如何在Android Java中從接口類初始化媒體播放器？

Question

我知道我必須調用onpause()或ondestroy()方法來停止媒體播放器我必須初始化媒體播放器，但在我的情況下，我調用html並使用android和html之間的接口作爲在html上調用android方法。我嘗試從接口有實例爲：

WebAppInterface wb= new WebAppInterface(mContext); 

，並在接口I初始化的媒體播放器：

public MediaPlayer mp = MediaPlayer.create(mContext,R.raw.sound); 

，並在主要活動中我嘗試initalize媒體播放器在oncreate()方法：

mediaplayer=wb.mp; 

MainActivity：

public class Ramadan extends Activity { 

private MediaPlayer mediaplayer; 

Context mContext; 

WebAppInterface wb= new WebAppInterface(mContext); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity); 


    //Call HTML Files 
    WebView myWebView = (WebView) findViewById(R.id.web_engine); 

    WebSettings webSettings = myWebView.getSettings(); 
    webSettings.setJavaScriptEnabled(true); 
    myWebView.loadUrl("file:///android_asset/index.html"); 

    // Intiate interface 

    myWebView.addJavascriptInterface(new WebAppInterface(this), "Android"); 
    mediaplayer=wb.mp; 


} 
@Override 
protected void onPause() 
{ 
    super.onPause(); 
    if(mediaplayer.isPlaying()) 
    mediaplayer.pause(); //stop the sound 
} 

@Override 
protected void onResume() 
{ 
    super.onResume(); 
    if(WebAppInterface.checked) 
    { 
     mediaplayer.start(); 
    } 
} 
@Override 
protected void onDestroy() 
{ 
    super.onDestroy(); 
    if(WebAppInterface.checked) 
    { 
     mediaplayer.stop(); 
    } 
} 

接口：

Context mContext; 
public MediaPlayer mp = MediaPlayer.create(mContext,R.raw.sound); 
public static boolean checked = false; 
/** Instantiate the interface and set the context */ 
WebAppInterface(Context c) { 
    mContext = c; 
} 


/** Show a toast from the web page */ 
@JavascriptInterface 
public void playsound(String value ) { 
    if (value.equals("on")) { 
     checked = true; 

     mp.setLooping(true); 
     mp.start(); 
    } 
    else 
    { 
     checked = false; 
     mp.stop(); 
    } 
} 
} 

的logcat：

07-01 10:56:20.421: E/AndroidRuntime(636): java.lang.RuntimeException: 
Unable to instantiate activity ComponentInfo{com.ramadan/com.ramadan.Ramadan}:  java.lang.NullPointerException 

Answer 1

看來你的媒體播放器時的onResume被稱爲尚未初始化。更多關於MediaPlayer State Diagram

[編輯-1

並嘗試mp初始化構造即內：

public MediaPlayer mp = null; 
public static boolean checked = false; 
/** Instantiate the interface and set the context */ 
WebAppInterface(Context c) { 
    mContext = c; 
    //Initialize mp here. 
    mp = MediaPlayer.create(mContext,R.raw.sound); 
} 

EDIT-1]

[編輯-2

初始化WebAppInterface一次，通過該參考addJavascriptInterface()。即：

WebAppInterface wb=null; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity); 


    //Call HTML Files 
    WebView myWebView = (WebView) findViewById(R.id.web_engine); 

    WebSettings webSettings = myWebView.getSettings(); 
    webSettings.setJavaScriptEnabled(true); 
    myWebView.loadUrl("file:///android_asset/index.html"); 

    //Initialise WebAppInterface and pass this ref.. 
    wb=new WebAppInterface(this); 
    myWebView.addJavascriptInterface(wb, "Android"); 
    mediaplayer=wb.mp; 
} 

EDIT-2]