Question

我想创建带有粘性标头的recycleview，我可以使带有标题的recyclerview正常运行，但是当我尝试使它们粘时，我的应用程序崩溃了。我尝试了here中的代码，它在ArrayList<String>上正常工作，但是当我在ArrayList<ItemObject>上尝试时，我的应用程序崩溃了。

这是我的日志。

2019-07-13 15:34:53.144 22566-22566/com.elytelabs.stickyrecyclerview E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.elytelabs.stickyrecyclerview, PID: 22566
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.Object.equals(java.lang.Object)' on a null object reference
    at com.elytelabs.stickyrecyclerview.RecyclerSectionItemDecoration.onDrawOver(RecyclerSectionItemDecoration.java:55)
    at androidx.recyclerview.widget.RecyclerView.draw(RecyclerView.java:4223)
    at android.view.View.updateDisplayListIfDirty(View.java:18168)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.updateDisplayListIfDirty(View.java:18159)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.updateDisplayListIfDirty(View.java:18159)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.updateDisplayListIfDirty(View.java:18159)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.updateDisplayListIfDirty(View.java:18159)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.updateDisplayListIfDirty(View.java:18159)
    at android.view.View.draw(View.java:18946)
    at android.view.ViewGroup.drawChild(ViewGroup.java:4240)
    at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4024)
    at android.view.View.draw(View.java:19221)
    at com.android.internal.policy.DecorView.draw(DecorView.java:791)
    at android.view.View.updateDisplayListIfDirty(View.java:18168)
    at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:685)
    at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:691)
    at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:799)
    at android.view.ViewRootImpl.draw(ViewRootImpl.java:3051)
    at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2846)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2399)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1432)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6861)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1026)
    at android.view.Choreographer.doCallbacks(Choreographer.java:838)
    at android.view.Choreographer.doFrame(Choreographer.java:769)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1012)
    at android.os.Handler.handleCallback(Handler.java:790)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:171)
    at android.app.ActivityThread.main(ActivityThread.java:6651)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:824)
2019-07-13 15:34:53.200 22566-22566/com.elytelabs.stickyrecyclerview I/Process: Sending signal. PID: 22566 SIG: 9

这是代码。

MainActivity

mRecyclerView = findViewById(R.id.recyclerview);
    mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
    adapter = new Adapter(this, getData());
    mRecyclerView.setAdapter(adapter);

    RecyclerSectionItemDecoration sectionItemDecoration =
            new RecyclerSectionItemDecoration(getResources().getDimensionPixelSize(R.dimen.header),
                    true,
                    getSectionCallback(getData()));
    mRecyclerView.addItemDecoration(sectionItemDecoration);

}

public ArrayList<ItemObject> getData() {
    ArrayList<ItemObject> list = new ArrayList <ItemObject>();
    list.add(new ItemObject("Header 1",true));
    list.add(new ItemObject("Example Text 1","Something Here 1"));
    list.add(new ItemObject("Example Text 2","Something Here 2 "));
    list.add(new ItemObject("Example Text 3","Something Here 3"));

    list.add(new ItemObject("Header 2",true));
    list.add(new ItemObject("Example Text 1","Something Here 1"));
    list.add(new ItemObject("Example Text 2","Something Here 2 "));
    list.add(new ItemObject("Example Text 3","Something Here 3"));

    list.add(new ItemObject("Header 3",true));
    list.add(new ItemObject("Example Text 1","Something Here 1"));
    list.add(new ItemObject("Example Text 2","Something Here 2 "));
    list.add(new ItemObject("Example Text 3","Something Here 3"));
    return list;
}

private RecyclerSectionItemDecoration.SectionCallback getSectionCallback(final List<ItemObject> list) {
    return new RecyclerSectionItemDecoration.SectionCallback() {
        @Override
        public boolean isSection(int position) {
            return list.get(position).isHeader();
        }

        @Override
        public String getSectionHeader(int position) {
            return list.get(position).getHeaderTitle();
        }

    };
}

}

这是我的RecyclerSectionItemDecoration

public class RecyclerSectionItemDecoration extends RecyclerView.ItemDecoration {

private final int             headerOffset;
private final boolean         sticky;
private final SectionCallback sectionCallback;

private View headerView;
private TextView header;

public RecyclerSectionItemDecoration(int headerHeight, boolean sticky, @NonNull SectionCallback sectionCallback) {
    headerOffset = headerHeight;
    this.sticky = sticky;
    this.sectionCallback = sectionCallback;
}

