如何有效地将可绘制文件夹中的位图加载到ListView中？

Question

如何将drawable文件夹中的图像加载到一个ListView快速发生且不使用大量RAM的方式？

Answer 1

注意:此帖子已弃用,请使用RecyclerView创建列表

我一直在玩把drawable文件夹中存储的一些(非常大的)图像加载到a中ListView,在这篇文章中我想分享我来的结果.也许(我希望如此)这会节省一些时间.我已经测试了我在几个Android 4+设备上发布的代码,我可以说它运行得相当顺利,并且使用的内存量相对较低.一些解释如下:

• 我们正在扩展 BaseAdapter
• 使用图像将在后台加载 AsyncTask
• 对于这种适配器来说,我们将使用一些自定义类的ArrayList<>参数化Objects.在我的应用程序中,此类称为武器
• 我们将根据屏幕尺寸缩放图像
• 我们将TextView在每个List Row中应用一种字体

您可以将此代码用于任何目的,并以任何方式对其进行修改.我唯一要求的是在声称某些东西不起作用之前正确测试代码.它相信,相信我.

如果您注意到任何复制 - 粘贴 - 编辑错误(因为我删除了一些与此小教程无关的代码),欢迎您提供反馈.

在发布代码之前,这是一个演示getView()方法逻辑的小状态图:

该Adapter课程的代码如下:我试图在评论中解释你需要的一切:

public class WeaponAdapter extends BaseAdapter implements View.OnClickListener {

private ArrayList<Weapon> items;
private LayoutInflater inflater = null;
private WeaponHolder weaponHolder;
private Weapon wp;
private Context c;
private Bitmap bmp;

/*--- a simple View Holder class ---*/
static class WeaponHolder {
    public TextView text;
    public ImageView image, addFav;
    public AsyncImageSetter mImageLoader;

}

/*--- Context and all weapons of specified class are passed here ---*/

public WeaponAdapter(ArrayList<Weapon> items, Context c) {
    this.items = (ArrayList<Weapon>) items;
    inflater = LayoutInflater.from(c);
    this.c = c;
}

@Override
public int getCount() {
    return items.size();

}

@Override
public Weapon getItem(int position) {
    return items.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    /*--- initialize our Weapon Object ---*/
    wp = items.get(position);

    if (convertView == null) {
        /*--- no View is available. Inflate our list item layout and init the Views we need ---*/
        convertView = inflater.inflate(R.layout.category_row, null);
        weaponHolder = new WeaponHolder();
        weaponHolder.text = (TextView) convertView
                .findViewById(R.id.tvCatText);
        weaponHolder.image = (ImageView) convertView
                .findViewById(R.id.imgCatImage);
        weaponHolder.addFav = (ImageView) convertView
                .findViewById(R.id.imgAddFav);
        convertView.setTag(weaponHolder);
    } else {
        weaponHolder = (WeaponHolder) convertView.getTag();
        /*--- if convertView is not null, cancel the current loading operation to 
         * improve performance and decrease RAM usage ---*/
        weaponHolder.mImageLoader.cancel();
    }
    /*--- load the image in background ---*/
    weaponHolder.mImageLoader = new AsyncImageSetter(c, weaponHolder.image,
            wp.getImage(), bmp, weaponHolder.text);
    weaponHolder.mImageLoader.execute();
    weaponHolder.text.setText(wp.getName());
    weaponHolder.addFav.setOnClickListener(this);
    return convertView;

}

@Override
public void onClick(View v) {


        // do any stuff here



}
   }

这是我们AsyncTask将在后台加载和设置图像.

注意:我的Weapon类有一个getImage()方法,它返回drawable对应于武器的resId Object.您可以按照适合自己的方式修改此部件.

public class AsyncImageSetter extends AsyncTask<Void, Void, Bitmap> {

private ImageView img;
private int image_resId;
private Bitmap bmp;
private Context c;
private boolean cancel = false;
private int sampleSize;
private TextView txtGunName;
private Typeface font;


public AsyncImageSetter(Context c, ImageView img, int image_ResId,
        Bitmap bmp, TextView txtGunName) {

    this.img = img;
    this.image_resId = image_ResId;
    this.bmp = bmp;
    this.c = c;
    this.txtGunName = txtGunName;

}

public void cancel() {
    cancel = true;
}

@Override
protected void onPreExecute() {
    /*--- we hide the Views from the user until the content is ready. This will prevent
     * the user from seeing an image being "transformed" into the next one (as a result of
     * View recycling) on slow devices. 
     */
    img.setVisibility(View.GONE);
    txtGunName.setVisibility(View.GONE);
    font = Typeface.createFromAsset(c.getAssets(), "b_reg.otf");
    super.onPreExecute();
}

@Override
protected Bitmap doInBackground(Void... params) {

    if (!cancel) {
        try {
            return decodeAndScale(bmp);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    return null;
}

@Override
protected void onPostExecute(Bitmap result) {

    img.setVisibility(View.VISIBLE);
    try {
        img.setImageBitmap(result);
    } catch (Exception e) {
        /*--- show an error icon in case something went wrong ---*/
        img.setImageResource(R.drawable.ic_warn);
    }

    txtGunName.setVisibility(View.VISIBLE);
    txtGunName.setTypeface(font);
    super.onPostExecute(result);
}

private Bitmap decodeAndScale(Bitmap bmp) {

    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inSampleSize = setSampleSize();

    return BitmapFactory.decodeResource(c.getResources(), image_resId,
            options);

}

private int setSampleSize() {

    // TODO add multiple screens check
/*--- modify this method to match your needs ---*/
    if (GetSettings.getScreenWidth((Activity) c) >= 320) {
        /*--- physical width >= 480px ---*/
        sampleSize = 2;
    }

    return sampleSize;
}}

您可能已经注意到我使用getScreenWidth()了GetSettings该类中的方法.它的代码非常简单,并返回表示设备屏幕宽度的dp值:

    public static int getScreenWidth(Activity a) {

    Display display = a.getWindowManager().getDefaultDisplay();
    DisplayMetrics outMetrics = new DisplayMetrics();
    display.getMetrics(outMetrics);

    float density = a.getResources().getDisplayMetrics().density;
    float dpWidth = outMetrics.widthPixels / density;

    return (int) dpWidth;

}

嗯,这就是全部,我希望这篇文章对某人有所帮助.干杯.

PS如果你肯定确定某些东西不起作用,很可能是因为你的内部应用程序结构与我使用的不同.在这种情况下,我建议您执行以下步骤:

1. 提出一个新问题,这样您就可以添加格式正确的代码和LogCat输出
2. 通过在我的帖子中添加评论来通知我.我很乐意帮你弄清楚什么是错的