我一直在 SO 线程中寻找答案，但无法从之前的讨论中找出我的问题。我有一个 ListView ，它加载了大约 50 张图像(它曾经是大约 100 张，但这几乎没有加载任何图像)。通过适配器从 api 端点获取我的 JSON 内容(包括图像 URL)后，我的代码将其放入 ListView 中。

目前， picasso 有 50 张图片，当我向下滚动提要时，它会一次加载一张图片。我觉得好像将滚动条固定在 ListView 中的一个项目上会使该图像加载速度更快。但是，当我向上滚动时，它会将占位符放回原处并再次重新加载图像。有办法解决这个问题吗？

public class MainActivity extends Activity {
    private List<Post> myPosts = new ArrayList<Post>();
    protected String[] mBlogPostTitles;
    public static final String TAG = MainActivity.class.getSimpleName();//prints name of class without package name

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if(isNetworkAvailable()) {
            GetBlogPostsTask getBlogPostsTask = new GetBlogPostsTask(); // new thread
            getBlogPostsTask.execute();// don't call do in background directly
        }else{
            Toast.makeText(this, "Network is unavailable", Toast.LENGTH_LONG).show();
        }
    }
    public boolean isNetworkAvailable() {
        ConnectivityManager manager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = manager.getActiveNetworkInfo();

        boolean isAvailable = false;

        if(networkInfo != null && networkInfo.isConnected()){
            isAvailable = true;
        }

        return isAvailable;
    }
    private void populateListView() {
        ArrayAdapter<Post> adapter = new MyListAdapter();
        ListView list = (ListView) findViewById(R.id.postsListView);
        list.setAdapter(adapter);
    }

    private class MyListAdapter extends ArrayAdapter<Post>{
        public MyListAdapter() {
            super(MainActivity.this, R.layout.item_view, myPosts);
        }

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

            // make sure we have a view to work with
            View itemView = convertView;
            if (itemView == null) {
                itemView = getLayoutInflater().inflate(R.layout.item_view, parent,false);
            }
            //find the post to work with
            Post currentPost = myPosts.get(position);
            Context context = itemView.getContext();

            String imageURL = currentPost.getImage();
            if(imageURL == null || imageURL.isEmpty()){
                ImageView imageView = (ImageView) itemView.findViewById(R.id.item_image);
                imageView.setVisibility(View.GONE);
            }else{
                ImageView imageView = (ImageView) itemView.findViewById(R.id.item_image);
                Picasso.with(context)
                        .load(imageURL)
                        .tag(context)
                        .placeholder(R.drawable.kanye8080s)
                        .error(R.drawable.stadiumarcadium)
                        .into(imageView);
                imageView.setVisibility(View.VISIBLE);
            }

            //Username
            TextView userText = (TextView) itemView.findViewById(R.id.item_txtUser);
            userText.setText(currentPost.getUser());

            //Time of post
            TextView timeText = (TextView) itemView.findViewById(R.id.item_txtTime);
            timeText.setText("" + currentPost.getTime());

            //The actual post
            TextView postText = (TextView) itemView.findViewById(R.id.item_txtPost);
            postText.setText("" + currentPost.getPost());

            //The actual post
            TextView likesText = (TextView) itemView.findViewById(R.id.item_txtLikes);
            likesText.setText("" + currentPost.getLikes());

            return itemView;
        }
    }

    private class GetBlogPostsTask extends AsyncTask<Object, Void, List> {

        @Override
        protected List doInBackground(Object[] params) {

            int responseCode = -1;//need to have this variable outside scope of try/catch block
            JSONObject jsonResponse = null;
            StringBuilder builder = new StringBuilder();
            HttpClient client = new DefaultHttpClient();
            HttpGet httpget = new HttpGet(""); /// api endpoint redacted

            try {

                HttpResponse response = client.execute(httpget);
                StatusLine statusLine = response.getStatusLine();
                responseCode = statusLine.getStatusCode();

                if(responseCode == HttpURLConnection.HTTP_OK){ //could have used just 200 value
                    HttpEntity entity = response.getEntity();
                    InputStream content = entity.getContent();
                    BufferedReader reader = new BufferedReader(new InputStreamReader(content));
                    String line;
                    while((line = reader.readLine()) != null){
                        builder.append(line);
                    }

                    jsonResponse = new JSONObject(builder.toString());

                    JSONArray jsonPosts = jsonResponse.getJSONArray("posts");
                    for(int i=0; i < jsonPosts.length(); i++ ){
                        JSONObject jsonPost = jsonPosts.getJSONObject(i);

                        int post_id = Integer.parseInt(jsonPost.getString("id"));
                        String post_user = jsonPost.getString("user");
                        String post_account = jsonPost.getString("account");
                        int post_time = Integer.parseInt(jsonPost.getString("time"));
                        String post_post = jsonPost.getString("post");
                        String post_image = jsonPost.getString("image");
                        int post_likes = Integer.parseInt(jsonPost.getString("likes"));

