android - 使用 AsyncTask 填充图像的 GridView 时滚动缓慢

Question

你好，我这里有点问题。我正在使用和异步任务来显示来自外部或内部存储的图像。现在它可以工作，但问题是，它非常缓慢并且在滚动时有点不稳定。我不知道为什么？请任何解决方案或想法如何做到这一点。

import java.io.IOException;
import java.util.ArrayList;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Debug;
import android.os.Environment;
import android.provider.MediaStore;
import android.view.Display;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.Window;
import android.view.WindowManager;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.GridView;
import android.widget.ImageView;
import android.widget.Toast;
import android.widget.AdapterView.OnItemClickListener;

public class Wallpaper extends Activity implements OnItemClickListener{

/*Instance variables*/
 private GridView grid;
 private ImageAdapter imageAdapter;
 private Display display;
 Cursor cursor;
 boolean inInternalStorage = false;



 public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.wallpaper_images);

        display = ((WindowManager) getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();

        setupViews();
        setProgressBarIndeterminateVisibility(true); 
        loadImages();
    }

 @Override
 protected void onStop(){
     super.onStop();
 }

 /*methods called from AsyncTask as appropriate when its querying and processing the images from the MediaStore*/

 private void loadImages() {
 final Object data = getLastNonConfigurationInstance();
 if(data == null){
     new LoadImagesFromSDCard().execute();
 }else {
     final LoadedImage[] photos = (LoadedImage[])data;
     if(photos.length == 0){

        new LoadImagesFromSDCard().execute();
     }
     for(LoadedImage photo:photos){
         addImage(photo);
    }
 }

}

private void addImage(LoadedImage... value) {

    for(LoadedImage photo: value){
        imageAdapter.addPhotos(photo);  
        imageAdapter.notifyDataSetChanged();
    }

}

private void setupViews() {

        grid = (GridView)findViewById(R.id.gridview);
        grid.setNumColumns(display.getWidth()/95);
        grid.setClipToPadding(false);
        imageAdapter = new ImageAdapter(getApplicationContext());
        grid.setAdapter(imageAdapter);
        grid.setOnItemClickListener(this);
}

protected void onDestroy() {
        super.onDestroy();
        final GridView gridview = grid;
        final int count = grid.getChildCount();
        ImageView v = null;
        for (int i = 0; i < count; i++) {
            v = (ImageView) grid.getChildAt(i);
            ((BitmapDrawable) v.getDrawable()).setCallback(null); 
        }

        unbindDrawables(findViewById(R.id.gridview));
        System.gc();
    }

/*public Object onRetainNonConfigurationInstance(){
    final GridView gridview = grid;
    final int count = grid.getChildCount();
    final LoadedImage[] list = new LoadedImage[count];

    for(int i = 0; i < count; i++){
        final ImageView v = (ImageView)grid.getChildAt(i);
        list[i] = new LoadedImage(((BitmapDrawable) v.getDrawable()).getBitmap());
    }
    return list;
}*/


/*utility method called that prevents screen from crashing when screen orientation changes*/
private void unbindDrawables(View view){
    if(view.getBackground() != null){
        view.getBackground().setCallback(null);
    }
    if(view instanceof ViewGroup){
        for(int i = 0; i < ((ViewGroup) view).getChildCount(); i++){
            unbindDrawables(((ViewGroup)view).getChildAt(i));
        }
        try{
            ((ViewGroup)view).removeAllViews();
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}

/*AsyncTask thats queries the MediaStore for images and creates thumbnail images from bitmaps*/
class LoadImagesFromSDCard extends AsyncTask<Object, LoadedImage, Object> {

    @Override
    protected Object doInBackground(Object... params) {

        Cursor cursor;
        Bitmap bitmap = null;
        Bitmap newbitmap = null;
        Uri uri = null;

        String [] img = {MediaStore.Images.Media._ID};

        String state = Environment.getExternalStorageState();

        if(Environment.MEDIA_MOUNTED.equals(state)){    

        cursor = getContentResolver().query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, img, 
          MediaStore.Images.Media.DATA + " like ? " , new String[]{ "%dcim%"}, null);
        } else {
         cursor = getContentResolver().query(MediaStore.Images.Media.INTERNAL_CONTENT_URI, img, null, null, null);
         inInternalStorage = true;
        }

        int column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media._ID);
        int size = cursor.getCount();

        if(size == 0){
            Toast.makeText(getApplicationContext(), "There are no Images on the sdcard", Toast.LENGTH_SHORT).show();

