viewLifecycleOwnerLiveData を使って Fragment の onDestroyView() で自動で null がセットされる ViewBinding 用の property delegates を作る

ViewBinding のドキュメントでは Fragment で使う時の実装はこのようになっています。

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1. class LoginFragment : Fragment() {  
2.   
3.     private var _binding: FragmentLoginBinding? = null  
4.     private val binding: FragmentLoginBinding  
5.         get() = _binding!!  
6.   
7.     override fun onCreateView(  
8.         inflater: LayoutInflater,  
9.         container: ViewGroup?,  
10.         savedInstanceState: Bundle?  
11.     ): View {  
12.         _binding = FragmentLoginBinding.inflate(inflater, container, false)  
13.         return binding.root  
14.     }  
15.   
16.     override fun onDestroyView() {  
17.         super.onDestroyView()  
18.         _binding = null  
19.     }  
20. }  

class LoginFragment : Fragment() { private var _binding: FragmentLoginBinding? = null private val binding: FragmentLoginBinding get() = _binding!! override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View { _binding = FragmentLoginBinding.inflate(inflater, container, false) return binding.root } override fun onDestroyView() { super.onDestroyView() _binding = null } } Fragment はそのライフサイクル中にViewが破棄されることがあるので onDestroyView() で View への参照を外しておく必要があります。

「Fragment.viewLifecycleOwnerLiveData で LiveData<LifecycleOwner?> が取れます」で紹介した viewLifecycleOwnerLiveData を使うと、onDestroyView() で null を代入する処理を自動でやってくれる ViewBinding 用の property delegates を作ることができます。

https://medium.com/@Zhuinden/simple-one-liner-viewbinding-in-fragments-and-activities-with-kotlin-961430c6c07c を参考に変更を加えています。

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1. class FragmentViewBindingDelegate<T : ViewBinding>(  
2.     val fragment: Fragment,  
3.     val viewBindingFactory: (View) -> T  
4. ) : ReadOnlyProperty<Fragment, T> {  
5.   
6.     private var binding: T? = null  
7.   
8.     private val viewLifecycleOwnerObserver = Observer<LifecycleOwner?> {  
9.         if (it == null) {  
10.             binding = null  
11.         }  
12.     }  
13.   
14.     private val observer = object : DefaultLifecycleObserver {  
15.   
16.         override fun onCreate(owner: LifecycleOwner) {  
17.             fragment.viewLifecycleOwnerLiveData.observeForever(viewLifecycleOwnerObserver)  
18.         }  
19.   
20.         override fun onDestroy(owner: LifecycleOwner) {  
21.             fragment.viewLifecycleOwnerLiveData.removeObserver(viewLifecycleOwnerObserver)  
22.             fragment.lifecycle.removeObserver(this)  
23.         }  
24.     }  
25.   
26.     init {  
27.         if (fragment.lifecycle.currentState != Lifecycle.State.DESTROYED) {  
28.             fragment.lifecycle.addObserver(observer)  
29.         }  
30.     }  
31.   
32.     override fun getValue(thisRef: Fragment, property: KProperty<*>): T {  
33.         val binding = binding  
34.         if (binding != null) {  
35.             return binding  
36.         }  
37.   
38.         val view = thisRef.view  
39.         checkNotNull(view) {  
40.             "Should get bindings when the view is not null."  
41.         }  
42.   
43.         return viewBindingFactory(view).also { this.binding = it }  
44.     }  
45. }  
46.   
47. fun <T : ViewBinding> Fragment.viewBinding(viewBindingFactory: (View) -> T) =  
48.     FragmentViewBindingDelegate(this, viewBindingFactory)  

class FragmentViewBindingDelegate<T : ViewBinding>( val fragment: Fragment, val viewBindingFactory: (View) -> T ) : ReadOnlyProperty<Fragment, T> { private var binding: T? = null private val viewLifecycleOwnerObserver = Observer<LifecycleOwner?> { if (it == null) { binding = null } } private val observer = object : DefaultLifecycleObserver { override fun onCreate(owner: LifecycleOwner) { fragment.viewLifecycleOwnerLiveData.observeForever(viewLifecycleOwnerObserver) } override fun onDestroy(owner: LifecycleOwner) { fragment.viewLifecycleOwnerLiveData.removeObserver(viewLifecycleOwnerObserver) fragment.lifecycle.removeObserver(this) } } init { if (fragment.lifecycle.currentState != Lifecycle.State.DESTROYED) { fragment.lifecycle.addObserver(observer) } } override fun getValue(thisRef: Fragment, property: KProperty<*>): T { val binding = binding if (binding != null) { return binding } val view = thisRef.view checkNotNull(view) { "Should get bindings when the view is not null." } return viewBindingFactory(view).also { this.binding = it } } } fun <T : ViewBinding> Fragment.viewBinding(viewBindingFactory: (View) -> T) = FragmentViewBindingDelegate(this, viewBindingFactory) これを使うと最初のコードはこうなります。

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1. class LoginFragment : Fragment() {  
2.   
3.     private val binding by viewBinding(FragmentLoginBinding::bind)  
4.   
5.     override fun onCreateView(  
6.         inflater: LayoutInflater,  
7.         container: ViewGroup?,  
8.         savedInstanceState: Bundle?  
9.     ): View {  
10.         return inflater.inflate(R.layout.fragment_login, container, false)  
11.     }  
12. }  

class LoginFragment : Fragment() { private val binding by viewBinding(FragmentLoginBinding::bind) override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View { return inflater.inflate(R.layout.fragment_login, container, false) } }