Notify Observer when item is added to List of LiveData

Question

I need to get an Observer event when the item is added to the List of LiveData. But as far as I understand the event receives only when I replace the old list with a new one

Answer 1

How about this?

public class ListLiveData<T> extends LiveData<List<T>> {
    public void addAll(List<T> items) {
        if (getValue() != null && items != null) {
            getValue().addAll(items);
            setValue(getValue());
        }
    }

    public void clear() {
        if (getValue() != null) {
            getValue().clear();
            setValue(getValue());
        }
    }

    @Override public void setValue(List<T> value) {
        super.setValue(value);
    }

    @Nullable @Override public List<T> getValue() {
        return super.getValue();
    }
}

// add changed listener
    mMessageList.observe(mActivity, new Observer() {
        @Override public void onChanged(@Nullable Object o) {
            notifyDataSetChanged();
        }
    });

Answer 2

I think this class will help you:

class ArrayListLiveData<T> : MutableLiveData<ArrayList<T>>()
{
    private val mArrayList = ArrayList<T>()

    init
    {
        set(mArrayList)
    }

    fun add(value: T)
    {
        mArrayList.add(value)
        notifyChanged()
    }

    fun add(index: Int, value: T)
    {
        mArrayList.add(index, value)
        notifyChanged()
    }

    fun addAll(value: ArrayList<T>)
    {
        mArrayList.addAll(value)
        notifyChanged()
    }

    fun setItemAt(index: Int, value: T)
    {
        mArrayList[index] = value
        notifyChanged()
    }

    fun getItemAt(index: Int): T
    {
        return mArrayList[index]
    }

    fun indexOf(value: T): Int
    {
        return mArrayList.indexOf(value)
    }

    fun remove(value: T)
    {
        mArrayList.remove(value)
        notifyChanged()
    }

    fun removeAt(index: Int)
    {
        mArrayList.removeAt(index)
        notifyChanged()
    }

    fun clear()
    {
        mArrayList.clear()
        notifyChanged()
    }

    fun size(): Int
    {
        return mArrayList.size
    }
}

and this extensions:

fun <T> MutableLiveData<T>.set(value: T)
    {
        if(AppUtils.isOnMainThread())
        {
            setValue(value)
        }
        else
        {
            postValue(value)
        }
    }

    fun <T> MutableLiveData<T>.get() : T
    {
        return value!!
    }

    fun <T> MutableLiveData<T>.notifyChanged()
    {
        set(get())
    }