iOS中ParseSwift SDK的复杂查询实现指南

parseswift 查询手册 介绍 在 基本查询 https //www back4app com/docs/ios/parse swift sdk/data objects/basic queries 指南中，我们介绍了一些构建查询和从 back4app 数据库中检索数据的基本方法。 parseswift sdk parseswift sdk 提供了构建更复杂查询的实用方法，以用于实际应用程序。 在本指南中，您将深入了解 query\<u> query\<u> （泛型）类，并查看您可以用来构建查询的所有方法。您将使用一个简单的数据库类（名为 profile profile ）和一些模拟数据来使用 swift 执行查询。 在开始之前，我们应该建立几个关于对象和数据类型的概念。在本指南中，我们将任何可以存储在数据库中的数据类型（如结构体）称为对象。在某些情况下，它们也被称为项目或元素。 属性是数据类型的成员（主要是结构体）。然而，在这里，我们通常称它们为字段。parseswift 使用术语“键”来表示属性的名称。因此，在 parseswift sdk parseswift sdk 中的任何方法中标记为“键”的任何参数表示其值必须与字段的名称匹配。 先决条件 要完成本教程，您需要： 一个 在 back4app 上创建的应用 。 一个基本的 ios 应用来测试查询 目标 探索不同的方法， query\<u> query\<u> 类提供的执行查询的方式。 query\<u> 类 在本指南中，我们将查询有关一个人联系信息。这些数据存储在 profile profile 类下，存储在 back4app 数据库中。为了通过 ios 应用程序检索个人资料，我们需要创建一个 profile profile 结构来包含这些信息。 1 import parseswift 2 3 4 struct profile parseobject { 5 6 7 // custom properties to organize the person's information 8 var name string? 9 var birthday date? 10 var numberoffriends int? 11 var favoritefoods \[string]? 12 var luckynumbers \[int]? 13 var lastloginlocation parsegeopoint? // a data type to store coordinates for geolocation 14 var isactive bool? 15 var membership pointer\<membership>? // more details about membership below 16 17 18 } 此外，我们使用对象 会员资格 会员资格 来展示 个人资料 个人资料 和 会员资格 会员资格 之间的查询是如何相互作用的。 会员资格 会员资格 结构的实现方式如下 1 import parseswift 2 3 4 struct membership parseobject { 5 6 7 // custom properties to organize the membership's information 8 var typedescription string? 9 var expirationdate date? 10 11 12 } 在 back4app 数据库上进行的任何查询操作都是通过通用类 query\<profile> query\<profile> 执行的，其中通用类型 个人资料 个人资料 （符合 parseswift parseswift 协议）是我们试图检索的对象。在实例化一个 query\<profile> query\<profile> 对象后，我们必须调用其 find( ) find( ) 方法之一（请参见 基本查询指南 https //www back4app com/docs/ios/parse swift sdk/data objects/basic queries ) 来处理查询返回的结果。 您可以在 query\<u> query\<u> 类别的 官方文档中查看更多信息 https //github com/parse community/parse swift 保存数据对象 通过以下代码片段，我们创建并保存一些数据以测试各种查询 1 let adamsandler = profile( 2 name "adam sandler", 3 birthday date(string "09/09/1996"), 4 numberoffriends 2, 5 favoritefoods \["lobster", "bread"], 6 luckynumbers \[2, 7], 7 lastloginlocation try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319), 8 isactive false, 9 membership nil 10 ) 11 12 let britneyspears = profile( 13 name "britney spears", 14 birthday date(string "12/02/1981"), 15 numberoffriends 52, 16 favoritefoods \["cake", "bread"], 17 luckynumbers \[22, 7], 18 lastloginlocation try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319), 19 isactive true, 20 membership nil 21 ) 22 23 let carsonkressley = profile( 24 name "carson kressley", 25 birthday date(string "03/27/1975"), 26 numberoffriends 12, 27 favoritefoods \["fish", "cookies"], 28 luckynumbers \[8, 7], 29 lastloginlocation try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319), 30 isactive true, 31 membership nil 32 ) 33 34 // we create and save a membership object 35 let premiummembership = membership(typedescription "premium", expirationdate date(string "10/10/2030")) 36 let savedpremiummembership = try! premiummembership save() // careful with the forced unwrap 37 38 let danaykroyd = profile( 39 name "dan aykroyd", 40 birthday date(string "07/01/1952"), 41 numberoffriends 66, 42 favoritefoods \["jam", "peanut butter"], 43 luckynumbers \[22, 77], 44 lastloginlocation try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319), 45 isactive true, 46 membership try? init(savedpremiummembership) 47 ) 48 49 let eddymurphy = profile( 50 name "eddie murphy", 51 birthday date(string "04/03/1961"), 52 numberoffriends 49, 53 favoritefoods \["lettuce", "pepper"], 54 luckynumbers \[6, 5], 55 lastloginlocation try? parsegeopoint(latitude 27 104919974838154, longitude 52 61428045237739), 56 isactive false, 57 membership nil 58 ) 59 60 let fergie = profile( 61 name "fergie", 62 birthday date(string "03/27/1975"), 63 numberoffriends 55, 64 favoritefoods \["lobster", "shrimp"], 65 luckynumbers \[12, 7], 66 lastloginlocation try? parsegeopoint(latitude 27 104919974838154, longitude 52 61428045237739), 67 isactive false, 68 membership nil 69 ) 70 71 do { 72 = try? adamsandler save() 73 = try? britneyspears save() 74 = try? carsonkressley save() 75 = try? danaykroyd save() 76 = try? eddymurphy save() 77 } catch { 78 // handle the errors here 79 } 一旦上述代码执行完毕，我们应该有一个小型数据库可以使用。 有关如何使用 parseswift 保存数据的更多信息，请访问 ios 数据对象 查询检索器 这些方法负责运行查询并检索其结果，始终存在于您的查询实现中。 