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！