viewDidUnload 中的内存管理 - 我应该将里面的数组和对象都置空吗？

Question

我有一个表格视图来加载来自互联网的新闻。我并试图将 viewDidUnload 中的所有属性归零。

- (void)viewDidUnload
{
    self.newsArray = nil;
    self.newsTableView = nil;
    self.indicatorView = nil;
//  self.iconDownLoader = nil;
    self.downloadArray = nil;

    [super viewDidUnload];
}

每次应用程序在 viewDidUnload 中崩溃时。如果我评论 self.iconDownLoader = nil; ，那就没问题了。那么谁能告诉我为什么会发生这种情况？感谢您。

---------------------NewsViewController.m------------------------ ---

//
//  NewsViewController.m
//
//  Created by on 18/01/12.
//  Copyright (c) 2012 __MyCompanyName__. All rights reserved.
//

#import "NewsViewController.h"
#import "ASIHTTPRequest.h"
#import "SBJson.h"
#import "NewsModel.h"
#import "NewsDetailViewController.h"

#define kCustomRowCount 6
#define IconPlaceHolder @"Spinner"

@implementation NewsViewController

@synthesize appDelegate, newsTableViewCell, newsTableView, indicatorView;
@synthesize iconDownLoader, newsArray, downloadArray;

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

        // setup appDelegate
        self.appDelegate = (SydneyAppDelegate *)[[UIApplication sharedApplication] delegate];

        // initial arrays
        self.newsArray = [[NSMutableArray alloc] init];
        self.downloadArray = [[NSMutableArray alloc] init];
    }
    return self;
}

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

#pragma mark - View lifecycle

- (void)viewDidLoad
{
    [super viewDidLoad];

    if(self.appDelegate.reachable) {
        [self getNews];
    }
    else
    {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"No Connection" message:@"No Internet connection. Please try again later." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
        [alert show];   
    }
}

- (void)viewDidUnload
{
    self.newsArray = nil;
    self.newsTableView = nil;
    self.indicatorView = nil;
//  self.iconDownLoader = nil;
    self.downloadArray = nil;

    [super viewDidUnload];
}

#pragma mark - ASIHTTPRequest

- (void) getNews
{
    NSURL *url = [NSURL URLWithString:@"http://ferrarimaseratisydney.com/api/getPublicNews.html"];
    ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
    [request setDelegate:self];
    [request startAsynchronous];
}

- (void) requestFinished:(ASIHTTPRequest *)request
{
    NSString *responseString = [request responseString];
    NSArray *json = [responseString JSONValue];

    for (id aNewsInJson in json)
    {
        NewsModel *aNews = [[NewsModel alloc] initWithJson:aNewsInJson];
        [self.newsArray addObject:aNews];
    }

    [self.indicatorView removeFromSuperview];

    [self.newsTableView reloadData];
}

- (void) requestFailed:(ASIHTTPRequest *)request
{
    NSError *error;
    error = [request error];
}

#pragma mark - Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{   
    [tableView deselectRowAtIndexPath:indexPath animated:YES];

    // Navigation logic may go here. Create and push another view controller.
    NewsDetailViewController *newsDetailViewController = [[NewsDetailViewController alloc] init];

    // transform news array
    newsDetailViewController.news = [self.newsArray objectAtIndex:indexPath.row];

    // Pass the selected object to the new view controller.
    [self.navigationController pushViewController:newsDetailViewController animated:YES];
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [self.newsArray count];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"NewsCellIdentifier";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        [[NSBundle mainBundle] loadNibNamed:@"NewsTableViewCell" owner:self options:nil];
        cell = self.newsTableViewCell;
        self.newsTableViewCell = nil;
    }

    // read from newsModel
    NewsModel *news = [self.newsArray objectAtIndex:indexPath.row];

    UILabel *label;
    label = (UILabel *)[cell viewWithTag:10];
    label.text = [NSString stringWithString:news.title];
    label = nil;
    label = (UILabel *)[cell viewWithTag:11];
    label.text = [NSString stringWithString:news.description];
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:12];
    imageView.image = news.image;

    if (news.image == nil)
    {
        imageView.image = [UIImage imageNamed:IconPlaceHolder];

        self.iconDownLoader = [[IconDownLoader alloc] init];
        self.iconDownLoader.url = news.imageUrl;
        self.iconDownLoader.delegate = self;
        self.iconDownLoader.indexPath = indexPath;
        if (self.appDelegate.ip4 == YES)
        {
            self.iconDownLoader.width = 300;
            self.iconDownLoader.height = 150;
        }
        else
        {
            self.iconDownLoader.width = 150;
            self.iconDownLoader.height = 75;
        }
        [self.downloadArray addObject:self.iconDownLoader];
        [self.iconDownLoader start];
    }
    return cell;
}

