java - 无条件布局膨胀: Should use View Holder pattern

Question

我是新手，我尝试膨胀项目行，但它会在滚动时减慢应用程序的速度。在下面的代码中，它说我需要使用 View Holder。

        row = inflater.inflate(mLayoutResourceId, parent, false);

我该如何解决这个问题？

package com.example.android.ontrack.adapters;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;

import com.example.android.ontrack.R;
import com.example.android.ontrack.models.Order;

public class OrdersList extends ArrayAdapter<Order> {

    Context mContext;
    int mLayoutResourceId;
    Order mData[] = null;

    public OrdersList(Context context, int resource, Order[] data) {
        super(context, resource, data);
        this.mContext = context;
        this.mLayoutResourceId = resource;
        this.mData = data;
    }


    @Override
    public Order getItem(int position) {
        return super.getItem(position);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View row;

        LayoutInflater inflater = LayoutInflater.from(mContext);
        row = inflater.inflate(mLayoutResourceId, parent, false);

            TextView nameOfSchool = row.findViewById(R.id.school_name_orders);
        TextView nameOfAgent = row.findViewById(R.id.agent_name);
        TextView orderId = row.findViewById(R.id.order_id);
        TextView netRevenue = row.findViewById(R.id.net_revenue);
        TextView orderQuantity = row.findViewById(R.id.total_quantity);
        TextView date = row.findViewById(R.id.date);

        Order orders = mData[position];

        nameOfSchool.setText(orders.nameOfSchool);
        nameOfAgent.setText(String.valueOf(orders.nameOfAgent));
        orderId.setText(String.valueOf(orders.orderId));
        netRevenue.setText(String.valueOf(orders.netRevenue));
        orderQuantity.setText(String.valueOf(orders.totalQuantity));
        date.setText(String.valueOf(orders.date));

        return row;
    }
}

Answer 1

您的问题出在这段代码中:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View row;

    LayoutInflater inflater = LayoutInflater.from(mContext);
    row = inflater.inflate(mLayoutResourceId, parent, false);

    ...
    return row;
}

警告消息表明，无论系统将什么传递给 getView() ，你总是打电话inflater.inflate()生成您的row .

首先要了解的是 convertView参数传递给getView()有时为空，有时不为空。 当它不为 null 时，它是一个“旧”(“回收”) View ，之前从 getView() 返回。换句话说，它与您自己膨胀的 View 类型完全相同，只是其中包含旧数据。

因此，您可以通过将上面的代码更改为以下内容来解决该问题:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View row;

    if (convertView == null) {
        LayoutInflater inflater = LayoutInflater.from(mContext);
        row = inflater.inflate(mLayoutResourceId, parent, false);
    } else {
        row = convertView;
    }

    ...
    return row;
}

第二个要理解的是findViewById()是慢的。不像慢几秒那样慢，但足够慢，如果你必须运行它数百次，就会导致你的 UI 滞后。因此，“ View 持有者模式”是一种避免每次都查找这些 View 的方法。

您要做的第一件事是创建一个类，其中包含行中每个 View 的字段。鉴于您发布的内容，看起来像这样:

private static class ViewHolder {

    private final TextView nameOfSchool;
    private final TextView nameOfAgent;
    private final TextView orderId;
    private final TextView netRevenue;
    private final TextView orderQuantity;
    private final TextView date;

    private ViewHolder(View row) {
        nameOfSchool = row.findViewById(R.id.school_name_orders);
        nameOfAgent = row.findViewById(R.id.agent_name);
        orderId = row.findViewById(R.id.order_id);
        netRevenue = row.findViewById(R.id.net_revenue);
        orderQuantity = row.findViewById(R.id.total_quantity);
        date = row.findViewById(R.id.date);
    }
}

现在您必须将其插入您的getView()代码。您将需要创建一个新的 ViewHolder每次你膨胀一个 View 时(即当 convertView 为空时)，你会想要重新使用现有的 ViewHolder否则。我们为此使用 View 的标签。

总的来说，它看起来像这样:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View row;
    ViewHolder holder;

    if (convertView == null) {
        LayoutInflater inflater = LayoutInflater.from(mContext);
        row = inflater.inflate(mLayoutResourceId, parent, false);
        holder = new ViewHolder(row);
        row.setTag(holder);       
    } else {
        row = convertView;
        holder = (ViewHolder) row.getTag();
    }

    Order orders = mData[position];

    holder.nameOfSchool.setText(orders.nameOfSchool);
    holder.nameOfAgent.setText(String.valueOf(orders.nameOfAgent));
    holder.orderId.setText(String.valueOf(orders.orderId));
    holder.netRevenue.setText(String.valueOf(orders.netRevenue));
    holder.orderQuantity.setText(String.valueOf(orders.totalQuantity));
    holder.date.setText(String.valueOf(orders.date));

    return row;
}