如何将参数从 Activity 传递给 Fragment

Question

我有一个通过采用者类实例化和填充 Fragment 的活动。我想将值/对象传递给这个 Fragment 采用者类，以便我可以指定布局等。

我曾尝试使用 setArgument 方法传递包，但我的代码爆炸了，不知道为什么。据我了解，我可以在采用者类的 onCreate 方法中收集包。我看到在注释代码后立即正确设置了参数，但遗憾的是在那之后它崩溃了，我不明白为什么。如果我删除捆绑代码，它不会崩溃。:(

下面是我的代码，有人知道我应该把捆绑代码放在哪里，注释掉了吗？

public class LoginScreen extends ActionBarActivity {
    private final String ARG_SELECTED_LAYOUT_ID = "selectedLayoutId";
private final int DEFAULT_LAYOUT = R.layout.layout_list;

private int mSelectedLayoutId;

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

    ActionBar actionBar = getSupportActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    actionBar.setDisplayShowTitleEnabled(false);
    actionBar.setDisplayShowHomeEnabled(false);

    mSelectedLayoutId = DEFAULT_LAYOUT;
    if (savedInstanceState != null) {
        mSelectedLayoutId = savedInstanceState.getInt(ARG_SELECTED_LAYOUT_ID);
    }

    addLayoutTab(
            actionBar, R.layout.layout_list, R.drawable.ic_list, "list");
    addLayoutTab(
            actionBar, R.layout.layout_grid, R.drawable.ic_grid, "grid");
    addLayoutTab(
            actionBar, R.layout.layout_staggered_grid, R.drawable.ic_staggered, "staggered");
    addLayoutTab(
            actionBar, R.layout.layout_spannable_grid, R.drawable.ic_spannable, "spannable");
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt(ARG_SELECTED_LAYOUT_ID, mSelectedLayoutId);
}

private void addLayoutTab(ActionBar actionBar, int layoutId, int iconId, String tag) {
    ActionBar.Tab tab = actionBar.newTab()
            .setText("")
            .setIcon(iconId)
            .setTabListener(new TabListener(layoutId, tag));
    actionBar.addTab(tab, layoutId == mSelectedLayoutId);
}

public class TabListener implements ActionBar.TabListener {
    private LayoutFragment mFragment;
    private final int mLayoutId;
    private final String mTag;


    public TabListener(int layoutId, String tag) {
        mLayoutId = layoutId;
        mTag = tag;
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
        mFragment = (LayoutFragment) getSupportFragmentManager().findFragmentByTag(mTag);



        if (mFragment == null) {
            mFragment = (LayoutFragment) LayoutFragment.newInstance(mLayoutId);
            ft.add(R.id.content, mFragment, mTag);

        } else {
            ft.attach(mFragment);
        }


        /*
            Bundle bundle = new Bundle();
            bundle.putInt("noTiles", 4);
            mFragment.setArguments(bundle);           
        */

        mSelectedLayoutId = mFragment.getLayoutId();
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
        if (mFragment != null) {
            ft.detach(mFragment);
        }
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
    }
}

}

逻辑猫

03-03 11:56:53.675  32507-32507/weebuns.predictionking E/AndroidRuntime? FATAL EXCEPTION: main
    Process: weebuns.predictionking, PID: 32507
    java.lang.RuntimeException: Unable to start activity ComponentInfo{weebuns.predictionking/weebuns.predictionking.LoginScreen}: java.lang.NullPointerException: Attempt to invoke virtual method 'void weebuns.predictionking.LayoutFragment.setArguments(android.os.Bundle)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void weebuns.predictionking.LayoutFragment.setArguments(android.os.Bundle)' on a null object reference

Answer 1

要将参数传递给您的片段，建议创建一个公共静态方法来实例化您的片段，例如：

public static MyFragment newInstance(String firstArg) {
        MyFragment f = new MyFragment ();
        Bundle args = new Bundle();
        args.putString("ARG1", firstArg);
        f.setArguments(args);
        return f;
    }

然后在如下onCreateView()方法中检索您的参数：

String arg = getArguments().getString("ARG1");

使用静态方法在其他片段或活动中使用您的参数实例化新片段，例如：

MyFragment f = MyFragment.newInstance(myArg);

Answer 2

如果没有看到 logcat 就很难说，但你肯定错过了commit()调用FragmentTransaction. 还要记住在调用之前将参数设置为片段ft.commit()。

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
    mFragment = (LayoutFragment) getSupportFragmentManager().findFragmentByTag(mTag);

    if (mFragment == null) {
        Bundle bundle = new Bundle();
        bundle.putInt("noTiles", 4);
        mFragment = (LayoutFragment) LayoutFragment.newInstance(mLayoutId);
        mFragment.setArguments(bundle);
        //ft.add(R.id.content, mFragment, mTag).commit();
        ft.add(R.id.content, mFragment, mTag);

    } else {
        ft.attach(mFragment);
    }

    mSelectedLayoutId = mFragment.getLayoutId();
}