IOS开发之资源文件的延迟加载方法

延迟加载指一些对象不是在应用和视图等初始化时创建，而是在用到它的时候创建。当应用中有一些对象并不经常使用时，延迟加载可以提高程序性能。
首先，我们要考虑的就是对资源文件的延迟加载。由于资源文件的访问涉及IO操作，这本身就会耗费一定的 CPU时间，如果文件比较大而且加载时机又不合适，就会造成内存浪费。无论是什么类型的文件，有些情况下采用延迟加载是很有必要的。
如图1所示的需求，南昌APP开发公司小编认为可以使用分屏控件（UIPageControl）左右滑动屏幕来浏览这3张图片。

图1 图片延迟加载实例

PageControlNavigation实例是没有采用延迟加载的实现代码，其中的ViewController代码如下：
class ViewController: UIViewController, UIScrollViewDelegate {
var page1 : UIImageView!
var page2 : UIImageView!
var page3 : UIImageView!

@IBOutlet weak var scrollView: UIScrollView!
@IBOutlet weak var pageControl: UIPageControl!

override func viewDidLoad() {
super.viewDidLoad()
self.scrollView.delegate = self
self.scrollView.contentSize
= CGSizeMake(self.view.frame.size.width*3, self.scrollView.
frame.size.height)
self.scrollView.frame = self.view.frame
var mainStoryboard = self.storyboard!
self.page1 = UIImageView(image: UIImage(named: "达芬奇-蒙娜丽莎.png"))
self.page1.frame = CGRectMake(0.0, 0.0, 320.0, 480.0)
self.page2 = UIImageView(image: UIImage(named: "罗丹-思想者.png"))
self.page2.frame = CGRectMake(320.0, 0.0, 320.0, 480.0)
self.page3 = UIImageView(image: UIImage(named: "保罗克利-肖像.png"))
self.page3.frame = CGRectMake(2 * 320.0, 0.0, 320.0, 480.0)
self.scrollView.addSubview(page1)
self.scrollView.addSubview(page2)
self.scrollView.addSubview(page3)

#import "ViewController.h"
@interface ViewController ()
@property (strong, nonatomic) UIImageView *page1;
@property (strong, nonatomic) UIImageView *page2;
@property (strong, nonatomic) UIImageView *page3;
@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (weak, nonatomic) IBOutlet UIPageControl *pageControl;
- (IBAction)changePage:(id)sender;
@end
@implementation ViewController
- (void)viewDidLoad

{

[

super viewDidLoad];
self.scrollView.delegate = self;
self.scrollView.contentSize
= CGSizeMake(self.view.frame.size.width*3,
self.scrollView.frame.size.height);
self.scrollView.frame = self.view.frame;

self.page1 = [[UIImageView alloc] initWithFrame:CGRectMake(0.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page1.image = [UIImage imageNamed:@"达芬奇-蒙娜丽莎.png"];

self.page2 = [[UIImageView alloc] initWithFrame:CGRectMake(320.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page2.image = [UIImage imageNamed:@"罗丹-思想者.png"];

}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}

func scrollViewDidScroll(scrollView: UIScrollView) {
var offset = scrollView.contentOffset
self.pageControl.currentPage = Int(offset.x) / 320
}
@IBAction func changePage(sender: AnyObject) {
UIView.animateWithDuration(0.3,animations : {
var whichPage = self.pageControl.currentPage
self.scrollView.contentOffset = CGPointMake
(320 * CGFloat(whichPage), 0)
})
}
}

self.page3 = [[UIImageView alloc] initWithFrame:CGRectMake(2 * 320.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page3.image = [UIImage imageNamed:@"保罗克利-肖像.png"];
[self.scrollView addSubview:self.page1];
[self.scrollView addSubview:self.page2];
[self.scrollView addSubview:self.page3];
}

- (void) scrollViewDidScroll: (UIScrollView *) aScrollView
{
CGPoint offset = aScrollView.contentOffset;
self.pageControl.currentPage = offset.x / 320.0f;
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
#pragma mark -
#pragma mark PageControl stuff
- (IBAction)changePage:(id)sender
{
[UIView animateWithDuration:0.3f animations:^{
NSInteger whichPage = self.pageControl.currentPage;
self.scrollView.contentOffset = CGPointMake(320.0f * whichPage, 0.0f);
}];
}
@end

我们是在viewDidLoad方法中一次加载全部3张图片，但是有的时候用户不一定会浏览后面的图片，他可能只看到第一张或第二张，后面的第三张没有去看，此时后面的两张图片仍然加载内存的话，会造成内存浪费。

这时候我们就可以采用资源文件的延迟加载功能，采用延迟加载实现时，ViewController的代码如下：

class ViewController: UIViewController, UIScrollViewDelegate {
var page1 : UIImageView!
var page2 : UIImageView!
var page3 : UIImageView!
@IBOutlet weak var scrollView: UIScrollView!
@IBOutlet weak var pageControl: UIPageControl!

