【iOS】ARC

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Transitioning to ARC Release Notes

Transitioning to ARC Release Notes

為啥會寫這篇，原因在於，寫其他文章，莫名其妙整理這些東西，就順便註記了一下大綱

  • ARC is supported in Xcode 4.2 for OS X v10.6 and v10.7 (64-bit applications) and for iOS 4 and iOS 5. Weak references are not supported in OS X v10.6 and iOS 4.

  • Under ARC, strong is the default for object types.

ARC Enforces New Rules

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- 不能明確的調用 `dealloc`，與直接使用 `retain`, `retainCount` 或者 `autorelease`。
	- 但一樣可以使用 `dealloc` 來管理資源，在 ARC 中客製化使用 `dealloc`，不需要使用 `[super delloc]`
	- 可以使用 `CFRetain`, `CFRelease` 與其他 **Core Foundation-style objects** 相關的功能 (參考 Managing Toll-Free Bridging)
- 不能使用 `NSAllocateObject`, `NSDeallocateObject`
	- 可使用 `alloc`，runtime 會自動幫你處理需要 deallocating 的物件
- You cannot use object pointers in C structures.
	- Rather than using a struct, you can create an Objective-C class to manage the data instead.
- 必須使用特殊的類別轉換來告訴編輯器物件的生命週期，在 Objective-C 物件與 Core Foundatin 類型中傳遞參數。
- 不能使用 `NSAutoreleasePool` 物件
- You cannot use memory zones.
	- 不能使用 NSZone，現在在 Objective-C 運行下，會自動被忽略

ARC 強化在命名方法上的限制

不能給予存取器開頭為 new 的物件名稱，若要給予一個開頭為 new 的物件名稱，則必須用 getter 指定不同的

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// Won't work:
@property NSString *newTitle; //Xcode 會給予錯誤
 
// Works:
@property (getter=theNewTitle) NSString *newTitle;

ARC Introduces New Lifetime Qualifiers

Property Attributes

  • weak / strong : 有別於 MRC，是 ARC 下的新的宣告屬性
    • Under ARC, strong is the default for object types.
    • strong = retain
    • weak 相似於 assign，不同之處在於當 instance 被釋放後，property value 會被設定為 nil (assign remaining as a dangling pointer.)

Variable Qualifiers

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__strong : 預設
__weak : 當未被使用後，會被設定為 nil
__unsafe_unretained : 當沒被使用，且沒被設為 nil，若引用對象被釋放後，the pointer is left dangling.
__autoreleasing : 用來代表當參數通過 id * 傳遞後，在 return 會被 autoreleased

使用方法 ClassName * qualifier variableName;

使用 __weak 測試

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NSString *ref = @"test";
NSString * __weak string = ref;
NSLog(@"string: %@", string);  // test
    
NSString * __weak string2 = [[NSString alloc] initWithFormat:@"First Name: 123"]; //no other strong reference
NSLog(@"string: %@", string2); // null

Use Lifetime Qualifiers to Avoid Strong Reference Cycles

參考官方範例 主要在講 manual reference counting 與 使用 __block qualifier 的調用

  • In manual reference counting mode, __block id x; has the effect of not retaining x.
  • In ARC mode, __block id x; defaults to retaining x (just like all other values).
  • To get the manual reference counting mode behavior under ARC, you could use __unsafe_unretained __block id x; 但建議用 __weak 取代(if you don’t need to support iOS 4 or OS X v10.6)
  • 範例一：沒使用 __block
  • 範例二：使用 __block
  • 範例三：使用 __weak 替代 __block
  • 範例四：特殊使用 __weak 方法

ARC Uses a New Statement to Manage Autorelease Pools

在 ARC 下，不能使用 NSAutoreleasePool ，但可以用 @autoreleasepool blocks 替代

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@autoreleasepool {
     // Code, such as a loop that creates a large number of temporary objects.
}

