基于SQLite的ContentProvider我们见得多了,但是我们在做Android应用时,有时候程序需要下载网络上的图片,这时候我们希望能够把图片缓存到客户端本地,下次再要显示该图片时就不用再从网络上下载了,直接从本地缓存读取,这就需要用到存储文件的ContentProvider 。
这里只关注如何通过ContentProvider缓存图片,对Android本地文件操作不熟悉的同学可以参考Android文件存储,其他内容就不介绍了。
在Mainfest文件中,我们定义的ContentProvider名称为FileProvider,最后别忘了添加权限android.permission.WRITE_EXTERNAL_STORAGE
- <?xml
version="1.0"
encoding="utf-8"?>
- <manifest
xmlns:android="http://schemas.android.com/apk/res/android"
- package="com.ipjmc.demo.fileprovider"
- android:versionCode="1"
- android:versionName="1.0"
> - <uses-sdk
android:minSdkVersion="8"
/> - <application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
> - <activity
android:label="@string/app_name"
android:name=".FileProviderActivity"
> - <intent-filter
> - <action
android:name="android.intent.action.MAIN"
/> - <category
android:name="android.intent.category.LAUNCHER"
/> - </intent-filter>
- </activity>
- <provider
android:name=".FileProvider"
android:authorities="com.ipjmc.demo.fileprovider"
/> - </application>
- <uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE"
/> - </manifest>
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.ipjmc.demo.fileprovider" android:versionCode="1" android:versionName="1.0" > <uses-sdk android:minSdkVersion="8" /> <application android:icon="@drawable/ic_launcher" android:label="@string/app_name" > <activity android:label="@string/app_name" android:name=".FileProviderActivity" > <intent-filter > <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <provider android:name=".FileProvider" android:authorities="com.ipjmc.demo.fileprovider" /> </application> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> </manifest>
FileContentProvider代码如下,其中openFile是必须实现的方法,已经对关键的代码给出了注释
- package com.ipjmc.demo.fileprovider;
- import java.io.BufferedOutputStream;
- import java.io.File;
- import java.io.FileNotFoundException;
- import java.io.FileOutputStream;
- import java.io.IOException;
- import java.io.InputStream;
- import java.io.OutputStream;
- import android.content.ContentProvider;
- import android.content.ContentValues;
- import android.content.res.AssetManager;
- import android.database.Cursor;
- import android.net.Uri;
- import android.os.ParcelFileDescriptor;
- public class FileProvider
extends ContentProvider { - /*
- * 为了简单起见,这里直接将asset/pic.png拷贝到了程序的ExternalFilesDir,实际中应该是从网络上下载图片到ExternalFilesDir。
- */
- @Override
- public boolean onCreate() {
- File file = new File(getContext().getExternalFilesDir(null),
"pic.png"); - if (!file.exists()) {
- AssetManager assetManager = getContext().getAssets();
- try {
- InputStream is = assetManager.open("pic.png");
- OutputStream os = new BufferedOutputStream(new FileOutputStream(file));
- byte [] buf = new
byte[1024];
- int len = 0;
- while ((len = is.read(buf)) >
0) { - os.write(buf, 0, len);
- }
- is.close();
- os.close();
- } catch (IOException e) {
- e.printStackTrace();
- return false;
- }
- }
- return true;
- }
- @Override
- public Cursor query(Uri uri, String[] projection, String selection,
- String[] selectionArgs, String sortOrder) {
- // TODO Auto-generated method stub
- return null;
- }
- @Override
- public String getType(Uri uri) {
- if (uri.toString().endsWith(".png")) {
- return "image/png";
- }
- return null;
- }
- /*
- * 就是做一次映射,返回uri指定的文件的文件描述符
- */
- @Override
- public ParcelFileDescriptor openFile(Uri uri, String mode)
throws FileNotFoundException { - if ("image/png".equals(getType(uri))) {
- File file = new File(getContext().getExternalFilesDir(null), uri.getPath());
- if (file.exists()) {
- return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
- }
- }
- throw new FileNotFoundException(uri.getPath());
- }
- @Override
- public Uri insert(Uri uri, ContentValues values) {
- // TODO Auto-generated method stub
- return null;
- }
- @Override
- public int delete(Uri uri, String selection, String[] selectionArgs) {
- // TODO Auto-generated method stub
- return 0;
- }
- @Override
- public int update(Uri uri, ContentValues values, String selection,
- String[] selectionArgs) {
- // TODO Auto-generated method stub
- return 0;
- }
- }
package com.ipjmc.demo.fileprovider; import java.io.BufferedOutputStream; import java.io.File; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException; import java.io.InputStream; import java.io.OutputStream; import android.content.ContentProvider; import android.content.ContentValues; import android.content.res.AssetManager; import android.database.Cursor; import android.net.Uri; import android.os.ParcelFileDescriptor; public class FileProvider extends ContentProvider { /* * 为了简单起见,这里直接将asset/pic.png拷贝到了程序的ExternalFilesDir,实际中应该是从网络上下载图片到ExternalFilesDir。 */ @Override public boolean onCreate() { File file = new File(getContext().getExternalFilesDir(null), "pic.png"); if (!file.exists()) { AssetManager assetManager = getContext().getAssets(); try { InputStream is = assetManager.open("pic.png"); OutputStream os = new BufferedOutputStream(new FileOutputStream(file)); byte [] buf = new byte[1024]; int len = 0; while ((len = is.read(buf)) > 0) { os.write(buf, 0, len); } is.close(); os.close(); } catch (IOException e) { e.printStackTrace(); return false; } } return true; } @Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { // TODO Auto-generated method stub return null; } @Override public String getType(Uri uri) { if (uri.toString().endsWith(".png")) { return "image/png"; } return null; } /* * 就是做一次映射,返回uri指定的文件的文件描述符 */ @Override public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException { if ("image/png".equals(getType(uri))) { File file = new File(getContext().getExternalFilesDir(null), uri.getPath()); if (file.exists()) { return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY); } } throw new FileNotFoundException(uri.getPath()); } @Override public Uri insert(Uri uri, ContentValues values) { // TODO Auto-generated method stub return null; } @Override public int delete(Uri uri, String selection, String[] selectionArgs) { // TODO Auto-generated method stub return 0; } @Override public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) { // TODO Auto-generated method stub return 0; } }
下面是如何在Activity中该ContentProvider,其中Activity的布局文件我就不贴了,就一个ImageView
- package com.ipjmc.demo.fileprovider;
- import java.io.FileNotFoundException;
- import java.io.InputStream;
- import android.app.Activity;
- import android.graphics.Bitmap;
- import android.graphics.BitmapFactory;
- import android.net.Uri;
- import android.os.Bundle;
- import android.widget.ImageView;
- public class FileProviderActivity
extends Activity { - public static
final Uri URI = Uri.parse("content://com.ipjmc.demo.fileprovider/pic.png");
- ImageView mImageView;
- /** Called when the activity is first created. */
- @Override
- public void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.main);
- mImageView = (ImageView) findViewById(R.id.image);
- try {
- //通过ContentResolver获取图片的输入流,再转化为Bitmap
- InputStream is = getContentResolver().openInputStream(URI);
- Bitmap bitmap = BitmapFactory.decodeStream(is);
- mImageView.setImageBitmap(bitmap);
- } catch (FileNotFoundException e) {
- // TODO Auto-generated catch block
- e.printStackTrace();
- }
- }
- }