使用 NavigationDrawer 切换时，如何只保留片段的一个实例？

Question

我的应用程序以 AddressFragment 开头。从 NavigationDrawer 我开始（除其他外）一个 new AddressFragment ：

getSupportFragmentManager().beginTransaction()
                .replace(R.id.container, new AddressFragment())
                .addToBackStack(null)
                .commit();

但我宁愿回到第一个实例。我怎么能这样做？

或者更笼统地说，我怎样才能知道片段的实例是否已经存在，然后启动它，如果存在，否则创建一个？

Answer 1

在创建片段时为其设置标签，然后您可以通过片段管理器找到它并相应地替换/创建。

FragmentManager fManager = getFragmentManager();
FragmentTransaction fTransaction = fManager.beginTransaction();
Fragment fragment = fManager.findFragmentByTag("uniqueTag");

// If fragment doesn't exist yet, create one
if (fragment == null) {
    fTransaction.add(R.id.fragment_list, new ListFrag(), "uniqueTag");
}
else { // re-use the old fragment
    fTransaction.replace(R.id.fragment_list, fragment, "uniqueTag");
}

Answer 2

第一步：

优化当前代码，让 Fragment 拥有自己的“TAG”

getSupportFragmentManager().beginTransaction()
                .replace(R.id.container, new AddressFragment())
                .addToBackStack(**AddressFragment.class.getName()**)
                .commit();

第二步：

在您的应用程序流程中，您需要确定片段是否存在：

public static boolean isFragmentInBackstack(final FragmentManager fragmentManager, final String fragmentTagName) {
    for (int entry = 0; entry < fragmentManager.getBackStackEntryCount(); entry++) {
        if (fragmentTagName.equals(fragmentManager.getBackStackEntryAt(entry).getName())) {
            return true;
        }
    }
    return false;
}

第三步：

执行片段操作

if (exists) {
    // Fragment exists, go back to that fragment
    //// you can also use POP_BACK_STACK_INCLUSIVE flag, depending on flow
    mFragmentManager.popBackStackImmediate(AddressFragment.class.getName(), 0);

} else {
    // Fragment doesn't exist
    // STEP 1 + additional backstack management
}

Answer 3

这是经过测试的答案，希望对您有所帮助 首先在 MainActivity 中全局设置这些字段

private static final int TIME_INTERVAL = 2000; 
private long mBackPressed;
private FragmentManager fm;
private FragmentTransaction ft;
private DrawerLayout drawer;

现在在 onNavigationItemSelected() 中这样实现

    @Override
public boolean onNavigationItemSelected(MenuItem item) {
Fragment fragment = null;
Class fragmentClass = null;
switch (item.getItemId()) {
case R.id.nav_home:
fragmentClass = Fragment.class;//this is MainAvctivty extends AppCompatActivity
break;
case R.id.nav_f1:
fragmentClass = FragmentOne.class;
break;
case R.id.nav_f2:
fragmentClass = FragmentTwo.class;
break;
case R.id.nav_f3:
fragmentClass = FragmentThree.class;
break;
case R.id.nav_f4:
fragmentClass = FragmentFour.class;
break;
case R.id.nav_f5:
fragmentClass = FragmentFive.class;
break;
default:
fragmentClass = Fragment.class;
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {e.printStackTrace();}
//Insert the fragment by replacing any existing fragment
fm = getSupportFragmentManager();
ft = fm.beginTransaction();
ft.replace(R.id.frame_container, fragment);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
ft.addToBackStack(null);
ft.commit();
item.setChecked(true);
setTitle(item.getTitle());
drawer.closeDrawers();
return true;
}

现在像下面这样处理onbackpressed

@Override
public void onBackPressed() {
if (drawer != null) {
fm = getSupportFragmentManager();
ft = fm.beginTransaction();
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else if (fm.getBackStackEntryCount() > 0) {
fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_CLOSE);
ft.commit();
} else if (mBackPressed + TIME_INTERVAL > System.currentTimeMillis()) {
super.onBackPressed();
} else {
Toast.makeText(this,"Press again to exit the app",Toast.LENGTH_SHORT).show();
}
mBackPressed = System.currentTimeMillis();
}
}

Answer 4

第一次创建带有标签的片段。

当你想替换它时，尝试获取带有标签的片段，如果它返回 null 你创建一个新的