find //this is the basic method for retrieving your query results 1 let query = profile query() // instantiate a query\<profile> class with no additional constraints 2 3 let profiles = try? query find() // executes the query synchronously returns and array of profiles it throws an error if something happened 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } first //retrieves the first profile instance that meets the query criteria 1 let query = profile query() 2 3 let profile = try? query first() // executes the query synchronously 4 5 query first { result in // executes the query asynchronously 6 // handle the result (of type result\<profile, parseerror>) 7 } count //retrieves the number of elemets found by the query 1 let query = profile query() // instantiate a query\<profile> class with no additional constraints 2 3 let profiles = try? query count() // executes the query synchronously 4 5 query count { result in // executes the query asynchronously 6 // handle the result (of type result\<int, parseerror>) 7 } distinct //runs the query and returns a list of unique values from the results and the specified field, name in this case 1 let query = profile query() // instantiate a query\<profile> class with no additional constraints 2 3 let profiles = try? query distinct("name") // executes the query synchronously 4 5 query distinct("name") { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } find all //retrieves a complete list of profile’s that satisfy this query 1 let query = profile query() 2 3 let profiles = try? query findall() // executes the query synchronously 4 5 query findall { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } using the objectid //when you need to retrieve an object with a specific objectid 1 let profile = try? profile(objectid "huyfn4yxfd") fetch() // retrieves synchronously the profile with the given objectid it throws an error if something happened 2 3 profile(objectid "huyfn4yxfd") fetch { // retrieves asynchronously the profile with the given objectid 4 // handle the result (of type result\<profile, parseerror>) 5 } 对字符串类型字段的约束查询 我们现在开始对查询应用特定的约束。首先，我们将施加约束，仅在 字符串 字符串 类型字段满足给定条件时选择对象。对 query\<profile> query\<profile> 对象施加的这种及其他类型的约束是通过 queryconstraint queryconstraint 对象完成的。下面和接下来的部分我们详细说明如何构建这样的约束。 equal //used when we need to select all the objects which have a field equal to a given string we use the == operator to construct the corresponding constraint on the left hand side goes the field’s name and on the right hand side goes the string to compare against 1 let constraint queryconstraint = "name" == "fergie" // selects all profiles with name equals to fergie 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } regular expression //used when one of the fields contains a given string 1 let constraint queryconstraint = containsstring(key "name", substring "ie") 2 // containsstring(key\ substring ) is a method provided by the parseswift sdk 3 let query = profile query(constraint) 4 5 let profiles = try? query find() // executes the query synchronously 6 7 query find { result in // executes the query asynchronously 8 // handle the result (of type result<\[profile], parseerror>) 9 } contains substring //retrieves the number of elemets found by the query 1 let query = profile query() // instantiate a query\<profile> class with no additional constraints 2 3 let profiles = try? query count() // executes the query synchronously 4 5 query count { result in // executes the query asynchronously 6 // handle the result (of type result\<int, parseerror>) 7 } matches text //similar to == but it has additional options like case sensite, language and diacritic sensitive 1 // careful with the force unwrap! 2 let constraint queryconstraint = try! matchestext(key "name", text "fergie", options \[ casesensitive false]) 3 // matchestext(key\ text ,options ) is a method provided by the parseswift sdk 4 5 let query = profile query(constraint) 6 7 let profiles = try? query find() // executes the query synchronously 8 9 query find { result in // executes the query asynchronously 10 // handle the result (of type result<\[profile], parseerror>) 11 } 带有可比较字段约束的查询 尝试选择某个数字类型字段必须等于、不同、大于或小于给定值的对象是非常常见的。