何时使用目标C中的属性？ [英] When to use properties in objective C?
问题描述
我在这里有一个问题：令人困惑的双重免费iPhone应用程序中的错误消息/内存泄漏我认为需要一个新问题来回答它。
I have a question here: Confusing double free error message/memory leak in iPhone app which I think needs a new question to answer it.
我感兴趣的代码就是那个问题但是我将在此处重新发布
The code I am interested in is in that question but I will re post it here
#import <UIKit/UIKit.h>
#import "MyManager.h"
@interface ListOfCarShares : UITableViewController <NSXMLParserDelegate>
{
NSURLConnection *connection;
NSMutableData *carsharexml;
NSMutableArray *ldestination;
NSMutableArray *ldeparts_from;
NSMutableArray *lcs_id;
NSMutableArray *ltime;
NSMutableString *currentElement;
NSMutableString *tdest;
NSMutableString *tfrom;
NSMutableString *ttime;
NSMutableString *tid;
}
-(void)fetchcarshares;
@property (nonatomic, assign) IBOutlet UITableViewCell *maincell;
@end
#import "ListOfCarShares.h"
@implementation ListOfCarShares
@synthesize maincell;
- (id)initWithStyle:(UITableViewStyle)style
{
self = [super initWithStyle:style];
if (self) {
// Custom initialization
}
return self;
}
- (void)parser:(NSXMLParser *)parser
didStartElement:(NSString *)elementName
namespaceURI:(NSString *)namespaceURI
qualifiedName:(NSString *)qualifiedName
attributes:(NSDictionary *)attributeDict
{
currentElement = [[elementName copy] autorelease];
if ([elementName isEqualToString:@"destination"])
{
//NSLog(@"found current conditions tag it reads %@",currentElement);
tdest = [[NSMutableString alloc] init];
}
if ([elementName isEqualToString:@"departs_from"])
{
tfrom = [[NSMutableString alloc] init];
}
if ([elementName isEqualToString:@"time"])
{
ttime = [[NSMutableString alloc] init];
}
if ([elementName isEqualToString:@"cs_id"])
{
tid = [[NSMutableString alloc] init];
}
}
- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string
{
if ([currentElement isEqualToString:@"destination"])
{
[tdest appendString:string];
}
if ([currentElement isEqualToString:@"departs_from"])
{
[tfrom appendString:string];
}
if ([currentElement isEqualToString:@"time"])
{
[ttime appendString:string];
}
if ([currentElement isEqualToString:@"cs_id"])
{
[tid appendString:string];
}
}
- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName
namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName
{
if ([currentElement isEqualToString:@"destination"])
{
[ldestination addObject:tdest];
[tdest release];
}
if ([currentElement isEqualToString:@"departs_from"])
{
[ldeparts_from addObject:tfrom];
[tfrom release];
}
if ([currentElement isEqualToString:@"time"])
{
[ltime addObject:ttime];
[ttime release];
}
if ([currentElement isEqualToString:@"cs_id"])
{
[lcs_id addObject:tid];
[tid release];
}
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
#pragma mark - View lifecycle
- (void)viewDidLoad
{
[super viewDidLoad];
// Uncomment the following line to preserve selection between presentations.
// self.clearsSelectionOnViewWillAppear = NO;
// Uncomment the following line to display an Edit button in the navigation bar for this view controller.
// self.navigationItem.rightBarButtonItem = self.editButtonItem;
}
- (void)viewDidUnload
{
[super viewDidUnload];
}
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
ldestination = [[NSMutableArray alloc] init];
ldeparts_from = [[NSMutableArray alloc] init];
ltime = [[NSMutableArray alloc] init];
lcs_id = [[NSMutableArray alloc] init];
carsharexml = [[NSMutableData alloc] init];
[self fetchcarshares];
}
- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
}
- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:animated];
[connection release];
[ldestination release];
[ldeparts_from release];
[ltime release];
[lcs_id release]; ///
[carsharexml release];
}
- (void)viewDidDisappear:(BOOL)animated
{
[super viewDidDisappear:animated];
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
// Return YES for supported orientations
return (interfaceOrientation == UIInterfaceOrientationPortrait);
}
#pragma mark - Table view data source
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
// Return the number of sections.
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
// Return the number of rows in the section.
return [ltime count];
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
//static NSString *CellIdentifier = @"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:nil];
if (cell == nil)
{
[[NSBundle mainBundle] loadNibNamed:@"carsharecell" owner:self options:nil];
}
// Configure the cell...
cell=maincell;
UILabel *from;
UILabel *dest;
UILabel *time;
from = (UILabel *)[cell viewWithTag:4];
dest = (UILabel *)[cell viewWithTag:5];
time = (UILabel *)[cell viewWithTag:6];
from.text=[ldeparts_from objectAtIndex:indexPath.row];
dest.text=[ldestination objectAtIndex:indexPath.row];
time.text=[ltime objectAtIndex:indexPath.row];
return cell;
}
/*
// Override to support conditional editing of the table view.
