android - View.SurfaceView, why its member, mSurfaceHolder, returns null from getSurface()? -

i studying android game development these days. come across problem surfaceview\surfaceholder. when read source code of view/surfaceview.java in android sdk 22, confused. following code:

 public class surfaceview extends mockview {     ...         public surfaceholder getholder() {         return msurfaceholder;     }      private surfaceholder msurfaceholder = new surfaceholder() {         ...          @override         public surface getsurface() { return null; }         @override         public canvas lockcanvas() { return null; }         ...      } } 

i know, msurfaceholder.getsurface()\lockcanvas important, returns null! so, think msurfaceholder may dealt other steps. have learned example surfaceview, didn't figout out special steps deal msurfaceholder, example's code following:

  public class surfaceviewtest extends activity {      string tag = "surfaceviewtest";     fastrenderview surfaceview = null;      @override     protected void oncreate(bundle savedinstancestate) {         super.oncreate(savedinstancestate);         //setcontentview(r.layout.activity_surface_view_test);         surfaceview = new fastrenderview(this);         setcontentview(surfaceview);     }      @override     protected void onresume() {         super.onresume();         surfaceview.doresume();     }      @override     public boolean oncreateoptionsmenu(menu menu) {         // inflate menu; adds items action bar if           // present.         getmenuinflater().inflate(r.menu.menu_surface_view_test, menu);         return true;     }      @override     public boolean onoptionsitemselected(menuitem item) {         // handle action bar item clicks here. action bar         // automatically handle clicks on home/up button, long         // specify parent activity in androidmanifest.xml.         int id = item.getitemid();          //noinspection simplifiableifstatement         if (id == r.id.action_settings) {             return true;         }          return super.onoptionsitemselected(item);     }      /**      * customized surfaceview class      */     public class fastrenderview extends surfaceview implements runnable {          boolean running = false;         surfaceholder holder = null;         thread thread = null;          public fastrenderview(context context) {             super(context);             // holder             // getholder() returns surfaceholder implementation,             // isn't null, contains nothing.             holder = getholder();             if (holder == null)                 log.e(tag, "failed valid holder");          }          public void doresume() {             thread = new thread(this);             thread.start();             running = true;         }          public random rand = new random();         @override         public void run() {             while (running) {                 // android sdk 22, surfaceview.java, can see,                 // holder's getsurface() returns null.                 // why? returns null, here exception                  // should thrown out!                 if (!holder.getsurface().isvalid()) {                     continue;                 }                  canvas canvas = holder.lockcanvas();                 if (canvas == null) {                     log.e(tag, "get invalid canvas draw");                     continue;                 }                 canvas.drawrgb(                     rand.nextint(255),                      rand.nextint(255),                      rand.nextint(255));                 holder.unlockcanvasandpost(canvas);                  // sleep                 try {                     thread.sleep(1000);                 }                 catch(exception e) {                     e.printstacktrace();                 }             }             log.i(tag, "running ...");         }          public void dopause() {              running = false;             while (true) {                 try {                     thread.join();                 } catch (exception e) {                     e.printstacktrace();                 }             }         }     } }  

thanks in advance!

i'm afraid refer wrong source code. code of surfaceview post layoutlib not real framework source code, source code of surfaceview here