懒惰下载图像到gridView

Question

在我的应用程序中，我需要从网址下载大量图片并将其显示在gridView中。 （可能在1-200张图片之间）。我不想一次下载所有图片。我读到了关于懒惰下载的问题，我的问题是：我可以只获得Json的一部分，在不同的线程中下载图片，并且只有当用户向下滚动 gridView时，我才会继续Json的其他部分，依此类推？

编辑：你好。我想在这个gridView中实现多选，我很难在适配器的getView（）方法中实现代码。这是我正在使用的示例：example。如何在getView（）方法中组合此代码：

public View getView(int position, View convertView, ViewGroup parent) {
        CheckableLayout l;
        ImageView i;

        if (convertView == null) {
            i = new ImageView(Grid3.this);
            i.setScaleType(ImageView.ScaleType.FIT_CENTER);
            i.setLayoutParams(new ViewGroup.LayoutParams(50, 50));
            l = new CheckableLayout(Grid3.this);
            l.setLayoutParams(new GridView.LayoutParams(GridView.LayoutParams.WRAP_CONTENT,
                    GridView.LayoutParams.WRAP_CONTENT));
            l.addView(i);
        } else {
            l = (CheckableLayout) convertView;
            i = (ImageView) l.getChildAt(0);
        }

        ResolveInfo info = mApps.get(position);
        i.setImageDrawable(info.activityInfo.loadIcon(getPackageManager()));

        return l;
    }


public class CheckableLayout extends FrameLayout implements Checkable {
    private boolean mChecked;

    public CheckableLayout(Context context) {
        super(context);
    }

    public void setChecked(boolean checked) {
        mChecked = checked;
        setBackgroundDrawable(checked ?
                getResources().getDrawable(R.drawable.blue)
                : null);
    }

    public boolean isChecked() {
        return mChecked;
    }

    public void toggle() {
        setChecked(!mChecked);
    }

}

我的getView（）代码：

public View getView(int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    ViewHolder holder;
    View vi = convertView;

    if(convertView == null) {
        vi = inflater.inflate(com.egedsoft.instaprint.R.layout.item_clickable, null);
        holder = new ViewHolder();
        holder.imgPhoto = (ImageView)vi.findViewById(com.egedsoft.instaprint.R.id.imageClickable);
        vi.setTag(holder);

    } else {
        holder = (ViewHolder) vi.getTag();
    }


    if (!arrayUrls.get(position).getThumbnailUrl().isEmpty()){
        imageLoader.DisplayImage(arrayUrls.get(position).getThumbnailUrl(), holder.imgPhoto);
    }


    return vi;
}

Answer 1

这是我在活动中获取多张照片的方法。您可以使用它的一部分来适应您的逻辑。我用它来从相册中获取Facebook图像。所以我的需求（我假设）与您的需求不同。但同样，逻辑可能对你有用。

注意：这将是漫长的。 ; - ）

这些是通过ACtivity使用的全局声明：

// HOLD THE URL TO MAKE THE API CALL TO
private String URL;

// STORE THE PAGING URL
private String pagingURL;

// FLAG FOR CURRENT PAGE
int current_page = 1;

// BOOLEAN TO CHECK IF NEW FEEDS ARE LOADING
Boolean loadingMore = true;

Boolean stopLoadingData = false;

这是获取初始图像集的代码块：

private class getPhotosData extends AsyncTask<Void, Void, Void> {

    @Override
    protected Void doInBackground(Void... arg0) {

        // CHANGE THE LOADING MORE STATUS TO PREVENT DUPLICATE CALLS FOR
        // MORE DATA WHILE LOADING A BATCH
        loadingMore = true;

        // SET THE INITIAL URL TO GET THE FIRST LOT OF ALBUMS
        URL = "https://graph.facebook.com/" + initialAlbumID
                + "/photos&access_token="
                + Utility.mFacebook.getAccessToken() + "?limit=10";

        try {

            HttpClient hc = new DefaultHttpClient();
            HttpGet get = new HttpGet(URL);
            HttpResponse rp = hc.execute(get);

            if (rp.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
                String queryAlbums = EntityUtils.toString(rp.getEntity());

                JSONObject JOTemp = new JSONObject(queryAlbums);

                JSONArray JAPhotos = JOTemp.getJSONArray("data");

                // IN MY CODE, I GET THE NEXT PAGE LINK HERE

                getPhotos photos;

                for (int i = 0; i < JAPhotos.length(); i++) {
                    JSONObject JOPhotos = JAPhotos.getJSONObject(i);
                    // Log.e("INDIVIDUAL ALBUMS", JOPhotos.toString());

                    if (JOPhotos.has("link")) {

                        photos = new getPhotos();

                        // GET THE ALBUM ID
                        if (JOPhotos.has("id")) {
                            photos.setPhotoID(JOPhotos.getString("id"));
                        } else {
                            photos.setPhotoID(null);
                        }

