在RecyclerView中實現定點刷新

時間 2019-11-06

標籤 java android git github 編程數組 app ide oop post 欄目 Java 简体版

原文原文鏈接

寫在前面

近期，在筆者開源的BindingListAdapter庫中出現了這樣的一個Issue。java

這實際上是一個在列表中比較常見的問題，在沒有找到比較好的解決辦法以前，確實都是經過整項刷新notifyDataChanged來保證數據顯示的正確性。到後來的notifyItemChanged和更佳的DiffUtil，說明開發者們一直都在想辦法來解決並優化它。android

但其實若是你使用DataBinding，作這個局部刷新或者說是定點刷新，就很簡單了，這多是大多數使用DataBinding的開發者並不知道的技巧。git

巧用ObservableField

能夠先看看實際的效果github

關鍵點就在於不要直接綁定具體的值到xml中，應先使用ObservableField包裹一層。編程

class ItemViewModel constructor( val data:String){
    //not
    //val count = data
    //val liked = false
    
    //should
    val count = ObservableField<String>(data)
    val liked = ObservableBoolean()
}

//partial_list_item.xml
<?xml version="1.0" encoding="utf-8"?>
<layout>
    <data>
        <variable
            name="item"
            type="io.ditclear.app.partial.PartialItemViewModel" />

        <variable
            name="presenter"
            type="io.ditclear.bindingadapterx.ItemClickPresenter" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:onClick="@{(v)->presenter.onItemClick(v,item)}"
        >
        <TextView
            android:text="@{item.count}" />
        <ImageView
            android:src="@{item.liked?@drawable/ic_action_liked:@drawable/ic_action_unlike}"/>
    </androidx.constraintlayout.widget.ConstraintLayout>

</layout>
複製代碼

當UI須要改變時，改變ItemViewModel的數據便可。數組

//activity

override fun onItemClick(v: View, item: PartialItemViewModel) {
    item.toggle()
}

// ItemViewModle
 fun toggle(){
        liked.set(!liked.get())

        count.set("$data ${if (liked.get()) "liked" else ""}")
    }
複製代碼

至於爲何？這就要說到DataBinding更新UI的原理了。app

DataBinding更新UI原理

當咱們在項目中運用DataBinding，並將xml文件轉換爲DataBinding形式以後。通過編譯build，會生成相應的binding文件，好比partial_list_item.xml就會生成對應的PartialListItemBinding文件，這是一個抽象類，還會有一個PartialListItemBindingImpl實現類實現具體的渲染UI的方法executeBindings。ide

當數據有改變的時候，就會從新調用executeBindings方法，更新UI，那怎麼作到的呢？oop

咱們先來看PartialListItemBinding的構造方法.post

public abstract class PartialListItemBinding extends ViewDataBinding {
  @NonNull
  public final ImageView imageView;

  @Bindable
  protected PartialItemViewModel mItem;

  @Bindable
  protected ItemClickPresenter mPresenter;

  protected PartialListItemBinding(DataBindingComponent _bindingComponent, View _root, int _localFieldCount, ImageView imageView) {
      //調用父類的構造方法
    super(_bindingComponent, _root, _localFieldCount);
    this.imageView = imageView;
  }
  //... 
 }
複製代碼

調用了父類ViewDataBinding的構造方法，並傳入了三個參數，這裏看第三個參數_localFieldCount，它表明xml中存在幾個ObservableField形式的數據，繼續追蹤.

protected ViewDataBinding(DataBindingComponent bindingComponent, View root, int localFieldCount) {
    this.mBindingComponent = bindingComponent;
    //考點1
    this.mLocalFieldObservers = new ViewDataBinding.WeakListener[localFieldCount];
    this.mRoot = root;
    if (Looper.myLooper() == null) {
        throw new IllegalStateException("DataBinding must be created in view's UI Thread");
    } else {
        if (USE_CHOREOGRAPHER) {
            //考點2
            this.mChoreographer = Choreographer.getInstance();
            this.mFrameCallback = new FrameCallback() {
                public void doFrame(long frameTimeNanos) {
                    ViewDataBinding.this.mRebindRunnable.run();
                }
            };
        } else {
            this.mFrameCallback = null;
            this.mUIThreadHandler = new Handler(Looper.myLooper());
        }

