java - 在哪里可以更好地全面、深入地理解Context参数的实例化？

Question

我试图解决这段代码中Context和Classes之间的联系以及我对这个概念的理解。 MainActivity.this在这里不起作用。它带有周围的代码。

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);中Context参数的整体思维模型。陈述。必须存在该行代码才能调用 databaseHandler 中的方法。类。

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、图形模型链接、如何 this.databaseHandler = new DatabaseHandler(MainActivity.this);声明被构建将不胜感激。

Answer 1

行:

this.databaseHandler = new DatabaseHandler(MainActivity.this);

期望构造函数中有一个有效的Context。 Activity 扩展了 ContextThemeWrapper，因此是要传递的有效对象，但该对象内的实际上下文必须有效。

看起来您已经从 Activity 复制了代码，该代码的所有代码都包含在 MainActivity 对象中，并且在创建 MainActivity 时从框架来看，上下文是有效的。

如果您将 Activity 实例化为常规类，则它将被创建，但 Context 将无效。 Android 框架必须实例化它，并在类的构造过程中为其提供有效的Context。您无法创建自己的有效Context，因为 Android 需要定义它。

这是一篇关于什么是 Context 的好文章:

What is 'Context' on Android?

我不知道有任何图形可以描述或解释它。它有点抽象，因为它涉及有关 Android 环境的许多信息，包括一些视觉方面(如屏幕信息)和非视觉方面(文件位置、安全性、权限等)。