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  • Android RecyclerView使用详解（三）

    在上一篇（RecyclerView使用详解（二））文章中介绍了RecyclerView的多Item布局实现，接下来要来讲讲RecyclerView的Cursor实现，相较于之前的实现，Cursor有更多的使用场景，也更加的常用，特别是配合LoaderManager和CursorLoader进行数据的缓存及加载显示，基于此我们来重点看看RecyclerView的CursorAdapter具体要怎么实现。

    ##一、CursorAdapter实现(配合LoaderManager和CursorLoader)

    如果之前你用过ListView实现过此功能(CursorAdapter)，那么你一定对下面这两个方法并不陌生

    [代码]java代码：

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    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        return null;
    }
     
    @Override
    public void bindView(View view, Context context, Cursor cursor) {
     
    }

    其中newView方法是用来创建Item布局的，bindView 方法是用来绑定当前View数据的，就相当于之前的getView方法拆成了两个方法实现。

    如果你用RecyclerView，你会发现CursorAdapter这个类没有了，既然没有了，那我们就自己仿照着ListView的CursorAdapter类来实现，具体的代码没什么大的出入，无非就是注册两个观察者去监听数据库数据的变化，但是有两个地方需要注意一下，一个就是hasStableIds() 这个方法RecyclerView.Adapter中不能复写父类的方法，需要在初始化的时候调用setHasStableIds(true); 来完成相同功能，第二个就是notifyDataSetInvalidated() 这个方法没有，统一修改成notifyDataSetChanged() 方法即可。

    [代码]java代码：

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    void init(Context context, Cursor c, int flags) {
            boolean cursorPresent = c != null;
            mCursor = c;
            mDataValid = cursorPresent;
            mContext = context;
            mRowIDColumn = cursorPresent ? c.getColumnIndexOrThrow("_id") : -1;
            if ((flags & FLAG_REGISTER_CONTENT_OBSERVER) == FLAG_REGISTER_CONTENT_OBSERVER) {
                mChangeObserver = new ChangeObserver();
                mDataSetObserver = new MyDataSetObserver();
            } else {
                mChangeObserver = null;
                mDataSetObserver = null;
            }
     
            if (cursorPresent) {
                if (mChangeObserver != null) c.registerContentObserver(mChangeObserver);
                if (mDataSetObserver != null) c.registerDataSetObserver(mDataSetObserver);
            }
     
            setHasStableIds(true);//这个地方要注意一下，需要将表关联ID设置为true
        }
     
        //************//
        public Cursor swapCursor(Cursor newCursor) {
            if (newCursor == mCursor) {
                return null;
            }
            Cursor oldCursor = mCursor;
            if (oldCursor != null) {
                if (mChangeObserver != null) oldCursor.unregisterContentObserver(mChangeObserver);
                if (mDataSetObserver != null) oldCursor.unregisterDataSetObserver(mDataSetObserver);
            }
            mCursor = newCursor;
            if (newCursor != null) {
                if (mChangeObserver != null) newCursor.registerContentObserver(mChangeObserver);
                if (mDataSetObserver != null) newCursor.registerDataSetObserver(mDataSetObserver);
                mRowIDColumn = newCursor.getColumnIndexOrThrow("_id");
                mDataValid = true;
                // notify the observers about the new cursor
                notifyDataSetChanged();
            } else {
                mRowIDColumn = -1;
                mDataValid = false;
                // notify the observers about the lack of a data set
                //There is no notifyDataSetInvalidated() method in RecyclerView.Adapter
                notifyDataSetChanged();//注意此处
            }
            return oldCursor;
        }
        //************//
        private class MyDataSetObserver extends DataSetObserver {
            @Override
            public void onChanged() {
                mDataValid = true;
                notifyDataSetChanged();
            }
     
            @Override
            public void onInvalidated() {
                mDataValid = false;
                //There is no notifyDataSetInvalidated() method in RecyclerView.Adapter
                notifyDataSetChanged();//注意此处
            }
        }

    怎么样，是不是很简单，没错，就是这么简单，这里是完整的BaseAbstractRecycleCursorAdapter代码,用法和ListView的CursorAdapter用法一致，具体的可以看看我的Recyclerview LoaderManager Provider

    ##二、Item的动画实现 RecyclerView本身就已经实现了ITEM的动画，只需要调用以下几个函数来增删Item即可出现默认动画。

    [代码]java代码：

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       notifyItemChanged(int)
    notifyItemInserted(int)
    notifyItemRemoved(int)
    notifyItemRangeChanged(int, int)
    notifyItemRangeInserted(int, int)
    notifyItemRangeRemoved(int, int)

    怎么样，是不是很轻松，如果你不满足系统默认动画，那么你可以自定义实现RecyclerView.ItemAnimator的接口方法，实现代码可以参考DefaultItemAnimator.当然，如果你不想自己实现，那么也没关系，这里有人已经写了开源库，你可以去看看recyclerview-animators,这里给出默认动画实现方式代码AnimFragment

    ##三、嵌套RecycleView

    一般是不推荐使用嵌套RecycleView的，和ListView是类似的，遇到这种需要嵌套的View一般都是想别的办法来规避，比如动态AddView，或者通过RecycleView的MultipleItemAdapter来实现，通过设置不同的ItemType布局不同的View，但是有时候会闲麻烦，想直接就用嵌套的方式来做，那么和ListView实现方式不同的是，ListView的实现一般都是继承ListView然后复写onMeasure方法，如下所示：

    [代码]java代码：

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    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int expandSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
        super.onMeasure(widthMeasureSpec, expandSpec);
    }

