android之显示数据库信息

关键字

ListView

adapter

MVC

在android开发中也使用到了MVC架构,其中的xml布局文件就是V,M就是我们定义好的javabean类,而控制器就是就是适配器类adapter

布局文件

采用线性布局的方式,ListView就是一个View容器,可以往里面放任何的View对象

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="xidian.dy.com.chujia.MainActivity">

<ListView
android:id="@+id/lv"
android:layout_width="match_parent"
android:layout_height="match_parent">

</ListView>
</LinearLayout>

javabean

package xidian.dy.com.chujia;

/**
* Created by dy on 2016/6/17.
*/
public class Person {
private String name;
private int salary;
private String phone;

public String getPhone() {
return phone;
}

public void setPhone(String phone) {
this.phone = phone;
}

public int getSalary() {
return salary;
}

public void setSalary(int salary) {
this.salary = salary;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

@Override
public String toString() {
return name +' ' + salary + ' ' + phone;
}
}

Activity

　　先将数据从数据库中读出来封装成一个javabean对象，然后在将所以的对象放到List容器中。使用系统提供的adapter对象来遍历list容器，将取得内容封装成一个View对象,返回给ListView。ListView每拿到一个View对象就将其显示出来。这里的为了使用方便将pater作为内部类来使用。

package xidian.dy.com.chujia;

import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private SQLiteDatabase db;
private List<Person> personList;

@Override
protected void onCreate(Bundle savedInstanceState) {
MainActivity mContext = MainActivity.this;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyDBOpenHelper myDBHelper = new MyDBOpenHelper(mContext, "my.db", null, 1);
db = myDBHelper.getWritableDatabase();
ContentValues values = new ContentValues();
for(int i=0; i<50; i++){
values.put("name", "data"+i);
values.put("salary", 1000+i);
values.put("phone","12345678901");
db.insert("person", null, values);
values.clear();
}

personList = new ArrayList<>();
Cursor cursor = db.query("person",new String[]{"name", "salary", "phone"},null,null,null,null,null,null);
while (cursor.moveToNext()){
Person person = new Person();
person.setName(cursor.getString(0));
person.setSalary(cursor.getInt(1));
person.setPhone(cursor.getString(2));
personList.add(person);
}
cursor.close();
ListView lv = (ListView) findViewById(R.id.lv);
if(lv != null)
lv.setAdapter(new MyAdapter());
}

class MyAdapter extends BaseAdapter{

//提供给系统调用,指定集合中有多少个元素
@Override
public int getCount() {
return personList.size();
}
//提供给系统调用,获取一个View来显示
@Override
public View getView(int position, View convertView, ViewGroup parent) {
　　　　　　　　//这个TextView将来要在MainActivity中显示,所以应将MainActivity.this作为TextView的上下文
TextView text = new TextView(MainActivity.this);
text.setText(personList.get(position).toString());
text.setTextSize(18);
return text;
}

@Override
public Object getItem(int position) {
return null;
}

@Override
public long getItemId(int position) {
return 0;
}
}
}

效果

布局优化

对上面的显示效果可以作出如下的优化



新增布局文件item_list.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="xidian.dy.com.chujia.MainActivity">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="30sp"/>
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="15sp"
/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textSize="15sp"
/>
</LinearLayout>
</RelativeLayout>

修改Activity

package xidian.dy.com.chujia;

import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private SQLiteDatabase db;
private List<Person> personList;

@Override
protected void onCreate(Bundle savedInstanceState) {
MainActivity mContext = MainActivity.this;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyDBOpenHelper myDBHelper = new MyDBOpenHelper(mContext, "my.db", null, 1);
db = myDBHelper.getWritableDatabase();
ContentValues values = new ContentValues();
for(int i=0; i<50; i++){
values.put("name", "data"+i);
values.put("salary", 1000+i);
values.put("phone","12345678901");
db.insert("person", null, values);
values.clear();
}

