如何在两个片段之间切换，而不必每次重新创建片段？

Question

我正在开发一个android应用程序，它使用导航抽屉在两个片段之间切换。然而，每次我切换时，片段都会被完全重新创建。

这是我主要活动的代码。

/* The click listener for ListView in the navigation drawer */
private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        selectItem(position);
    }
}

private void selectItem(int position) {
    android.support.v4.app.Fragment fragment;
    String tag;
    android.support.v4.app.FragmentManager; fragmentManager = getSupportFragmentManager();

    switch(position) {
        case 0:
            if(fragmentManager.findFragmentByTag("one") != null) {
                fragment = fragmentManager.findFragmentByTag("one");
            } else {
                fragment = new OneFragment();
            }
            tag = "one";
            break;
        case 1:
            if(fragmentManager.findFragmentByTag("two") != null) {
                fragment = fragmentManager.findFragmentByTag("two");
            } else {
                fragment = new TwoFragment();
            }
            tag = "two";
            break;
    }

    fragment.setRetainInstance(true);
    fragmentManager.beginTransaction().replace(R.id.container, fragment, tag).commit();

    // update selected item and title, then close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mNavTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

我设置了一些调试日志记录，每次调用selectItem时，都会销毁一个片段，而创建另一个片段。

有什么方法可以防止碎片被重新创建，而只是重复使用它们吗？

Answer 1

替换方法会销毁您的片段。一种解决方法是将它们设置为可见性。GONE，另一种（不太容易的）方法是将它们保存在变量中。如果您这样做，请确保您不会左右泄漏内存。

Answer 2

使用带有标签的附加/分离方法：

分离将破坏视图hirachy，但保持状态，就像在后台一样；这将使“不可见”片段具有更小的内存占用。但请注意，您需要正确实现片段生命周期（您首先应该这样做）

从UI分离给定片段。这与将其放在后堆栈上时的状态相同：片段从UI中删除，但其状态仍由片段管理器主动管理。进入此状态时，其视图层次结构将被破坏。

第一次添加片段时

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.add(android.R.id.content, new MyFragment(),MyFragment.class.getSimpleName());
t.commit();

然后拆下它

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.detach(MyFragment.class.getSimpleName());
t.commit();

并再次连接，如果切换回，状态将保持不变

FragmentTransaction t = getSupportFragmentManager().beginTransaction();
t.attach(getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()));
t.commit();

但是你总是要检查片段是否已经添加，如果没有，则添加它，否则只需附加它：

if (getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()) == null) {
    FragmentTransaction t = getSupportFragmentManager().beginTransaction();
    t.add(android.R.id.content, new MyFragment(), MyFragment.class.getSimpleName());
    t.commit();
} else {
    FragmentTransaction t = getSupportFragmentManager().beginTransaction();
    t.attach(getSupportFragmentManager().findFragmentByTag(MyFragment.class.getSimpleName()));
    t.commit();
}

Answer 3

在@meredrica指出replace（）会破坏片段之后，我又回顾了FragmentManager文档。这就是我想出的解决方案，似乎奏效了。

/* The click listener for ListView in the navigation drawer */
private class DrawerItemClickListener implements ListView.OnItemClickListener {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        selectItem(position);
    }
}

private void selectItem(int position) {
    android.support.v4.app.FragmentManager; fragmentManager = getSupportFragmentManager();

    switch(position) {
        case 0:
            if(fragmentManager.findFragmentByTag("one") != null) {
                //if the fragment exists, show it.
                fragmentManager.beginTransaction().show(fragmentManager.findFragmentByTag("one")).commit();
            } else {
                //if the fragment does not exist, add it to fragment manager.
                fragmentManager.beginTransaction().add(R.id.container, new OneFragment(), "one").commit();
            }
            if(fragmentManager.findFragmentByTag("two") != null){
                //if the other fragment is visible, hide it.
                fragmentManager.beginTransaction().hide(fragmentManager.findFragmentByTag("two")).commit();
            }
            break;
        case 1:
            if(fragmentManager.findFragmentByTag("two") != null) {
                //if the fragment exists, show it.
                fragmentManager.beginTransaction().show(fragmentManager.findFragmentByTag("two")).commit();
            } else {
                //if the fragment does not exist, add it to fragment manager.
                fragmentManager.beginTransaction().add(R.id.container, new TwoFragment(), "two").commit();
            }
            if(fragmentManager.findFragmentByTag("one") != null){
                //if the other fragment is visible, hide it.
                fragmentManager.beginTransaction().hide(fragmentManager.findFragmentByTag("one")).commit();
            }
            break;
    }

    // update selected item and title, then close the drawer
    mDrawerList.setItemChecked(position, true);
    setTitle(mNavTitles[position]);
    mDrawerLayout.closeDrawer(mDrawerList);
}

我也添加了这一点，但我不确定这是否必要。

@Override
public void onDestroy() {
    super.onDestroy();
    FragmentManager fragmentManager = getSupportFragmentManager();
    if(fragmentManager.findFragmentByTag("one") != null){
        fragmentManager.beginTransaction().remove(fragmentManager.findFragmentByTag("one")).commit();
    }
    if(fragmentManager.findFragmentByTag("two") != null){
        fragmentManager.beginTransaction().remove(fragmentManager.findFragmentByTag("two")).commit();
    }
}