android点击返回时触发的事件

 private void dialog(){    AlertDialog.Builder builder = new Builder(PPTSoftwareActivity.this);builder.setMessage("确认要退出么？");builder.setTitle("提示");builder.setPositiveButton("确定", new DialogInterface.OnClickListener() {public void onClick(DialogInterface dialog, int which) {// TODO Auto-generated method stubdialog.dismiss();finish();}});builder.setNegativeButton("取消", new DialogInterface.OnClickListener() {public void onClick(DialogInterface dialog, int which) {// TODO Auto-generated method stubdialog.dismiss();}});builder.show();    }

@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {// TODO Auto-generated method stubif(keyCode == KeyEvent.KEYCODE_BACK){dialog();}return false;}