InfiniteFragmentStatePagerAdapter

InfiniteFragmentStatePagerAdapter.kt

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.support.v4.view.PagerAdapter
import android.view.View
import android.view.ViewGroup

abstract class InfiniteFragmentStatePagerAdapter(
        private val fragmentManager: FragmentManager
) : PagerAdapter() {
    companion object {
        private const val KEY_FRAGMENT_STATES = "states"
    }

    private var currentTransaction: FragmentTransaction? = null
    private var currentPrimaryItem: Fragment? = null
    private val savedStates = HashMap<String, Fragment.SavedState>()
    private val fragments = HashMap<String, Fragment>()

    abstract fun getItemCount(): Int
    abstract fun getItem(position: Int): Fragment
    abstract fun getItemIdentifier(position: Int): String
    abstract fun getItemTitle(position: Int): String

    @SuppressLint("CommitTransaction")
    override final fun instantiateItem(container: ViewGroup, position: Int): Any {
        val identifier = getItemIdentifier(getInternalItemPosition(position))
        fragments[identifier]?.let { return it }

        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction()
        }

        val fragment = getItem(getInternalItemPosition(position)).apply {
            savedStates[identifier]?.let { setInitialSavedState(it) }
            setMenuVisibility(false)
            userVisibleHint = false
        }
        fragments[identifier] = fragment
        currentTransaction?.add(container.id, fragment)
        return fragment
    }

    @SuppressLint("CommitTransaction")
    override final fun destroyItem(container: ViewGroup, position: Int, obj: Any) {
        val identifier = getItemIdentifier(getInternalItemPosition(position))
        val fragment = obj as Fragment

        if (currentTransaction == null) {
            currentTransaction = fragmentManager.beginTransaction()
        }

        fragments.remove(identifier)
        if (fragment.isAdded) {
            savedStates[identifier] = fragmentManager.saveFragmentInstanceState(fragment)
        } else {
            savedStates.remove(identifier)
        }

        currentTransaction?.remove(fragment)
    }

    override final fun setPrimaryItem(container: ViewGroup, position: Int, obj: Any?) {
        val fragment = obj as? Fragment
        if (fragment !== currentPrimaryItem) {
            currentPrimaryItem?.run {
                setMenuVisibility(false)
                userVisibleHint = false
            }
            currentPrimaryItem = fragment?.apply {
                setMenuVisibility(true)
                userVisibleHint = true
            }
        }
    }

    override final fun finishUpdate(container: ViewGroup) {
        currentTransaction?.commitNowAllowingStateLoss()
        currentTransaction = null
    }

    override final fun isViewFromObject(view: View, obj: Any) = (obj as Fragment).view === view

    override final fun saveState(): Parcelable {
        val state = Bundle()
        if (savedStates.isNotEmpty()) {
            state.putSerializable(KEY_FRAGMENT_STATES, savedStates)
        }
        fragments.keys.forEach { identifier ->
            fragments[identifier]?.takeIf { it.isAdded }?.let {
                fragmentManager.putFragment(state, identifier, it)
            }
        }
        return state
    }

    override final fun restoreState(state: Parcelable?, loader: ClassLoader?) {
        val bundle = (state as? Bundle)?.apply { classLoader = loader } ?: return

        fragments.clear()
        savedStates.clear()

        @Suppress("UNCHECKED_CAST")
        savedStates.putAll((bundle.getSerializable(KEY_FRAGMENT_STATES) as HashMap<String, Fragment.SavedState>))
        savedStates.keys.forEach { identifier ->
            fragmentManager.getFragment(bundle, identifier)?.let {
                it.setMenuVisibility(false)
                fragments[identifier] = it
            }
        }
    }

    // アイテムが変更された際に呼ばれ、移動後のアイテムの位置を返すメソッド。
    // 常時 POSITION_NONE を返した場合は更新時にすべてのFragmentが再生成されることになるため、
    // アイテムの動的な変更が頻繁に発生する場合は適切な位置を返し、かつ不要なSaveStateを破棄することが望ましい。
    override final fun getItemPosition(obj: Any?) = POSITION_NONE

    override final fun getCount() = if (getItemCount() == 0) 0 else Integer.MAX_VALUE

    override final fun getPageTitle(position: Int) = getItemTitle(getInternalItemPosition(position))

    // この値で ViewPager#setCurrentItem(int, boolean) を呼び出して、初期位置がちょうど真ん中になるようにする
    fun getInitialPosition() = (count.toDouble() / 2 / getItemCount()).toInt() * getItemCount()

    private fun getInternalItemPosition(position: Int) = if (getItemCount() == 0) 0 else position % getItemCount()
}