How to unit test this line of LayoutInflater.from() in android

Question

Hi Am building a simple RecycleView Adapter and am trying to test all the methods of the adapter but the onCreateViewHolder is been dificult for me.

 @Override
public NewsViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.fragment_news,parent,false);
return new NewsViewHolder(v);
}

I Try using mockito to build a mock of the viewGroup class and return a spy of the MockContext when getContext() is call but it seems to be that because am returning a MockContext LayoutInflater.from() return a null pointer exception.

This is my test

 @Test
public void testOnCreateViewHolder() throws Exception {
    ViewGroup vg = mock(ViewGroup.class);
    MockContext mockContext = new MockContext();
    MockContext spyContext = spy(mockContext);
    when(vg.getContext()).thenReturn(spyContext);


    NewsViewHolder vh = adapter.onCreateViewHolder(vg, 0);
    Assert.assertNotNull("Response cant be null",vh);
}

Thanks in advance.

Answer 1

I came across this question when I was the same problem. I ended up solving it by myself.

Suppose that you have a simple onCreateViewHolder like this:

@Override
public TeamsViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    return new NewsViewHolder(LayoutInflater
            .from(parent.getContext())
            .inflate(R.layout.fragment_news_view_holder, parent, false)
    );
}

Use PowerMock to mock the static instance of LayoutInflater. In the snippet below, I have annotated using numbered comments all the steps you need to take in order to make this work:

import android.test.mock.MockContext;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import static junit.framework.Assert.assertEquals;
import static junit.framework.Assert.assertNotNull;
import static org.mockito.Matchers.anyInt;
import static org.mockito.Matchers.eq;
import static org.mockito.Mockito.when;
import static org.powermock.api.mockito.PowerMockito.mockStatic;

// 1. signal JUnit to run this test PowerMockRunner
@RunWith(PowerMockRunner.class)

// 2. signal PowerMock to prepare the static instance of LayoutInflater for testing
@PrepareForTest({LayoutInflater.class})
public class NewsRecyclerViewAdapterTest {

    // 3. LayoutInflater.from(context) returns an inflater, 
    // so we need to mock that one
    @Mock
    LayoutInflater mockInflater;

    @Mock
    View mockView;

    @Mock
    ViewGroup mockParent;

    private int dummyTestId;

    private MockContext mockContext;

    private NewsRecyclerViewAdapter adapter;

    @Before
    public void setUp() throws Exception {

        MockitoAnnotations.initMocks(this);

        dummyTestId = 0x10000;

        // 4. mock the static LayoutInflater in "LayoutInflater.from()" 
        // so that we can ask it to return a mockInflater that we moked above
        mockStatic(LayoutInflater.class);

        mockContext = new MockContext();

        adapter = new NewsRecyclerViewAdapter(Arrays.asList(new NewsItem(), new NewsItem(), new NewsItem()));
    }

    @Test
    public void onCreateViewHolderShouldReturnAValidViewHolder() throws Exception {

        // 5. mock the context that comes from parent ViewGroup
        when(mockParent.getContext()).thenReturn(mockContext);

        // 6. mock the inflater that is returned by LayoutInflater.from()
        when(LayoutInflater.from(mockContext)).thenReturn(mockInflater);

        // 7. pass anyInt() as a resource id to care of R.layout.fragment_news_view_holder in onCreateViewHolder()
        when(mockInflater.inflate(anyInt(), eq(mockParent), eq(false))).thenReturn(mockView);

        // call onCreateViewHolder() to act
        NewsViewHolder viewHolder = adapter.onCreateViewHolder(mockParent, dummyTestId);

        // OKAY straightfoward right?
        assertNotNull(viewHolder);

        // this is not very important but I recommend it,
        // it just returns the view sent to NewsViewHolder 
        // and verify it against the mockView that you inflated above
        assertEquals(viewHolder.getItemView(), mockView);
    }

}