RecyclerView使用詳解（三）

時間  2019-11-29
標籤 recyclerview 使用 詳解 简体版
原文   原文鏈接

在上一篇（RecyclerView使用詳解（二））文章中介紹了RecyclerView的多Item佈局實現，接下來要來說講RecyclerView的Cursor實現，相較於以前的實現，Cursor有更多的使用場景，也更加的經常使用，特別是配合LoaderManager和CursorLoader進行數據的緩存及加載顯示，基於此咱們來重點看看RecyclerView的CursorAdapter具體要怎麼實現。java

1、CursorAdapter實現(配合LoaderManager和CursorLoader)

若是以前你用過ListView實現過此功能(CursorAdapter)，那麼你必定對下面這兩個方法並不陌生android

@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方法是用來建立Item佈局的，bindView 方法是用來綁定當前View數據的，就至關於以前的getView方法拆成了兩個方法實現。git

若是你用RecyclerView，你會發現CursorAdapter這個類沒有了，既然沒有了，那咱們就本身仿照着ListView的CursorAdapter類來實現，具體的代碼沒什麼大的出入，無非就是註冊兩個觀察者去監聽數據庫數據的變化，可是有兩個地方須要注意一下，一個就是hasStableIds() 這個方法RecyclerView.Adapter中不能複寫父類的方法，須要在初始化的時候調用setHasStableIds(true); 來完成相同功能，第二個就是notifyDataSetInvalidated() 這個方法沒有，統一修改爲notifyDataSetChanged() 方法便可。github

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);//這個地方要注意一下，須要將表關聯ID設置爲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();//注意此處
        }
        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();//注意此處
        }
    }

怎麼樣，是否是很簡單，沒錯，就是這麼簡單，這裏是完整的BaseAbstractRecycleCursorAdapter代碼,用法和ListView的CursorAdapter用法一致，具體的能夠看看個人Recyclerview LoaderManager Provider數據庫

2、Item的動畫實現

RecyclerView自己就已經實現了ITEM的動畫，只須要調用如下幾個函數來增刪Item便可出現默認動畫。緩存

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

怎麼樣，是否是很輕鬆，若是你不知足系統默認動畫，那麼你能夠自定義實現RecyclerView.ItemAnimator的接口方法，實現代碼能夠參考DefaultItemAnimator.固然，若是你不想本身實現，那麼也不要緊，這裏有人已經寫了開源庫，你能夠去看看recyclerview-animators,這裏給出默認動畫實現方式代碼AnimFragmentruby

3、嵌套RecycleView

通常是不推薦使用嵌套RecycleView的，和ListView是相似的，遇到這種須要嵌套的View通常都是想別的辦法來規避，好比動態AddView，或者經過RecycleView的MultipleItemAdapter來實現，經過設置不一樣的ItemType佈局不一樣的View，可是有時候會閒麻煩，想直接就用嵌套的方式來作，那麼和ListView實現方式不一樣的是，ListView的實現通常都是繼承ListView而後複寫onMeasure方法，以下所示：app

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

可是RecycleView的實現方式再也不是繼承RecycleView來作，而是經過修改LayoutManager的方式，即經過繼承LinearLayoutManager GridLayoutManager StaggeredGridLayoutManager來修改子控件的測量，下面給出主要代碼：ide

FullyLinearLayoutManager函數

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. \nwidthMode " + widthMode
                + " \nheightMode " + heightSpec
                + " \nwidthSize " + widthSize
                + " \nheightSize " + heightSize
                + " \ngetItemCount() " + 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 動態添加時報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

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 動態添加時報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();
            }
        }
    }

4、效果圖以下：

Item默認動畫效果

嵌套ScrollView效果

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