ViewBinding與RecycleView（一）

時間 2020-04-23

標籤 viewbinding recycleview 简体版

原文原文鏈接

如何使用

在Android Studio 3.6的穩定版本中，咱們就能夠使用ViewBinding替代findViewByIdjava

另外關於ViewBinding 與Kotlin Android Extensions 的區分這裏很少作介紹，
能夠參考下stackoverflow中的討論json

ViewBinding如何使用？若是是Kotlin DSL的話這樣添加：app

android {
        ...
        viewBinding.isEnabled = true

}

不然：ide

android {
        ...
        viewBinding {
            enabled = true
        }
    }

簡單例子

<?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"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.google.android.material.tabs.TabLayout
        app:layout_constraintTop_toTopOf="parent"
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:tabMode="fixed" />

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/vp2"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tabs" />
</androidx.constraintlayout.widget.ConstraintLayout>

而後在activity中：ui

private lateinit var mBinding: ActivityTabBinding

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mBinding = ActivityTabBinding.inflate(layoutInflater)
        setContentView(mBinding.root)
        attachTabsOnViewPager2()
}

在app/buildle/generated/data_binding_base_class_source_out/...目錄看下生成的ActivityTabBinding類this

// Generated by view binder compiler. Do not edit!
package tt.reducto.instantsearch.databinding;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.constraintlayout.widget.ConstraintLayout;
import androidx.viewbinding.ViewBinding;
import androidx.viewpager2.widget.ViewPager2;
import com.google.android.material.tabs.TabLayout;
import java.lang.NullPointerException;
import java.lang.Override;
import java.lang.String;
import tt.reducto.instantsearch.R;

public final class ActivityTabBinding implements ViewBinding {
  @NonNull
  private final ConstraintLayout rootView;

  @NonNull
  public final TabLayout tabs;

  @NonNull
  public final ViewPager2 vp2;

  private ActivityTabBinding(@NonNull ConstraintLayout rootView, @NonNull TabLayout tabs,
      @NonNull ViewPager2 vp2) {
    this.rootView = rootView;
    this.tabs = tabs;
    this.vp2 = vp2;
  }

  @Override
  @NonNull
  public ConstraintLayout getRoot() {
    return rootView;
  }

  @NonNull
  public static ActivityTabBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, null, false);
  }

  @NonNull
  public static ActivityTabBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    View root = inflater.inflate(R.layout.activity_tab, parent, false);
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }

  @NonNull
  public static ActivityTabBinding bind(@NonNull View rootView) {
    // The body of this method is generated in a way you would not otherwise write.
    // This is done to optimize the compiled bytecode for size and performance.
    String missingId;
    missingId: {
      TabLayout tabs = rootView.findViewById(R.id.tabs);
      if (tabs == null) {
        missingId = "tabs";
        break missingId;
      }
      ViewPager2 vp2 = rootView.findViewById(R.id.vp2);
      if (vp2 == null) {
        missingId = "vp2";
        break missingId;
      }
      return new ActivityTabBinding((ConstraintLayout) rootView, tabs, vp2);
    }
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}

與RecycleView結合

關注下ActivityTabBinding類中的inflate方法是否是跟咱們RecycleView中的onCreateViewHolder方法特別像？建立View root時都不會將其添加到父對象ViewGroup上，通常咱們建立ViewHolder像這樣：google

class CategoryViewHolder constructor(itemView: View) :
    RecyclerView.ViewHolder(itemView) {
    constructor(parent: ViewGroup) :
            this(LayoutInflater.from(parent.context).inflate(R.layout.category_item, parent, false))

    fun bind(category: Category) {
        itemView.categoryName.text = category.name
        itemView.categoryID.text = " "
    }
}

因此咱們能夠給自定義ViewHolder類傳入ViewBinding引用 :代理

open class BaseBindingViewHolder<T : ViewBinding> private constructor(val mBinding: T) :
    RecyclerView.ViewHolder(mBinding.root) {
    //
    constructor(
        parent: ViewGroup,
        creator: (inflater: LayoutInflater, root: ViewGroup, attachToRoot: Boolean) -> T
    ) : this(creator(LayoutInflater.from(parent.context), parent, false))

}

咱們再給ViewGroup提供一個擴展方法省去ViewHolder在onCreateViewHolder中的建立：code

fun <T : ViewBinding> ViewGroup.getViewHolder(
    creator: (inflater: LayoutInflater, root: ViewGroup, attachToRoot: Boolean) -> T
): BaseBindingViewHolder<T> = BaseBindingViewHolder(this, creator)

利用ViewBinding 一個簡單的Adapter就這樣:

CategoryItemBinding是根據xml文件自動生成的

class CategoryAdapter : RecyclerView.Adapter<BaseBindingViewHolder<CategoryItemBinding>>() {
    private var list: List<Category> = listOf()
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BaseBindingViewHolder<CategoryItemBinding> {
        return parent.getViewHolder(CategoryItemBinding::inflate)
    }

    override fun onBindViewHolder(holder:  BaseBindingViewHolder<CategoryItemBinding>, position: Int) {
        holder.mBinding.categoryName.text = list[position].name
    }

    fun setItem(list: List<Category>) {
        this.list = list
        notifyDataSetChanged()
    }

    override fun getItemCount(): Int = list.size
}

綜上，這些是比較簡單的操做..

自定義kotlin屬性委託

kotlin源碼中的實現判空的委託屬性：

/**
 * Standard property delegates.
 */
public object Delegates {

   /**
     * Returns a property delegate for a read/write property with a non-`null` value that is initialized not during
     * object construction time but at a later time. Trying to read the property before the initial value has been
     * assigned results in an exception.
     *
     * @sample samples.properties.Delegates.notNullDelegate
     */
    public fun <T : Any> notNull(): ReadWriteProperty<Any?, T> = NotNullVar()
    
    ......
    
}

private class NotNullVar<T : Any>() : ReadWriteProperty<Any?, T> {
    private var value: T? = null

    public override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        return value ?: throw IllegalStateException("Property ${property.name} should be initialized before get.")
    }

    public override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        this.value = value
    }
}

其中 NotNullVar 繼承了 ReadWriteProperty，並實現了他的兩個方法，而Delegates.notNull() 屬於委託屬性。

看一個自定義委託findViewById的例子：

class MainActivity : AppCompatActivity(){

    private val etSearch : FixedKeyboardEditText by bindView(R.id.et_search)

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        etSearch.setText("test")
    }
}

fun <T: View> bindView( id : Int): FindView<T> = FindView(id)

class FindView<T : View >(val id:Int) : ReadOnlyProperty<Any?, T> {

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {

        if(this.value == null) {
            this.value = (thisRef as Activity).findViewById<T>(id)
        }
        return this.value?:throw RuntimeException("can not find this view")
    }

    var value : T? = null
}

若是咱們把itemView與數據源的綁定經過自定義委託來代理，那是否是會方便不少？？