java - Android多线程预加载imageview(uri)

Question

我编写了一个简单的程序来在 ListView 中显示图像，但是当我滚动列表时，它非常慢。我想创建一个新线程来处理 imageview 的加载。我在论坛和文档中阅读，但我无法理解，您可以在单独的线程上编写简单的代码，也许使用asyntask，来管理什么(在单独的线程中预加载图像并仅在加载时显示)？(我打算不使用毕加索或其他)

现在在我的代码中，当我按下菜单项时，我将 ImageView (uri)添加到线性布局。这是我的代码:

主要 Activity :

public class MainActivity extends AppCompatActivity {



ImageView image;
LinearLayout background;


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

    background=(LinearLayout)findViewById(R.id.ll);


}

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


    return true;
}



@Override
public boolean onOptionsItemSelected(MenuItem item) {

    int id = item.getItemId();
    if (id == R.id.action_settings) {


        //open gallery
        Intent i = new Intent(Intent.ACTION_PICK,android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
        i.setType("image/* video/*");
         startActivityForResult(i, 0);





         return true;
    }
    return super.onOptionsItemSelected(item);
}

protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    // Here we need to check if the activity that was triggers was the Image Gallery.
    // If it is the requestCode will match the LOAD_IMAGE_RESULTS value.
    // If the resultCode is RESULT_OK and there is some data we know that an image was picked
    if (requestCode == 0 && resultCode == RESULT_OK && null != data) {
        // Let's read picked image data - its URI
        Uri selectedImage = data.getData();
        // Get the real path from the Uri
        // Let's read picked image path using content resolver  
        String[] filePathColumn = { MediaStore.Images.Media.DATA };

        Cursor cursor = getContentResolver().query(selectedImage,
                filePathColumn, null, null, null);
        cursor.moveToFirst();

        int columnIndex = cursor.getColumnIndex(filePathColumn[0]);
        String picturePath = cursor.getString(columnIndex);
        cursor.close();



        image=new ImageView(getApplicationContext());

        image.setImageURI(selectedImage);


        background.addView(image);



    }

}
}

activity_main.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.provathreadimmagini.MainActivity" >

              <ScrollView
                 android:id="@+id/s"
                 android:layout_width="match_parent"
                 android:layout_height="match_parent"
                 android:layout_above="@+id/linearLayout1"
                 android:layout_alignParentLeft="true"
                 android:layout_alignParentTop="true"
                  android:fillViewport="true"
                     >

                 <LinearLayout
                     android:id="@+id/ll"
                     android:layout_width="match_parent"
                     android:layout_height="wrap_content"
                     android:gravity="center"
                     android:orientation="vertical"
                      android:paddingBottom="20dp"  >



                 </LinearLayout>
             </ScrollView>


</RelativeLayout>

谢谢

Answer 1

The idea is that the UI thread (main thread of the application) will not get stuck waiting every time you want to access your files and search for a specific image in order to load it.

For this case you can use (for example) an AsyncTask.

AsyncTask allow you to perform background operations (on a background thread) that will not cause your UI thread freezing.

正如您在文档中看到的，您应该创建一个扩展 AsyncTask 的类:

// In this example first param (URL) indicates the parameter you want to pass. 
// The second param is the progress unit you want to publish while it is computing
// The third is the result you wish yo send to the UI thread
public class BackgroundTask extends AsyncTask<URL, Integer, Long> {

}

现在，您必须重写至少一个方法:

// Long, URL - as stated above
protected Long doInBackground(URL... urls) {

}

为了完成此操作，您可能需要扩展结果函数

// In this function you should call your UI thread to set the image 
protected void onPostExecute(Long result) {
     showDialog("Downloaded " + result + " bytes");
 }

对于您的示例，我建议您将 onActivityResult 代码移至新的 doInBackground 方法中。

编辑:

在您的主要 Activity 中:

protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    BackgroundTask task = new BackgroundTask(mImage);
    task.execute(data.getData());

在BackgroundTask类中:

protected Long doInBackground(Uri... uris) {
    String[] filePathColumn = { MediaStore.Images.Media.DATA };

    Cursor cursor = getContentResolver().query(uris[0],
            filePathColumn, null, null, null);
    cursor.moveToFirst();

    int columnIndex = cursor.getColumnIndex(filePathColumn[0]);
    String picturePath = cursor.getString(columnIndex);
    cursor.close();

    mImage.setImageURI(uris[0]);
}