java - 实现 AsyncTask 的正确方法是什么？静态或非静态嵌套类？

Question

Android 示例中的“登录”将 AsyncTask 实现为非静态内部类。然而，根据 Commonsguys 的说法，这个类应该是静态的并且使用对外部 Activity 的弱引用 see this .

那么实现AsyncTask的正确方法是什么？静态还是非静态？

Commonsguy 实现
https://github.com/commonsguy/cw-android/tree/master/Rotation/RotationAsync/

来自 Google 的登录示例

package com.example.asynctaskdemo;

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.annotation.TargetApi;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Build;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.View;
import android.view.inputmethod.EditorInfo;
import android.widget.EditText;
import android.widget.TextView;

/**
 * Activity which displays a login screen to the user, offering registration as
 * well.
 */
public class LoginActivity extends Activity {
    /**
     * A dummy authentication store containing known user names and passwords.
     * TODO: remove after connecting to a real authentication system.
     */
    private static final String[] DUMMY_CREDENTIALS = new String[] { "foo@example.com:hello", "bar@example.com:world" };

    /**
     * The default email to populate the email field with.
     */
    public static final String EXTRA_EMAIL = "com.example.android.authenticatordemo.extra.EMAIL";

    /**
     * Keep track of the login task to ensure we can cancel it if requested.
     */
    private UserLoginTask mAuthTask = null;

    // Values for email and password at the time of the login attempt.
    private String mEmail;
    private String mPassword;

    // UI references.
    private EditText mEmailView;
    private EditText mPasswordView;
    private View mLoginFormView;
    private View mLoginStatusView;
    private TextView mLoginStatusMessageView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_login);

        // Set up the login form.
        mEmail = getIntent().getStringExtra(EXTRA_EMAIL);
        mEmailView = (EditText) findViewById(R.id.email);
        mEmailView.setText(mEmail);

        mPasswordView = (EditText) findViewById(R.id.password);
        mPasswordView.setOnEditorActionListener(new TextView.OnEditorActionListener() {
            @Override
            public boolean onEditorAction(TextView textView, int id, KeyEvent keyEvent) {
                if (id == R.id.login || id == EditorInfo.IME_NULL) {
                    attemptLogin();
                    return true;
                }
                return false;
            }
        });

        mLoginFormView = findViewById(R.id.login_form);
        mLoginStatusView = findViewById(R.id.login_status);
        mLoginStatusMessageView = (TextView) findViewById(R.id.login_status_message);

        findViewById(R.id.sign_in_button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                attemptLogin();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.activity_login, menu);
        return true;
    }

    /**
     * Attempts to sign in or register the account specified by the login form.
     * If there are form errors (invalid email, missing fields, etc.), the
     * errors are presented and no actual login attempt is made.
     */
    public void attemptLogin() {
        if (mAuthTask != null) {
            return;
        }

        // Reset errors.
        mEmailView.setError(null);
        mPasswordView.setError(null);

        // Store values at the time of the login attempt.
        mEmail = mEmailView.getText().toString();
        mPassword = mPasswordView.getText().toString();

        boolean cancel = false;
        View focusView = null;

        // Check for a valid password.
        if (TextUtils.isEmpty(mPassword)) {
            mPasswordView.setError(getString(R.string.error_field_required));
            focusView = mPasswordView;
            cancel = true;
        }
        else if (mPassword.length() < 4) {
            mPasswordView.setError(getString(R.string.error_invalid_password));
            focusView = mPasswordView;
            cancel = true;
        }

        // Check for a valid email address.
        if (TextUtils.isEmpty(mEmail)) {
            mEmailView.setError(getString(R.string.error_field_required));
            focusView = mEmailView;
            cancel = true;
        }
        else if (!mEmail.contains("@")) {
            mEmailView.setError(getString(R.string.error_invalid_email));
            focusView = mEmailView;
            cancel = true;
        }

        if (cancel) {
            // There was an error; don't attempt login and focus the first
            // form field with an error.
            focusView.requestFocus();
        }
        else {
            // Show a progress spinner, and kick off a background task to
            // perform the user login attempt.
            mLoginStatusMessageView.setText(R.string.login_progress_signing_in);
            showProgress(true);
            mAuthTask = new UserLoginTask();
            mAuthTask.execute((Void) null);
        }
    }