personList = new ArrayList<>();
Cursor cursor = db.query("person",new String[]{"name", "salary", "phone"},null,null,null,null,null,null);
while (cursor.moveToNext()){
Person person = new Person();
person.setName(cursor.getString(0));
person.setSalary(cursor.getInt(1));
person.setPhone(cursor.getString(2));
personList.add(person);
}
cursor.close();
ListView lv = (ListView) findViewById(R.id.lv);
if(lv != null)
lv.setAdapter(new MyAdapter());
}

class MyAdapter extends BaseAdapter{

//提供给系统调用,指定集合中有多少个元素
@Override
public int getCount() {
return personList.size();
}
//提供给系统调用,获取一个View来显示
@Override
public View getView(int position, View convertView, ViewGroup parent) {
//将一个布局文件转为一个View对象
View v =  View.inflate(MainActivity.this, R.layout.item_list,null);
//设置该对象中的值
TextView txt1 = (TextView) v.findViewById(R.id.name);
TextView txt2 = (TextView) v.findViewById(R.id.salary);
TextView txt3 = (TextView) v.findViewById(R.id.phone);
Person p = personList.get(position);
if (txt1 != null)
txt1.setText(p.getName());
if (txt2 != null)
//因为p.getSalary()是int类型,所以应转为字符串类型
txt2.setText(String.valueOf(p.getSalary()));
if (txt3 != null )
txt3.setText(p.getPhone());
//将一个布局嵌套到另一个中
return v;
}

@Override
public Object getItem(int position) {
return null;
}

@Override
public long getItemId(int position) {
return 0;
}
}
}

效果

优化

　　当我们在滑动屏幕的时候，每出现一个新的条目,适配器中getView方法就会被调用一次。在每次调用的时候都会创建一个View对象,这样非常消耗内存。但是getView中有一个参数convertView,这个一个缓存对象。当一个条目被移除屏幕的时候，该条目对应的View对象就会被缓存起来。当getView方法被调用的时候,我们可以获取缓存对象，从而避免去创建新的对象了。

package xidian.dy.com.chujia;

import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private SQLiteDatabase db;
private List<Person> personList;

@Override
protected void onCreate(Bundle savedInstanceState) {
MainActivity mContext = MainActivity.this;
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyDBOpenHelper myDBHelper = new MyDBOpenHelper(mContext, "my.db", null, 1);
db = myDBHelper.getWritableDatabase();
ContentValues values = new ContentValues();
for(int i=0; i<50; i++){
values.put("name", "data"+i);
values.put("salary", 1000+i);
values.put("phone","12345678901");
db.insert("person", null, values);
values.clear();
}

personList = new ArrayList<>();
Cursor cursor = db.query("person",new String[]{"name", "salary", "phone"},null,null,null,null,null,null);
while (cursor.moveToNext()){
Person person = new Person();
person.setName(cursor.getString(0));
person.setSalary(cursor.getInt(1));
person.setPhone(cursor.getString(2));
personList.add(person);
}
cursor.close();
ListView lv = (ListView) findViewById(R.id.lv);
if(lv != null)
lv.setAdapter(new MyAdapter());
}

class MyAdapter extends BaseAdapter{

//提供给系统调用,指定集合中有多少个元素
@Override
public int getCount() {
return personList.size();
}
//提供给系统调用,获取一个View来显示
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View v;
if(convertView == null)
//将一个布局文件转为一个View对象
v = View.inflate(MainActivity.this, R.layout.item_list, null);
else
//使用缓存中的View对象,避免重新创建
v=convertView;
//设置该对象中的值
TextView txt1 = (TextView) v.findViewById(R.id.name);
TextView txt2 = (TextView) v.findViewById(R.id.salary);
TextView txt3 = (TextView) v.findViewById(R.id.phone);
Person p = personList.get(position);
if (txt1 != null)
txt1.setText(p.getName());
if (txt2 != null)
//因为p.getSalary()是int类型,所以应转为字符串类型
txt2.setText(String.valueOf(p.getSalary()));
if (txt3 != null )
txt3.setText(p.getPhone());
//将一个布局嵌套到另一个中
return v;
}

@Override
public Object getItem(int position) {
return null;
}

@Override
public long getItemId(int position) {
return 0;
}
}
}