using Microsoft.EntityFrameworkCore;

namespace TodoList.Application.Common.Models;

public class PaginatedList<T>
{
    public List<T> Items { get; }
    public int PageNumber { get; }
    public int TotalPages { get; }
    public int TotalCount { get; }

    public PaginatedList(List<T> items, int count, int pageNumber, int pageSize)
    {
        PageNumber = pageNumber;
        TotalPages = (int)Math.Ceiling(count / (double)pageSize);
        TotalCount = count;
        Items = items;
    }

    // 增加屬性表示是否有前一頁
    public bool HasPreviousPage => PageNumber > 1;
    // 增加屬性表示是否有后一頁
    public bool HasNextPage => PageNumber < TotalPages;

    // 分頁結果構建輔助方法
    public static async Task<PaginatedList<T>> CreateAsync(IQueryable<T> source, int pageNumber, int pageSize)
    {
        var count = await source.CountAsync();
        // 注意我們給的請求中pageNumber是從1開始的
        var items = await source.Skip((pageNumber - 1) * pageSize).Take(pageSize).ToListAsync();

        return new PaginatedList<T>(items, count, pageNumber, pageSize);
    }
}

添加對于分頁結果的Mapping Profile

在Application/Common/Mappings中新增一個類用于實現(xiàn)關于分頁結果的擴展方法：

MappingExtensions.cs

using AutoMapper;
using AutoMapper.QueryableExtensions;
using Microsoft.EntityFrameworkCore;
using TodoList.Application.Common.Models;

namespace TodoList.Application.Common.Mappings;

public static class MappingExtensions
{
    public static Task<PaginatedList<TDestination>> PaginatedListAsync<TDestination>(this IQueryable<TDestination> queryable, int pageNumber, int pageSize)
    {
        return PaginatedList<TDestination>.CreateAsync(queryable, pageNumber, pageSize);   
    }

    public static Task<List<TDestination>> ProjectToListAsync<TDestination>(this IQueryable queryable, IConfigurationProvider configuration)
    {
        return queryable.ProjectTo<TDestination>(configuration).ToListAsync();
    }
}

創(chuàng)建分頁查詢請求

為了演示分頁查詢的應用，我們新增一個允許分頁查詢TodoItem的Query：

GetTodoItemsWithPaginationQuery.cs

using System.Linq;
using AutoMapper;
using AutoMapper.QueryableExtensions;
using MediatR;
using TodoList.Application.Common.Interfaces;
using TodoList.Application.Common.Mappings;
using TodoList.Application.Common.Models;
using TodoList.Application.TodoItems.Specs;
using TodoList.Domain.Entities;

namespace TodoList.Application.TodoItems.Queries.GetTodoItems;

public class GetTodoItemsWithPaginationQuery : IRequest<PaginatedList<TodoItemDto>>
{
    public Guid ListId { get; set; }
    public int PageNumber { get; set; } = 1;
    public int PageSize { get; set; } = 10;
}

public class GetTodoItemsWithPaginationQueryHandler : IRequestHandler<GetTodoItemsWithPaginationQuery, PaginatedList<TodoItemDto>>
{
    private readonly IRepository<TodoItem> _repository;
    private readonly IMapper _mapper;

    public GetTodoItemsWithPaginationQueryHandler(IRepository<TodoItem> repository, IMapper mapper)
    {
        _repository = repository;
        _mapper = mapper;
    }

    public async Task<PaginatedList<TodoItemDto>> Handle(GetTodoItemsWithPaginationQuery request, CancellationToken cancellationToken)
    {
        return await _repository
            .GetAsQueryable(x => x.ListId == request.ListId)
            .OrderBy(x => x.Title)
            .ProjectTo<TodoItemDto>(_mapper.ConfigurationProvider)
            .PaginatedListAsync(request.PageNumber, request.PageSize);
    }
}

創(chuàng)建查詢Controller

TodoItemController.cs

// 對于查詢來說，一般參數(shù)是來自查詢字符串的，所以這里用[FromQuery]
[HttpGet]
public async Task<ApiResponse<PaginatedList<TodoItemDto>>> GetTodoItemsWithPagination([FromQuery] GetTodoItemsWithPaginationQuery query)
{
    return ApiResponse<PaginatedList<TodoItemDto>>.Success(await _mediator.Send(query));
}