RuntimeException：无法在 onPreExecute() 中创建处理程序

Question

我收到了java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()，而且，我知道，这意味着我正在尝试在不是 UI 线程的线程中进行一些 UI 修改。让我担心的是，我在 onPreExecute() 的 AsyncTask 中遇到了异常，这就是 UI 线程：

public class CreateDatabaseTask extends AsyncTask<String, Void, Boolean> {

    private static final String TAG = "CreateDatabaseTask";

    private SQLiteHelper helper;
    private ProgressDialog dialog;

    private Context context;

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

    @Override
    protected void onPreExecute() {

        helper = new SQLiteHelper(context);
        dialog = new ProgressDialog(context);  // <--- Just here!

        if (!helper.checkExists()) {
            dialog.setMessage("Iniciando por primera vez...");
            dialog.show();
        }
    }

    @Override
    protected Boolean doInBackground(String... params) throws SQLException {
    ...
    }

}

我正在自定义 SQLiteOpenHelper 类中的 public void onUpgrade(...) 中调用 CreateDatabaseTask 构造函数：

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        try {
            deleteDatabase();
        } catch (IOException e) {
            e.printStackTrace();
        }

        CreateDatabaseTask task = new CreateDatabaseTask(context);  //<--- I'm calling the constructor here
        task.execute();
    }

那么，这个AsyncTask 是在欺骗我还是我误会了什么？

Answer 1

ProgressDialog 在Constructor 中获得Context，但您在onPreExecute 中使用了this。相反，您应该写 YourActivity.this(您的活动的相应名称)

注意：

此错误的原因可能是尝试通过不是Activity 的Context 显示应用程序dialog

---

试试这个方法

public class CreateDatabaseTask extends AsyncTask<String, Void, Boolean> {

private static final String TAG = "CreateDatabaseTask";

private SQLiteHelper helper;
private ProgressDialog dialog;

private Context context;

public CreateDatabaseTask(Context context) {
    this.context = context;
    dialog = new ProgressDialog(YourActivity.this); //context should be instance of the Activity  
}

@Override
protected void onPreExecute() {

    helper = new SQLiteHelper(context);
    

    if (!helper.checkExists()) {
        dialog.setMessage("Iniciando por primera vez...");
        dialog.show();
    }
}

@Override
protected Boolean doInBackground(String... params) throws SQLException {
...
}

}