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;
- }*/
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";
- }
- }
/** * 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;
- }