        }else {

        }

        for(int i = 0; i < size; i++){
            cursor.moveToPosition(i);
            int ImageId = cursor.getInt(column_index);

            if(inInternalStorage == true){
             uri = Uri.withAppendedPath(MediaStore.Images.Media.INTERNAL_CONTENT_URI, "" + ImageId);
         }else {
             uri = Uri.withAppendedPath(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, "" + ImageId);
         }


         try {
             BitmapFactory.Options options = new BitmapFactory.Options();
             options.inSampleSize=4;


             bitmap = BitmapFactory.decodeStream(getContentResolver().openInputStream(uri), null, options);

             if(bitmap != null){
                 newbitmap = Bitmap.createScaledBitmap(bitmap, 100, 100, true);
                 bitmap.recycle();
             }
             if(newbitmap != null){
                 publishProgress(new LoadedImage(newbitmap));
             }
         }catch(IOException e){

         }
        }
        cursor.close();
        return null;    
    }

    @Override
    public void onProgressUpdate(LoadedImage... value){
        addImage(value);
    }

       @Override
        protected void onPostExecute(Object result) {
            setProgressBarIndeterminateVisibility(false);
        }
}

private static class LoadedImage {
    Bitmap mBitmap;

    LoadedImage(Bitmap bitmap) {
        mBitmap = bitmap;
    }

    public Bitmap getBitmap() {
        return mBitmap;
    }
}

/*Image Adapter to populate grid view of images*/

public class ImageAdapter extends BaseAdapter {
    private Context mContext;
    private ArrayList<LoadedImage> photos = new ArrayList<LoadedImage>();


    public ImageAdapter(Context context){
        this.mContext = context;
    }

    public void addPhotos(LoadedImage photo){
        photos.add(photo);
    }

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


    @Override
    public Object getItem(int position) {
        return position;
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        System.gc();
        ImageView image;
        ViewHolder holder;

        if(convertView == null){
            LayoutInflater inflater = (LayoutInflater)mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.image_list,null);
            holder = new ViewHolder();
            holder.image = (ImageView)convertView.findViewById(R.id.image_list_id);
            convertView.setTag(holder); 
        } else{
            holder = (ViewHolder)convertView.getTag();
        }   
        holder.image.setLayoutParams(new GridView.LayoutParams(100, 100));
        holder.image.setImageBitmap(photos.get(position).getBitmap());
        return convertView;
    }
}

static class ViewHolder {
   ImageView image;
}

@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
    Cursor cursor = null;
    int image_column_index = 0;
    String[] proj = {MediaStore.Images.Media.DATA};

    String state = Environment.getExternalStorageState();

  if(Environment.MEDIA_MOUNTED.equals(state)){

    cursor = managedQuery(MediaStore.Images.Media.EXTERNAL_CONTENT_URI,proj, MediaStore.Images.Media.DATA + " like ? ", new String[]{"%dcim%"},null);

  }else{
      cursor = managedQuery(MediaStore.Images.Media.INTERNAL_CONTENT_URI,proj,null, null,null);
  }
    cursor.moveToPosition(position);
    image_column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);


    String info = cursor.getString(image_column_index);
    Intent imageviewer  = new Intent(getApplicationContext(), ViewImage.class);
           imageviewer.putExtra("pic_name", info);
           startActivity(imageviewer);

}

}

我想我所追求的是一种在我调用 publishProgress() 之前允许 doBackground() 运行一段时间的方法。就像 android 中的图库应用程序一样。任何关于为什么这可能很慢或如何解决这个问题的想法将不胜感激，因为我已经坚持了一段时间如何提高性能。

Answer 1

最大的瓶颈是从设备(内部或外部)访问图像。因此，您会希望内存中有尽可能多的图像(当然是在合理范围内)。您可以通过多种方式做到这一点:

• 首先加载大约 18 张图像(如您所说，在显示之前进行预缓冲)。检查以确保图片数量不超过您选择的数量。

• 创建一个“thumbs.db”类型的文件，用于存储 100x100 像素的位图缩略图。这将允许更快的读取，因为您只需要读取一个文件然后提取每个位图。如果用户单击图像，则从存储中请求它。此方法需要更多工作，但能够非常快速地加载缩略图。您可能必须设计自己的简单文件头，例如:

   <file header (size of file, number of images)>
   <image header (img id, size in bytes)>
   <image bitmap data>
   <image header (img id, size in bytes)>
   <image bitmap data>
   <image header (img id, size in bytes)>
   <image bitmap data>
   ...