如何让我的 ArrayAdapter 遵循 ViewHolder 模式? [英] How can I make my ArrayAdapter follow the ViewHolder pattern?

问题描述

这是我的 ArrayAdapter.我想通过遵循 ViewHolder 模式来提高效率:

Here is my ArrayAdapter. I would like to make this more efficient by following the ViewHolder pattern:

http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/view/List14.html

但我不确定如何实现这一点.

but am not sure how to accomplish this.

更新:ViewHolder 模式

private class QuoteAdapter extends ArrayAdapter<Quote> {

    private ArrayList<Quote> items;
    // used to keep selected position in ListView
    private int selectedPos = -1; // init value for not-selected

    public QuoteAdapter(Context context, int textViewResourceId, ArrayList<Quote> items) {
        super(context, textViewResourceId, items);
        this.items = items;
    }

    public void setSelectedPosition(int pos) {
        selectedPos = pos;
        // inform the view of this change
        notifyDataSetChanged();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        ViewHolder holder; // to reference the child views for later actions

        if (v == null) {
            LayoutInflater vi = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.mainrow, null);

            // cache view fields into the holder
            holder = new ViewHolder();
            holder.nameText = (TextView) v.findViewById(R.id.nameText);
            holder.priceText = (TextView) v.findViewById(R.id.priceText);
            holder.changeText = (TextView) v.findViewById(R.id.changeText);

            // associate the holder with the view for later lookup
            v.setTag(holder);
        }
        else {
            // view already exists, get the holder instance from the view
            holder = (ViewHolder)v.getTag();
        }

        // change the row color based on selected state
        if (selectedPos == position) {
            v.setBackgroundResource(R.drawable.stocks_selected_gradient);
            holder.nameText.setTextColor(Color.WHITE);
            holder.priceText.setTextColor(Color.WHITE);
            holder.changeText.setTextColor(Color.WHITE);
        } else {
            v.setBackgroundResource(R.drawable.stocks_gradient);
            holder.nameText.setTextAppearance(getApplicationContext(), R.style.BlueText);
            holder.priceText.setTextAppearance(getApplicationContext(), R.style.BlueText);
            holder.changeText.setTextAppearance(getApplicationContext(), R.style.BlueText);
        }

        Quote q = items.get(position);
        if (q != null) {
            if (holder.nameText != null) {
                holder.nameText.setText(q.getSymbol());
            }
            if (holder.priceText != null) {
                holder.priceText.setText(q.getLastTradePriceOnly());
            }
            if (holder.changeText != null) {
                try {
                    float floatedChange = Float.valueOf(q.getChange());
                    if (floatedChange < 0) {
                        if (selectedPos != position)
                            holder.changeText.setTextAppearance(getApplicationContext(), R.style.RedText); // red
                    } else {
                        if (selectedPos != position)
                            holder.changeText.setTextAppearance(getApplicationContext(), R.style.GreenText); // green
                    }
                } catch (NumberFormatException e) {
                    System.out.println("not a number");
                } catch (NullPointerException e) {
                    System.out.println("null number");
                }
                holder.changeText.setText(q.getChange() + " (" + q.getPercentChange() + ")");
            }
        }
        return v;
    }
}

推荐答案

ViewHolder 基本上是一个静态类实例，您在创建视图时将其与视图关联，缓存您在运行时查找的子视图.如果视图已存在，则检索持有者实例并使用其字段而不是调用 findViewById.

The ViewHolder is basically a static class instance that you associate with a view when it's created, caching the child views you're looking up at runtime. If the view already exists, retrieve the holder instance and use its fields instead of calling findViewById.

就你而言:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View v = convertView;
    ViewHolder holder; // to reference the child views for later actions

    if (v == null) {
        LayoutInflater vi = 
            (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        v = vi.inflate(R.layout.mainrow, null);
        // cache view fields into the holder
        holder = new ViewHolder();
        holder.nameText = (TextView) v.findViewById(R.id.nameText);
        holder.priceText = (TextView) v.findViewById(R.id.priceText);
        holder.changeText = (TextView) v.findViewById(R.id.changeText);
        // associate the holder with the view for later lookup
        v.setTag(holder);
    }
    else {
        // view already exists, get the holder instance from the view
        holder = (ViewHolder) v.getTag();
    }
    // no local variables with findViewById here

    // use holder.nameText where you were 
    // using the local variable nameText before

    return v;
}

// somewhere else in your class definition
static class ViewHolder {
    TextView nameText;
    TextView priceText;
    TextView changeText;
}

警告:我没有尝试编译这个，所以请谨慎对待.

caveat: I didn't try to compile this, so take with a grain of salt.

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