Android fragments 为什么在Android测试中尝试从fragmentManager获取片段时会出现NullPointerException？

你好，我是Android测试的新手。在UI测试期间，我试图检查我的应用程序中是否有可见的按钮。我写了一些东西：

@RunWith(AndroidJUnit4.class)
public class MainActivityTest {
@Rule
public ActivityTestRule<MainActivity> mRule = new ActivityTestRule<>(MainActivity.class);

@Before
public void setUp() {
}

@Test
public void clickOnProductTest() {
    if (isRegisterClosed()) {
        openRegister();
    }
    onView(withText("Food")).perform(click());
    onView(withText("Mineral water")).perform(click());
}

private boolean isRegisterClosed() {
    MainActivity activity = mRule.getActivity();
    FragmentManager fragmentManager = activity.getFragmentManager();

    Fragment f = fragmentManager.findFragmentById(R.id.current_order_fragment);

    View v = f.getView();

    Button b = (Button) v.findViewById(R.id.orderOpenRegister);
    return b.getVisibility() == View.VISIBLE;
}

private void openRegister() {
    onView(withId(R.id.orderOpenRegister)).perform(click());
}

此按钮位于当前\u顺序\u片段中，其ID为orderOpenRegister

编辑 我发现，我应该添加以下行：

fragmentManager.executePendingTransactions（）

所以我的方法看起来像：

private boolean isRegisterClosed() {
        FragmentManager fragmentManager = activity.getFragmentManager();
        fragmentManager.executePendingTransactions();
        Fragment f  = fragmentManager
                      .findFragmentById(R.id.current_order_fragment);
        View v = f.getView();
        Button b = (Button) v.findViewById(R.id.orderOpenRegister);
        return b.getVisibility() == View.VISIBLE;
}

---

但如果我这样做，我需要在UI线程中运行这个测试。有人知道如何在UI线程上运行测试吗？

您可以在UI线程中运行测试，如下所示

Instrumentation instrumentation = InstrumentationRegistry.getInstrumentation();

instrumentation.runOnMainSync(new Runnable() {
  @Override
  public void run() { //your test
  }
});

Instrumentation instrumentation = InstrumentationRegistry.getInstrumentation();

instrumentation.runOnMainSync(new Runnable() {
  @Override
  public void run() { //your test
  }
});