java - Android项目->MainAcitvity.java - R是一个 undefined variable

Question

这是我创建 Android 项目时生成的 MainActivity.java 类。有一个变量 R 没有在任何地方定义。我根本没有修改这个类。这个 R 变量应该分配给什么？

/**
 * Used to store the last screen title. For use in
 * {@link #restoreActionBar()}.
 */
private CharSequence mTitle;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mNavigationDrawerFragment = (NavigationDrawerFragment) getSupportFragmentManager()
            .findFragmentById(R.id.navigation_drawer);
    mTitle = getTitle();

    // Set up the drawer.
    mNavigationDrawerFragment.setUp(R.id.navigation_drawer,
            (DrawerLayout) findViewById(R.id.drawer_layout));
}

@Override
public void onNavigationDrawerItemSelected(int position) {
    // update the main content by replacing fragments
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager
            .beginTransaction()
            .replace(R.id.container,
                    PlaceholderFragment.newInstance(position + 1)).commit();
}

public void onSectionAttached(int number) {
    switch (number) {
    case 1:
        mTitle = getString(R.string.title_section1);
        break;
    case 2:
        mTitle = getString(R.string.title_section2);
        break;
    case 3:
        mTitle = getString(R.string.title_section3);
        break;
    }
}

public void restoreActionBar() {
    ActionBar actionBar = getSupportActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
    actionBar.setDisplayShowTitleEnabled(true);
    actionBar.setTitle(mTitle);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    if (!mNavigationDrawerFragment.isDrawerOpen()) {
        // Only show items in the action bar relevant to this screen
        // if the drawer is not showing. Otherwise, let the drawer
        // decide what to show in the action bar.
        getMenuInflater().inflate(R.menu.main, menu);
        restoreActionBar();
        return true;
    }
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    /**
     * Returns a new instance of this fragment for the given section number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        return rootView;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        ((MainActivity) activity).onSectionAttached(getArguments().getInt(
                ARG_SECTION_NUMBER));
        }
    }
}

Answer 1

此R 是 IDE 自动生成的类的名称。当您打算使用它时，您应该在 java 文件的导入部分中导入该类。

import your.app.pack.R;

其中 your.app.pack 是您真实包名称的占位符(通常由点分隔的三部分组成)

当您在 res 文件夹中声明资源实体(例如 id、字符串、drawable 等)时，IDE 会自动为其分配一个整数标识符。 java代码中的那些标识符将寻址到这些资源。

注意:如果您在导入后仍然遇到与此 R 相关的错误，请尝试清理您的项目并重新构建(或重建它)