- (BOOL)tableView:(UITableView *)tableView canEditRowAtIndexPath:(NSIndexPath *)indexPath
{
// Return NO if you do not want the specified item to be editable.
return YES;
}
*/
/*
// Override to support editing the table view.
- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
if (editingStyle == UITableViewCellEditingStyleDelete) {
// Delete the row from the data source
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
}
else if (editingStyle == UITableViewCellEditingStyleInsert) {
// Create a new instance of the appropriate class, insert it into the array, and add a new row to the table view
}
}
*/
/*
// Override to support rearranging the table view.
- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath
{
}
*/
/*
// Override to support conditional rearranging of the table view.
- (BOOL)tableView:(UITableView *)tableView canMoveRowAtIndexPath:(NSIndexPath *)indexPath
{
// Return NO if you do not want the item to be re-orderable.
return YES;
}
*/
-(void)fetchcarshares
{
MyManager *sharedManager = [MyManager sharedManager];
NSString *urlString = [NSString stringWithFormat:@"http://url/get.php?username=%@&password=%@",sharedManager.user,sharedManager.passw];
NSURL *url = [NSURL URLWithString:urlString];
NSURLRequest *req = [NSURLRequest requestWithURL:url];
connection = [[NSURLConnection alloc] initWithRequest:req delegate:self startImmediately:YES];
}
-(void) connection:(NSURLConnection *)conn didReceiveData:(NSData *)data
{
[carsharexml appendData:data];
}
-(void) connectionDidFinishLoading:(NSURLConnection *)conn
{
NSString *xmlcheck = [[NSString alloc] initWithData:carsharexml encoding:NSUTF8StringEncoding];
NSLog(@"%@",xmlcheck);
[xmlcheck release];
NSXMLParser *parser = [[NSXMLParser alloc] initWithData: carsharexml];
[parser setDelegate:self];
[parser parse];
[parser release];
[[self tableView] reloadData];
}
-(CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
return 102;
}
#pragma mark - Table view delegate
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
}
-(void)dealloc
{
[super dealloc];
}
@end
我只定义了一个属性在.h文件中。回答这个问题的人似乎认为我有双重免费错误的原因是因为我的变量没有 @property 。
I only have one property defined in the .h file. The people who answered that question seem to think that the reason I am having double free error is due to the fact I don't have @property for my variables.
我有很多代码几乎与此相同，我没有问题。
I have lots of code practically identical to this and I don't have a problem.
我的问题是
- 我什么时候应该使用房产？
- 我应该在这里使用房产吗？为什么？
谢谢
推荐答案
从技术上讲，您只需要为可以从其他类访问的值使用属性，但许多人发现所有指针的使用（保留）属性更容易 - 键入实例变量，以便保留更自动。 （然后使用 self.propertyName = xxx; 表示设置， self.propertyName = nil; 用于释放 dealloc 。）
You technically only need to use properties for values that are intended to be accessible from other classes, but many find it easier to use (retained) properties for all pointer-type instance variables so that the retaining is a bit more automatic. (And then use self.propertyName = xxx; notation for setting and self.propertyName = nil; for releasing in dealloc.)
是的，你可以手动进行保留和释放，但这样做很乏味所以，当你进行快速编辑时，你往往会搞砸。但是，您必须注意的一件事是将保留（不是简单的自动恢复）值（例如 alloc / init 值）分配给 self.xxx 属性。如果你不以某种方式缓解它，这将导致双重保留。
Yes, you can do the retains and releases "manually", but it's a hair tedious to do so, and you tend to muck things up when you make "quick edits". The one thing you have to watch out for, though, is assigning a retained (not simply autoretained) value (such as your alloc/init values) to a self.xxx property. This will result in double retain, if you don't mitigate it somehow.
另一件事，如果你不使用属性，总是 nil release 之后的指针值。这可以防止您意外使用释放的值，并且它可以防止您执行双重发布。
Another thing to do, if you don't use properties, is to always nil a pointer value after you release it. This prevents you from accidentally using the released value and and it prevents you from doing a double release.
（注意使用像我上面描述的懒惰技术并不是糟糕的编程，而不是完美地搞清楚所有内容。大约98％的编程都在调试，你可以做任何事情来防止错误或让它们更容易找到是善良。）
(Note that it's in no way "bad programming" to use "lazy" techniques like I described above, vs "perfectly" figuring out everything. About 98% of programming is debugging, and anything you can do to prevent bugs or make them easier to find is goodness.)
（我还要注意，上述代码中的问题似乎主要是你没有 nil 发布它们后的 tdest 等指针。而你的 if 测试应该检查一下指针在使用之前已被占用。）
(I'll also note that your problem in the above code appears to be mainly that you do not nil thetdest et al pointers after releasing them. And your if tests should likely check to see if the pointer has been nilled before using it.)
已添加：请注意，上述内容适用于ARC之前的程序。使用ARC，规则会发生重大变化。
Added: Note that the above applies to pre-ARC programs. With ARC the "rules" change substantially.
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