How to correctly invoke activity methods from the adapter?

A somewhat unusual implementation is proposed in another answer .. I will write a more classic solution.

For a start, what’s wrong with calling the activation methods from the adapter and why it’s bad. The fact is that the activator is a kontroller of what is displayed on the screen and it should act as the initiator of the actions of the widgets located on it (the adapter is part of the widget for displaying the list). If you turn to an abstract example, then when you call the activation methods from the adapter, you get something like the fact that the mailbox "sucks" the apartment with you, so that you can check your mail, instead of you going there to look. This solution does not look very much.
The general sense is that the control should initiate actions, and not events affect the control (the remote control programs in the TV, not the program to switch channels)

The methods of interaction with the adapter depend on the type of interaction - whether an event occurs in the widget (for example, click on an element or in an abstract example - whether the mail arrived in the mailbox), which needs to be activated or activated, some information / action is required from the widget, for example, the data displayed in a widget or an update in the list.

In the first case, a callback mechanism is used (callback interface). In the abstract example, we will install a sensor in the box that coordinates the presence of mail.
This mechanism is designed so that by calling the interface method in one place of the code, it is automatically called in another place that implements this callback interface. In this case, the arguments received in the method of communication initiator are transferred to the receiver method and you can transfer data associated with the event (for example, record ID).

Typical callback.

In the adapter:

 class SomeAdapter extends RecyclerView.Adapter <Data, SomeAdapter.ItemHolder> { // создаем сам интерфейс и указываем метод и передаваемые им аргументы // View на котором произошло событие и позиция этого View interface OnCardClickListener { void onCardClick(View view, int position); } // создаем поле объекта-колбэка private static OnCardClickListener mListener; public SomeAdapter (Data data) { // конструктор адаптера } @Override public ItemHolder onCreateViewHolder(ViewGroup parent, int viewType) { .... return new ItemHolder(v); } @Override public void onBindViewHolder( ItemHolder holder, int position) { ..... } // метод-сеттер для привязки колбэка к получателю событий public void setOnCardClickListener(OnCardClickListener listener) { mListener = listener; } public static class ItemHolder extends RecyclerView.ViewHolder implements View.OnClickListener{ CardView mCard; public ItemHolder(View v) { super(v); mCard = (CardView) v.findViewById(R.id.bell_card); // само событие, которое нужно передать (клик на элементе) mCard.setOnClickListener(this); } // обработчик события @Override public void onClick(View v) { // вызов метода-колбэка и назначение передаваемых значений аргументам int position = getAdapterPosition(); mListener.onCardClick(v, position); } } } 

In activit:

 // имплементируем наш интерфейс обратного вызова в активити public class SomeActiviy extends Activity implements SomeAdapter.OnCardClickListener { Data data; ... @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mAdapter = new SomeAdapter(data); // подписываем нашу активити на события колбэка mAdapter.setOnCardClickListener(this); mList.setAdapter(mAdapter); } // метод, который получит события из нашего колбэка @Override public void onCardClick(View view,final int pos) { // здесь обрабатываем событие, значения View и pos // равны значениям, присвоенным в адаптере } } 

The second option is much simpler, we just create methods in the adapter that will return the data we need or will cause certain actions in the adapter when they are required to be activated. That is, it’s not the adapter that calls activation methods, but it activates the adapter methods when it needs information / actions from it:

in adapter:

 class SomeAdapter extends RecyclerView.Adapter <Data, SomeAdapter.ItemHolder> { // метод обновления списка по новому набору данных public void notifyData(Data result) { mData = result; notifyDataSetChanged(); } // метод получения размера списка public int getSize() { return mData.size(); } } 

In activit:

 public class SomeActiviy extends Activity { Data data; ... @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); mAdapter = new SomeAdapter(data); mList.setAdapter(mAdapter); // активити требуется размер списка int size = mAdapter.getSize(); // активити требуется обновить список с вновь полученными данными Data data = getData(); mAdapter.notifyData(data); } }