ContentProvider的意義

ContentProvider的類注釋如下：

Content providers are one of the primary building blocks of Android applications, providing content to applications. They encapsulate data and provide it to applications through the single ContentResolver interface. A content provider is only required if you need to share data between multiple applications. For example, the contacts data is used by multiple applications and must be stored in a content provider. If you don't need to share data amongst multiple applications you can use a database directly via android.database.sqlite.SQLiteDatabase

內(nèi)容提供者是Android應(yīng)用程序的主要構(gòu)建塊之一呻拌，為應(yīng)用程序提供內(nèi)容。它們封裝數(shù)據(jù)并通過單一的ContentResolver接口提供給應(yīng)用程序凝果。如果需要在多個應(yīng)用程序之間共享數(shù)據(jù)耘拇，則只需要內(nèi)容提供程序。例如污它，聯(lián)系人數(shù)據(jù)由多個應(yīng)用程序使用剖踊，必須存儲在內(nèi)容提供者中。如果您不需要在多個應(yīng)用程序之間共享數(shù)據(jù)衫贬，您可以直接通過數(shù)據(jù)庫使用數(shù)據(jù)庫德澈。

由此可見，ContentProvider適合進(jìn)程間的數(shù)據(jù)共享固惯，你的應(yīng)用可以通過ContentProvider將需要共享的部分?jǐn)?shù)據(jù)提供給其他應(yīng)用梆造。你也可以通過ContentProvider獲取手機(jī)的圖片、音視頻葬毫、通訊錄等數(shù)據(jù)

ContentProvider實(shí)現(xiàn)

需要重寫以下的幾個主要方法

• onCreate 初始化創(chuàng)建ContentProvider
• query 返回查詢數(shù)據(jù)
• insert 插入數(shù)據(jù)
• update 更新數(shù)據(jù)
• delete 刪除數(shù)據(jù)
• getType

getType方法很特別镇辉，基本用不上，但是它具體的作用又是什么呢贴捡，來看看它的注釋

Implement this to handle requests for the MIME type of the data at the
given URI. The returned MIME type should start with vnd.android.cursor.item for a single record, or vnd.android.cursor.dir/ for multiple items.
根據(jù)訪問的URI忽肛，返回對應(yīng)的MIME類型的字符串，這個字符串需要以vnd.android.cursor.item（返回單條數(shù)據(jù)記錄）或者vnd.android.cursor.dir/（返回多條數(shù)據(jù)記錄） 開頭

看到這些方法是不是很熟悉烂斋？跟數(shù)據(jù)庫操作一毛一樣啊屹逛，其實(shí)在這個時候我們可以將ContentProvider理解成一個共享數(shù)據(jù)庫础废，我們只要在ContentProvider初始化的時候，實(shí)例化一個可讀寫的數(shù)據(jù)庫罕模，在以上的方法中對這個數(shù)據(jù)庫進(jìn)行增刪改查的操作就可以了评腺。當(dāng)你的數(shù)據(jù)庫有多個表的時候，這些增刪改查又是如何區(qū)分不同的表呢淑掌？其實(shí)這些方法都有一個Uri的參數(shù)回調(diào)蒿讥，我們可以通過實(shí)例UriMatcher來區(qū)分用戶操作的到底是哪個表。

示例

創(chuàng)建一個DBOpenHelper

public static class TestDBOpenHelper extends SQLiteOpenHelper{
    public static final String DB_NAME = "test.db";
    public static final int VERSION=1;
    public static final String TABLE_NAME = "person";
    public static final String TABLE_NAME2 = "class";
    public static final String CREATE_TABLE = "create table if not exists "+TABLE_NAME+"(_id integer, name TEXT, desc TEXT)";
    public static final String CREATE_TABLE2= "create table if not exists "+TABLE_NAME2+"(id integer, className TEXT, desc TEXT)";

    public TestDBOpenHelper(Context context){

        super(context,DB_NAME,null,VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(CREATE_TABLE);
        db.execSQL(CREATE_TABLE2);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

ContentProvider

public class TestContentProvider extends ContentProvider {

        private static final UriMatcher mUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        public static final String AUTHORITY = "com.gzkit.dailysample.TestContentProvider";  //授權(quán)
        public static final Uri PERSON_CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/person");
        public static final Uri CLASS_CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/class");

        private static final int TABLE_CODE_PERSON = 2;
        private static final int TABLE_CODE_CLASS = 3;

        static {
            //關(guān)聯(lián)不同的 URI 和 code抛腕，便于后續(xù) getType
            mUriMatcher.addURI(AUTHORITY, "person", TABLE_CODE_PERSON);
            mUriMatcher.addURI(AUTHORITY, "class", TABLE_CODE_CLASS);
        }

            //數(shù)據(jù)庫
        private SQLiteDatabase mDatabase;
        private Context mContext;
        private String mTable;

        private void initProvider(){

            mContext = getContext();
            mTable = TestDBOpenHelper.TABLE_NAME;
            mDatabase = new TestDBOpenHelper(mContext).getWritableDatabase();
                //插入一條初始數(shù)據(jù)
            mDatabase.execSQL("insert into "+mTable+" values(1,'test','good boy') ");


        }


        @Override
        public boolean onCreate() {

            initProvider();
            return false;
        }

        @Nullable
        @Override
        public Cursor query(@NonNull Uri uri, @Nullable String[] projection, @Nullable String selection, @Nullable String[] selectionArgs, @Nullable String sortOrder) {

            String tableName = getTableName(uri);


            return mDatabase.query(tableName,projection,selection,selectionArgs,null,sortOrder,null);
        }

