封装和使用ViewBinding

封装和使用 ViewBinding，该替代 Kotlin synthetic 和 ButterKnife

ViewBinding 的基础用法

首先要在 module 的 build.gradle 文件配置开启 ViewBinding：

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android {
    ...
    viewBinding {
        enabled = true
    }

    buildFeatures {
        viewBinding true
    }
}

这样该模块下每个 XML 文件都生成一个对应的绑定类，每个绑定类会包含根视图以及具有 ID 的所有视图的引用。绑定类的命名是：将 XML 文件的名称转换为驼峰命名，并在末尾添加 “Binding” 。

比如现在有 activity_app.xml 文件：

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<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context=".AppActivity">

  <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Heihei"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

如果不想生成某个布局的绑定类，可以在根视图添加 tools:viewBindingIgnore=”true” 属性。

那这个绑定类的对象怎么实例化呢？该类会生成相关的 inflate 静态方法，调用该方法即可获得绑定对象。

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class AppActivity : AppCompatActivity() {

  private lateinit var binding: ActivityAppBinding

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityAppBinding.inflate(layoutInflater)
    setContentView(binding.root)
  }
}

在 Fragment 使用有点不同，由于 Fragment 的存在时间比其视图长，需要在 onDestroyView() 方法中清除对绑定类实例的所有引用，所以写起来会有点麻烦。

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class HomeFragment : Fragment() {
  private var _binding: HomeFragmentBinding? = null
  private val binding get() = _binding!!

  override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?
  ): View {
    _binding = ResultProfileBinding.inflate(inflater, container, false)
    return binding.root
  }

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    binding.tvHelloWorld.text = "Hello Android!"
  }

  override fun onDestroyView() {
    super.onDestroyView()
    _binding = null
  }
}

还有在 Adapter 的使用，因为布局不是只创建一次，而是每有一项数据就会创建，不能像上面那样在 Adapter 里写一个 binding 全局变量，这样 binding 只会得到最后一次创建的视图。所以 binding 对象应该是给 ViewHolder 持有。

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class TextAdapter(
  private val list: List<String>
) : RecyclerView.Adapter<TextAdapter.TextViewHolder>() {
  
  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): TextViewHolder {
    val binding = ItemTextBinding.inflate(LayoutInflater.from(parent.context), parent, false)
    return TextViewHolder(binding)
  }

  override fun onBindViewHolder(holder: TextViewHolder, position: Int) {
    val content = list[position]
    holder.binding.tvContent.text = content
  }

  override fun getItemCount() = list.size

  class TextViewHolder(val binding : ItemTextBinding) : RecyclerView.ViewHolder(binding.root)
}

封装方法

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inline fun <reified VB : ViewBinding> Activity.inflate() = lazy {
  inflateBinding<VB>(layoutInflater).apply { setContentView(root) }
}

inline fun <reified VB : ViewBinding> Dialog.inflate() = lazy {
  inflateBinding<VB>(layoutInflater).apply { setContentView(root) }
}

@Suppress("UNCHECKED_CAST")
inline fun <reified VB : ViewBinding> inflateBinding(layoutInflater: LayoutInflater) =
  VB::class.java.getMethod("inflate", LayoutInflater::class.java).invoke(null, layoutInflater) as VB

Activity 的用法，在 Dialog 使用是类似的。

Fragment 的封装不一样，首先生成方法 bind()，只需传个 View。另外还需要释放 binding 对象，不能用延时委托改用属性委托

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inline fun <reified VB : ViewBinding> Fragment.bindView() =
  FragmentBindingDelegate(VB::class.java)
  
inline fun Fragment.doOnDestroyView(crossinline block: () -> Unit) =
  viewLifecycleOwner.lifecycle.addObserver(object : LifecycleObserver {
    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    fun onDestroyView() {
      block.invoke()
    }
  })

class FragmentBindingDelegate<VB : ViewBinding>(
  private val clazz: Class<VB>
) : ReadOnlyProperty<Fragment, VB> {

  private var binding: VB? = null

  @Suppress("UNCHECKED_CAST")
  override fun getValue(thisRef: Fragment, property: KProperty<*>): VB {
    if (binding == null) {
      binding = clazz.getMethod("bind", View::class.java)
        .invoke(null, thisRef.requireView()) as VB
      thisRef.doOnDestroyView { binding = null }
    }
    return binding!!
  }
}

如果还有其它释放操作要在 binding 销毁前执行，需要写在 doOnDestroyView() 方法里

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class HomeFragment : Fragment(R.layout.fragment_home) {

  private val binding: FragmentHomeBinding by bindView()

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    binding.tvHelloWorld.text = "Hello Android!"
    doOnDestroyView {
      // 在 binding 对象销毁前进行释放操作
    }
  }
}

列表的封装，binding 对象是给 ViewHolder 持有，所以 BindingViewHolder 来接收 binding

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class BindingViewHolder<VB : ViewBinding>(val binding: VB) : RecyclerView.ViewHolder(binding.root)

反射进行实例化

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inline fun <reified T : ViewBinding> newBindingViewHolder(parent: ViewGroup): BindingViewHolder<T> {
  val method = T::class.java.getMethod("inflate", LayoutInflater::class.java, ViewGroup::class.java, Boolean::class.java)
  val binding = method.invoke(null, LayoutInflater.from(parent.context), parent, false) as T
  return BindingViewHolder(binding)
}

onCreateViewHolder 调用封装的方法创建 BindingViewHolder 对象，然后在 onBindViewHolder 方法通过 holder 持有的 binding 拿到控件

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class TextAdapter(
  private val list: List<String>
) : RecyclerView.Adapter<BindingViewHolder<ItemTextBinding>>() {
  
  override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) =
    newBindingViewHolder<ItemTextBinding>(parent)

  override fun onBindViewHolder(holder: BindingViewHolder<ItemTextBinding>, position: Int) {
    val content = list[position]
    holder.binding.tvContent.text = content
  }

  override fun getItemCount() = list.size
}

基类封装

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public abstract class BaseBindingActivity<VB extends ViewBinding> extends AppCompatActivity {

  private VB binding;

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = ViewBindingUtil.inflateWithGeneric(this, getLayoutInflater());
    setContentView(binding.getRoot());
  }

  public VB getBinding() {
    return binding;
  }
}

class MainActivity extends BaseBindingActivity<ActivityMainBinding> {

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getBinding().tvHelloWorld.setText("Hello Android!");
  }
}

混淆配置

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-keepclassmembers class * implements androidx.viewbinding.ViewBinding {
  public static * inflate(android.view.LayoutInflater);
  public static * inflate(android.view.LayoutInflater, android.view.ViewGroup, boolean);
  public static * bind(android.view.View);
}