这是通过使用以下可比较操作来实现的 equal //use this constraint when you need to select objects where a certain number type field is equal to a given value 1 let constraint queryconstraint = "numberoffriends" == 2 // selects all profiles with number of friends = 2 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } not equal //the opposite constraint of equals to and it also works with date type fields 1 let constraint queryconstraint = "numberoffriends" != 2 // selects all profiles where the number of friends is different from 2 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } less than //to construct this constraint we use the < operator on the left hand side goes the field’s name and on the right hand side goes the value to compare against 1 let constraint queryconstraint = "numberoffriends" < 49 // selects all profiles with a maximum of 48 friends 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } less than or equal //to construct this constraint we use the <= operator on the left hand side goes the field’s name (also referred as key) and on the right hand side goes the value to compare against 1 let constraint queryconstraint = "numberoffriends" <= 49 // selects all profiles with a maximum of 49 friends 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } greater than //once again, for this constraint we use another operator to construct it, the > operator 1 let constraint queryconstraint = "birthday" > date(string "08/19/1980") // selects all profiles who born after 08/19/1980 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } greater than or equal //last but not least, the greater than or equal constraint are constructed using the >= operator 1 let constraint queryconstraint = "numberoffriends" >= 20 // selects all profiles with at least 20 number of friends 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } 这些约束也适用于 日期 日期 类型字段。例如，我们可以有 1 let constraint queryconstraint = "birthday" > date(string "08/19/1980") // selects all profiles who were born after 08/19/1980 2 let query = profile query(constraint) 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } 对于 布尔 布尔 类型字段，我们有 等于 和 不等于 选项。例如 1 let constraint queryconstraint = "isactive" == true 2 let query = profile query(constraint) // selects all active profiles 3 4 let profiles = try? query find() // executes the query synchronously 5 6 query find { result in // executes the query asynchronously 7 // handle the result (of type result<\[profile], parseerror>) 8 } 涉及地理位置的约束查询 对于包含位置数据的字段（即，类型为 parsegeopoint parsegeopoint ）我们有以下查询选项。 near //in order to select objects where one of their parsegeopoint type fields is near to another given geopoint, we use 1 guard let geopoint = try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319) else { 2 return 3 } 4 5 let query = profile query(near(key "lastloginlocation", geopoint geopoint)) 6 // near(key\ geopoint ) is a method provided by the parseswift sdk 7 8 let profiles = try? query find() // executes the query synchronously 9 10 query find { result in // executes the query asynchronously 11 // handle the result (of type result<\[profile], parseerror>) 12 } contains //given an object with a parsepolygon type field, in order to select them in a query depending on what geopoints the field contains, we can use the following constraint 1 struct myobject parseswift { 2 3 4 var apolygon parsepolygon? 5 6 7 } 8 9 guard let geopoint = try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319) else { 10 return 11 } 12 13 let query = myobject query(polygoncontains(key "apolygon", point geopoint)) 14 // polygoncontains(key\ point ) is a method provided by the parseswift sdk 15 16 let myobjects = try? query find() // executes the query synchronously 17 18 query find { result in // executes the query asynchronously 19 // handle the result (of type result<\[myobject], parseerror>) 20 } within geo box //used to select objects where a parsegeopoint type field is inside of a given geopoint box this box is described by its northeastpoint and southwestpoint geopoints 1 guard let southwestpoint = try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319), 2 let northeastpoint = try? parsegeopoint(latitude 37 28412167489413, longitude 121 91268034622319) else { 3 return 4 } 5 6 let query = profile query(withingeobox(key "lastloginlocation", fromsouthwest southwestpoint, tonortheast northeastpoint)) 7 // withingeobox(key\ fromsouthwest\ tonortheast ) is a method provided by the parseswift sdk 8 9 let profiles = try? query find() // executes the query synchronously 10 11 query find { result in // executes the query asynchronously 12 // handle the result (of type result<\[profile], parseerror>) 13 } within km/mph //used to select objects according to whether or not a parsegeopoint type field value is near a given geopoint additionally, we should provide the maximum distance for how far the field value can be from the geopoint 1 guard let geopoint = try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319) else { 2 return 3 } 4 5 let query1 = profile query(withinkilometers(key "lastloginlocation", geopoint geopoint, distance 100)) 6 // withinkilometers(key\ geopoint\ distance ) is a method provided by the parseswift sdk 7 8 let query2 = profile query(withinmiles(key "lastloginlocation", geopoint geopoint, distance 100)) 9 // withinmiles(key\ geopoint\ distance ) is a method provided by the parseswift sdk 10 11 // execute the corresponding query within polygon //used to select objects with a parsegeopoint type field in a given polygon the vertices of the polygon are represented by parsegeopoint objects 1 // create the vertices of the polygon (4 in this case) 2 guard let geopoint1 = try? parsegeopoint(latitude 37 48412167489413, longitude 122 11268034622319), 3 let geopoint2 = try? parsegeopoint(latitude 37 48412167489413, longitude 121 91268034622319), 4 let geopoint3 = try? parsegeopoint(latitude 37 38412167489413, longitude 121 91268034622319), 5 let geopoint4 = try? parsegeopoint(latitude 37 38412167489413, longitude 122 01268034622319) else { 6 return 7 } 8 9 let constraint queryconstraint = withinpolygon( 10 key "lastloginlocation", 11 points \[geopoint1, geopoint2, geopoint3, geopoint4] 12 ) 13 // withinpolygon(key\ points ) is a method provided by the parseswift sdk 14 15 let query = profile query(constraint) 16 17 let profiles = try? query find() // executes the query synchronously 18 19 query find { result in // executes the query asynchronously 20 // handle the result (of type result<\[profile], parseerror>) 21 } within radians //similar to ‘within kilometers’ or ‘within miles’ but the distance is expressed in radians 1 guard let geopoint = try? parsegeopoint(latitude 37 48412167489413, longitude 122 11268034622319) else { 2 return 3 } 4 5 let query = profile query(withinradians(key "lastloginlocation", geopoint geopoint, distance 1 5)) 6 // withinradians(key\ geopoint\ distance ) is a method provided by the parseswift sdk 7 8 let profiles = try? query find() // executes the query synchronously 9 10 query find { result in // executes the query asynchronously 11 // handle the result (of type result<\[profile], parseerror>) 12 } 涉及数组类型字段的约束查询 当我们想要检索的对象需要满足其任何数组类型字段的给定条件时， parseswift sdk parseswift sdk 提供了以下替代方案来实现这一点 contained in //add a constraint to the query that requires a particular field to be contained in the provided array 1 let constraint queryconstraint = containedin(key "luckynumbers", array \[2, 7]) 2 // containedby(key\ array ) is a method provided by the parseswift sdk 3 4 let query = profile query(constraint) 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } contained by //add a constraint to the query that requires a particular field to be contained by the provided array it retrieves objects where all the elements of their array type field match 1 let constraint queryconstraint = containedby(key "luckynumbers", array \[2, 7]) 2 // containedby(key\ array ) is a method provided by the parseswift sdk 3 4 let query = profile query(constraint) 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } contains all //add a constraint to the query that requires a particular array type field to contain every element of the provided array 1 let constraint queryconstraint = containsall(key "luckynumbers", array \[2, 7]) 2 // containsall(key\ array ) is a method provided by the parseswift sdk 3 4 let query = profile query(constraint) 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } 高级查询 到目前为止，我们介绍了可以应用于特定字段的主要条件，以便从 back4app 数据库中选择对象，使用 parseswift sdk parseswift sdk 。在本节中，我们将继续进行查询的组合和高级查询。 exists //it is used to select objects depending on the value of a given field not being nil 1 let constraint queryconstraint = exists(key "membership") 2 // exists(key ) is a method provided by the parseswift sdk 3 4 let query = profile query(constraint) // will retrieve all profiles which have a membership 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } does not exists //it is used to select objects depending on the value of a given field being nil 1 let constraint queryconstraint = doesnotexist(key "membership") 2 // doesnotexist(key ) is a method provided by the parseswift sdk 3 4 let query = profile query(constraint) // will retrieve all profiles which do not have a membership 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } matches key in query //when we need to retrieve objects using the result from a different query as a condition, we make use of the method doesnotmatchkeyinquery(key\ querykey\ query ) to construct the corresponding condition to illustrate this in more detail, we first introduce a new object friend