        @Nullable
        @Override
        public String getType(@NonNull Uri uri) {
            int match = mUriMatcher.match(uri);
            String MIME = "";
            switch (match) {
                case TABLE_CODE_PERSON:

                    MIME = "vnd.android.cursor.dir/multi";
                    break;
                case TABLE_CODE_CLASS:
                    MIME = "vnd.android.cursor.item/single";

                    break;
            }

            return MIME;
        }

        @Nullable
        @Override
        public Uri insert(@NonNull Uri uri, @Nullable ContentValues values) {
            String tableName = getTableName(uri);

            mDatabase.insert(tableName,null,values);
            //發(fā)送數(shù)據(jù)變動的通知
            mContext.getContentResolver().notifyChange(uri,null);
            return null;
        }

        @Override
        public int delete(@NonNull Uri uri, @Nullable String selection, @Nullable String[] selectionArgs) {
            return 0;
        }

        @Override
        public int update(@NonNull Uri uri, @Nullable ContentValues values, @Nullable String selection, @Nullable String[] selectionArgs) {
            return 0;
        }

        /**
         * CRUD 的參數(shù)是 Uri芋绸，根據(jù) Uri 獲取對應(yīng)的表名
         *
         * @param uri
         * @return
         */
        private String getTableName(final Uri uri) {
            String tableName = "";
            int match = mUriMatcher.match(uri);
            switch (match){
                case TABLE_CODE_PERSON:
                    tableName = TestDBOpenHelper.TABLE_NAME;

                    break;

                case TABLE_CODE_CLASS:
                    tableName = TestDBOpenHelper.TABLE_NAME2;

                    break;
            }
            return tableName;
        }

在AndroidManifest中注冊ContentProvider

提供ContentProvider的APP按實(shí)際需要加入以下權(quán)限

<permission android:name="com.gzkit.dailysample.permission.READ_CONTENT"
    android:protectionLevel="normal"
    />
<permission android:name="com.gzkit.dailysample.permission.WRITE_CONTENT"
    android:protectionLevel="normal"
    />
<permission android:name="com.gzkit.dailysample.provider.permission"
    android:protectionLevel="normal"
    />

如果provider不存在這些權(quán)限的屬性，聲明這些權(quán)限則不是必要的

application節(jié)點(diǎn)下

<provider
   android:name="com.gzkit.dailysample.bean.TestContentProvider"
    android:authorities="com.gzkit.dailysample.TestContentProvider"
    android:exported="true"
    android:readPermission="com.gzkit.dailysample.permission.READ_CONTENT"
    android:writePermission="com.gzkit.dailysample.permission.WRITE_CONTENT"
    android:permission="com.gzkit.dailysample.provider.permission"
    android:process=":provider"
/>

• name 自定義的類
• authorities 定義授權(quán)的名稱兽埃，在自定義的ContentProvider中要用到
• exported 外部是否能調(diào)用
• process 進(jìn)程名
• readPermission 讀取的權(quán)限名侥钳，必須和前面的permission節(jié)點(diǎn)名一致
• writePermission 寫入的權(quán)限名，必須和前面的permission節(jié)點(diǎn)名一致
• permission 讀寫的權(quán)限名柄错，必須和前面的permission節(jié)點(diǎn)名一致

值得注意的是舷夺，readPermission和writePermission的優(yōu)先級比permission高。

有以下幾種情況

• 所有權(quán)限都不聲明售貌，其他應(yīng)用無須聲明權(quán)限就可以自由讀寫
• 只聲明了permission给猾，則其他應(yīng)用需要聲明對應(yīng)的權(quán)限，即可讀寫
• 只聲明readPermission颂跨，則其他應(yīng)用需要聲明此權(quán)限敢伸，才可以讀取數(shù)據(jù)，但仍然可以寫入數(shù)據(jù)
• 只聲明writePermission恒削，和上面相反
• 聲明readPermission或writePermission池颈，同時聲明了permission，則其他應(yīng)用需要聲明對應(yīng)的權(quán)限钓丰，可以讀寫躯砰。

ContentProvider聲明了哪種權(quán)限，調(diào)用的那方就必須聲明對應(yīng)的權(quán)限

調(diào)用方需要聲明對應(yīng)的權(quán)限

<!--ContentProvider需要的權(quán)限-->
<uses-permission android:name="com.gzkit.dailysample.permission.READ_CONTENT"/>
<uses-permission android:name="com.gzkit.dailysample.permission.WRITE_CONTENT"/>
<uses-permission android:name="com.gzkit.dailysample.provider.permission"/>

使用ContentProvider

在activity或service中通過getContentResolver()獲取resolver接口進(jìn)行數(shù)據(jù)的增刪查改

public static final String AUTHORITY = "com.gzkit.dailysample.TestContentProvider";  //授權(quán)
public static final Uri PERSON_CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/person");
public static final Uri CLASS_CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/class");


public void insert(){
        ContentValues contentValues = new ContentValues();
        contentValues.put("_id",1);
        contentValues.put("name","test here");
        contentValues.put("desc","here is desc");
        getContentResolver().insert(PERSON_CONTENT_URI,contentValues);
}

public void query(){
        Cursor cursor =  getContentResolver().query(CLASS_CONTENT_URI,null,null,null,null);
        Toast.makeText(mContext, "查詢結(jié)果數(shù)："+cursor.getCount(), Toast.LENGTH_SHORT).show();
        cursor.close();
}

下一篇携丁，將會講到如何利用ContentProvider讀取手機(jī)的多媒體資料