                        myPosts.add(new Post(post_id, post_user, post_account, post_time, post_post, post_image, "profile picture here", post_likes));
                    }
                }else{
                    Log.i(TAG, "Unsuccessful HTTP Response Code: " + responseCode);
                }
            }
            catch (MalformedURLException e){
                Log.e(TAG, "Exception caught");
            }
            catch (IOException e){
                Log.e(TAG, "Exception caught");
            }
            catch (Exception e){//must be in this order, this is the last, general catch
                Log.e(TAG, "Exception caught", e);
            }

            return null;
        }
        @Override
        protected void onPostExecute(List result) {
            // call populateListView method here
            populateListView();
            super.onPostExecute(result);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

编辑:

我已将我的代码更新为一个 View 持有者模式，创建了两个单独的 View (一个用于带有图像的帖子，一个用于带有文本的帖子)并且还包括 picasso 的新 scroll detection capabilities .

我看到一些图像加载速度有所提高，至少当 View 在滚动时聚焦时，图像现在更有可能加载。但是，在向上滚动那些曾经加载过的相同图像时，它们会消失。感觉 Picasso 一次只加载 4-5 张图像并替换已加载的图像以腾出空间。我更新的代码如下:

public class MainActivity extends Activity {
    private List<Post> myPosts = new ArrayList<Post>();
    protected String[] mBlogPostTitles;
    public static final String TAG = MainActivity.class.getSimpleName();//prints name of class without package name

    ...

    private void populateListView() {
        Activity activity = MainActivity.this;

        ArrayAdapter<Post> adapter = new MyListAdapter();
        ListView list = (ListView) findViewById(R.id.postsListView);
        list.setAdapter(adapter);
        list.setOnScrollListener(new SampleScrollListener(activity));
    }

    private class MyListAdapter extends ArrayAdapter<Post>{
        public MyListAdapter() {
            super(MainActivity.this, R.layout.item_view, myPosts);
        }

        @Override
        public int getViewTypeCount() {
            return 2;
        }

        @Override
        public int getItemViewType(int position) {
            String imageURL = myPosts.get(position).getImage();
            if(imageURL == null || imageURL.isEmpty()){
                return 1; // text based
            }else{
                return 0; // image based
            }
        }

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

            PostViewHolder holder;

            int type = getItemViewType(position);

            View itemView = convertView;

            // make sure we have a view to work with
            if (itemView == null) {
                holder = new PostViewHolder();
                if(type == 1) {
                    itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
                } else {
                    itemView = getLayoutInflater().inflate(R.layout.image_post_view, parent, false);
                    holder.image = (ImageView) itemView.findViewById(R.id.item_image);
                }
                holder.user = (TextView) itemView.findViewById(R.id.item_txtUser);
                holder.time = (TextView) itemView.findViewById(R.id.item_txtTime);
                holder.post = (TextView) itemView.findViewById(R.id.item_txtPost);
                holder.likes = (TextView) itemView.findViewById(R.id.item_txtLikes);

                itemView.setTag(holder);
            } else {
                holder = (PostViewHolder) itemView.getTag();
            }

            //find the post to work with
            Post currentPost = myPosts.get(position);

            if(type != 1) {
                Context context = itemView.getContext();
                String imageURL = currentPost.getImage();

                Picasso.with(context).setIndicatorsEnabled(true);
                //Picasso.with(context).setLoggingEnabled(true);
                Picasso.with(context)
                        .load(imageURL)
                        .tag(context)
                        .placeholder(R.drawable.kanye8080s)
                        //.skipMemoryCache()
                        .error(R.drawable.stadiumarcadium)
                        .fit()
                        .into(holder.image);
            }
            //Username
            holder.user.setText(currentPost.getUser());

            //Time of post
            holder.time.setText("" + currentPost.getTime());

            //The actual post
            holder.post.setText(currentPost.getPost());

            //Likes for the post
            holder.likes.setText("" + currentPost.getLikes());

            return itemView;
        }
    }
    public class SampleScrollListener implements AbsListView.OnScrollListener {
        private final Context context;

        public SampleScrollListener(Context context) {
            this.context = context;
        }

        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {
            final Picasso picasso = Picasso.with(context);
            if (scrollState == SCROLL_STATE_IDLE || scrollState == SCROLL_STATE_TOUCH_SCROLL) {
                picasso.resumeTag(context);
            } else {
                picasso.pauseTag(context);
            }
        }

        @Override
        public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,
                             int totalItemCount) {
            // Do nothing.
        }
    }
    ...
}

问题出在哪里？我应该以某种方式在缓存中预加载这些图像吗？虽然我已经研究过 Picasso 的新优先级功能，但我是否应该告诉 Picasso 以某种方式按照它们在我的 ListView 中出现的顺序加载图像？有任何想法吗？如何在向上滚动时“保留”已加载的图像？

最佳答案

使用 picasso 调整大小

 Picasso.with(context)
.load(imageURL)
.tag(context)
.placeholder(R.drawable.kanye8080s)
.error(R.drawable.stadiumarcadium)
.into(imageView)
.resize(x,y);

//这肯定有帮助

关于java - picasso 在 ListView 中向上滚动时不断重新加载图像，加载缓慢，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/27771646/