟動自定義服務的過程，可以參考Android系統在新程式中啟動自定義服務過程（startService）的原理分析一文。