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);

    int pos = parent.getChildAdapterPosition(view);
    if (sectionCallback.isSection(pos)) {
        outRect.top = headerOffset;
    }
}

@Override
public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
    super.onDrawOver(c, parent, state);

    if (headerView == null) {
        headerView = inflateHeaderView(parent);
        header = (TextView) headerView.findViewById(R.id.tv_header);
        fixLayoutSize(headerView, parent);
    }

    CharSequence previousHeader = "";
    for (int i = 0; i < parent.getChildCount(); i++) {
        View child = parent.getChildAt(i);
        final int position = parent.getChildAdapterPosition(child);

        CharSequence title = sectionCallback.getSectionHeader(position);
        header.setText(title);
        if (!previousHeader.equals(title) || sectionCallback.isSection(position)) {
            drawHeader(c, child, headerView);
            previousHeader = title;
        }
    }
}

private void drawHeader(Canvas c, View child, View headerView) {
    c.save();
    if (sticky) {
        c.translate(0, Math.max(0, child.getTop() - headerView.getHeight()));
    } else {
        c.translate(0, child.getTop() - headerView.getHeight());
    }
    headerView.draw(c);
    c.restore();
}

private View inflateHeaderView(RecyclerView parent) {
    return LayoutInflater.from(parent.getContext())
            .inflate(R.layout.view_header, parent, false);
}

private void fixLayoutSize(View view, ViewGroup parent) {
    int widthSpec = View.MeasureSpec.makeMeasureSpec(parent.getWidth(),
            View.MeasureSpec.EXACTLY);
    int heightSpec = View.MeasureSpec.makeMeasureSpec(parent.getHeight(),
            View.MeasureSpec.UNSPECIFIED);

    int childWidth = ViewGroup.getChildMeasureSpec(widthSpec,
            parent.getPaddingLeft() + parent.getPaddingRight(),
            view.getLayoutParams().width);
    int childHeight = ViewGroup.getChildMeasureSpec(heightSpec,
            parent.getPaddingTop() + parent.getPaddingBottom(),
            view.getLayoutParams().height);

    view.measure(childWidth, childHeight);

    view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight());
}

public interface SectionCallback {

    boolean isSection(int position);

    CharSequence getSectionHeader(int position);
}

}

这是我的模特

class ItemObject {
private String pageName;
private String pageUrl;
private  String headerTitle;

private boolean isHeader ;

public ItemObject(String pageName , String pageUrl) {
    this.pageName = pageName;
    this.pageUrl = pageUrl;
}

public ItemObject(String headerTitle, boolean isHeader) {
    this.headerTitle = headerTitle;
    this.isHeader = isHeader;
}

public String getHeaderTitle() {
    return headerTitle;
}


public String getPageName() {
    return pageName;
}

public String getPageUrl() {
    return pageUrl;
}

public boolean isHeader() {
    return isHeader;
}

}

Answer 1

问题出在onDrawOver方法中，我们可以将其隔离为以下代码：

    CharSequence previousHeader = "";
    for (int i = 0; i < parent.getChildCount(); i++) {
        View child = parent.getChildAt(i);
        final int position = parent.getChildAdapterPosition(child);

        CharSequence title = sectionCallback.getSectionHeader(position);
        header.setText(title);
        if (!previousHeader.equals(title) || sectionCallback.isSection(position)) {
            drawHeader(c, child, headerView);
            previousHeader = title;
        }
    }

stacktrace告诉我们它发生在我们称为equals的行中，这是因为目标对象是null。因此，我们推断previousHeader在某些时候必须为空。

它已初始化为""，所以这一定是由于在这一行中分配了null：

        previousHeader = title;

换句话说，sectionCallback.getSectionHeader(position)必须在某个时候返回null。

那该怎么办？

要视情况而定。

如果将“节标题”设为null不正确，请找出原因，并加以解决。

如果正确（或者无论如何都要处理！），那么可以防止NPE的最小更改就是更改

        previousHeader = title;

到

        previousHeader = (title == null) ? "" : title;

Answer 2

在RecyclerSectionItemDecoration中编辑代码

 if (headerView == null) {
    headerView = inflateHeaderView(parent);
    header = (TextView) headerView.findViewById(R.id.tv_header);
    fixLayoutSize(headerView, parent);
}

收件人

    headerView = inflateHeaderView(parent);
    header = (TextView) headerView.findViewById(R.id.tv_header);
    fixLayoutSize(headerView, parent);

如果无法使用Canvas c，我跟踪它，但没有任何参考。

粘性标题Recyclerview无法与ArrayList <ItemObject>一起使用的原因是什么？

2 个答案: