如何通过 menuitem 调用另一个活动？ Android Studio 错误：预期方法调用

Question

我对 Android Studio 完全陌生，刚刚学习了面向对象的编程。 我的项目要求我在开源代码上构建一些东西。 我在菜单中添加了一个新菜单项，并希望在用户单击具有 id:plot 的菜单项后开始另一个活动。 我按照互联网上的食谱进行操作，但出现错误。 它说“预期方法调用”，这意味着终端片段可能是一种方法。 我很困惑，因为我认为这是一门课（一项活动）。 在这种情况下，谁能指导我开始一项活动（转到下一页进行不同的活动）？ 最简单的方法是什么？ 非常感激。

我在 onOptionsItemSelected(MenuItem item) 中添加了这个。

if (id == R.id.plot){
            Intent intent = new Intent(TerminalFragment.this, MainActivity2.class);
            startActivity(intent);
        }

TerminalFragment.java（完整代码）

package de.kai_morich.simple_bluetooth_le_terminal;

import android.app.Activity;
import android.app.AlertDialog;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.text.Editable;
import android.text.Spannable;
import android.text.SpannableStringBuilder;
import android.text.method.ScrollingMovementMethod;
import android.text.style.ForegroundColorSpan;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class TerminalFragment extends Fragment implements ServiceConnection, SerialListener {

    private MenuItem menuItem;
    private enum Connected { False, Pending, True }

    private String deviceAddress;
    private SerialService service;

    private TextView receiveText;
    private TextView sendText;
    private TextUtil.HexWatcher hexWatcher;

    private Connected connected = Connected.False;
    private boolean initialStart = true;
    private boolean hexEnabled = false;
    private boolean pendingNewline = false;
    private String newline = TextUtil.newline_crlf;

    /*
     * Lifecycle
     */
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
        //Register with activity
        // You must inform the system that your app bar fragment is participating in the population of the options menu.
        // tells the system that your fragment would like to receive menu-related callbacks.
        setRetainInstance(true);
        deviceAddress = getArguments().getString("device");
    }

    @Override
    public void onDestroy() {
        if (connected != Connected.False)
            disconnect();
        getActivity().stopService(new Intent(getActivity(), SerialService.class));
        super.onDestroy();
    }

    @Override
    public void onStart() {
        super.onStart();
        if(service != null)
            service.attach(this);
        else
            getActivity().startService(new Intent(getActivity(), SerialService.class)); // prevents service destroy on unbind from recreated activity caused by orientation change
    }

    @Override
    public void onStop() {
        if(service != null && !getActivity().isChangingConfigurations())
            service.detach();
        super.onStop();
    }

    @SuppressWarnings("deprecation") // onAttach(context) was added with API 23. onAttach(activity) works for all API versions
    @Override
    public void onAttach(@NonNull Activity activity) {
        super.onAttach(activity);
        getActivity().bindService(new Intent(getActivity(), SerialService.class), this, Context.BIND_AUTO_CREATE);
    }

    @Override
    public void onDetach() {
        try { getActivity().unbindService(this); } catch(Exception ignored) {}
        super.onDetach();
    }

    @Override
    public void onResume() {
        super.onResume();
        if(initialStart && service != null) {
            initialStart = false;
            getActivity().runOnUiThread(this::connect);
        }
    }

    @Override
    public void onServiceConnected(ComponentName name, IBinder binder) {
        service = ((SerialService.SerialBinder) binder).getService();
        service.attach(this);
        if(initialStart && isResumed()) {
            initialStart = false;
            getActivity().runOnUiThread(this::connect);
        }
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
        service = null;
    }

    /*
     * UI
     */
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_terminal, container, false);
        receiveText = view.findViewById(R.id.receive_text);                          // TextView performance decreases with number of spans
        receiveText.setTextColor(getResources().getColor(R.color.colorRecieveText)); // set as default color to reduce number of spans
        receiveText.setMovementMethod(ScrollingMovementMethod.getInstance());

        sendText = view.findViewById(R.id.send_text);
        hexWatcher = new TextUtil.HexWatcher(sendText);
        hexWatcher.enable(hexEnabled);
        sendText.addTextChangedListener(hexWatcher);
        sendText.setHint(hexEnabled ? "HEX mode" : "");

        View sendBtn = view.findViewById(R.id.send_btn);
        sendBtn.setOnClickListener(v -> send(sendText.getText().toString()));
        return view;
    }