    /**
     * Shows the progress UI and hides the login form.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB_MR2)
    private void showProgress(final boolean show) {
        // On Honeycomb MR2 we have the ViewPropertyAnimator APIs, which allow
        // for very easy animations. If available, use these APIs to fade-in
        // the progress spinner.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR2) {
            int shortAnimTime = getResources().getInteger(android.R.integer.config_shortAnimTime);

            mLoginStatusView.setVisibility(View.VISIBLE);
            mLoginStatusView.animate().setDuration(shortAnimTime).alpha(show ? 1 : 0).setListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    mLoginStatusView.setVisibility(show ? View.VISIBLE : View.GONE);
                }
            });

            mLoginFormView.setVisibility(View.VISIBLE);
            mLoginFormView.animate().setDuration(shortAnimTime).alpha(show ? 0 : 1).setListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    mLoginFormView.setVisibility(show ? View.GONE : View.VISIBLE);
                }
            });
        }
        else {
            // The ViewPropertyAnimator APIs are not available, so simply show
            // and hide the relevant UI components.
            mLoginStatusView.setVisibility(show ? View.VISIBLE : View.GONE);
            mLoginFormView.setVisibility(show ? View.GONE : View.VISIBLE);
        }
    }

    /**
     * Represents an asynchronous login/registration task used to authenticate
     * the user.
     */
    public class UserLoginTask extends AsyncTask<Void, Void, Boolean> {
        @Override
        protected Boolean doInBackground(Void... params) {
            // TODO: attempt authentication against a network service.

            try {
                // Simulate network access.
                Thread.sleep(2000);
            }
            catch (InterruptedException e) {
                return false;
            }

            for (String credential : DUMMY_CREDENTIALS) {
                String[] pieces = credential.split(":");
                if (pieces[0].equals(mEmail)) {
                    // Account exists, return true if the password matches.
                    return pieces[1].equals(mPassword);
                }
            }

            // TODO: register the new account here.
            return true;
        }

        @Override
        protected void onPostExecute(final Boolean success) {
            mAuthTask = null;
            showProgress(false);

            if (success) {
                finish();
            }
            else {
                mPasswordView.setError(getString(R.string.error_incorrect_password));
                mPasswordView.requestFocus();
            }
        }

        @Override
        protected void onCancelled() {
            mAuthTask = null;
            showProgress(false);
        }
    }
}

如果取决于特定情况，那么使用 HttpClient 从 Internet 加载 ListView 项(文本 + 加上位图)，我应该如何实现我的 AsyncTask？

Answer 1

实现 AsyncTask 的方式没有单一的“正确”方式。但这是我的两分钱:

此类旨在在 Activity 的上下文中执行“轻量级”工作。这就是为什么它在 UI 线程中运行 onPreExecute、onProgressUpdate、onPostExecute 方法，以便他们可以快速访问字段并更新 GUI。任何可能需要较长时间才能完成且并非旨在更新特定 Activity 的任务都应移至服务。

这些方法主要用于更新 GUI。由于 GUI 与 Activity 实例相关(字段可能声明为私有(private)成员变量)，因此将 AsyncTask 实现为非静态嵌套类更方便。这也是我认为最自然的方式。

如果任务要在其他 Activity 中重用，我认为应该允许它有自己的类。老实说，我不喜欢静态嵌套类，尤其是内部 View 。如果它是一个类，则意味着它在概念上与 Activity 不同。如果它是静态的，则意味着它与该 Activity 的具体实例无关。但是由于它们是嵌套的，这些类在视觉上位于父类中，使其更难阅读，并且在项目包资源管理器中可能不会被注意到，因为它只显示文件。尽管比内部类耦合少，但这并不是那么有用:如果类发生变化，您必须将整个父文件合并/提交给版本控制。如果您在哪里重用它，那么您必须在任何地方以 Parent.Nested 的形式访问它。因此，为了不将其他 Activity 耦合到 Parent 类，您可能希望重构它并将嵌套类提取到它自己的文件中。

所以对我来说，问题是内部类与顶级类。