我正在尝试解决此代码中Context和Classes之间的连接以及我对此概念的理解。 MainActivity.this在这里不起作用。它附带了周围的代码。
package com.Table;
import android.content.Context;
import android.graphics.Color;
import android.os.AsyncTask;
import android.util.Log;
import android.view.Gravity;
import android.view.View;
import android.widget.HorizontalScrollView;
import android.widget.RelativeLayout;
import android.widget.ScrollView;
import android.widget.TableLayout;
import android.widget.TableRow;
import android.widget.TextView;
import android.widget.Toast;
import com.example.tablefreezepane.DatabaseHandler;
import com.example.tablefreezepane.MainActivity;
import java.util.ArrayList;
import java.util.List;
public class AsyncInsertData extends AsyncTask<String, String, String> {
DatabaseHandler databaseHandler;
String type;
long timeElapsed;
protected AsyncInsertData(String type){
this.type = type;
this.databaseHandler = new DatabaseHandler(MainActivity.this);
}
// @type - can be 'normal' or 'fast'
//@Override
//protected void onPreExecute() {
// super.onPreExecute();
// tvStatus.setText("Inserting " + editTextRecordNum.getText() + " records...");
//}
@Override
protected String doInBackground(String... aurl) {
try {
// get number of records to be inserted
int insertCount = 20;
// empty the table
databaseHandler.deleteRecords();
// keep track of execution time
long lStartTime = System.nanoTime();
if (type.equals("normal")) {
databaseHandler.insertNormal(insertCount);
} else {
databaseHandler.insertFast(insertCount);
}
// execution finised
long lEndTime = System.nanoTime();
// display execution time
timeElapsed = lEndTime - lStartTime;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
protected void onPostExecute(String unused) {
Toast.makeText(getApplicationContext(),"This is an Android Toast Message", Toast.LENGTH_LONG).show();
//tvStatus.setText("Done " + choice + " inserting " + databaseHandler.countRecords() + " records into table: [" + this.databaseHandler.tableName + "]. Time elapsed: " + timeElapsed / 1000000 + " ms.");
}
}
阅读或复制代码不是问题。问题存在于this.databaseHandler = new DatabaseHandler(MainActivity.this);语句中Context参数的整体思维模型中。行代码必须在那里才能调用databaseHandler类中的方法。
package com.example.tablefreezepane;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteStatement;
import android.widget.TextView;
public class DatabaseHandler extends SQLiteOpenHelper {
// for our logs
public static final String TAG = "DatabaseHandler.java";
public TextView tvstatus;
// database version
private static final int DATABASE_VERSION = 7;
// database name
protected static final String DATABASE_NAME = "NinjaDatabase2";
// table details
public String tableName = "locations";
public String fieldObjectId = "id";
public String fieldObjectName = "name";
public String fieldObjectDescription = "description";
// constructor
public DatabaseHandler(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
// creating table
@Override
public void onCreate(SQLiteDatabase db) {
String sql = "";
this.tvstatus.setText("Creating table");
sql += "CREATE TABLE " + tableName;
sql += " ( ";
sql += fieldObjectId + " INTEGER PRIMARY KEY AUTOINCREMENT, ";
sql += fieldObjectName + " TEXT, ";
sql += fieldObjectDescription + " TEXT ";
sql += " ) ";
db.execSQL(sql);
this.tvstatus.setText("Table created...");
// create the index for our INSERT OR REPLACE INTO statement.
// this acts as the WHERE name="name input" AND description="description input"
// if that WHERE clause is true, I mean, it finds the same name and description in the database,
// it will be REPLACEd.
// ELSE, what's in the database will remain and the input will be INSERTed (new record)
String INDEX = "CREATE UNIQUE INDEX locations_index ON "
+ tableName + " (name, description)";
db.execSQL(INDEX);
}
/*
* When upgrading the database, it will drop the current table and recreate.
*/
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
String sql = "DROP TABLE IF EXISTS " + tableName;
db.execSQL(sql);
onCreate(db);
}
// insert data using transaction and prepared statement
public void insertFast(int insertCount) {
// you can use INSERT only
String sql = "INSERT OR REPLACE INTO " + tableName + " ( name, description ) VALUES ( ?, ? )";
SQLiteDatabase db = this.getWritableDatabase();
/*
* According to the docs http://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html
* Writers should use beginTransactionNonExclusive() or beginTransactionWithListenerNonExclusive(SQLiteTransactionListener)
* to start a transaction. Non-exclusive mode allows database file to be in readable by other threads executing queries.
*/
//db.beginTransactionNonExclusive();
db.beginTransaction();
SQLiteStatement stmt = db.compileStatement(sql);
for(int x=1; x<=insertCount; x++){
stmt.bindString(1, "Name # " + x);
stmt.bindString(2, "Description # " + x);
stmt.execute();
stmt.clearBindings();
}
db.setTransactionSuccessful();
db.endTransaction();
}
// inserts the record without using transaction and prepare statement
public void insertNormal(int insertCount){
try{
SQLiteDatabase db = this.getWritableDatabase();
for(int x=1; x<=insertCount; x++){
ContentValues values = new ContentValues();
values.put(fieldObjectName, "Name # " + x);
values.put(fieldObjectDescription, "Description # " + x);
db.insert(tableName, null, values);
}
db.close();
}catch(Exception e){
e.printStackTrace();
}
}
// deletes all records
public void deleteRecords(){
SQLiteDatabase db = this.getWritableDatabase();
db.execSQL("delete from "+ tableName);
db.close();
}
// count records
public int countRecords(){
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery("SELECT count(*) from " + tableName, null);
cursor.moveToFirst();
int recCount = cursor.getInt(0);
cursor.close();
db.close();
return recCount;
}
}
任何指导，图形模型的链接，this.databaseHandler = new DatabaseHandler(MainActivity.this);声明的构建方式都将受到赞赏。
答案 0 :(得分：2)
该行：
this.databaseHandler = new DatabaseHandler(MainActivity.this);
期望构造函数中有效Context。 Activity扩展ContextThemeWrapper，因此是一个有效的传递对象，但对象中的实际上下文必须有效。
看起来您已复制Activity中包含MainActivity对象中所有此代码的代码，并且从框架创建MainActivity时，上下文有效。
如果您将Activity实例化为普通类，则会创建它，但Context无效。 Android框架必须实例化它，并且在构造类时，会给它一个有效的Context。您无法创建自己的有效Context，因为Android需要对其进行定义。
这是关于Context是什么的好文章：
我不知道描述或解释它的图形。它有点抽象，因为它涉及大量有关Android环境的信息，包括一些可视方面（如屏幕信息）以及非可视方面（文件位置，安全性，权限等）。