java - 从我的DialogFragment中的MainActivity访问ArrayList 遇到麻烦

Question

我需要能够从MainActivity.java类中的ArrayList(ScannedDevicesArrayList)中传递ScannedDevicesListItemDialog.java类索引。

我尝试在MainActivity.java类中创建一个函数(getScannedDevicesArrayList)以返回ArrayList，然后在对话框的onClick中调用该函数。但是，在运行程序时，只要单击对话框中的某个项目，应用程序就会崩溃。

如何在对话框中从MainActivity.java引用此ArrayList以便能够执行简单的操作，例如在单击对话框中的项目时将其索引打印到控制台？

MainActivity.java

package com.august.customtisensortagclient;

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.AlertDialog;
import android.app.Dialog;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothManager;
import android.bluetooth.le.BluetoothLeScanner;
import android.bluetooth.le.ScanCallback;
import android.bluetooth.le.ScanResult;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.AsyncTask;
import android.support.v4.app.DialogFragment;
import android.support.v4.app.Fragment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.method.ScrollingMovementMethod;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;

import java.io.StringBufferInputStream;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {


    BluetoothManager mBluetoothManager;
    BluetoothAdapter mBluetoothAdapter;
    BluetoothLeScanner mBluetoothScanner;
    TextView console;
    Button clear_button;
    public ArrayList<String> scannedDevicesArrayList = new ArrayList<String>();
    int scannedDevicesArrayListTop = -1;
    private final static int REQUEST_ENABLE_BT = 1;
    private static final int PERMISSION_REQUEST_COARSE_LOCATION = 1;
    ArrayAdapter<String> myArrayAdapter;
    ListView list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        console = (TextView) findViewById(R.id.Console);
        console.append("Console is working...\n");
        registerClearButton();




        // Functional code

        setupBluetooth();
        startScanning();
        registerScannedDevicesListCallback();

        // Test code


    }

    public void clearConsole(){
        console.setText("Console is working...\n");
        scannedDevicesArrayList.clear();
        myArrayAdapter.notifyDataSetChanged();
        scannedDevicesArrayListTop = -1;
    }

    private void registerScannedDevicesListCallback() {
        list = (ListView) findViewById(R.id.myListView);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                TextView textView = (TextView) view;
                DialogFragment newFragment = new ScannedDevicesListItemDialog();
                newFragment.show(getSupportFragmentManager(), "ScannedDevicesListItemDialog");

            }
        });
    }

    private void registerClearButton(){
        clear_button = (Button) findViewById(R.id.clear_button);
        clear_button.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                clearConsole();
            }
        });
    }

    public void addDeviceToScannedDevicesList() {
        list = (ListView) findViewById(R.id.myListView);
        myArrayAdapter = new ArrayAdapter<String>(this,
                R.layout.my_list_view_items, scannedDevicesArrayList);
        list.setAdapter(myArrayAdapter);
    }

    public void setupBluetooth() {
        // Get BluetoothAdapter
        final BluetoothManager bluetoothManager =
                (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        mBluetoothAdapter = bluetoothManager.getAdapter();
        // Get BluetoothLeScanner
        mBluetoothScanner = mBluetoothAdapter.getBluetoothLeScanner();
        // Make sure Bluetooth is enabled
        if (mBluetoothAdapter != null && !mBluetoothAdapter.isEnabled()) {
            Intent enableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableIntent, REQUEST_ENABLE_BT);
        }
        // Make sure location is enabled
        if (this.checkSelfPermission(Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            final AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle("This app needs location access");
            builder.setMessage("Please grant location access so this app can detect peripherals.");
            builder.setPositiveButton(android.R.string.ok, null);
            builder.setOnDismissListener(new DialogInterface.OnDismissListener() {
                @Override
                public void onDismiss(DialogInterface dialog) {
                    requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION},
                            PERMISSION_REQUEST_COARSE_LOCATION);
                }
            });
            builder.show();
        }
    }

