何時使用self.在網上搜索或者論壇裏的回覆大多都是簡簡單單的說這與objc的存取方法有關,如何與存取方式有關究竟他們之間的是什麼樣的關係就很少有同學回答了。下面以代碼來說明問題:
創建一個Student類,繼承NSObject類,代碼:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <Foundation/Foundation.h>
2:
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>@<span class="kwrd" style="margin: 0px; padding: 0px;">interface</span> Student : NSObject{
4:
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span> NSString *idd;
6: NSString *name;
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>}
8: @property (nonatomic, retain) NSString *idd;
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span>@property (nonatomic, retain) NSString *name;
10:
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span>@end
.m文件 代碼:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <span class="str" style="margin: 0px; padding: 0px;">"Student.h"</span>
2:
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>@implementation Student
4: @synthesize idd,name;
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span>
6: - (void)dealloc
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>{
8: [idd release];
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span> [name release];
10: [super dealloc];
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span>}
12:
<span class="lnum" style="margin: 0px; padding: 0px;"> 13: </span>
14: @end
使用@propety @synthesize實現Student的成員屬性的set get方法。通常我們在其他類裏訪問Student的成員屬性的做法:
獲取student的名字通過student.name,給名字賦值[student setName:@“jordy”]; 其中student是Student類對象,如果在Student類內部訪問其成員屬性使用[self setName:@”jordy”], 訪問使用self.name;
注意:上述的代碼,由於wordpress的原因,代碼中的字符會自動保存爲中文格式。你在使用時記得改爲英文格式。
在Student.h和Student.m文件,是我們習慣性的寫法,但似乎還是不能解釋什麼加self和不加self的區別,請看下面代碼,是另一種習慣性的寫法,還以Student類爲例:
.h文件 代碼:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <Foundation/Foundation.h>
2:
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>@<span class="kwrd" style="margin: 0px; padding: 0px;">interface</span> Student : NSObject{
4:
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span> NSString *_idd;
6: NSString *_name;
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>}
8: @property (nonatomic, retain) NSString *idd;
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span>@property (nonatomic, retain) NSString *name;
10:
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span>@end
.m文件 代碼:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <span class="str" style="margin: 0px; padding: 0px;">"Student.h"</span>
2:
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>@implementation Student
4: @synthesize idd = _idd;
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span>@synthesize name = _name;
6:
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>- (<span class="kwrd" style="margin: 0px; padding: 0px;">void</span>)dealloc
8: {
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span> [_idd release];
10: _idd = nil;
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span> [_name release];
12: _name = nil;
<span class="lnum" style="margin: 0px; padding: 0px;"> 13: </span> [super dealloc];
14: }
<span class="lnum" style="margin: 0px; padding: 0px;"> 15: </span>
16:
<span class="lnum" style="margin: 0px; padding: 0px;"> 17: </span>@end
可以注意到上述代碼,與之前的代碼,在.h文件name變量改寫爲了_name;在.m文件中@sythesize的寫法也發生了變化。
如果通過方法self._name獲取屬性的值,xcode編譯器會提示錯誤,其實這也就說明了,我們通常使用self.name實際使用的是student類name的get方法,同理name的set方法亦是如此。
接下來從內存管理來說明使用self.和不使用self的區別:
ViewController.h文件,使用Student類,代碼如下:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <UIKit/UIKit.h>
2: @class Student;
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>
4: @interface ViewController : UIViewController{
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span>
6: Student *_student;
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>}
8:
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span>@property (nonatomic, retain) Student *student;
10:
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span>@end
ViewController.m文件,代碼:
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>#import <span class="str" style="margin: 0px; padding: 0px;">"ViewController.h"</span>
2: #import "Student.h"
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>
4: @implementation ViewController
<span class="lnum" style="margin: 0px; padding: 0px;"> 5: </span>@synthesize student = _student;
6:
<span class="lnum" style="margin: 0px; padding: 0px;"> 7: </span>- (<span class="kwrd" style="margin: 0px; padding: 0px;">void</span>)didReceiveMemoryWarning
8: {
<span class="lnum" style="margin: 0px; padding: 0px;"> 9: </span> [super didReceiveMemoryWarning];
10: }
<span class="lnum" style="margin: 0px; padding: 0px;"> 11: </span>
12: #pragma mark - View lifecycle
<span class="lnum" style="margin: 0px; padding: 0px;"> 13: </span>
14: - (void)viewDidLoad
<span class="lnum" style="margin: 0px; padding: 0px;"> 15: </span>{
16: [super viewDidLoad];
<span class="lnum" style="margin: 0px; padding: 0px;"> 17: </span>}
18:
<span class="lnum" style="margin: 0px; padding: 0px;"> 19: </span>- (<span class="kwrd" style="margin: 0px; padding: 0px;">void</span>) dealloc
20: {
<span class="lnum" style="margin: 0px; padding: 0px;"> 21: </span> [_student release];
22: _student = nil;
<span class="lnum" style="margin: 0px; padding: 0px;"> 23: </span> [super dealloc];
24: }
其它的方法沒有使用到,所以這裏就不在顯示了。
在ViewController.m的viewDidLoad方法中創建一個Student類的對象
<span class="lnum" style="margin: 0px; padding: 0px;"> 1: </span>Student *mystudent = [[Student alloc] init];
2: self.student = mystudent;
<span class="lnum" style="margin: 0px; padding: 0px;"> 3: </span>[mystudent release];
這是相信有人會有疑問了,問什麼創建student對象要這麼複雜,似乎直接使用self.student = [[Student alloc] init]; 也沒有問題,不加self有時也是挺正常的呀?
接下來就需要從內存角度來分析它們之間的區別了:
1、加self的方式:
Student *mystudent = [[Student alloc] init]; //mystudent 對象 retainCount = 1;
self.student = mystudent; //student 對象 retainCount = 2;
[mystudent release]; //student 對象 retainCount = 1;
retainCount指對象引用計數,student的property 是retain 默認使用self.student引用計數+1。
2、不加self的方式
Student *mystudent = [[Student alloc] init]; //mystudent 對象 retainCount = 1;
student = mystudent; //student 對象 retainCount = 1;
[mystudent release]; //student 對象內存已釋放,如果調用,會有異常
3、加self直接賦值方式
self.student = [[Student alloc] init]; //student 對象 retainCount = 2;容易造成內存泄露
由於objective-c內存管理是根據引用計數處理的,當一個對象的引用計數爲零時,gcc纔會釋放該內存。
@interface CustomClass : UIViewController { NSString *str } @property (retain, nonatomic) NSString *str; @implementation CustomClass @synthesize str; -(void)viewDidLoad { //方法一 用alloc必須手動釋放一次 self.str = [[NSString alloc]initWithString:@"my str"]; [str release]; //方法二 用類方法不用 self.str = [NSString stringWithString:@"my str"]; 以後調用時直接使用str,不必使用self.str [str appendString:@"\n"]; } //在dealloc中必須釋放 - (void)dealloc { //方法一 [str release]; str = nil; //方法二 self.str = nil; [super dealloc]; }