which has a property numberofcontacts of type int 1 struct friend parseswift { 2 3 var numberofcontacts int? 4 5 } does not match key in query //this case is the opposite of matches key in query it is useful when we want to select all the profile objects where their numberoffriends field does not match with the numberofcontacts field in all friend objects the method we use to compose this query is doesnotmatchkeyinquery(key\ querykey\ query ) 1 let friendquery = friend query() // selects all friends objects 2 3 let constraint queryconstraint = doesnotmatchkeyinquery( 4 key "numberoffriends", 5 querykey "numberofcontacts", 6 query friendquery 7 ) 8 // doesnotmatchkeyinquery(key\ querykey\ query ) is a method provided by the parseswift sdk 9 10 let query = profile query(constraint) 11 12 let profiles = try? query find() // executes the query synchronously 13 14 query find { result in // executes the query asynchronously 15 // handle the result (of type result<\[profile], parseerror>) 16 } 查询排序 对查询结果进行排序是开始显示或操作这些结果之前的关键。 parseswift sdk parseswift sdk 提供了以下选项来实现这一点。 ascending //sort the results in ascending order, overwrites previous orderings multiple fields can be used to solve ordering ties by calling the order( ) method on a query\<profile> object, we obtain a new query which implements the given order option the argument for the order( ) method is an array of query\<profile> order items these enumerations contain the name of the field to be used for the sort algorithm for ascending order we use query\<profile> order ascending("nameofthefield") 1 let query = profile query() order(\[ ascending("numberoffriends")]) 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } descending //similar to the ascending order, the only diference is that instead of using query\<profile> order ascending("nameofthefield") we should use query\<profile> order descending("nameofthefield") 1 let query = profile query() order(\[ descending("numberoffriends")]) 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } text score //this kind of sorting procedure relies on a score metric in order to use this option, the profile object must conform to the parsequeryscorable protocol once the protocol is implemented, we call the sortbytextscore() method on the query\<profile> object to obtain a new one that implements the required sorting method 1 extension profile parsequeryscorable { 2 var score double? { 3 // add the corresponding implementation 4 } 5 } 6 7 let query = profile query() sortbytextscore() 8 9 let profiles = try? query find() // executes the query synchronously 10 11 query find { result in // executes the query asynchronously 12 // handle the result (of type result<\[profile], parseerror>) 13 } 字段选择 根据查询时所需的数据，这可能需要更多或更少的时间。在某些情况下，可能只需从对象中检索某些字段并忽略不必要的字段。此外，通过仅选择我们在查询中需要的字段，我们可以避免过度获取并提高获取过程的性能。 exclude //calling the exclude( ) method on a query returns a new query that will exclude the fields passed (in variadic format) as the argument 1 let query = profile query() exclude("name", "numberoffriends") 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } include //it is used to retrieve the fields whose data type conforms to the parseobject protocol (i e , objects stored in the same database under its corresponding class name) calling the include( ) method on a query returns a new query that will include the fields passed (in variadic format) as the argument for instance, the membership field in profile will not be fetched unless we include it manually 1 let query = profile query() include("membership") 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } include all //when we need to retrieve the fields that conforms to the parseobject we call the includeall() method on the query 1 let query = profile query() includeall() 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } select //it returns only the specified fields in the returned objects calling the select( ) method on a query returns a new query which will select only the fields passed (in variadic format) as argument 1 let query = profile query() select("name", "birthday") 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } 分页 在大型数据库中，分页是查询和处理大量结果的基本功能。 parseswift sdk parseswift sdk 提供以下方法来管理这些情况。 