Android获取本地图片缩略图终极解决方案

QAQ学Android真的还是要在项目中获得锻炼，脱离实际一切都是耍流氓哼唧~！

花了一下午时间搞定了项目中要实现的：获取本地图片缩略图并显示在ListView上的，并且点击要能获得该图片文件路径功能，下面先上效果图：



作为一个新手，大概碰到这种需求的思路就是：

首先，递归遍历本地所有文件，然后按文件后缀名找出所有的图片文件，更好的方式是在媒体库里查找所有的图片(系统已经帮你过滤好了所有的图片文件直接去调用就阔以了)，再通过得到的文件对象file显示图像。

当然这种处理结果就是大概1~2张图片就直接OOM了。（反正我手机里图片都是至少上MB的...）。

所以呢，必须对图片进行压缩，于是我又在网上找到了一个比较好的图片压缩方法(这里没有引用转载地址了，1是因为是昨天找到的现在已经找不到网址了，2是因为很多篇博客都是相同的方法相同的注释，我也不知道到底谁才是原作...)：

/*    *//**
* 根据指定的图像路径和大小来获取缩略图
* 此方法有两点好处：
*     1. 使用较小的内存空间，第一次获取的bitmap实际上为null，只是为了读取宽度和高度，
*        第二次读取的bitmap是根据比例压缩过的图像，第三次读取的bitmap是所要的缩略图。
*     2. 缩略图对于原图像来讲没有拉伸，这里使用了2.2版本的新工具ThumbnailUtils，使
*        用这个工具生成的图像不会被拉伸。
* @param imagePath 图像的路径
* @param width 指定输出图像的宽度
* @param height 指定输出图像的高度
* @return 生成的缩略图
*//*
private Bitmap getImageThumbnail(String imagePath, int width, int height) {
Bitmap bitmap = null;
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
// 获取这个图片的宽和高，注意此处的bitmap为null
bitmap = BitmapFactory.decodeFile(imagePath, options);
options.inJustDecodeBounds = false; // 设为 false
// 计算缩放比
int h = options.outHeight;
int w = options.outWidth;
int beWidth = w / width;
int beHeight = h / height;
int be = 1;
if (beWidth < beHeight) {
be = beWidth;
} else {
be = beHeight;
}
if (be <= 0) {
be = 1;
}
options.inSampleSize = be;
// 重新读入图片，读取缩放后的bitmap，注意这次要把options.inJustDecodeBounds 设为 false
bitmap = BitmapFactory.decodeFile(imagePath, options);
// 利用ThumbnailUtils来创建缩略图，这里要指定要缩放哪个Bitmap对象
bitmap = ThumbnailUtils.extractThumbnail(bitmap, width, height,
ThumbnailUtils.OPTIONS_RECYCLE_INPUT);
return bitmap;
}*/

方法很简单，然而可以发现这个方法其实是做了这么几件事情：

1、根据传入的图片路径构造bitmap，得到原图的宽高.

2、计算图片合适的缩放比.

3、利用ThumbnailUtils来创建缩略图，然后返回这个最终缩放以后的bitmap.

尽管像方法中说的那样：使用较小的内存空间，第一次获取的bitmap实际上为null，只是为了读取宽度和高度， 第二次读取的bitmap是根据比例压缩过的图像，第三次读取的bitmap是所要的缩略图。

然而整个方法还是开辟了3个bitmap对象的内存区域，这还不考虑ThumbnailUtils.extractThumbnail()方法所耗费的时空间。

所以当我使用了这个方法实现了在ListView中遍历本地图片的时候，上下滑起来是灰常卡滴(这里就不贴gif图了有兴趣想知道的朋友可以自己尝试一下)

