ProgressDialog不显示

一般用ProgressDialog来提示用户等待耗时操作，如图中代码块

IsVideoQualityWithFace();

如果不在单独线程中执行的话，ProgressDialog将不会显示

	AlertDialog.Builder builder = new AlertDialog.Builder(context);
		builder.setTitle("提示")
				.setMessage( msg )
				.setNegativeButton("取消", new DialogInterface.OnClickListener() {
					@Override
					public void onClick(DialogInterface dialog, int which) {
						dialog.dismiss();
					}
				})
				.setPositiveButton("确定", new DialogInterface.OnClickListener() {
					@Override
					public void onClick(DialogInterface dialog, int which) {
						if(mCamera != null){
							mCamera.stopRecordVideo();
						}
						dialog.dismiss();
						mDlgProgressFaceCheck = new ProgressDialog( CameraActivity.this);
						if(mDlgProgressFaceCheck!=null){
							mDlgProgressFaceCheck.setTitle("温馨提示");
							mDlgProgressFaceCheck.setMessage("正在检测视频质量...");
							mDlgProgressFaceCheck.setCancelable(false);
							mDlgProgressFaceCheck.setCanceledOnTouchOutside(false);
							mDlgProgressFaceCheck.show();
						}
						new Thread(new Runnable(){  
		                    public void run() {  
								 IsVideoQualityWithFace(); 
		                    }                     
		                }).start();  
 
					}
				});		
		AlertDialog ad = builder.create();
		ad.show();