                        // GET THE ALBUM NAME
                        if (JOPhotos.has("name")) {
                            photos.setPhotoName(JOPhotos.getString("name"));
                        } else {
                            photos.setPhotoName(null);
                        }

                        // GET THE ALBUM COVER PHOTO
                        if (JOPhotos.has("picture")) {
                            photos.setPhotoPicture(JOPhotos
                                    .getString("picture"));
                        } else {
                            photos.setPhotoPicture(null);
                        }

                        // GET THE PHOTO'S SOURCE
                        if (JOPhotos.has("source")) {
                            photos.setPhotoSource(JOPhotos
                                    .getString("source"));
                        } else {
                            photos.setPhotoSource(null);
                        }

                        arrPhotos.add(photos);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {

        // SET THE ADAPTER TO THE GRIDVIEW
        gridOfPhotos.setAdapter(adapter);

        // CHANGE THE LOADING MORE STATUS
        loadingMore = false;
    }

}

这是为了检测用户何时滚动到最后并获取新的图像集：

// ONSCROLLLISTENER
gridOfPhotos.setOnScrollListener(new OnScrollListener() {

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {

    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem,
            int visibleItemCount, int totalItemCount) {
        int lastInScreen = firstVisibleItem + visibleItemCount;
        if ((lastInScreen == totalItemCount) && !(loadingMore)) {

            if (stopLoadingData == false) {
                // FETCH THE NEXT BATCH OF FEEDS
                new loadMorePhotos().execute();
            }

        }
    }
});

最后，这就是我获取下一组图像的方式：

private class loadMorePhotos extends AsyncTask<Void, Void, Void> {

    @Override
    protected Void doInBackground(Void... arg0) {

        // SET LOADING MORE "TRUE"
        loadingMore = true;

        // INCREMENT CURRENT PAGE
        current_page += 1;

        // Next page request
        URL = pagingURL;

        try {

            HttpClient hc = new DefaultHttpClient();
            HttpGet get = new HttpGet(URL);
            HttpResponse rp = hc.execute(get);

            if (rp.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
                String queryAlbums = EntityUtils.toString(rp.getEntity());
                // Log.e("PAGED RESULT", queryAlbums);

                JSONObject JOTemp = new JSONObject(queryAlbums);

                JSONArray JAPhotos = JOTemp.getJSONArray("data");

                // IN MY CODE, I GET THE NEXT PAGE LINK HERE

                getPhotos photos;

                for (int i = 0; i < JAPhotos.length(); i++) {
                    JSONObject JOPhotos = JAPhotos.getJSONObject(i);
                    // Log.e("INDIVIDUAL ALBUMS", JOPhotos.toString());

                    if (JOPhotos.has("link")) {

                        photos = new getPhotos();

                        // GET THE ALBUM ID
                        if (JOPhotos.has("id")) {
                            photos.setPhotoID(JOPhotos.getString("id"));
                        } else {
                            photos.setPhotoID(null);
                        }

                        // GET THE ALBUM NAME
                        if (JOPhotos.has("name")) {
                            photos.setPhotoName(JOPhotos.getString("name"));
                        } else {
                            photos.setPhotoName(null);
                        }

                        // GET THE ALBUM COVER PHOTO
                        if (JOPhotos.has("picture")) {
                            photos.setPhotoPicture(JOPhotos
                                    .getString("picture"));
                        } else {
                            photos.setPhotoPicture(null);
                        }

                        // GET THE ALBUM'S PHOTO COUNT
                        if (JOPhotos.has("source")) {
                            photos.setPhotoSource(JOPhotos
                                    .getString("source"));
                        } else {
                            photos.setPhotoSource(null);
                        }

                        arrPhotos.add(photos);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {

        // get listview current position - used to maintain scroll position
        int currentPosition = gridOfPhotos.getFirstVisiblePosition();

        // APPEND NEW DATA TO THE ARRAYLIST AND SET THE ADAPTER TO THE
        // LISTVIEW
        adapter = new PhotosAdapter(Photos.this, arrPhotos);
        gridOfPhotos.setAdapter(adapter);

        // Setting new scroll position
        gridOfPhotos.setSelection(currentPosition + 1);

        // SET LOADINGMORE "FALSE" AFTER ADDING NEW FEEDS TO THE EXISTING
        // LIST
        loadingMore = false;
    }

}

这是SET和GET从上述查询中收集的数据的帮助类：

public class getPhotos {

    String PhotoID;

    String PhotoName;

    String PhotoPicture;

    String PhotoSource;

    // SET THE PHOTO ID
    public void setPhotoID(String PhotoID)  {
        this.PhotoID = PhotoID;
    }

    // GET THE PHOTO ID
    public String getPhotoID()  {
        return PhotoID;
    }

