内容提供者

CSDN话题挑战赛第2期
参赛话题：面试宝典

1. Uri

Uri：统一资源定位符。url也是uri的一种

[scheme:][//authority][path][?query][#fragment]
↓
↓
[scheme:][//host:port][path][?query][#fragment]

• path可以有多个，每个用/连接，比如
  scheme://authority/path1/path2/path3?query#fragment

• query参数可以带有对应的值，也可以不带，如果带对应的值用=表示
  scheme://authority/path1/path2/path3?id = 1#fragment，这里有一个参数id，它的值是1

• query参数可以有多个，每个用&连接

  scheme://authority/path1/path2/path3?id = 1&name = mingming&old#fragment

• 在android中，除了scheme、authority是必须要有的，其它的几个path、query、fragment，它们每一个可以选择性的要或不要，但顺序不能变

String str = "http://www.csdn.com:8080/path/filename.htm?netrow=10&path=32&id=4#monk";
Uri uri = Uri.parse(str);
// 获取path部分
List<String> list = uri.getPathSegments();
for(String item:list){
    L.i("tag","item:"+item);
}
/*
item:path
item:filename.htm
*/

// 获取query部分
String query1 = uri.getQueryParameter("netrow");// 10
String query2 = uri.getQueryParameter("path")// 4

2. 创建Provider

安卓四大组件之一，平时用的不多，比较流行的用法是三方库的初始化，例如集成facebook

其工作方式是通过uri来标识其它应用要访问的数据，通过ContentResolver实现增、删、改、查的操作，还可以通过ContentObserver监听数据的变化

class MyContentProvider : ContentProvider() {
    companion object {
        private const val AUTHORITY = "com.demo.myprovider"
        private const val MATCH_CODE = 100
        private val NOTIFY_URI = Uri.parse("content://$AUTHORITY/student")
        
        private val uriMatcher = UriMatcher(UriMatcher.NO_MATCH)
    }
    
    init {
        uriMatcher.addURI(AUTHORITY,"student", MATCH_CODE)
    }

    // SQLite数据库
    private var dao:Dao?=null

    override fun onCreate(): Boolean {
        dao = Dao.getInstance(context);
        return false;
    }
    override fun query(uri: Uri, projection: Array<out String>?, selection: String?, selectionArgs: Array<out String>?, sortOrder: String?): Cursor? {
        val match = uriMatcher.match(uri)
        return if (match== MATCH_CODE) dao.queryStudent()else null
    }
    override fun getType(uri: Uri): String? {
        return null
    }
     override fun insert(uri: Uri, values: ContentValues?): Uri? {
        if (uriMatcher.match(uri) == MATCH_CODE) {
            dao.insertStudent(values)
            context?.contentResolver?.notifyChange(NOTIFY_URI, null)
        }
        return null
    }
    override fun delete(uri: Uri, selection: String?, selectionArgs: Array<out String>?): Int {
        TODO("Not yet implemented")
    }

    override fun update(uri: Uri, values: ContentValues?, selection: String?, selectionArgs: Array<out String>?): Int {
        TODO("Not yet implemented")
    }

<provider
      android:authorities="com.demo.myprovider"
      android:name=".MyContentProvider"
      android:exported="true"/>

3. 操作Provider

其它应用操作本应用的ContentProvider

companion object{
    private const val AUTHORITY = "com.demo.myprovider"
    private val NOTIFY_URI = Uri.parse("content://$AUTHORITY/student")
}

var handler :Handler = object : Handler(Looper.getMainLooper()){
    override fun handleMessage(msg: Message) {

    }
}

private fun test(){
    contentResolver.registerContentObserver(NOTIFY_URI,true, object :ContentObserver(handler){
        override fun onChange(selfChange: Boolean, uri: Uri?) {
            super.onChange(selfChange, uri)
            val msg = Message.obtain()
            msg.obj = uri
            handler.sendMessage(msg)
        }
    })
}

通过ContentProvider来操作数据

 fun onClick(v: View) {
     val id: Int = v.getId()
     if (id == R.id.btn_query) {
         val cursor: Cursor? = contentResolver.query(STUDENT_URI, null, null, null, null, null)
         if (cursor != null && cursor.getCount() > 0) {
             while (cursor.moveToNext()) {
                 val name: String = cursor.getString(cursor.getColumnIndex("name"))
                 val age: Int = cursor.getInt(cursor.getColumnIndex("age"))
                 val desc: String = cursor.getString(cursor.getColumnIndex("desc"))
                 Log.e(javaClass.simpleName, "Student:name = $name,age = $age,desc = $desc")
             }
             cursor.close()
         }
     } else if (id == R.id.btn_insert) {
         val contentValues = ContentValues()
         contentValues.put("name", "insert")
         contentValues.put("age", "-100")
         contentValues.put("desc", "i am insert")
         contentResolver.insert(STUDENT_URI, contentValues)
     } else if (id == R.id.btn_del) {
         contentResolver.delete(STUDENT_URI, null, null)
     }
 }

ContenProvider的用法大体如上

4. FileProvider

Android7.0新特性，对于7.0及以上的设备而言，Google官方禁止使用公开file://的形式对外暴露uri，否则会报FileUriExposedException异常，需要使用content://类型进行Uri的分享，并且需要提供临时的文件访问权限Intent.FLAG_GRANT_READ_URI_PERMISSION和Intent.FLAG_GRANT_WRITE_URI_PERMISSION。官方给出的方案是使用FileProvider进行分享

 <provider
      android:authorities="com.demo.myapplication.fileProvider"
      android:name="androidx.core.content.FileProvider"
      android:grantUriPermissions="true"
      android:exported="false" >
     <meta-data 
           android:name="android.support.FILE_PROVIDER_PATHS"
           android:resource="@xml/file_paths" />
provider>

authorities：签名认证，自定义，获取uri时保持一致即可

name：使用官方提供的FileProvider，也可以自定义继承FileProvider

grantUriPermissions：临时访问权限(READ 和 WRITE)

meta-data：配置可以访问的文件的路径配置信息，需要使用xml文件配置。name固定写法。

<resources>
    <paths>
        
        <files-path
            name="my_files"
            path="mazaiting/"/>
        
        <cache-path
            name="my_cache"
            path="mazaiting/"/>
        
        <external-files-path
            name="external-files-path"
            path="mazaiting/"/>
        
        <external-cache-path
            name="name"
            path="mazaiting/" />
        
        <external-path
            name="my_external_path"
            path="mazaiting/"/>
        
        <external-path
            name="files_root"
            path="Android/data/<包名>/"/>
        
        <external-path
            name="external_storage_root"
            path="."/>
    paths>
resources>

用安装apk举个FileProvider使用的例子

val intent = Intent(Intent.ACTION_VIEW)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
    val uri = FileProvider.getUriForFile(this,"${BuildConfig.APPLICATION_ID}.fileProvider",File(path))
    intent.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
    intent.setDataAndType(uri,"application/vnd.android.package-archive")
}else{
    intent.setDataAndType(Uri.parse("file://$path"),"application/vnd.android.package-archive")
}
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
startActivity(intent)