    但是RecycleView的实现方式不再是继承RecycleView来做，而是通过修改LayoutManager的方式，即通过继承LinearLayoutManager GridLayoutManager StaggeredGridLayoutManager来修改子控件的测量，下面给出主要代码：

    FullyLinearLayoutManager

    [代码]java代码：

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    private int[] mMeasuredDimension = new int[2];
     
        @Override
        public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state,
                              int widthSpec, int heightSpec) {
     
            final int widthMode = View.MeasureSpec.getMode(widthSpec);
            final int heightMode = View.MeasureSpec.getMode(heightSpec);
            final int widthSize = View.MeasureSpec.getSize(widthSpec);
            final int heightSize = View.MeasureSpec.getSize(heightSpec);
     
            Log.i(TAG, "onMeasure called. widthMode " + widthMode
                    + " heightMode " + heightSpec
                    + " widthSize " + widthSize
                    + " heightSize " + heightSize
                    + " getItemCount() " + getItemCount());
     
            int width = 0;
            int height = 0;
            for (int i = 0; i < getItemCount(); i++) {
                measureScrapChild(recycler, i,
                        View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                        View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                        mMeasuredDimension);
     
                if (getOrientation() == HORIZONTAL) {
                    width = width + mMeasuredDimension[0];
                    if (i == 0) {
                        height = mMeasuredDimension[1];
                    }
                } else {
                    height = height + mMeasuredDimension[1];
                    if (i == 0) {
                        width = mMeasuredDimension[0];
                    }
                }
            }
            switch (widthMode) {
                case View.MeasureSpec.EXACTLY:
                    width = widthSize;
                case View.MeasureSpec.AT_MOST:
                case View.MeasureSpec.UNSPECIFIED:
            }
     
            switch (heightMode) {
                case View.MeasureSpec.EXACTLY:
                    height = heightSize;
                case View.MeasureSpec.AT_MOST:
                case View.MeasureSpec.UNSPECIFIED:
            }
     
            setMeasuredDimension(width, height);
        }
     
        private void measureScrapChild(RecyclerView.Recycler recycler, int position, int widthSpec,
                                       int heightSpec, int[] measuredDimension) {
            try {
                View view = recycler.getViewForPosition(0);//fix 动态添加时报IndexOutOfBoundsException
     
                if (view != null) {
                    RecyclerView.LayoutParams p = (RecyclerView.LayoutParams) view.getLayoutParams();
     
                    int childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec,
                            getPaddingLeft() + getPaddingRight(), p.width);
     
                    int childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec,
                            getPaddingTop() + getPaddingBottom(), p.height);
     
                    view.measure(childWidthSpec, childHeightSpec);
                    measuredDimension[0] = view.getMeasuredWidth() + p.leftMargin + p.rightMargin;
                    measuredDimension[1] = view.getMeasuredHeight() + p.bottomMargin + p.topMargin;
                    recycler.recycleView(view);
                }
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
            }
        }

    FullyGridLayoutManager

    [代码]java代码：

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    private int[] mMeasuredDimension = new int[2];
     
        @Override
        public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
            final int widthMode = View.MeasureSpec.getMode(widthSpec);
            final int heightMode = View.MeasureSpec.getMode(heightSpec);
            final int widthSize = View.MeasureSpec.getSize(widthSpec);
            final int heightSize = View.MeasureSpec.getSize(heightSpec);
     
            int width = 0;
            int height = 0;
            int count = getItemCount();
            int span = getSpanCount();
            for (int i = 0; i < count; i++) {
                measureScrapChild(recycler, i,
                        View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                        View.MeasureSpec.makeMeasureSpec(i, View.MeasureSpec.UNSPECIFIED),
                        mMeasuredDimension);
     
                if (getOrientation() == HORIZONTAL) {
                    if (i % span == 0) {
                        width = width + mMeasuredDimension[0];
                    }
                    if (i == 0) {
                        height = mMeasuredDimension[1];
                    }
                } else {
                    if (i % span == 0) {
                        height = height + mMeasuredDimension[1];
                    }
                    if (i == 0) {
                        width = mMeasuredDimension[0];
                    }
                }
            }
     
            switch (widthMode) {
                case View.MeasureSpec.EXACTLY:
                    width = widthSize;
                case View.MeasureSpec.AT_MOST:
                case View.MeasureSpec.UNSPECIFIED:
            }
     
            switch (heightMode) {
                case View.MeasureSpec.EXACTLY:
                    height = heightSize;
                case View.MeasureSpec.AT_MOST:
                case View.MeasureSpec.UNSPECIFIED:
            }
     
            setMeasuredDimension(width, height);
        }
     
        private void measureScrapChild(RecyclerView.Recycler recycler, int position, int widthSpec,
                                       int heightSpec, int[] measuredDimension) {
            if (position < getItemCount()) {
                try {
                    View view = recycler.getViewForPosition(0);//fix 动态添加时报IndexOutOfBoundsException
                    if (view != null) {
                        RecyclerView.LayoutParams p = (RecyclerView.LayoutParams) view.getLayoutParams();
                        int childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec,
                                getPaddingLeft() + getPaddingRight(), p.width);
                        int childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec,
                                getPaddingTop() + getPaddingBottom(), p.height);
                        view.measure(childWidthSpec, childHeightSpec);
                        measuredDimension[0] = view.getMeasuredWidth() + p.leftMargin + p.rightMargin;
                        measuredDimension[1] = view.getMeasuredHeight() + p.bottomMargin + p.topMargin;
                        recycler.recycleView(view);
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }

    ##四、效果图如下：

    Item默认动画效果

    嵌套ScrollView效果

    最后给出代码下载地址–>Demo Code
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  • 原文地址：https://www.cnblogs.com/android-blogs/p/5690855.html
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