我在 Kotlin 中重新实现了现有的解决方案，以便它允许您返回 aString而不是 along作为项目 ID。您可以在此处或下方找到它：
import android.annotation.SuppressLint
import android.os.Bundle
import android.os.Parcelable
import android.support.v4.app.Fragment
import android.support.v4.app.FragmentManager
import android.support.v4.app.FragmentTransaction
import android.view.View
import android.view.ViewGroup
import java.util.HashSet
import java.util.LinkedHashMap
/**
* A PagerAdapter that can withstand item reordering. See
* https://issuetracker.google.com/issues/36956111.
*
* @see android.support.v4.app.FragmentStatePagerAdapter
*/
abstract class MovableFragmentStatePagerAdapter(
private val manager: FragmentManager
) : NullablePagerAdapter() {
private var currentTransaction: FragmentTransaction? = null
private var currentPrimaryItem: Fragment? = null
private val savedStates = LinkedHashMap<String, Fragment.SavedState>()
private val fragmentsToItemIds = LinkedHashMap<Fragment, String>()
private val itemIdsToFragments = LinkedHashMap<String, Fragment>()
private val unusedRestoredFragments = HashSet<Fragment>()
/** @see android.support.v4.app.FragmentStatePagerAdapter.getItem */
abstract fun getItem(position: Int): Fragment
/**
* @return a unique identifier for the item at the given position.
*/
abstract fun getItemId(position: Int): String
/** @see android.support.v4.app.FragmentStatePagerAdapter.startUpdate */
override fun startUpdate(container: ViewGroup) {
check(container.id != View.NO_ID) {
"ViewPager with adapter $this requires a view id."
}
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.instantiateItem */
override fun instantiateItem(container: ViewGroup, position: Int): Any {
val itemId = getItemId(position)
val f = itemIdsToFragments[itemId]
if (f != null) {
unusedRestoredFragments.remove(f)
return f
}
if (currentTransaction == null) {
// We commit the transaction later
@SuppressLint("CommitTransaction")
currentTransaction = manager.beginTransaction()
}
val fragment = getItem(position)
fragmentsToItemIds.put(fragment, itemId)
itemIdsToFragments.put(itemId, fragment)
val fss = savedStates[itemId]
if (fss != null) {
fragment.setInitialSavedState(fss)
}
fragment.setMenuVisibility(false)
fragment.userVisibleHint = false
currentTransaction!!.add(container.id, fragment)
return fragment
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.destroyItem */
override fun destroyItem(container: ViewGroup, position: Int, fragment: Any) {
(fragment as Fragment).destroy()
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.setPrimaryItem */
override fun setPrimaryItem(container: ViewGroup, position: Int, fragment: Any?) {
fragment as Fragment?
if (fragment !== currentPrimaryItem) {
currentPrimaryItem?.let {
it.setMenuVisibility(false)
it.userVisibleHint = false
}
fragment?.setMenuVisibility(true)
fragment?.userVisibleHint = true
currentPrimaryItem = fragment
}
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.finishUpdate */
override fun finishUpdate(container: ViewGroup) {
if (!unusedRestoredFragments.isEmpty()) {
for (fragment in unusedRestoredFragments) fragment.destroy()
unusedRestoredFragments.clear()
}
currentTransaction?.let {
it.commitAllowingStateLoss()
currentTransaction = null
manager.executePendingTransactions()
}
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.isViewFromObject */
override fun isViewFromObject(view: View, fragment: Any): Boolean =
(fragment as Fragment).view === view
/** @see android.support.v4.app.FragmentStatePagerAdapter.saveState */
override fun saveState(): Parcelable? = Bundle().apply {
putStringArrayList(KEY_FRAGMENT_IDS, ArrayList<String>(savedStates.keys))
putParcelableArrayList(
KEY_FRAGMENT_STATES,
ArrayList<Fragment.SavedState>(savedStates.values)
)
for ((f, id) in fragmentsToItemIds.entries) {
if (f.isAdded) {
manager.putFragment(this, "$KEY_FRAGMENT_STATE$id", f)
}
}
}
/** @see android.support.v4.app.FragmentStatePagerAdapter.restoreState */
override fun restoreState(state: Parcelable?, loader: ClassLoader?) {
if ((state as Bundle?)?.apply { classLoader = loader }?.isEmpty == false) {
state!!
fragmentsToItemIds.clear()
itemIdsToFragments.clear()
unusedRestoredFragments.clear()
savedStates.clear()
val fragmentIds: List<String> = state.getStringArrayList(KEY_FRAGMENT_IDS)
val fragmentStates: List<Fragment.SavedState> =
state.getParcelableArrayList(KEY_FRAGMENT_STATES)
for ((index, id) in fragmentIds.withIndex()) {
savedStates.put(id, fragmentStates[index])
}
for (key: String in state.keySet()) {
if (key.startsWith(KEY_FRAGMENT_STATE)) {
val itemId = key.substring(KEY_FRAGMENT_STATE.length)
manager.getFragment(state, key)?.let {
it.setMenuVisibility(false)
fragmentsToItemIds.put(it, itemId)
itemIdsToFragments.put(itemId, it)
}
}
}
unusedRestoredFragments.addAll(fragmentsToItemIds.keys)
}
}
private fun Fragment.destroy() {
if (currentTransaction == null) {
// We commit the transaction later
@SuppressLint("CommitTransaction")
currentTransaction = manager.beginTransaction()
}
val itemId = fragmentsToItemIds.remove(this)
itemIdsToFragments.remove(itemId)
if (itemId != null) {
savedStates.put(itemId, manager.saveFragmentInstanceState(this))
}
currentTransaction!!.remove(this)
}
private companion object {
const val KEY_FRAGMENT_IDS = "fragment_keys_"
const val KEY_FRAGMENT_STATES = "fragment_states_"
const val KEY_FRAGMENT_STATE = "fragment_state_"
}
}
还有 Java 部分：
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.view.PagerAdapter;
import android.view.ViewGroup;
/**
* A PagerAdapter whose {@link #setPrimaryItem} is overridden with proper nullability annotations.
*/
public abstract class NullablePagerAdapter extends PagerAdapter {
@Override
public void setPrimaryItem(@NonNull ViewGroup container,
int position,
@Nullable Object object) {
// `object` is actually nullable. It's even in the dang source code which is hilariously
// ridiculous:
// `mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null);`
}
}