Sticky Header Recyclerview 不使用 ArrayList<ItemObject> 为什么会这样？ [关闭]答案

【问题标题】：Sticky Header Recyclerview Not working with ArrayList<ItemObject> why so? [closed]Sticky Header Recyclerview 不使用 ArrayList<ItemObject> 为什么会这样？ [关闭]
【发布时间】：2019-07-13 10:28:21
【问题描述】：

我想创建带有 Sticky headers 的 recycleview，我可以使用 header 创建 recyclerview，它工作得非常好，但是当我尝试让它们变得有粘性时，我的应用程序崩溃了。我尝试了here 的代码，它与ArrayList<String> 一起工作正常，但是当我用ArrayList<ItemObject> 尝试它时，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

这里是代码。

主活动

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);
}

}

这是我的模型

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;
}

}

【问题讨论】：

标签： java android arraylist android-recyclerview

【解决方案1】：

问题出在onDrawOver方法中，我们可以将其隔离到这段代码中：

    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 告诉我们它出现在我们调用equals 的行中，这是因为目标对象是null。所以我们推断previousHeader 在某些时候必须为空。

它被初始化为""，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。

那么解决办法是什么？

视情况而定。

如果“节标题”为 null 不正确，请找出发生这种情况的原因并进行修复。
如果它是正确的（或者如果你无论如何都必须处理它！）那么阻止 NPE 的最小更改就是更改
```
        previousHeader = title;
```
到
```
        previousHeader = (title == null) ? "" : title;
```

【讨论】：

【解决方案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);
}

到

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

如果检查 Canvas c 不起作用，我会跟踪它，但从未得到任何参考。

【讨论】：

if (someObject == null) 永远不会抛出异常。如果someObject 为空，if (someObject.something == null) 可能会抛出异常，但这是不同的。你的假设是错误的。
是的，你是对的！，但没有必要检查使用 if .Possibly Exception throw Canvas c null object @Zoe
错误点在这一行 if (!previousHeader.equals(title) || sectionCallback.isSection(position)) { drawHeader(c, child, headerView);上一个标题 = 标题； }
(!previousHeader.equals(title) || sectionCallback.isSection(position)) 没有异常，drawHeader(c, child, headerView) 出现异常只有@John