    }
}
複製代碼

經過《觀察》，發現其根據localFieldCount初始化了一個WeakListener數組，名爲mLocalFieldObservers。另外一個重點是初始化了一個mFrameCallback，在回調中執行了mRebindRunnable.run。

當生成的PartialListItemBindingImpl對象調用executeBindings方法時，經過updateRegistration會對mLocalFieldObservers數組中的內容進行賦值。

隨之生成的是相應的WeakPropertyListener，來看看它的定義。

private static class WeakPropertyListener extends Observable.OnPropertyChangedCallback implements ObservableReference<Observable> {
    final WeakListener<Observable> mListener;

    public WeakPropertyListener(ViewDataBinding binder, int localFieldId) {
        mListener = new WeakListener<Observable>(binder, localFieldId, this);
    }

 	//...
    @Override
    public void onPropertyChanged(Observable sender, int propertyId) {
        ViewDataBinding binder = mListener.getBinder();
        if (binder == null) {
            return;
        }
        Observable obj = mListener.getTarget();
        if (obj != sender) {
            return; // notification from the wrong object?
        }
        //劃重點
        binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
    }
}
複製代碼

當ObservableField的值有改變的時候，onPropertyChanged會被調用，而後就會回調binder(即binding對象)的handleFieldChange方法，繼續觀察。

private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
    if (!this.mInLiveDataRegisterObserver) {
        boolean result = this.onFieldChange(mLocalFieldId, object, fieldId);
        if (result) {
            this.requestRebind();
        }

    }
}
複製代碼

若是值有改變，result爲true，接着requestRebind方法被執行。

protected void requestRebind() {
    if (this.mContainingBinding != null) {
        this.mContainingBinding.requestRebind();
    } else {
        synchronized(this) {
            if (this.mPendingRebind) {
                return;
            }

            this.mPendingRebind = true;
        }

        if (this.mLifecycleOwner != null) {
            State state = this.mLifecycleOwner.getLifecycle().getCurrentState();
            if (!state.isAtLeast(State.STARTED)) {
                return;
            }
        }
		//劃重點
        if (USE_CHOREOGRAPHER) { // SDK_INT >= 16
            this.mChoreographer.postFrameCallback(this.mFrameCallback);
        } else {
            this.mUIThreadHandler.post(this.mRebindRunnable);
        }
    }

}
複製代碼

在上述代碼最後，能夠看到sdk版本16以上會執行

this.mChoreographer.postFrameCallback(this.mFrameCallback);，16如下則是經過Handler。

關於postFrameCallBack，給的註釋是Posts a frame callback to run on the next frame.，簡單理解就是發生在下一幀即16ms以後的回調。

關於Choreographer，推薦閱讀 Choreographer 解析

但無論如何，最終都是調用mRebindRunnable.run，來看看對它的定義。

/** * Runnable executed on animation heartbeat to rebind the dirty Views. */
private final Runnable mRebindRunnable = new Runnable() {
    @Override
    public void run() {
        synchronized (this) {
            mPendingRebind = false;
        }
        processReferenceQueue();

        if (VERSION.SDK_INT >= VERSION_CODES.KITKAT) {
            // Nested so that we don't get a lint warning in IntelliJ
            if (!mRoot.isAttachedToWindow()) {
                // Don't execute the pending bindings until the View
                // is attached again.
                mRoot.removeOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
                mRoot.addOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
                return;
            }
        }
        //劃重點
        executePendingBindings();
    }
};
複製代碼

其實就是在下一幀的時候再執行了一次executePendingBindings方法，到這裏，DataBinding更新UI的邏輯咱們也就所有打通了。

寫在最後

筆者已經使用了DataBinding好幾年的時間，深切的體會到了它對於開發效率的提高，決不下於Kotlin，用好了它就是劍客最鋒利的寶劍，削鐵如泥，用很差便自損八百。爲此，上一年我專門寫了一篇DataBinding實用指南講了講本身的使用經驗，同時開源了運用DataBinding來簡化RecyclerView適配器的BindingListAdapter，但願更多的開發者喜歡它。

GitHub示例：github.com/ditclear/Bi…