    @Override
    public void onCreateOptionsMenu(@NonNull Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.menu_terminal, menu);
        menu.findItem(R.id.hex).setChecked(hexEnabled);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.clear) {
            receiveText.setText("");
            return true;
        } if (id == R.id.plot){
            Intent intent = new Intent(TerminalFragment.this, MainActivity2.class);
            startActivity(intent);
        }else if (id == R.id.newline) {
            String[] newlineNames = getResources().getStringArray(R.array.newline_names);
            String[] newlineValues = getResources().getStringArray(R.array.newline_values);
            int pos = java.util.Arrays.asList(newlineValues).indexOf(newline);
            AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
            builder.setTitle("Newline");
            builder.setSingleChoiceItems(newlineNames, pos, (dialog, item1) -> {
                newline = newlineValues[item1];
                dialog.dismiss();
            });
            builder.create().show();
            return true;
        } else if (id == R.id.hex) {
            hexEnabled = !hexEnabled;
            sendText.setText("");
            hexWatcher.enable(hexEnabled);
            sendText.setHint(hexEnabled ? "HEX mode" : "");
            item.setChecked(hexEnabled);
            return true;
        } else {
            return super.onOptionsItemSelected(item);
        }
    }

    /*
     * Serial + UI
     */
    private void connect() {
        try {
            BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
            BluetoothDevice device = bluetoothAdapter.getRemoteDevice(deviceAddress);
            status("connecting...");
            connected = Connected.Pending;
            SerialSocket socket = new SerialSocket(getActivity().getApplicationContext(), device);
            service.connect(socket);
        } catch (Exception e) {
            onSerialConnectError(e);
        }
    }

    private void disconnect() {
        connected = Connected.False;
        service.disconnect();
    }

    private void send(String str) {
        if(connected != Connected.True) {
            Toast.makeText(getActivity(), "not connected", Toast.LENGTH_SHORT).show();
            return;
        }
        try {
            String msg;
            byte[] data;
            if(hexEnabled) {
                StringBuilder sb = new StringBuilder();
                TextUtil.toHexString(sb, TextUtil.fromHexString(str));
                TextUtil.toHexString(sb, newline.getBytes());
                msg = sb.toString();
                data = TextUtil.fromHexString(msg);
            } else {
                msg = str;
                data = (str + newline).getBytes();
            }
            SpannableStringBuilder spn = new SpannableStringBuilder(msg + '\n');
            spn.setSpan(new ForegroundColorSpan(getResources().getColor(R.color.colorSendText)), 0, spn.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
            receiveText.append(spn);
            service.write(data);
        } catch (Exception e) {
            onSerialIoError(e);
        }
    }

    private void receive(byte[] data) {
        if(hexEnabled) {
            receiveText.append("Hello" + TextUtil.toHexString(data) + '\n');
        } else {
            String msg = new String(data);
            if(newline.equals(TextUtil.newline_crlf) && msg.length() > 0) {
                // don't show CR as ^M if directly before LF
                msg = msg.replace(TextUtil.newline_crlf, TextUtil.newline_lf);
                // special handling if CR and LF come in separate fragments
                if (pendingNewline && msg.charAt(0) == '\n') {
                    Editable edt = receiveText.getEditableText();
                    if (edt != null && edt.length() > 1)
                        edt.replace(edt.length() - 2, edt.length(), "");
                }
                pendingNewline = msg.charAt(msg.length() - 1) == '\r';
            }
            receiveText.append(TextUtil.toCaretString(msg, newline.length() != 0)); //print out data
        }
    }

    private void status(String str) {
        SpannableStringBuilder spn = new SpannableStringBuilder(str + '\n');
        spn.setSpan(new ForegroundColorSpan(getResources().getColor(R.color.colorStatusText)), 0, spn.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
        receiveText.append(spn);
    }

    /*
     * SerialListener
     */
    @Override
    public void onSerialConnect() {
        status("connected");
        connected = Connected.True;
    }

    @Override
    public void onSerialConnectError(Exception e) {
        status("connection failed: " + e.getMessage());
        disconnect();
    }

    @Override
    public void onSerialRead(byte[] data) {
        receive(data);
    }

    @Override
    public void onSerialIoError(Exception e) {
        status("connection lost: " + e.getMessage());
        disconnect();
    }

}

menu_terminal.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/plot"
        android:title="PLOTDATA"
        app:showAsAction="always" />
    <item
        android:id="@+id/clear"
        android:icon="@drawable/ic_delete_white_24dp"
        android:title="Clear"
        app:showAsAction="always" />
    <item
        android:id="@+id/newline"
        android:title="Newline"
        app:showAsAction="never" />
    <item
        android:id="@+id/hex"
        android:title="HEX Mode"
        android:checkable="true"
        app:showAsAction="never" />
</menu>

Answer 1

在这里， TerminalFragment是一个片段，而不是一个活动。 因此，您应该使用getActivity()而不是在new Intent()中使用TerminalFragment.this 。

所以，最终的代码看起来像这样：

Intent intent = new Intent(getActivity(), MainActivity2.class);
startActivity(intent);

你也可以检查一下： Intent from Fragment to Activity