    // SET THE PHOTO NAME
    public void setPhotoName(String PhotoName)  {
        this.PhotoName = PhotoName;
    }

    // GET THE PHOTO NAME
    public String getPhotoName()    {
        return PhotoName;
    }

    // SET THE PHOTO PICTURE
    public void setPhotoPicture(String PhotoPicture)    {
        this.PhotoPicture = PhotoPicture;
    }

    // GET THE PHOTO PICTURE
    public String getPhotoPicture() {
        return PhotoPicture;
    }

    // SET THE PHOTO SOURCE
    public void setPhotoSource(String PhotoSource)  {
        this.PhotoSource = PhotoSource;
    }

    // GET THE PHOTO SOURCE
    public String getPhotoSource()  {
        return PhotoSource;
    }
}

如果您还需要adapter代码，请与我们联系。我在适配器中使用了Fedor's Lazy Loading方法。

呼。希望这有任何帮助。如果您还有其他问题，请随时提出。 ： - ）

编辑：添加了适配器代码：

public class PhotosAdapter extends BaseAdapter {

    private Activity activity;

    ArrayList<getPhotos> arrayPhotos;

    private static LayoutInflater inflater = null;
    ImageLoader imageLoader; 

    public PhotosAdapter(Activity a, ArrayList<getPhotos> arrPhotos) {

        activity = a;

        arrayPhotos = arrPhotos;

        inflater = (LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        imageLoader = new ImageLoader(activity.getApplicationContext());
    }

    public int getCount() {
        return arrayPhotos.size();
    }

    public Object getItem(int position) {
        return arrayPhotos.get(position);
    }

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

    public View getView(final int position, View convertView, ViewGroup parent) {

        ViewHolder holder;

        View vi = convertView;
        if(convertView == null) {
            vi = inflater.inflate(R.layout.photos_item, null);

            holder = new ViewHolder();

            holder.imgPhoto = (ImageView)vi.findViewById(R.id.grid_item_image);

            vi.setTag(holder);
          } else {
            holder = (ViewHolder) vi.getTag();
        }


        if (arrayPhotos.get(position).getPhotoPicture() != null){
            imageLoader.DisplayImage(arrayPhotos.get(position).getPhotoPicture(), holder.imgPhoto);
        }
        return vi;
    }

    static class ViewHolder {
        ImageView imgPhoto;

    }
}

编辑：添加了加载时显示进度的步骤：

向您的XML添加ProgressBar，您可以在其下方使用GridView。如果引起任何问题，请尽量使用重量。

<LinearLayout
    android:id="@+id/linlaProgressBar"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:orientation="horizontal" >

    <ProgressBar
        style="@style/Spinner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="2dp" />
</LinearLayout>

在您的Java中，将Linearlayout linlaProgressBar声明为全局并将其投放到onCreate()并将其可见性设置为linlaProgressBar.setVisibility(View.GONE);

在onPreExecute()中使用它：

@Override
protected void onPreExecute() {
    // SHOW THE BOTTOM PROGRESS BAR (SPINNER) WHILE LOADING MORE PHOTOS
    linlaProgressBar.setVisibility(View.VISIBLE);
}

最后在onPostExecute()

中添加

// HIDE THE BOTTOM PROGRESS BAR (SPINNER) AFTER LOADING MORE ALBUMS
linlaProgressBar.setVisibility(View.GONE);

Answer 2

您可以从Android文档中查看Using adapter Views和GridView。 最重要的是适配器调用方法getView只传递屏幕上显示的条目的位置，并在用户滚动时询问不同的位置。

简单的方法是使用和AsyncTask在适配器的getView方法上下载所需的图片。

有一个example

Answer 3

根据经验，在合理消耗内存的同时实现平滑滚动（和整体响应）很棘手。

首先寻找现有解决方案是个好主意，例如，从这里开始： Lazy load of images in ListView

我们最终获得了定制的专有解决方案。它是一个后台线程，它将下载请求和下载以及高速缓存排在外部存储上，只显示仍然可见的图像。当新图像到达时，视图会得到通知，并决定何时通知适配器更新。

它还节省了带宽，这在某些情况下很重要。

Answer 4

我发现 IceMAN的回答非常有用，但我也建议避免使用两个AsyncTasks，你可以轻松完成。

您需要创建一个通用方法来获取所需的数据，您可以在其中创建if / else条件（作为示例）：

 <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/bg"
android:orientation="vertical" >
<ScrollView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" >

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_marginTop="5dp"
        android:background="@android:color/transparent"
        android:orientation="vertical" >

        <WebView
            android:id="@+id/webRootOperations"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:visibility="gone"
            android:background="@android:color/transparent" />

 </LinearLayout>
 </ScrollView>
</LinearLayout>

addMovies 是onScroll方法中的布尔值。 在AsyncTask中提供查询URL中的当前页面并且瞧 - 您使代码更小：）