Cursor这个类是Android开发者难以避免的，比如数据库、ContentResolver内容的读取，但通过这个类读取内容非常的繁琐，针对要读取的每一个字段都会有这样一段代码：
int idIndex = cursor.getColumnIndex("id"); //获取字段对应的列index（列index通常并不需要每次都获取）
if(idIndex >= 0){ //判断列index的合法性String id = cursor.getString(idIndex); //获取对应列的内容
}
这种代码基本没法复用，而且还都是纯手工代码，自动生成比较麻烦，我希望可以像用json映射那样，每个字段/列一行代码就完成这个任务，所以本文就仿照以前解构Bundle一样，来解构Cursor（完整实现差不多100行）。
实现效果
以MediaStore读取照片为例，先编写内容要映射到的Java数据类（重点在于其中的CursorContract）：
public class SystemMedia implements Serializable {private long id;private String data;private long size;private String displayName;private String mimeType;private long dateAdded;private long dateModified;private long bucketId;private String bucketDisplayName;private String album;private int height;private int width;private int orientation;public interface CursorContract { //重点：这个类声明映射的合约，需要提供一个同样参数的构造方法以方便使用SystemMedia consume(@Key(MediaStore.MediaColumns._ID) long id,@Key(MediaStore.MediaColumns.DATA) String data,@Key(MediaStore.MediaColumns.SIZE) long size,@Key(MediaStore.MediaColumns.DISPLAY_NAME) String displayName,@Key(MediaStore.MediaColumns.MIME_TYPE) String mimeType,@Key(MediaStore.MediaColumns.DATE_ADDED) long dateAdded,@Key(MediaStore.MediaColumns.DATE_MODIFIED) long dateModified,@Key(MediaStore.MediaColumns.BUCKET_ID) long bucketId,@Key(MediaStore.MediaColumns.BUCKET_DISPLAY_NAME) String bucketDisplayName,@Key(MediaStore.MediaColumns.HEIGHT) int height,@Key(MediaStore.MediaColumns.WIDTH) int width,@Key(MediaStore.MediaColumns.ALBUM) String album,@Key(MediaStore.MediaColumns.ORIENTATION) int orientation);}public SystemMedia(long id, String data, long size, String displayName, String mimeType, long dateAdded, long dateModified, long bucketId, String bucketDisplayName, int height, int width, String album, int orientation) {this.id = id;this.data = data;this.size = size;this.displayName = displayName;this.mimeType = mimeType;this.dateAdded = dateAdded;this.dateModified = dateModified;this.bucketId = bucketId;this.bucketDisplayName = bucketDisplayName;this.height = height;this.width = width;this.album = album;this.orientation = orientation;}public SystemMedia() {}//省略 getter 和 setter
}
然后我们的查询代码就变成了：
public void query(Context context) {try (Cursor cursor = context.getContentResolver().query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, null,null, null, null)) {if (cursor != null) {List<SystemMedia> result = new ArrayList<>();while (cursor.moveToNext()) {SystemMedia media = (SystemMedia) CursorAdapter.withCursor(cursor, SystemMedia.CursorContract.class, SystemMedia::new);result.add(media);}}}
}
这样就结束了。
API说明
- CursorAdapter.withCursor 方法
- 第一个Cursor参数：目标
Cursor对象 - 第二个Class参数：解构的合约接口，需要是单方法的函数式接口，类名和方法名随意，该方法的参数需要使用
Key注解 - 第三个T参数：合约接口的实现类，完成由所有字段到对象的转换，通常为全属性的构造方法的方法引用，比如
SystemMedia::new
- Key 注解
用于标注参数对应的字段名（Cursor列名）
完整实现（差不多100行）
其中用到的Memorizer工具见Java小技巧：创建带缓存的过程
public abstract class CursorAdapter<T> {abstract T read(Cursor cursor, int index);public static <T> Object withCursor(Cursor cursor, Class<T> clazz, T t) {List<Pair<Integer, CursorAdapter<?>>> list = CursorAdapter.adapterExtractor.apply(cursor).apply(clazz);Method[] methods = clazz.getMethods();if (methods.length == 1) {Object[] args = list.stream().map(pair -> pair.first >= 0 ? pair.second.read(cursor, pair.first) : null).toArray();try {return methods[0].invoke(t, args);} catch (IllegalAccessException | InvocationTargetException e) {throw new RuntimeException(e);}} else {throw new IllegalStateException("methods length is not 1, current is " + methods.length);}}static final Function<Cursor, Function<Class<?>, List<Pair<Integer, CursorAdapter<?>>>>> adapterExtractor = Memorizer.weakMemorize(cursor -> Memorizer.memorize(clazz -> {Method[] methods = clazz.getMethods();if (methods.length == 1) {Method desMethod = methods[0];Annotation[][] parameterAnnotations = desMethod.getParameterAnnotations();Type[] parameterTypes = desMethod.getGenericParameterTypes();if (parameterTypes.length == parameterAnnotations.length) {List<Pair<Integer, CursorAdapter<?>>> adapterList = new LinkedList<>();for (int i = 0; i < parameterTypes.length; i++) {Type parameterType = parameterTypes[i];Optional<Pair<Integer, CursorAdapter<?>>> pairOptional = Arrays.stream(parameterAnnotations[i]).filter(annotation -> annotation instanceof Key).map(annotation -> (Key) annotation).findFirst().map(key -> new Pair<>(cursor.getColumnIndex(key.value()), CursorAdapter.adapterBuilder.apply(parameterType)));if (pairOptional.isPresent()) {adapterList.add(pairOptional.get());} else {throw new IllegalStateException("every parameter must contains a Key annotation");}}return adapterList;} else {throw new IllegalStateException("parameters length is not equal to annotations length");}} else {throw new IllegalArgumentException("methods size must be 1, current is " + methods.length);}}));private static final Function<Type, CursorAdapter<?>> adapterBuilder = Memorizer.memorize(type -> {if (int.class.equals(type) || Integer.class.equals(type)) {return create(Cursor::getInt);} else if (float.class.equals(type) || Float.class.equals(type)) {return create(Cursor::getFloat);} else if (long.class.equals(type) || Long.class.equals(type)) {return create(Cursor::getLong);} else if (short.class.equals(type) || Short.class.equals(type)) {return create(Cursor::getShort);} else if (String.class.equals(type)) {return create(Cursor::getString);} else if (double.class.equals(type) || Double.class.equals(type)) {return create(Cursor::getDouble);} else if (type instanceof GenericArrayType) {Type componentType = ((GenericArrayType) type).getGenericComponentType();if (byte.class.equals(componentType)) {return create(Cursor::getBlob);} else {throw new IllegalStateException("unsupported componentType:" + componentType);}} else {throw new IllegalArgumentException("unsupported type : " + type);}});private static <T> CursorAdapter<T> create(BiFunction<Cursor, Integer, T> reader) {return new CursorAdapter<T>() {@OverrideT read(Cursor cursor, int index) {return reader.apply(cursor, index);}};}
}