Finaly，在踏破铁鞋无觅处之后，终于找到了最终解决方法，也是一开始忽略的方法：

原来一直不知道的Thumbnails类，才是解决问题的关键。

在Android系统中也有对应的thumbnails文件，下面是百度百科对它的描述：



然后在MediaStore媒体库类中，也是有Thumbnails这么一个内部类的：

/**
* This class allows developers to query and get two kinds of thumbnails:
* MINI_KIND: 512 x 384 thumbnail
* MICRO_KIND: 96 x 96 thumbnail
*/
public static class Thumbnails implements BaseColumns {
public static final Cursor query(ContentResolver cr, Uri uri, String[] projection) {
return cr.query(uri, projection, null, null, DEFAULT_SORT_ORDER);
}

public static final Cursor queryMiniThumbnails(ContentResolver cr, Uri uri, int kind,
String[] projection) {
return cr.query(uri, projection, "kind = " + kind, null, DEFAULT_SORT_ORDER);
}

public static final Cursor queryMiniThumbnail(ContentResolver cr, long origId, int kind,
String[] projection) {
return cr.query(EXTERNAL_CONTENT_URI, projection,
IMAGE_ID + " = " + origId + " AND " + KIND + " = " +
kind, null, null);
}

/**
* This method cancels the thumbnail request so clients waiting for getThumbnail will be
* interrupted and return immediately. Only the original process which made the getThumbnail
* requests can cancel their own requests.
*
* @param cr ContentResolver
* @param origId original image id
*/
public static void cancelThumbnailRequest(ContentResolver cr, long origId) {
InternalThumbnails.cancelThumbnailRequest(cr, origId, EXTERNAL_CONTENT_URI,
InternalThumbnails.DEFAULT_GROUP_ID);
}

/**
* This method checks if the thumbnails of the specified image (origId) has been created.
* It will be blocked until the thumbnails are generated.
*
* @param cr ContentResolver used to dispatch queries to MediaProvider.
* @param origId Original image id associated with thumbnail of interest.
* @param kind The type of thumbnail to fetch. Should be either MINI_KIND or MICRO_KIND.
* @param options this is only used for MINI_KIND when decoding the Bitmap
* @return A Bitmap instance. It could be null if the original image
*         associated with origId doesn't exist or memory is not enough.
*/
public static Bitmap getThumbnail(ContentResolver cr, long origId, int kind,
BitmapFactory.Options options) {
return InternalThumbnails.getThumbnail(cr, origId,
InternalThumbnails.DEFAULT_GROUP_ID, kind, options,
EXTERNAL_CONTENT_URI, false);
}

/**
* This method cancels the thumbnail request so clients waiting for getThumbnail will be
* interrupted and return immediately. Only the original process which made the getThumbnail
* requests can cancel their own requests.
*
* @param cr ContentResolver
* @param origId original image id
* @param groupId the same groupId used in getThumbnail.
*/
public static void cancelThumbnailRequest(ContentResolver cr, long origId, long groupId) {
InternalThumbnails.cancelThumbnailRequest(cr, origId, EXTERNAL_CONTENT_URI, groupId);
}

/**
* This method checks if the thumbnails of the specified image (origId) has been created.
* It will be blocked until the thumbnails are generated.
*
* @param cr ContentResolver used to dispatch queries to MediaProvider.
* @param origId Original image id associated with thumbnail of interest.
* @param groupId the id of group to which this request belongs
* @param kind The type of thumbnail to fetch. Should be either MINI_KIND or MICRO_KIND.
* @param options this is only used for MINI_KIND when decoding the Bitmap
* @return A Bitmap instance. It could be null if the original image
*         associated with origId doesn't exist or memory is not enough.
*/
public static Bitmap getThumbnail(ContentResolver cr, long origId, long groupId,
int kind, BitmapFactory.Options options) {
return InternalThumbnails.getThumbnail(cr, origId, groupId, kind, options,
EXTERNAL_CONTENT_URI, false);
}

/**
* Get the content:// style URI for the image media table on the
* given volume.
*
* @param volumeName the name of the volume to get the URI for
* @return the URI to the image media table on the given volume
*/
public static Uri getContentUri(String volumeName) {
return Uri.parse(CONTENT_AUTHORITY_SLASH + volumeName +
"/images/thumbnails");
}

/**
* The content:// style URI for the internal storage.
*/
public static final Uri INTERNAL_CONTENT_URI =
getContentUri("internal");

/**
* The content:// style URI for the "primary" external storage
* volume.
*/
public static final Uri EXTERNAL_CONTENT_URI =
getContentUri("external");