    // Device scan callback.
    private ScanCallback leScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            while (!scannedDevicesArrayList.contains(result.getDevice().getAddress()) && (result.getRssi() > -55)){
                    scannedDevicesArrayList.add(result.getDevice().getAddress());
                    scannedDevicesArrayListTop++;
                    addDeviceToScannedDevicesList();
                    console.append(Integer.toString(result.getRssi()));
            }
        }
    };

    public ArrayList<String> getScannedDevicesArrayList(){
        return scannedDevicesArrayList;
    }

    public int getScannedDevicesArrayListTop() {
        return scannedDevicesArrayListTop;
    }

    public void startScanning() {
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() {
                mBluetoothScanner.startScan(leScanCallback);
            }
        });
    }

    public void stopScanning() {
        AsyncTask.execute(new Runnable() {
            @Override
            public void run() {
                mBluetoothScanner.stopScan(leScanCallback);
            }
        });
    }

    @Override
    public void onRequestPermissionsResult(int requestCode,
                                           String permissions[], int[] grantResults) {
        switch (requestCode) {
            case PERMISSION_REQUEST_COARSE_LOCATION: {
                if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    System.out.println("coarse location permission granted");
                } else {
                    final AlertDialog.Builder builder = new AlertDialog.Builder(this);
                    builder.setTitle("Functionality limited");
                    builder.setMessage("Since location access has not been granted, " +
                            "this app will not be able to discover beacons when in the background.");
                    builder.setPositiveButton(android.R.string.ok, null);
                    builder.setOnDismissListener(new DialogInterface.OnDismissListener() {

                        @Override
                        public void onDismiss(DialogInterface dialog) {
                        }

                    });
                    builder.show();
                }
                return;
            }
        }
    }


}

ScannedDevicesListItemDialog.java

package com.august.customtisensortagclient;

import android.app.AlertDialog;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.DialogFragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import java.util.ArrayList;


public class ScannedDevicesListItemDialog extends DialogFragment {


    String[] options = {"Left", "Right"};


    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        builder.setTitle("Connect as Left or Right?")
                .setItems(options, new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        MainActivity mainActivity = new MainActivity();
                        ArrayList<String> thisScannedDevicesArrayList = mainActivity.getScannedDevicesArrayList();
                        ((MainActivity)getActivity()).console.setText("Worked");
                        ((MainActivity)getActivity()).console.setText(thisScannedDevicesArrayList.get(0));
                    }
                });
        return builder.create();
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        tools:layout_editor_absoluteX="16dp"
        tools:layout_editor_absoluteY="0dp">

        <TextView
            android:id="@+id/Console"
            android:layout_width="272dp"
            android:layout_height="46dp"
            android:layout_alignParentBottom="true"
            android:layout_centerHorizontal="true"
            android:layout_marginBottom="200dp" />

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="wrap_content"
            android:layout_height="70dp"
            android:layout_alignParentBottom="true"
            android:scaleType="fitCenter"
            app:srcCompat="@mipmap/gruneisen_logo" />

        <Button
            android:id="@+id/clear_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_alignStart="@+id/Console"
            android:layout_marginBottom="131dp"
            android:text="CLEAR" />

        <ListView
            android:id="@+id/myListView"
            android:layout_width="match_parent"
            android:layout_height="379dp" />
    </RelativeLayout>
</android.support.constraint.ConstraintLayout>

my_list_view_items.xml

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:textSize="44sp">

</TextView>

Answer 1

MainActivity mainActivity = new MainActivity();

不是您访问 Activity 的方式。这也不是您访问已创建的类的方式，但这是另一个问题。

使用该代码，您将创建MainActivity的新实例，而不会获得当前存在的实例。这意味着您的ArrayList为null，因为它从未初始化。

要访问您的列表，请使用 getActivity()函数，该函数将获取Fragment的父Activity的实例，并将其转换为MainActivity:

MainActivity mainActivity = (MainActivity) getActivity();