Android UI滞后于设置图像

Question

I'm trying to create a simple app to learn about navigation drawers and setting images. I based this app on the template that Android Studio created for me.

I read that getting the drawable can be a long process so I made an Asynctask for it. Despite this, my UI still lags and I get the message that frames are being skipped due to too much work on the main thread. I figured out that the following line is causing those issues:

pic.setImageDrawable(drawable);    //(in onPostExecute)

I did all the processing in the background and this line should just be setting the drawable. Why is this lagging so much?

I read somewhere that setting the dimensions might make a difference. Right now I have both dimensions set to match_parent and the image is centered in the view. I don't want to give it a definite size.

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

private ImageView pic;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    pic = (ImageView) findViewById(R.id.pic);

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);

    //set first item to be selected and set its image
    navigationView.getMenu().getItem(0).setChecked(true);
    new ImageSetter().execute();
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    try {
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    } catch (NullPointerException e) {
        Log.e("back pressed", e.getStackTrace().toString());
    }
}

@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();
    new ImageSetter(id).execute();

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

/**
 * class for getting the image drawable in the background and then setting it
 */
private class ImageSetter extends AsyncTask<Integer, Void, Drawable> {

    int id = //image1;

    public ImageSetter() {
    }

    public ImageSetter(int id) {
        this.id = id;
    }

    /**
     * Get drawable in background
     * Precondition: id has already been set
     * @param params
     * @return drawable of the picture to set
     */
    @Override
    protected Drawable doInBackground(Integer... params) {
        int pic = R.drawable.b1;

        switch (id) {
           //set pic
        }

        return getResources().getDrawable(pic);
    }

    // Once complete, see if ImageView is still around and set drawable
    @Override
    protected void onPostExecute(Drawable drawable) {
        if (pic != null) {
            pic.setImageDrawable(drawable);
        }
    }
}

}

Answer 1

The code looks fine to me. using setImageDrawable() shouldn't be lagging too bad and does need to be run in the UI thread. You have offloaded the task to another thread, which is good.

How big is the source image that you are fetching? Android is notoriously bad at handling large images...

If your source image is too big, I would look at Loading Large Bitmaps Efficiently from the Android developer site. This will give you some tips on handling Bitmaps.