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 = 10;    [self.view addSubview:self.imageView];    [self.imageView setTranslatesAutoresizingMaskIntoConstraints: NO];        //创建前一张按钮    UIButton *prevButton = [[UIButton alloc] init];    prevButton.frame = CGRectMake(0, 20, 300, 20);    [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(0, 20, 300, 20);    [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:@20, @"VDist", @5, @"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:0 metrics:metrics views:views]];    [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-Padding-[prevButton]-(>=0)-[nextButton(==prevButton)]-Padding-|" options:0 metrics:metrics views:views]];        [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-Padding-[imageView]-Padding-|" options:0 metrics:metrics views:views]];    [contraits addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-VDist-[header]-Padding-[imageView]-(>=VDist)-|" options:0 metrics:metrics views:views]];    //垂直居中    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:prevButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:nextButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];        [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