override func viewDidLoad() {
super.viewDidLoad()
self.scrollView.delegate = self

self.scrollView.contentSize
= CGSizeMake(self.view.frame.size.width*3, self.scrollView.
frame.size.height)
self.scrollView.frame = self.view.frame
var mainStoryboard = self.storyboard!
self.page1 = UIImageView(image: UIImage(named:
"达芬奇-蒙娜丽莎.png")) ①

self.page1.frame = CGRectMake(0.0, 0.0, 320.0, 480.0)

self.scrollView.addSubview(page1)

}

#import "ViewController.h"
@interface ViewController ()
@property (strong, nonatomic) UIImageView *page1;
@property (strong, nonatomic) UIImageView *page2;
@property (strong, nonatomic) UIImageView *page3;

@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (weak, nonatomic) IBOutlet UIPageControl *pageControl;

- (IBAction)changePage:(id)sender;
-(void)loadImage:(NSInteger)nextPage;
@end

@implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
self.scrollView.delegate = self;

self.scrollView.contentSize = CGSizeMake(self.view.frame.size.width*3,

self.scrollView.frame.size.height);

self.scrollView.frame = self.view.frame;

override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
func scrollViewDidScroll(scrollView: UIScrollView) {
var offset = scrollView.contentOffset
self.pageControl.currentPage = Int(offset.x) / 320
self.loadImage(self.pageControl.currentPage + 1)
}
@IBAction func changePage(sender: AnyObject) {
UIView.animateWithDuration(0.3,animations : {
var whichPage = self.pageControl.currentPage
self.scrollView.contentOffset = CGPointMake
(320 * CGFloat(whichPage), 0)
self.loadImage(self.pageControl.currentPage + 1)
})
}
func loadImage(nextPage: Int) {

if nextPage == 1 && self.page2 == nil {
self.page2 = UIImageView(image: UIImage(named: "罗丹-思想者.png"))
self.page2.frame = CGRectMake(320.0, 0.0, 320.0, 480.0)
self.scrollView.addSubview(page2)
}

if nextPage == 2 && self.page3 == nil {

self.page3 = UIImageView(image: UIImage(named: "保罗克利-肖像.png"))

self.page3.frame = CGRectMake(2 * 320.0, 0.0, 320.0, 480.0)

self.scrollView.addSubview(page3)

}
}
}

self.page1 = [[UIImageView alloc] initWithFrame:CGRectMake(0.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page1.image = [UIImage imageNamed:@"达芬奇-蒙娜丽莎.png"]; ①

[self.scrollView addSubview:self.page1];

}
- (void) scrollViewDidScroll: (UIScrollView *) aScrollView
{
CGPoint offset = aScrollView.contentOffset;
self.pageControl.currentPage = offset.x / 320.0f;
[self loadImage:self.pageControl.currentPage + 1];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
#pragma mark -
#pragma mark PageControl stuff
- (IBAction)changePage:(id)sender
{
[UIView animateWithDuration:0.3f animations:^{
NSInteger whichPage = self.pageControl.currentPage;
self.scrollView.contentOffset = CGPointMake(320.0f * whichPage, 0.0f);
[self loadImage:self.pageControl.currentPage + 1];
}];
}
-(void)loadImage:(NSInteger)nextPage
{
if (nextPage == 1 && self.page2 == nil) {

self.page2 = [[UIImageView alloc] initWithFrame:CGRectMake(320.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page2.image = [UIImage imageNamed:@"罗丹-思想者.png"];
[self.scrollView addSubview:self.page2];
}
if (nextPage == 2 && self.page3 == nil) {

self.page3 = [[UIImageView alloc] initWithFrame:CGRectMake(2 * 320.0f, 0.0f,

self.view.frame.size.width, self.view.frame.size.height)];

self.page3.image = [UIImage imageNamed:@"保罗克利-肖像.png"];
[self.scrollView addSubview:self.page3];
}
}
@end

我们重新修改了这个实例，在viewDidLoad方法中只加载第一张图片，见第①行代码。如果用户滑动屏幕或点击分屏控件进入第二个屏幕，则调用loadImage：方法加载第二张图片，类似地，如果要进入第三个屏幕，则调用 loadImage：方法加载第三张图片。

在这两种实现方式中，LazyLoadPageControlNavigation实现了延迟加载。很显然，LazyLoadPageControlNavigation 的延迟加载友好很多。那么，两者究竟有多大的差别，这是可以量化的。通过Instruments工具的Allocations模板，可以分析ViewController视图控制器加载时内存占用方面的差别。图2是无延迟加载实现案例的Allocations模板跟踪，图3是采用延迟加载实现案例的Allocations模板跟踪。

图2 无延迟加载实现案例的Allocations模板跟踪

图3 使用延迟加载实现案例的Allocations模板跟踪

如图2所示，界面启动用时，内存占用马上达到8.12MB。如图3所示，界面启动用时，内存占用3.08MB，当我们滑动到第二和第三屏幕时，内存占用达到8.06MB，内存变化会有明显的两个阶梯。

综上所述，延迟加载的优势很明显。如果一定会访问到资源文件，则延迟加载这些资源文件时，内存占用方面就没有优势了，但是在界面加载速度方面还是有优势的。
以上便是南昌APP开发公司-百恒网络为大家介绍的关于资源文件的延迟加载方法，希望对大家有所帮助！此外，如有需要APP开发、网站建设、微信开发等方面的服务的朋友，欢迎来电和我们联系，百恒随时为您服务！