存储文件的ContentProvider

       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>

        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;
	}
}

        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();
		}
        
    }
}