Stack Variables Are Initialized with nil

ARC 下，strong, weak, autoreleasing 的堆疊變數初始化都為 nil

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NSString *name;
NSLog(@"name: %@", name);// nil

Use Compiler Flags to Enable and Disable ARC

ARC 下，對某個檔案

  • 使用 -fobjc-arc 的旗標，開啟 ARC
  • 使用 -fno-objc-arc，來取消 ARC

ARC is supported in Xcode 4.2 and later OS X v10.6 and later (64-bit applications) and for iOS 4 and later. Weak references are not supported in OS X v10.6 and iOS 4. There is no ARC support in Xcode 4.1 and earlier.

Managing Toll-Free Bridging

(管理 無損橋接)

在多數的 Cocoa application 中，需要使用 Core Foundation-style 的物件，無論是直接 Core Foundation framework (如 CFArrayRef or CFMutableDictionaryRef) 或者 採用 Core Foundation 的 frameworks 如 Core Graphics (CGColorSpaceRed 與 CGGradientRef)

因編輯器不會自動化的管理 Core Foundation 物件的生命週期，所以必須呼叫使用 CFRetain 與 CFRelease 來處理使用 Core Foundation 記憶體的規則 see Memory Management Programming Guide for Core Foundation

如果轉換 Objective-C 與 Core Foundation，需要告知編輯器相關的定義 (defined in objc/runtime.h) 或者 Core Foundation-style 巨集 (defined in NSObject.h):

  • __bridge transfers a pointer between Objective-C and Core Foundation with no transfer of ownership.
  • __bridge_retained or CFBridgingRetain casts an Objective-C pointer to a Core Foundation pointer and also transfers ownership to you. You are responsible for calling CFRelease or a related function to relinquish ownership of the object.
  • __bridge_transfer or CFBridgingRelease moves a non-Objective-C pointer to Objective-C and also transfers ownership to ARC. ARC is responsible for relinquishing ownership of the object.

EX:

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- (void)logFirstNameOfPerson:(ABRecordRef)person {
 
    NSString *name = (NSString *)ABRecordCopyValue(person, kABPersonFirstNameProperty);
    NSLog(@"Person's first name: %@", name);
    [name release];
}

轉換為

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- (void)logFirstNameOfPerson:(ABRecordRef)person {
 
    NSString *name = (NSString *)CFBridgingRelease(ABRecordCopyValue(person, kABPersonFirstNameProperty));
    NSLog(@"Person's first name: %@", name);
}

The Compiler Handles CF Objects Returned From Cocoa Methods

轉換成適當的類別，不能直接使用 UIColor 放入 NSMutableArray 中，要轉換成 CGColor。

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NSMutableArray *colors = [NSMutableArray arrayWithObject:(id)[[UIColor darkGrayColor] CGColor]];
[colors addObject:(id)[[UIColor lightGrayColor] CGColor]];

Cast Function Parameters Using Ownership Keywords

接下來的範例中，陣列 array 透過 CGGradientCreateWithColors 進行轉換

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NSArray *colors = <#An array of colors#>;
CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)colors, locations);

完整的範例如下

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- (void)drawRect:(CGRect)rect {
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    CGFloat locations[2] = {0.0, 1.0};
    NSMutableArray *colors = [NSMutableArray arrayWithObject:(id)[[UIColor darkGrayColor] CGColor]];
    [colors addObject:(id)[[UIColor lightGrayColor] CGColor]];
    CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)colors, locations);
    CGColorSpaceRelease(colorSpace);  // Release owned Core Foundation object.
    CGPoint startPoint = CGPointMake(0.0, 0.0);
    CGPoint endPoint = CGPointMake(CGRectGetMaxX(self.bounds), CGRectGetMaxY(self.bounds));
    CGContextDrawLinearGradient(ctx, gradient, startPoint, endPoint,
                                kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
    CGGradientRelease(gradient);  // Release owned Core Foundation object.
}

Common Issues While Converting a Project

Frequently Asked Questions