Android 练手之旅——RecyclerView(三）Observable

[soapgu]

引言

上回解決了ItemSource和ItemTemplate的设置和绑定问题。
但是没有解决数据动态更新的问题

RecyclerView.Adapter的更新机制

• 全局更新 notifyDataSetChanged()
• 数据新增 notifyItemRangeInserted(positionStart, itemCount)
• 数据移除 notifyItemRangeRemoved(positionStart, itemCount)
• 数据更新 notifyItemRangeChanged(positionStart, itemCount)
• 数据移动 notifyItemMoved(fromPosition, toPosition)

部分参考DiffUtil and data binding with RecyclerView

必须在合适的场景使用合适的api（比如notifyDataSetChanged虽然一招鲜吃遍天下鲜，肯定性能低下）

ShadowAdapter 的Observable对象改造

 private static class WeakReferenceOnListChangedCallback<T> extends ObservableList.OnListChangedCallback<ObservableList<T>> {
        final WeakReference<ShadowAdapter<T>> adapterRef;

        WeakReferenceOnListChangedCallback(ShadowAdapter<T> adapter) {
            this.adapterRef = new WeakReference<>(adapter);
        }

        @Override
        public void onChanged(ObservableList sender) {
            ShadowAdapter<T> adapter = adapterRef.get();
            if (adapter == null) {
                return;
            }
            //Utils.ensureChangeOnMainThread();
            adapter.notifyDataSetChanged();
        }

        @Override
        public void onItemRangeChanged(ObservableList sender, final int positionStart, final int itemCount) {
            ShadowAdapter<T> adapter = adapterRef.get();
            if (adapter == null) {
                return;
            }
            //Utils.ensureChangeOnMainThread();
            adapter.notifyItemRangeChanged(positionStart, itemCount);
        }

        @Override
        public void onItemRangeInserted(ObservableList sender, final int positionStart, final int itemCount) {
            ShadowAdapter<T> adapter = adapterRef.get();
            if (adapter == null) {
                return;
            }
            //Utils.ensureChangeOnMainThread();
            adapter.notifyItemRangeInserted(positionStart, itemCount);
        }

        @Override
        public void onItemRangeMoved(ObservableList sender, final int fromPosition, final int toPosition, final int itemCount) {
            ShadowAdapter<T> adapter = adapterRef.get();
            if (adapter == null) {
                return;
            }
            //Utils.ensureChangeOnMainThread();
            for (int i = 0; i < itemCount; i++) {
                adapter.notifyItemMoved(fromPosition + i, toPosition + i);
            }
        }

        @Override
        public void onItemRangeRemoved(ObservableList sender, final int positionStart, final int itemCount) {
            ShadowAdapter<T> adapter = adapterRef.get();
            if (adapter == null) {
                return;
            }
            //Utils.ensureChangeOnMainThread();
            adapter.notifyItemRangeRemoved(positionStart, itemCount);
        }
    }

新增一个OnListChangedCallback的类，里面完成所有相关的数据变化通知

 @Override
    public void onAttachedToRecyclerView(@NonNull RecyclerView recyclerView) {
        super.onAttachedToRecyclerView(recyclerView);
        if (this.recyclerView == null && items instanceof ObservableList) {
            callback = new WeakReferenceOnListChangedCallback<>(this);
            ((ObservableList<T>) items).addOnListChangedCallback(callback);
        }
        this.recyclerView = recyclerView;
    }

    @Override
    public void onDetachedFromRecyclerView(@NonNull RecyclerView recyclerView) {
        super.onDetachedFromRecyclerView(recyclerView);
        if (this.recyclerView != null && items instanceof ObservableList) {
            ((ObservableList<T>) items).removeOnListChangedCallback(callback);
            callback = null;
        }
        this.recyclerView = null;
    }

