获取活动实例

Question

请原谅我的简单问题,我是初学java和android开发者.当调用surfaceChanged时,如何在setCameraDisplayOrientation中获取Activity的实例？

public class MyActivity extends Activity
{
    private Camera mCamera;
    private CameraPreview mPreview;
    public final int cameraId = 0;
    public Activity activity = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

    activity = this; 

        // Create an instance of Camera
        mCamera = getCameraInstance();

        // Create our Preview view and set it as the content of our activity.
        mPreview = new CameraPreview(this, mCamera);
        FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
        preview.addView(mPreview);
    }

    public void setCameraDisplayOrientation(Activity activity,
                        int cameraId, android.hardware.Camera camera) {

    }

    public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
    private SurfaceHolder mHolder;
    private Camera mCamera;
    ...
    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        ...
        setCameraDisplayOrientation(activity, cameraId, mCamera);
        ....
    }
    }
}

Answer 1

这是一种使用静态变量来避免内存泄漏的方法:对将在onCreate(Bundle)方法中设置的Activity实例进行静态弱引用.

1. 在您的二级课程中写下如下内容:

   public Class SecondClass {
       private static WeakReference<Activity> mActivityRef;
       public static void updateActivity(Activity activity) {
           mActivityRef = new WeakReference<Activity>(activity);
       }
   

2. 然后在Activity类的onCreate(Bundle)方法中:

   @Override
   onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       SecondClass.updateActivity(this);
       ...
   }
   

3. 以这种方式使用活动实例:

   mActivityRef.get()
   

Answer 2

Activity a = (Activity) getContext();

只要您将当前活动作为构造函数中的上下文传递,就像您已经在做的那样.

Answer 3

我只是在我的主要活动中设置一个变量，就像这样......public static Activity activity = this;然后我可以从任何地方使用：MainActivity.activity。

您也可以在 onCreate() 方法中设置它，只需像这样public static Activity activity;在主活动类的顶部设置变量，然后在 onCreate() 方法中添加activity = this;任何地方即可。

例如，这适用于扩展 Activity 的任何类，public class MainActivity extends Activity但是您可以从任何类调用变量，即使它们不扩展 Activity。

希望这可以帮助。