在Swift中，如何从第三个ViewController访问第二个ViewController中的数据？

Question

我正在尝试运行下面的代码，以便我的ThirdViewController可以镜像它的＆＃34; myOrder＆＃34;变量基于SecondViewController中的变量，但我得到一个运行时错误＆＃34; Swift动态强制转换失败＆＃34;当调试器到达我用注释标记的行时＃34;错误在这里＆＃34;下面。我相信我正在使用Xcode 6.1.1和Swift 1.1。为什么SecondViewController不被接受为有效类型？

  import android.Manifest;
  import android.app.Activity;
  import android.app.AlertDialog;
  import android.content.Context;
  import android.content.DialogInterface;
  import android.content.Intent;
  import android.content.SharedPreferences;
  import android.content.pm.PackageManager;
  import android.net.Uri;
  import android.os.Bundle;
  import android.provider.Settings;
  import android.support.v4.app.ActivityCompat;
  import android.support.v4.content.ContextCompat;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;

  public class MainActivity extends AppCompatActivity {

public static final int MY_PERMISSIONS_REQUEST_CAMERA = 100;
public static final String ALLOW_KEY = "ALLOWED";
public static final String CAMERA_PREF = "camera_pref";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    if (ContextCompat.checkSelfPermission(this,
            Manifest.permission.CAMERA)
            != PackageManager.PERMISSION_GRANTED) {

        if (getFromPref(this, ALLOW_KEY)) {

            showSettingsAlert();

        } else if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.CAMERA)
                != PackageManager.PERMISSION_GRANTED) {
            // Should we show an explanation?
            if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                    Manifest.permission.CAMERA)) {
                showAlert();
            } else {
                // No explanation needed, we can request the permission.
                ActivityCompat.requestPermissions(this,
                        new String[]{Manifest.permission.CAMERA},
                        MY_PERMISSIONS_REQUEST_CAMERA);
            }
        }
    } else {
        openCamera();
    }

}

public static void saveToPreferences(Context context, String key, 
                                                 Boolean allowed) {
    SharedPreferences myPrefs = context.getSharedPreferences
                                (CAMERA_PREF, Context.MODE_PRIVATE);
    SharedPreferences.Editor prefsEditor = myPrefs.edit();
    prefsEditor.putBoolean(key, allowed);
    prefsEditor.commit();
}

public static Boolean getFromPref(Context context, String key) {
    SharedPreferences myPrefs = context.getSharedPreferences
                                (CAMERA_PREF, Context.MODE_PRIVATE);
    return (myPrefs.getBoolean(key, false));
}

private void showAlert() {
    AlertDialog alertDialog = new AlertDialog.Builder(this).create();
    alertDialog.setTitle("Alert");
    alertDialog.setMessage("App needs to access the Camera.");
    alertDialog.setButton(AlertDialog.BUTTON_NEGATIVE, "DONT ALLOW",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                    finish();
                }
            });
    alertDialog.setButton(AlertDialog.BUTTON_POSITIVE, "ALLOW",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                    ActivityCompat.requestPermissions(MainActivity.this,
                            new String[]{Manifest.permission.CAMERA},
                            MY_PERMISSIONS_REQUEST_CAMERA);

                }
            });
    alertDialog.show();
}

private void showSettingsAlert() {
    AlertDialog alertDialog = new AlertDialog.Builder(this).create();
    alertDialog.setTitle("Alert");
    alertDialog.setMessage("App needs to access the Camera.");
    alertDialog.setButton(AlertDialog.BUTTON_NEGATIVE, "DONT ALLOW",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                    //finish();
                }
            });
    alertDialog.setButton(AlertDialog.BUTTON_POSITIVE, "SETTINGS",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                    startInstalledAppDetailsActivity(MainActivity.this);

                }
            });
    alertDialog.show();
}


@Override
public void onRequestPermissionsResult
              (int requestCode, String permissions[], int[] grantResults) {
    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_CAMERA: {
            for (int i = 0, len = permissions.length; i < len; i++) {
                String permission = permissions[i];
                if (grantResults[i] == PackageManager.PERMISSION_DENIED) {
                    boolean showRationale = 
                       ActivityCompat.shouldShowRequestPermissionRationale
                                                        (this, permission);
                    if (showRationale) {
                        showAlert();
                    } else if (!showRationale) {
                        // user denied flagging NEVER ASK AGAIN
                        // you can either enable some fall back,
                        // disable features of your app
                        // or open another dialog explaining
                        // again the permission and directing to
                        // the app setting
                        saveToPreferences(MainActivity.this, ALLOW_KEY, true);
                    } 
                }
            }
        }

        // other 'case' lines to check for other
        // permissions this app might request

    }
}

public static void startInstalledAppDetailsActivity(final Activity context) {
    if (context == null) {
        return;
    }
    final Intent i = new Intent();
    i.setAction(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
    i.addCategory(Intent.CATEGORY_DEFAULT);
    i.setData(Uri.parse("package:" + context.getPackageName()));
    i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    i.addFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);
    i.addFlags(Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
    context.startActivity(i);
}

private void openCamera() {
    Intent intent = new Intent("android.media.action.IMAGE_CAPTURE");
    startActivity(intent);
}

}