PagedList<Object>用于Android很酷的分页库。使问题尽可能少: 如果我有一个字符串列表，比如

  List<String> stringList; // it consists of 200 strings

我要转换stringList输入 PagedList<String>喜欢

  PagedList<String> pagedStringList;

如果我有一个 PagedList<Object>如何将其转换为 List<Object> ？ 我经历了this for reference

如果我尝试反过来......

如何转换 List<Object>进入DataSource.Factory<Integer, Object> ..这样我就可以间接地将它转换成PagedList<> ？

From DataSource.Factory<Integer, Object> I can convert to PagedList<> but how can I convert list into PagedList<> ?

最佳答案

我将从更简单的部分开始，如果你想将 PagedList<> 转换为 List<>，你可以使用 snapshot() 方法，正如你在文档中看到的那样返回:

Returns an immutable snapshot of the PagedList in its current state.A PagedList snapshot is simply an immutable shallow copy of the current state of the PagedList as a List.

现在，对于另一种方式，您需要使用 PagedList.Builder，为此您需要一个 DataSource 来创建 PagedList。因此，管理您的静态列表的非常基本的数据源将如下所示:

StringDataSource.kt

class StringDataSource(val provider: StringListProvider) : PageKeyedDataSource<Int, String>() {
    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Int, String>) {
        val list = provider.getStringList(0, params.requestedLoadSize)
        callback.onResult(list, 1, 2)
    }

    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, String>) {
        val list = provider.getStringList(params.key, params.requestedLoadSize)
        callback.onResult(list, params.key + 1)
    }

    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<Int, String>) {
        val list = provider.getStringList(params.key, params.requestedLoadSize)
        val nextIndex = if (params.key > 1) params.key - 1 else null
        callback.onResult(list, nextIndex)
    }

    companion object {
        const val PAGE_SIZE = 20
    }
}

如您所见，您需要重写 loadInitial、loadAfter 和 loadBefore 这三个方法，以便告诉数据源如何加载列表中的大量数据。出于组织目的，我已将您的列表包装在提供者类中。

StringListProvider.kt

class StringListProvider(val list: List<String>) {


    fun getStringList(page: Int, pageSize: Int): List<String> {

        val initialIndex = page * pageSize
        val finalIndex = initialIndex + pageSize
        //TODO manage index out of range
        return list.subList(initialIndex, finalIndex)
    }
}

要使用数据源生成 PagedList，您需要使用 PagedList.Config.Builder 构建一个简单的配置:

    val myConfig = PagedList.Config.Builder()
            .setInitialLoadSizeHint(PAGE_SIZE)
            .setPageSize(PAGE_SIZE)
            .build()

然后使用数据源将其传递给 PagedList.Builder:

    val pagedStrings: PagedList<String> = PagedList.Builder<Int, String>(StringDataSource(StringListProvider(originalStrings)), myConfig)
            .setInitialKey(0)
            .build()

现在您在 pagedStrings 中有了一个 PagedList。

Java 中同样的事情:

StringListProvider.java

public class StringListProvider {

    private List<String> list;

    public StringListProvider(List<String> list) {
        this.list = list;
    }

    public List<String> getStringList(int page, int pageSize) {
        int initialIndex = page * pageSize;
        int finalIndex = initialIndex + pageSize;

        //TODO manage out of range index

        return list.subList(initialIndex, finalIndex);
    }
}

StringDataSource.java

public class StringDataSource extends PageKeyedDataSource<Integer, String> {

    public static final int PAGE_SIZE = 20;
    private StringListProvider provider;

    public StringDataSource(StringListProvider provider) {
        this.provider = provider;
    }

    @Override
    public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull LoadInitialCallback<Integer, String> callback) {
        List<String> result = provider.getStringList(0, params.requestedLoadSize);
        callback.onResult(result, 1, 2);
    }

    @Override
    public void loadBefore(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, String> callback) {
        List<String> result = provider.getStringList(params.key, params.requestedLoadSize);
        Integer nextIndex = null;

        if (params.key > 1) {
            nextIndex = params.key - 1;
        }
        callback.onResult(result, nextIndex);
    }

    @Override
    public void loadAfter(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, String> callback) {
        List<String> result = provider.getStringList(params.key, params.requestedLoadSize);
        callback.onResult(result, params.key + 1);
    }
}

配置生成

    PagedList.Config myConfig = new PagedList.Config.Builder()
            .setInitialLoadSizeHint(PAGE_SIZE)
            .setPageSize(PAGE_SIZE)
            .build();

列表转换

    List<String> myList = new ArrayList<>();
    StringListProvider provider = new StringListProvider(myList);
    StringDataSource dataSource = new StringDataSource(provider);
    PagedList<String> pagedStrings = new PagedList.Builder<Integer, String>(dataSource, myConfig)
            .setInitialKey(0)
            .build();

关于android - 如何将 List<Object> 转换为 PagedList<Object> ，反之亦然？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50030049/