#pragma mark - IconDownLoaderDelegate

- (void)iconDownLoadFinsh:(NSData *)imageData row:(NSIndexPath *)indexPath {

    UITableViewCell *cell = [self.newsTableView cellForRowAtIndexPath:indexPath];
    UIImageView *imageView = (UIImageView *)[cell viewWithTag:12];
    if (imageData != 0)
    {
        imageView.image = [UIImage imageWithData:imageData];
    }
    else
    {
        imageView.image = [UIImage imageNamed:@"icon57"];
    }
    NewsModel *newsModel = [self.newsArray objectAtIndex:indexPath.row];
    newsModel.image = [UIImage imageWithData:imageData];
}

@end

------------------------IconDownLoader.m------------------------- -----

//
//  IconDownLoader.m
//
//  Created by on 24/11/11.
//  Copyright (c) 2011 __MyCompanyName__. All rights reserved.
//

#import "IconDownLoader.h"
#import "ASIHTTPRequest.h"

@implementation IconDownLoader

@synthesize delegate = _delegate;
@synthesize url = _url;
@synthesize indexPath = _indexPath;
@synthesize width = _width;
@synthesize height = _height;
@synthesize request = _request;

- (void)start {

    NSString *originalString = @"width=%s&height=%s";
    NSString *newString = [NSString stringWithFormat:@"width=%d&height=%d&type=jpg", self.width, self.height];

    NSString *resizedURL = [self.url stringByReplacingOccurrencesOfString:originalString withString:newString];

    NSURL *url = [NSURL URLWithString:[resizedURL stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
    _request = [ASIHTTPRequest requestWithURL:url];
    if (_indexPath) {
        _request.userInfo = [NSDictionary dictionaryWithObject:_indexPath forKey:@"indexPath"];
    }
    [_request setDelegate:self];
    [_request startAsynchronous];
}

- (void)requestFinished:(ASIHTTPRequest *)request {

    NSInteger statusCode = request.responseStatusCode;
    switch (statusCode) {
        case 401: // Not Authorized: either you need to provide authentication credentials, or the credentials provided aren't valid.
            break;

        case 200: {
            NSData *responseData = [request responseData];
            if (!responseData) {
                UIAlertView *alertView;
                alertView = [[UIAlertView alloc] initWithTitle:@"Oops" message:[NSString stringWithFormat:@"Download failed in row %d", _indexPath.row] delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
                return;
            }
            [_delegate iconDownLoadFinsh:responseData row:[request.userInfo objectForKey:@"indexPath"]];
        }
            break;

        default:{

        }
    }
}

- (void)dealloc {
    if (_request != nil) {
        [_request clearDelegatesAndCancel];
    }
}

@end

Answer 1

一般来说，在 viewDidUnload 中，您应该只释放对您拥有的 nib 对象的所有引用并将其清零。

也就是说，如果 model 对象消耗大量内存，您也可以在 viewDidUnload 中销毁它们。您应该记住，viewDidUnload 是 viewDidLoad 的对应项，因此一个好的经验法则是仅销毁您在 viewDidUnload 中创建的那些对象viewDidLoad。您还应该记住，当视图控制器被释放时，viewDidUnload不会被调用——只有当它的视图被释放时。

在您的情况下，我不会仅仅因为您在 init... 中创建 newsArray 和 downloadArray 就释放它们。我只需向他们发送 removeAllObjects 即可。

至于崩溃，每次单元格需要图像时，您都会创建一个新的共享图标下载器，这有点尴尬。如果您确实需要共享下载器实例，则不应为每个单元重新创建它。看起来您希望将创建的所有下载器保留在 ivar 数组中，因为每个下载器都是一次性的，并且负责加载单个图像。

Answer 2

这里没有足够的信息可以告诉你，但可能你已经在代码的其他部分直接释放了 iconDownloader ，当时如果不将引用设置为 nil ，我们就看不到。

然后在 viewDidUnload 中您尝试释放无效引用。

对于@synthesize，使用：

@synthesize iconDownLoader = _iconDownloader;

然后修复所有编译器警告以使用 self.icondownloder 而不是 iconDownloader，并消除所有“release”的使用（假设您的属性被标记为保留，它应该是）。

事实上，也许您的整个问题是该属性不是保留属性，因此您创建了 iconDOwnloader 并且它会立即释放。