[android] Getting activity from context in android

This one has me stumped.

I need to call an activity method from within a custom layout class. The problem with this is that I don't know how to access the activity from within the layout.

ProfileView

public class ProfileView extends LinearLayout
{
    TextView profileTitleTextView;
    ImageView profileScreenImageButton;
    boolean isEmpty;
    ProfileData data;
    String name;

    public ProfileView(Context context, AttributeSet attrs, String name, final ProfileData profileData)
    {
        super(context, attrs);
        ......
        ......
    }

    //Heres where things get complicated
    public void onClick(View v)
    {
        //Need to get the parent activity and call its method.
        ProfileActivity x = (ProfileActivity) context;
        x.activityMethod();
    }
}

ProfileActivity

public class ProfileActivityActivity extends Activity
{
    //In here I am creating multiple ProfileViews and adding them to the activity dynamically.

    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.profile_activity_main);
    }

    public void addProfilesToThisView()
    {
        ProfileData tempPd = new tempPd(.....)
        Context actvitiyContext = this.getApplicationContext();
        //Profile view needs context, null, name and a profileData
        ProfileView pv = new ProfileView(actvitiyContext, null, temp, tempPd);
        profileLayout.addView(pv);
    }
}

As you can see above, I am instantiating the profileView programatically and passing in the activityContext with it. 2 questions:

1. Am i passing the correct context into the Profileview?
2. How do I get the containing activity from the context?

Context may be an Application, a Service, an Activity, and more.

Normally the context of Views in an Activity is the Activity itself so you may think you can just cast this Context to Activity but actually you can't always do it, because the context can also be a ContextThemeWrapper in this case.

ContextThemeWrapper is used heavily in the recent versions of AppCompat and Android (thanks to the android:theme attribute in layouts) so I would personally never perform this cast.

So short answer is: you can't reliably retrieve an Activity from a Context in a View. Pass the Activity to the view by calling a method on it which takes the Activity as parameter.

This method should be helpful..!

public Activity getActivityByContext(Context context){

if(context == null){
    return null;
    }

else if((context instanceof ContextWrapper) && (context instanceof Activity)){
        return (Activity) context;
    }

else if(context instanceof ContextWrapper){
        return getActivity(((ContextWrapper) context).getBaseContext());
    }

return null;

    }

I hope this helps.. Merry coding!

This is something that I have used successfully to convert Context to Activity when operating within the UI in fragments or custom views. It will unpack ContextWrapper recursively or return null if it fails.

public Activity getActivity(Context context)
{
    if (context == null)
    {
        return null;
    }
    else if (context instanceof ContextWrapper)
    {
        if (context instanceof Activity)
        {
            return (Activity) context;
        }
        else
        {
            return getActivity(((ContextWrapper) context).getBaseContext());
        }
    }

    return null;
}

And in Kotlin:

tailrec fun Context.activity(): Activity? = when {
  this is Activity -> this
  else -> (this as? ContextWrapper)?.baseContext?.activity()
}

I used convert Activity

Activity activity = (Activity) context;

If you like to call an activity method from within a custom layout class(non-Activity Class).You should create a delegate using interface.

It is untested and i coded it right . but i am conveying a way to achieve what you want.

First of all create and Interface

interface TaskCompleteListener<T> {
   public void onProfileClicked(T result);
}



public class ProfileView extends LinearLayout
{
    private TaskCompleteListener<String> callback;
    TextView profileTitleTextView;
    ImageView profileScreenImageButton;
    boolean isEmpty;
    ProfileData data;
    String name;

    public ProfileView(Context context, AttributeSet attrs, String name, final ProfileData profileData)
    {
        super(context, attrs);
        ......
        ......
    }
    public setCallBack( TaskCompleteListener<String> cb) 
    {
      this.callback = cb;
    }
    //Heres where things get complicated
    public void onClick(View v)
    {
        callback.onProfileClicked("Pass your result or any type");
    }
}

And implement this to any Activity.

and call it like

ProfileView pv = new ProfileView(actvitiyContext, null, temp, tempPd);
pv.setCallBack(new TaskCompleteListener
               {
                   public void onProfileClicked(String resultStringFromProfileView){}
               });

how about some live data callback,

class ProfileView{
    private val _profileViewClicked = MutableLiveData<ProfileView>()
    val profileViewClicked: LiveData<ProfileView> = _profileViewClicked
}

class ProfileActivity{

  override fun onCreateView(...){

    profileViewClicked.observe(viewLifecycleOwner, Observer { 
       activityMethod()
    })
  }

}

1. No
2. You can't

There are two different contexts in Android. One for your application (Let's call it the BIG one) and one for each view (let's call it the activity context).

A linearLayout is a view, so you have to call the activity context. To call it from an activity, simply call "this". So easy isn't it?

When you use

this.getApplicationContext();

You call the BIG context, the one that describes your application and cannot manage your view.

A big problem with Android is that a context cannot call your activity. That's a big deal to avoid this when someone begins with the Android development. You have to find a better way to code your class (or replace "Context context" by "Activity activity" and cast it to "Context" when needed).

Regards.

Just to update my answer. The easiest way to get your Activity context is to define a static instance in your Activity. For example

public class DummyActivity extends Activity
{
    public static DummyActivity instance = null;

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        // Do some operations here
    }

    @Override
    public void onResume()
    {
        super.onResume();
        instance = this;
    }

    @Override
    public void onPause()
    {
        super.onPause();
        instance = null;
    }
}

And then, in your Task, Dialog, View, you could use that kind of code to get your Activity context:

if (DummyActivity.instance != null)
{
    // Do your operations with DummyActivity.instance
}

Never ever use getApplicationContext() with views.

It should always be activity's context, as the view is attached to activity. Also, you may have a custom theme set, and when using application's context, all theming will be lost. Read more about different versions of contexts here.

an Activity is a specialization of Context so, if you have a Context you already know which activity you intend to use and can simply cast a into c; where a is an Activity and c is a Context.

Activity a = (Activity) c;