ListView中Item高度问题

一般情况下Item布局中写法如下

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="80dp">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="item" />

</android.support.constraint.ConstraintLayout>

但是上面的写法在运行之后并不能达到ListView的每个Item都是80dp的效果，而是所有的Item高度自适应了，这是什么原因呢？来看看源码。ListView在layout的时候会调用一个方法用于获取View，这个方法就是makeAndAddView(...),部分源码如下：

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
        boolean selected) {
    ...
    ...
    // Make a new view for this position, or convert an unused view if
    // possible.  获取View
    final View child = obtainView(position, mIsScrap);

    // This needs to be positioned and measured.
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

    return child;
}

可以看到makeAndAddView中调用了obtainView方法，这个方法是ListView父类AbsListView中的方法，部分源码如下：

View obtainView(int position, boolean[] outMetadata) {
    ...
    ...
    ...
    //设置Item的LayoutParams
    setItemViewLayoutParams(child, position);

    if (AccessibilityManager.getInstance(mContext).isEnabled()) {
        if (mAccessibilityDelegate == null) {
            mAccessibilityDelegate = new ListItemAccessibilityDelegate();
        }
        if (child.getAccessibilityDelegate() == null) {
            child.setAccessibilityDelegate(mAccessibilityDelegate);
        }
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);

    return child;
}

再继续看setItemViewLayoutParams方法，源码如下：

 private void setItemViewLayoutParams(View child, int position) {
    //这里的child指的是item，也就是上面xml中的 ...ConstraintLayout
    final ViewGroup.LayoutParams vlp = child.getLayoutParams();
    LayoutParams lp;
    if (vlp == null) {
        lp = (LayoutParams) generateDefaultLayoutParams();
    } else if (!checkLayoutParams(vlp)) {
        lp = (LayoutParams) generateLayoutParams(vlp);
    } else {
        lp = (LayoutParams) vlp;
    }

    if (mAdapterHasStableIds) {
        lp.itemId = mAdapter.getItemId(position);
    }
    lp.viewType = mAdapter.getItemViewType(position);
    lp.isEnabled = mAdapter.isEnabled(position);
    if (lp != vlp) {
      child.setLayoutParams(lp);
    }
}

setItemViewLayoutParams中的vlp默认是为null的，所以会进入到第一个if语句中的generateDefaultLayoutParams方法，如下：

 //生成一个AbsListView.LayoutParams，宽：MATCH_PARENT，高：WRAP_CONTENT
 protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
    return new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.WRAP_CONTENT, 0);
}

generateDefaultLayoutParams方法会生成一个宽为 MATCH_PARENT，高为WRAP_CONTENT的AbsListView.LayoutParams，所以你在item的xml中设置的高度没有生效，因为ListView默认给每个 Item设置了LayoutParams

解决方法

• 在xml里面多嵌套一次布局

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content">

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="80dp">

    <TextView
        android:id="@+id/tv_item"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="item" />
</RelativeLayout>
</android.support.constraint.ConstraintLayout>

• 或者在Adapter中的getView方法里面手动给convertView设置一个AbsListView.LayoutParams

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder = null;
        if (null == convertView) {
            holder = new ViewHolder();
            convertView = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.item_demo1, null, false);
            //设置convertView的LayoutParams
            convertView.setLayoutParams(new AbsListView.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT, 300)
            );
            holder.textView = convertView.findViewById(R.id.tv_item);
            convertView.setTag(holder);
            System.out.println(convertView.getLayoutParams() == null);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.textView.setText(list.get(position).getDesc());
        return convertView;
    }
  

参考链接