1,ARC 是编译器做的事情（在编译期间插入内存管理相关代码）
ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time
2，即使是局部指针变量（属性变量、局部变量 are strong by default），所指对象引用计数也将＋1
ARC ensures that oldLastName is not deallocated before the NSLog statement.
- (void)takeLastNameFrom:(Person *)person {
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NSString *oldLastname = [self lastName]; |
[self setLastName:[person lastName]]; |
NSLog(@"Lastname changed from %@ to %@", oldLastname, [self lastName]); |
} |
3，ARC仅仅维护了内存管理，其它的资源还是要手动置空或释放
（记得scrollview滑动时候退出页面 某些系统(因老的sdk中delegate为assign？)会crash）
You do not have to (indeed you cannot) release instance variables, but you may need to invoke [systemClassInstance setDelegate:nil] on system classes and other code that isn’t compiled using ARC.
4，new是可以作为属性字段名的，需要重写get方法
// Won't work: |
@property NSString *newTitle; |
// Works: |
@property (getter=theNewTitle) NSString *newTitle; |
5，正确的书写格式 ClassName * qualifier variableName; 不过编译器也兼容了非标准形式
MyClass * __weak myWeakReference; |
MyClass * __unsafe_unretained myUnsafeReference; |
6，注意 __weak 的使用（__weak的设计是为了解引用）
NSString * __weak string = [[NSString alloc] initWithFormat:@"First Name: %@", [self firstName]]; |
NSLog(@"string: %@", string); |
string是个空值
7，避免强引用 Use Lifetime Qualifiers to Avoid Strong Reference Cycles
避免循环引用 Two better options are to either use __weak, or set the __block value to nil to break the retain cycle.
example 7-1
MyViewController *myController = [[MyViewController alloc] init…];
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// ...
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MyViewController * __weak weakMyViewController = myController;
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myController.completionHandler = ^(NSInteger result) {
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[weakMyViewController dismissViewControllerAnimated:YES completion:nil];
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};
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example 7-2
For non-trivial cycles, however, you should use:(对于重要的回路或循环 应该对weak加strong)
这里讨论一个问题：
（1）循环引用是编译时 插入相关代码 进行内存管理的
(2) 块内的 MyViewController *strongMyController = weakMyController; 仅在运行时起作用 （引起引用计数+1）
(3) 尽管仅是在block执行期间引起对象的引用计数+1,仍旧会引起延迟释放的问题（本该立刻释放的，延迟释放了，这也是不能容忍的）
(4) For non-trivial cycles 重要的回路 究竟是指啥？如果该block确定定在主线程执行 貌似没啥可后怕；当线程重重入、并发的时候 貌似有问题，
但是 线程重重入、并发 可通过加锁解决问题；MyViewController *strongMyController = weakMyController；这句话的目的 究竟很在？
留个疑问在此！！！
MyViewController *myController = [[MyViewController alloc] init…];
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// ...
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MyViewController * __weak weakMyController = myController;
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myController.completionHandler = ^(NSInteger result) {
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MyViewController *strongMyController = weakMyController;
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if (strongMyController) {
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// ...
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[strongMyController dismissViewControllerAnimated:YES completion:nil];
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// ...
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}
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else {
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// Probably nothing...
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}
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};
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8，outlets 修饰的属性 除了顶层的需要strong 子试图使用week修饰
The patterns for declaring outlets in iOS and OS X change with ARC and become consistent across both platforms. The pattern you should typicallyadopt is: outlets should be weak, except for those from File’s Owner to top-level objects in a nib file (or a storyboard scene) which should be strong.
9，临时变量初始化默认值为nil （不包括NSInteger、int等）Stack Variables Are Initialized with nil
Using ARC, strong, weak, and autoreleasing stack variables are now implicitly initialized with nil
好习惯：手动指定初始值 如 int i =0；另外 这int i =0 也是必须的；因为初始值并不是0 是不确定的
10，编译器并没有自动化管理引用计数 Core Foundation下对象
The compiler does not automatically manage the lifetimes of Core Foundation objects
__bridge
__bridge_retained or CFBridgingRetain ／CFRelease
__bridge_transfer CFBridgingRelease