java - 应用程序在设备上停止(无终端错误)- 不显示 ListView

Question

我尝试在Android中制作一个简单的ListView。

activity_labas.xml:

    <TextView
        android:id="@+id/message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginLeft="@dimen/activity_horizontal_margin"
        android:layout_marginStart="@dimen/activity_horizontal_margin"
        android:layout_marginTop="@dimen/activity_vertical_margin"
        android:text="@string/title_home"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
    android:id="@+id/text1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
        android:text="@string/title_home2"/>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="0dp"
        android:layout_marginStart="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

labas.java:

package com.labas.holiday.labas;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.MenuItem;
import android.widget.ListView;
import android.widget.TextView;

public class labas extends AppCompatActivity {

    private TextView mTextMessage;
    private TextView mTextMessage2;

    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.navigation_home:
                    mTextMessage.setText(R.string.title_home);


                    return true;
                case R.id.navigation_dashboard:
                    mTextMessage.setText(R.string.title_dashboard);
                    mTextMessage2.setText(R.string.title_dashboard2);
                    return true;
                case R.id.navigation_notifications:
                    mTextMessage.setText(R.string.title_notifications);
                    return true;
            }
            return false;
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_labas);

        mTextMessage2 = (TextView) findViewById(R.id.message);
        mTextMessage = (TextView) findViewById(R.id.message);
        Log.i("myTag2", "STARTINGBOOTOM");
        Intent myIntent = new Intent(labas.this, ListViewLoader.class);
        //myIntent.putExtra("key", value); //Optional parameters
        labas.this.startActivity(myIntent);

        //setContentView(R.layout);
        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

}

ListViewLoader

package com.labas.holiday.labas;

import android.app.ListActivity;
import android.app.LoaderManager;
import android.content.CursorLoader;
import android.content.Loader;
import android.database.Cursor;
import android.os.Bundle;
import android.provider.ContactsContract;
import android.support.constraint.ConstraintLayout;
import android.util.Log;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.SimpleCursorAdapter;

public class ListViewLoader extends ListActivity
        implements LoaderManager.LoaderCallbacks<Cursor> {
    // This is the Adapter being used to display the list's data
    SimpleCursorAdapter mAdapter;

    // These are the Contacts rows that we will retrieve
    static final String[] PROJECTION = new String[] {ContactsContract.Data._ID,
            ContactsContract.Data.DISPLAY_NAME};

    // This is the select criteria
    static final String SELECTION = "((" +
            ContactsContract.Data.DISPLAY_NAME + " NOTNULL) AND (" +
            ContactsContract.Data.DISPLAY_NAME + " != '' ))";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i("myTag", "STAAAAAAAAAAAAAAAAAARRRRRRRRRRRRRRRRRTTTTTTTT");
        // Create a progress bar to display while the list loads
        ProgressBar progressBar = new ProgressBar(this);
        progressBar.setLayoutParams(new ConstraintLayout.LayoutParams(ConstraintLayout.LayoutParams.WRAP_CONTENT,
                ConstraintLayout.LayoutParams.WRAP_CONTENT));
        progressBar.setIndeterminate(true);
        getListView().setEmptyView(progressBar);

        // Must add the progress bar to the root of the layout
        ViewGroup root = (ViewGroup) findViewById(android.R.id.content);
        root.addView(progressBar);

        // For the cursor adapter, specify which columns go into which views
        String[] fromColumns = {ContactsContract.Data.DISPLAY_NAME};
        int[] toViews = {android.R.id.text1}; // The TextView in simple_list_item_1

        // Create an empty adapter we will use to display the loaded data.
        // We pass null for the cursor, then update it in onLoadFinished()
        mAdapter = new SimpleCursorAdapter(this,
                android.R.layout.simple_list_item_1, null,
                fromColumns, toViews, 0);
        setListAdapter(mAdapter);

        // Prepare the loader.  Either re-connect with an existing one,
        // or start a new one.
        getLoaderManager().initLoader(0, null, this);
    }


    @Override
    public Loader<Cursor> onCreateLoader(int i, Bundle bundle) {
        return new CursorLoader(this, ContactsContract.Data.CONTENT_URI,
                PROJECTION, SELECTION, null, null);
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
        mAdapter.swapCursor(cursor);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
        mAdapter.swapCursor(null);
    }
}

AndroidManifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.labas.holiday.labas">
    <uses-permission android:name="android.permission.READ_CONTACTS" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".labas"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name="ListViewLoader" android:label="@string/title_activity_test"/>
        <activity android:name="Testing" android:label="@string/title_activity_test"/>
    </application>
    </manifest>

结果应该是应用程序中的联系人列表。

目前，代码在 IDE 中不会引发任何错误，但会在设备上加载应用程序时停止。在 Debug模式下，它会在“return new CursorLoader(this, ContactsContract.Data.CONTENT_URI,PROJECTION, SELECTION, null, null);”处崩溃。

我没有看到空指针(我不会打赌^^)。

Answer 1

光标无法检索数据的原因之一是您缺少代码的运行时权限，您需要在添加 list 的同时请求运行时权限。

类似于:

private static final int INITIAL_REQUEST_NUMBER=1337;
private static final String[] INITIAL_PERMESSION={android.Manifest.permission.READ_CONTACTS};
ActivityCompat.requestPermissions(this, INITIAL_PERMESSION, INITIAL_REQUEST_NUMBER);

了解更多详情:https://developer.android.com/training/permissions/requesting.html

您需要确保用户在运行游标加载程序之前授予您权限，因此最好在运行加载程序之前请求 Activity 权限，这样如果用户拒绝权限(在首次运行期间)，您将取消运行加载程序。

即使在第一次运行期间授予用户权限，用户也可以撤销权限，因此最好在每次运行加载程序之前检查权限是否已授予，如果撤销，您将取消加载程序。

希望许可是您需要考虑的唯一问题。