如何在两个片段之间切换，而无需每次都重新创建片段? [英] How can I switch between two fragments, without recreating the fragments each time?

问题描述

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.但是，每次切换时，片段都会完全重新创建.

I'm working on an android application, that uses a navigation drawer to switch between two fragments. However, each time I switch, the fragment is completely recreated.

这是我主要活动的代码.

Here is the code from my main activity.

/* 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，每次调用 selectItem 时，都会销毁一个片段，同时创建另一个片段.

I've set up some debug logging, and every time selectItem is called, one fragment is destroyed, while the other is created.

有什么办法可以防止这些片段被重新创建，而是直接重用它们吗?

Is there any way to prevent the fragments from being recreated, and just reuse them instead?

推荐答案

在@meredrica 指出 replace() 会破坏片段之后，我又回到了 FragmentManager 文档.这是我想出的解决方案，似乎有效.

After @meredrica pointed out that replace() destroys the fragments, I went back through the FragmentManager documentation. This is the solution I've come up with, that seems to be working.

/* 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，但我不确定是否有必要.

I also added this bit, but I'm not sure if it's necessary or not.

@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();
    }
}

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