iOS: 学习笔记, 使用performSelectorOnMainThread及时刷新UIImageView

在iOS中, 界面刷新在主线程中进行, 这导致NSURLSession远程下载图片使用UIImageView直接设置Image并不能及时刷新界面.

下面的代码演示了如何使用 performSelectorOnMainThread: withObject:  waitUntilDone: 方法来及时刷新图片

1. 创建iOS空应用程序(Empty Application).

2. 加入一个控制器类. 在YYAppDelegate.m中修改

#import "MainViewController.h"

@implementation YYAppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];

    self.window.rootViewController = [[MainViewController alloc] initWithNibName:nil bundle:nil];

    [self.window makeKeyAndVisible];
    return YES;
}

3. 修改MainViewController.m文件

 //
 //  MainViewController.m
 //  UIByCodeDemo0602_ImageView
 //
 //  Created by yao_yu on 14-6-3.
 //  Copyright (c) 2014年 yao_yu. All rights reserved.
 //

 #import "MainViewController.h"

 @interface MainViewController ()

 @property(nonatomic, strong)UILabel *header;
 @property(nonatomic, strong)UIImageView *imageView;
 @property(nonatomic, strong)UIImage *imagedata;

 @end

 @implementation MainViewController

 - (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
 {
     self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
     if (self) {
         // Custom initialization
     }
     return self;
 }

 - (void)viewDidLoad
 {
     [super viewDidLoad];
     self.imagedata = nil;

     //创建标题标签
     self.header = [[UILabel alloc] init];
     self.header.text = @"示意图";
     self.header.textAlignment = NSTextAlignmentCenter;
     [self.view addSubview: self.header];
     [self.header setTranslatesAutoresizingMaskIntoConstraints: NO];

     //创建图片视图
     self.imageView = [[UIImageView alloc] init];
     [self.imageView setBackgroundColor: [UIColor blueColor]];
     [self.imageView setImage: [UIImage imageWithContentsOfFile:@"/Users/yao_yu/Documents/aaa/3002302_.png"]];
     self.imageView.layer.cornerRadius = ;
     [self.view addSubview:self.imageView];
     [self.imageView setTranslatesAutoresizingMaskIntoConstraints: NO];

     //创建前一张按钮
     UIButton *prevButton = [[UIButton alloc] init];
     prevButton.frame = CGRectMake(, , , );
     [prevButton setBackgroundColor:[UIColor redColor]];
     [prevButton setTitle:@"前一张" forState:UIControlStateNormal];
     [prevButton addTarget:self action:@selector(onShowPrevImage:) forControlEvents:UIControlEventTouchUpInside];
     [self.view addSubview: prevButton];
     [prevButton setTranslatesAutoresizingMaskIntoConstraints: NO];

     //创建后一张按钮
     UIButton *nextButton = [[UIButton alloc] init];
     nextButton.frame = CGRectMake(, , , );
     [nextButton setBackgroundColor:[UIColor redColor]];
     [nextButton setTitle:@"后一张" forState:UIControlStateNormal];
     [nextButton addTarget:self action:@selector(onShowNextImage:) forControlEvents:UIControlEventTouchUpInside];
     [self.view addSubview: nextButton];
     [nextButton setTranslatesAutoresizingMaskIntoConstraints: NO];

     //约束
     NSMutableArray *contraits = [NSMutableArray array];
     NSDictionary *metrics = [NSDictionary dictionaryWithObjectsAndKeys:@, @"VDist", @, @"Padding", nil];
     UILabel *header = self.header;
     UIImageView *imageView = self.imageView;
     NSDictionary *views = NSDictionaryOfVariableBindings(header, imageView, prevButton, nextButton);

     [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-Padding-[header]-Padding-|" options: metrics:metrics views:views]];
     [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-Padding-[prevButton]-(>=0)-[nextButton(==prevButton)]-Padding-|" options: metrics:metrics views:views]];

     [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-Padding-[imageView]-Padding-|" options: metrics:metrics views:views]];
     [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-VDist-[header]-Padding-[imageView]-(>=VDist)-|" options: metrics:metrics views:views]];
     //垂直居中
     [self.view addConstraint:[NSLayoutConstraint constraintWithItem:prevButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier: constant:]];
     [self.view addConstraint:[NSLayoutConstraint constraintWithItem:nextButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier: constant:]];

     [self.view addConstraints:contraits];

 }

 - (void)didReceiveMemoryWarning
 {
     [super didReceiveMemoryWarning];
 }

 -(void)onShowPrevImage:(id)sender
 {
     NSURL *URL = [NSURL URLWithString:@"http://img.gtimg.cn/images/hq_parts/hushen/stocks/300230.png"];
     NSURLRequest *request = [NSURLRequest requestWithURL:URL];
     NSURLSession *session = [NSURLSession sharedSession];
     NSURLSessionDataTask *task = [session dataTaskWithRequest:request
                                             completionHandler:
                                   ^(NSData *data, NSURLResponse *response, NSError *error) {
                                       self.imageView.image = nil;
                                       self.imagedata = [UIImage imageWithData:data];
                                       [self performSelectorOnMainThread:@selector(updateMyImage) withObject:nil  waitUntilDone:NO];
                                   }];

     [task resume];

 }

 -(void)onShowNextImage:(id)sender
 {
     NSURL *URL = [NSURL URLWithString:@"http://img.gtimg.cn/images/hq_parts/hushen/stocks/300023.png"];
     NSURLRequest *request = [NSURLRequest requestWithURL:URL];
     NSURLSession *session = [NSURLSession sharedSession];
     NSURLSessionDataTask *task = [session dataTaskWithRequest:request
                                             completionHandler:
                                   ^(NSData *data, NSURLResponse *response, NSError *error) {
                                       self.imageView.image = nil;
                                       self.imagedata = [UIImage imageWithData:data];
                                       [self performSelectorOnMainThread:@selector(updateMyImage) withObject:nil  waitUntilDone:NO];
                                   }];

     [task resume];
 }

 - (void)updateMyImage
 {
     if (!self.imageView.image)
         self.imageView.image = self.imagedata;
     return;
 }

 @end

4. 运行