在ShadowAdapter文件中实现加载和卸载RecyclerView重载回调
在加载的时候吧钩子加上，在卸载的时候把钩子去掉

BindingAdapter改造

BindingAdapter潜在性能问题

偶然调试过程中发现BindingAdapter的静态函数会被反复调用，并不符合我的预期
发现我每次列表加元素也会执行，这就有点不可理解了。加了断点

找到绑定自动生成代码ActivitySearchBindingImpl extends ActivitySearchBinding

private boolean onChangeDatacontextStringItems(androidx.databinding.ObservableList<java.lang.String> DatacontextStringItems, int fieldId) {
        if (fieldId == BR._all) {
            synchronized(this) {
                    mDirtyFlags |= 0x2L;
            }
            return true;
        }
        return false;
    }

这个回调，任意元素的变化都会调用mDirtyFlags 会赋值为2.
然后看调用的地方

@Override
    protected void executeBindings() {
        long dirtyFlags = 0;
        synchronized(this) {
            dirtyFlags = mDirtyFlags;
            mDirtyFlags = 0;
        }
        com.soapdemo.photohunter.viewmodels.SearchViewModel datacontext = mDatacontext;
        androidx.databinding.ObservableList<java.lang.String> datacontextStringItems = null;

        if ((dirtyFlags & 0x7L) != 0) {



                if (datacontext != null) {
                    // read datacontext.stringItems
                    datacontextStringItems = datacontext.getStringItems();
                }
                updateRegistration(1, datacontextStringItems);
        }
        // batch finished
        if ((dirtyFlags & 0x4L) != 0) {
            // api target 1

            this.buttonAdd.setOnClickListener(mCallback4);
            this.buttonClear.setOnClickListener(mCallback3);
        }
        if ((dirtyFlags & 0x7L) != 0) {
            // api target 1

            com.soapdemo.photohunter.util.BindingAdapters.setItems(this.dataList, datacontextStringItems, R.layout.text_row_item, (java.lang.String)null);
        }
    }

((dirtyFlags & 0x7L) != 0) 让我想到Linux文件的权限777，
二进制的三位任意的变化都会触发绑定函数执行。虽然没法完全理解框架的意图。我的函数需要做如下改造

@BindingAdapter(value = {"itemsSource","itemTemplate","variableName"},requireAll = false)
    public static <T> void setItems(RecyclerView recyclerView , List<T> itemsSource , @LayoutRes int itemTemplate, String variableName ){
        ShadowAdapter<T> adapter;

        @SuppressWarnings("unchecked")
        ShadowAdapter<T> oldAdapter = (ShadowAdapter<T>)recyclerView.getAdapter();
        if( oldAdapter == null ){
            ItemTemplate template = ItemTemplate.of( itemTemplate, variableName );
            adapter = new ShadowAdapter<>(template);
        }
        else {
            adapter = oldAdapter;
        }
        adapter.setItems(itemsSource);
        if (oldAdapter != adapter) {
            recyclerView.setAdapter(adapter);
        }
    }

public void setItems(List<T> items) {
        if (this.items == items) {
            return;
        }
        // If a recyclerview is listening, set up listeners. Otherwise wait until one is attached.
        // No need to make a sound if nobody is listening right?
        if (recyclerView != null) {
            if (this.items instanceof ObservableList) {
                ((ObservableList<T>) this.items).removeOnListChangedCallback(callback);
                callback = null;
            }
            if (items instanceof ObservableList) {
                callback = new WeakReferenceOnListChangedCallback<>(this);
                ((ObservableList<T>) items).addOnListChangedCallback(callback);
            }
        }
        this.items = items;
        notifyDataSetChanged();
    }

1 adapter 增加判断是否已经赋值
2 判断items是否重复赋值

Challenge for end

1. UI端已经完成了虚拟化，但是数据层的虚拟化怎么完成
2. 怎么完成一个无限的列表
3. 怎么做到一边加载一边翻滚，配合动画效果等
   这些问题有些难度，目前这阶段有点超纲了，等学习完成其他的目标后再来回顾这些问题

代码参考出处

一个很复杂的绑定库
binding-collection-adapter