limit //determines the maximum number of results a query is able to return, the default value is 100 we call the limit( ) method on the query in question to change this value 1 let query = profile query() limit(2) 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } skip //together with the previous option, the skip option allows us to paginate the results we call the skip( ) method on the query in question to change this value 1 let query = profile query() skip(2) 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } with count //when we need to know in advance the number of results retrieved from a query, we use the withcount(completion ) method instead of the usual find( ) in this way, the result is composed by the objects found together with an integer indicating the length of the array 1 let query = profile query() 2 3 query withcount { result in // executes the query asynchronously 4 // handle the result (of type result<(\[profile], int), parseerror>) 5 } 复合查询 这些方法将创建复合查询，可以组合多个 query\<profile> query\<profile> 实例以实现更复杂的结果。 and //compose a compound query that is the and of the passed queries this is accomplished by first instantiating the queries to be used for the and operation the and(queries ) method provided by the parseswift sdk allows us to perform the and operation and embed the result in a queryconstraint object this constraint is then used to instantiate the final query to be used for retrieveing the results 1 let query1 = profile query("numberoffriends" > 10) 2 let query2 = profile query("numberoffriends" < 50) 3 4 let query = profile query(and(queries query1, query2)) 5 // and(queries ) is a method provided by the parseswift sdk it takes an array of queries and applies the and op on them 6 7 let profiles = try? query find() // executes the query synchronously 8 9 query find { result in // executes the query asynchronously 10 // handle the result (of type result<\[profile], parseerror>) 11 } nor //in this case, we apply a nor operation on a set of queries we use the nor(queries ) method provided by the parseswift sdk to perform such operation we handle the result in a similar way as the and operation 1 let query1 = profile query("numberoffriends" > 10) 2 let query2 = profile query("numberoffriends" < 50) 3 4 let query = profile query(nor(queries query1, query2)) 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } or //similar to previous cases, we apply an or operation on a set of queries we use the or(queries ) method provided by the parseswift sdk to perform such operation we handle the result in a similar way as the and (or nor) operation 1 let query1 = profile query("numberoffriends" > 10) 2 let query2 = profile query("numberoffriends" < 50) 3 4 let query = profile query(or(queries query1, query2)) 5 6 let profiles = try? query find() // executes the query synchronously 7 8 query find { result in // executes the query asynchronously 9 // handle the result (of type result<\[profile], parseerror>) 10 } 数据库相关 这些方法与数据库偏好设置和操作相关。 aggregate //executes an aggregate query, retrieving objects over a set of input values to use this feature, instead of executing the query with find( ), we call the aggregate( completion ) the first argument of this function is a dictionary containing the information about the pipeline for more details, refer to mongodb documentation on aggregate 1 let query = profile query() 2 3 query aggregate(\[\["pipeline" 5]]) { result in 4 // handle the result (of type result<\[profile], parseerror>) 5 } explain //investigates the query execution plan, related to mongodb explain operation this option requires a new object (conforming to the decodable protocol) to handle the results 1 struct anycodable decodable { 2 // implement the corresponding properties 3} 4 5let query = profile query() 6 7let results = try? query findexplain() // executes the option synchronously 8 9// instantiate the completion block explicitely in order for the method findexplain(completion ) to infer the type of the returning results 10 let completion (result<\[anycodable], parseerror>) > void = { result in 11 // handle the result 12 } 13 14 // executes the option asynchronously 15 query findexplain(completion completion) readpreference //when using a mongodb replica set, use this method to choose from which replica the objects will be retrieved the possible values are primary (default), primary preferred, secondary, secondary preferred, or nearest 1 let query = profile query() readpreference("nearest") 2 3 let profiles = try? query find() // executes the query synchronously 4 5 query find { result in // executes the query asynchronously 6 // handle the result (of type result<\[profile], parseerror>) 7 } 结论 使用 parseswift sdk parseswift sdk , 我们能够理解如何构建和从 back4app 数据库中检索对象。通过这本食谱，您应该能够执行具有非常灵活约束的查询，并根据您的用例管理结果。有关上述任何主题的更多详细信息，您可以参考 parseswift 仓库 https //github com/parse community/parse swift