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java – What is the recommended way to launch a DialogFragment from a ViewModel?

Posted by: admin June 15, 2020 Leave a comment

Questions:

I have a list objects in a Recyclerview. When long-pressing an item I want to show a dialog with data from the item clicked.

The Recyclerview is using data binding for each item and I am able to display data from the selected item using Log when long-pressing.

When trying to show a dialog, however, you need to get to the Activity, which is not recommended to use in the ViewModel object.

So how can I show the dialog?

Thanks, Ove

How to&Answers:

Conceptually a ViewModel strikes me as the wrong place to launch a Dialog from. To do it more cleanly I would pass the RecyclerView.ViewHolder into the layout, and have a method on the ViewHolder that triggers a custom listener on your RecyclerView.Adapter. Then whoever subscribes to that listener (Activity/Fragment) can launch the Dialog. May seem a little roundabout, but I don’t think a ViewModel of a list item should have knowledge or control of its environment.

Here is an example. This is a general pattern for handling RecyclerView item clicks with data binding and a ViewModel. This is not a complete example, just the code to highlight this specific pattern.

Layout:

<layout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    >
    <data>
    <variable
        name="viewHolder"
        type="com.example.ViewHolder"
        />
    <variable
        name="viewModel"
        type="com.example.ViewModel"
        />
    </data>

    <com.example.View
        android:layout_width="match_parent"
        android:layout_height="24dp"
        android:onClick="@{() -> viewHolder.onClick(viewModel)}"
        />
</layout>

Adapter:

class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
    public interface SelectionListener {
        void onSelectionChanged(int newPosition, ViewModel viewModel);
    }

    private @NonNull WeakReference<SelectionListener> selectionListener =
            new WeakReference<>(null);

    public void setSelectionListener(@Nullable SelectionListener listener) {
        selectionListener = new WeakReference<>(listener);
    }

    public class ViewHolder extends RecyclerView.ViewHolder<ViewBinding> {
        ViewHolder(ViewBinding binding) {
            super(binding.getRoot());

            binding.setViewHolder(this);
            binding.setViewModel(new ViewModel());
        }

        public void onClick(ViewModel viewModel) {
            SelectionListener listener = selectionListener.get();
            if (listener != null) {
                listener.onSelectionChanged(getAdapterPosition(), viewModel);
            }
        }
    }
}

Answer:

See the Variables section of the official documentation of the Data Binding Library. There you find a variable context you can use.

A special variable named context is generated for use in binding expressions as needed. The value for context is the Context from the root View’s getContext(). The context variable will be overridden by an explicit variable declaration with that name.

Basically you could just pass it to another variable like the viewModel to show the dialog from there.

android:onClick="@{v -> viewModel.showDialog(context)}"

Answer:

So you can use the context of item for example itemView.getContext() to show AlertDialog

Answer:

The hint from Bayoudh led me in the right direction, but I’m posting this to put the pieces together. Below is a cardview that is clickable. Since my ViewModel holds no reference to the activity we have to pass the view in question as a parameter.

<android.support.v7.widget.CardView
        android:id="@+id/cardviewContact"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="@dimen/text_margin_0.5x"
        android:layout_marginRight="@dimen/text_margin_0.5x"
        android:layout_marginTop="@dimen/text_margin_0.5x"
        android:background="?attr/selectableItemBackground"
        android:clickable="true"
        android:minHeight="50dp"
        card_view:cardCornerRadius="4dp"
        android:onClick="@{(view) -> viewModel.onClick(view)}" >

The android:onClick="@{(view) -> viewModel.onClick(view)}" statement takes the current view as a parameter so you can use it in the ViewModel to get context with view.getContext() as Bayoudh states.