Swift 学习中的一点体会，不断更新中。。。

随着Xcode 8的发布，swift 3.0终于来了。又有一大批api名字发生了变化。但是感觉3.0之后的变化应该会小些，因此再重新仔细学习一下。

1. 关于swift引入的Computed Properties概念。

今天看了Salesforce的sdk，发现了如下的demo代码：

#import "SObjectData.h"@interface SampleRequestSObjectData : SObjectData@property (nonatomic, copy) NSString *name;@property (nonatomic, copy) NSString *contactId;@property (nonatomic, copy) NSString *contactName;@property (nonatomic, copy) NSString *productId;@property (nonatomic, copy) NSString *productName;@property (nonatomic, copy) NSString *deliveryDate;@property (nonatomic, copy) NSNumber *quantity;@property (nonatomic, copy) NSString *status;@property (nonatomic, copy) NSDictionary *authorizedUsers;@property (nonatomic, copy) NSArray *userRecords;@property (nonatomic, copy) NSArray *attachments;@end////  SampleRequestSObjectData.m//  SFDCOfflinePoc////  Created by PAULO VITOR MAGACHO DA SILVA on 1/24/16.//  Copyright © 2016 Topcoder Inc. All rights reserved.//#import "SampleRequestSObjectData.h"#import "SampleRequestSObjectDataSpec.h"#import "SObjectData+Internal.h"#import <SmartSync/SFSmartSyncConstants.h>@implementation SampleRequestSObjectData+ (SObjectDataSpec *)dataSpec {    static SampleRequestSObjectDataSpec *sDataSpec = nil;    if (sDataSpec == nil) {        sDataSpec = [[SampleRequestSObjectDataSpec alloc] init];    }    return sDataSpec;}#pragma mark - Property getters / setters- (NSString *)name {    return [self nonNullFieldValue:kSampleRequestNameField];}- (void)setName:(NSString *)name {    [self updateSoupForFieldName:kSampleRequestNameField fieldValue:name];}- (NSString *)contactId {    return [self nonNullFieldValue:kSampleRequestContactField];}- (void)setContactId:(NSString *)contactId {    [self updateSoupForFieldName:kSampleRequestContactField fieldValue:contactId];}- (NSString *)contactName {    return [[self nonNullFieldValue:kSampleRequestContactQueryField] objectForKey:@"Name"];}- (NSString *)productId {    return [self nonNullFieldValue:kSampleRequestProductField];}- (void)setProductId:(NSString *)productId {    [self updateSoupForFieldName:kSampleRequestProductField fieldValue:productId];}- (NSString *)productName {    return [[self nonNullFieldValue:kSampleRequestProductQueryField] objectForKey:@"Name"];}- (NSString *)deliveryDate {    return [self nonNullFieldValue:kSampleRequestDeliveryDateField];}- (void)setDeliveryDate:(NSString *)deliveryDate {    [self updateSoupForFieldName:kSampleRequestDeliveryDateField fieldValue:deliveryDate];}- (NSString *)quantity {    return [self nonNullFieldValue:kSampleRequestQuantityField];}- (void)setQuantity:(NSString *)quantity {    [self updateSoupForFieldName:kSampleRequestQuantityField fieldValue:quantity];}- (NSString *)status {    return [self nonNullFieldValue:kSampleRequestStatusField];}- (void)setStatus:(NSString *)status {    [self updateSoupForFieldName:kSampleRequestStatusField fieldValue:status];}- (NSDictionary *)authorizedUsers {    return [self nonNullFieldValue:kSampleRequestAuthorizedUsersField];}- (void)setAuthorizedUsers:(NSDictionary *)authorizedUsers {    [self updateSoupForFieldName:kSampleRequestAuthorizedUsersField fieldValue:authorizedUsers];}- (NSArray *)attachments {    return [self nonNullFieldValue:kSampleRequestAttachmentsField];}- (void)setAttachments:(NSArray *)attachment {    [self updateSoupForFieldName:kSampleRequestAttachmentsField fieldValue:attachment];}- (NSArray *) userRecords {    int totalSize = [[self.authorizedUsers objectForKey:@"totalSize"] intValue];    ) {        return [self.authorizedUsers objectForKey:@"records"];    }    return nil;}@end

通过代码，可以看出，声明的几个@property 其实没有对每个property对应的真实变量进行读写操作，而是通过自定义的get，set方法对其他的数据进行操作，但是系统会不会自动建立那些没用的真实变量呢？从代码上是看不出了。而且从声明的地方也看不出这些property的特殊性，不注意还真容易误解。

swift的Computed Properties，解决了这个问题：

struct Rect {    var origin = Point()    var size = Size()    var center: Point {        get {            let centerX = origin.x + (size.width / )            let centerY = origin.y + (size.height / )            return Point(x: centerX, y: centerY)        }        set(newCenter) {            origin.x = newCenter.x - (size.width / )            origin.y = newCenter.y - (size.height / )        }    }}

这里的center，有自定义的get，set方法，就明确地指出了它的目的不是为了保存一个值，而是“provide a getter and an optional setter to retrieve and set other properties and values indirectly.”

这个写法就比以前的oc写法要好上很多啊！