/**
* The default sort order for this table
*/
public static final String DEFAULT_SORT_ORDER = "image_id ASC";

/**
* The data stream for the thumbnail
* <P>Type: DATA STREAM</P>
*/
public static final String DATA = "_data";

/**
* The original image for the thumbnal
* <P>Type: INTEGER (ID from Images table)</P>
*/
public static final String IMAGE_ID = "image_id";

/**
* The kind of the thumbnail
* <P>Type: INTEGER (One of the values below)</P>
*/
public static final String KIND = "kind";

public static final int MINI_KIND = 1;
public static final int FULL_SCREEN_KIND = 2;
public static final int MICRO_KIND = 3;
/**
* The blob raw data of thumbnail
* <P>Type: DATA STREAM</P>
*/
public static final String THUMB_DATA = "thumb_data";

/**
* The width of the thumbnal
* <P>Type: INTEGER (long)</P>
*/
public static final String WIDTH = "width";

/**
* The height of the thumbnail
* <P>Type: INTEGER (long)</P>
*/
public static final String HEIGHT = "height";
}
}

我们可以从中找到几个可用于Cursor查找的重要参数：

/**
* The data stream for the thumbnail
* <P>Type: DATA STREAM</P>
*/
public static final String DATA = "_data";

/**
* The original image for the thumbnal
* <P>Type: INTEGER (ID from Images table)</P>
*/
public static final String IMAGE_ID = "image_id";

/**
* The content:// style URI for the "primary" external storage
* volume.
*/
public static final Uri EXTERNAL_CONTENT_URI =
getContentUri("external");

有了这三个参数，我们就可以很轻松从本地媒体库中获得图片缩略图的ID和路径。

//先得到缩略图的URL和对应的图片id
Cursor cursor = cr.query(
Thumbnails.EXTERNAL_CONTENT_URI,
new String[]{
Thumbnails.IMAGE_ID,
Thumbnails.DATA
},
null,
null,
null);

这里的缩略图ID有什么用呢？我们从它的注释中可以很明显地得到：

/**

* The original image for the thumbnal

* <P>Type: INTEGER (ID from Images table)</P>

ID from Images table！！！这个ID是跟多媒体库中的images表的ID相对应的，由此，我们可以通过这个id来设置cursor的查找条件，从而找出images表中对应的真正的图片文件的路径！

从而完美地实现了文章开头的功能需求。

下面是完整的代码：

获得一个HashMap参数的ArrayList,HashMap项的键"thumbnail_path"对应真实图片路径值，键"image_id_path"对应缩略图路径值，有了这两个路径，想干嘛干嘛了2333

/**
* 得到本地图片文件
* @param context
* @return
*/
public static ArrayList<HashMap<String,String>> getAllPictures(Context context) {
ArrayList<HashMap<String,String>> picturemaps = new ArrayList<>();
HashMap<String,String> picturemap;
ContentResolver cr = context.getContentResolver();
//先得到缩略图的URL和对应的图片id
Cursor cursor = cr.query(
Thumbnails.EXTERNAL_CONTENT_URI,
new String[]{
Thumbnails.IMAGE_ID,
Thumbnails.DATA
},
null,
null,
null);
if (cursor.moveToFirst()) {
do {
picturemap = new HashMap<>();
picturemap.put("image_id_path",cursor.getInt(0)+"");
picturemap.put("thumbnail_path",cursor.getString(1));
picturemaps.add(picturemap);
} while (cursor.moveToNext());
cursor.close();
}
//再得到正常图片的path
for (int i = 0;i<picturemaps.size();i++) {
picturemap = picturemaps.get(i);
String media_id = picturemap.get("image_id_path");
cursor = cr.query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
new String[]{
MediaStore.Images.Media.DATA
},
MediaStore.Audio.Media._ID+"="+media_id,
null,
null
);
if (cursor.moveToFirst()) {
do {
picturemap.put("image_id",cursor.getString(0));
picturemaps.set(i,picturemap);
} while (cursor.moveToNext());
cursor.close();
}
}
return picturemaps;
}