涉及到的知識(shí)點(diǎn)有：消息轉(zhuǎn)發(fā)TODO，Method-SwizzlingTODO，動(dòng)態(tài)方法TODO，KVO(暫無(wú))，AOP等。
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概述

Aspects是一個(gè)輕量級(jí)的、支持Objective-C&Swift語(yǔ)言的AOP實(shí)現(xiàn)。可以把Aspects當(dāng)做Method-SwizzlingTODO的一種應(yīng)用。它允許你在每個(gè)類或每個(gè)實(shí)例已有方法中添加代碼，可以設(shè)置切入點(diǎn)為after /instead/before等。比起常規(guī)的Method-SwizzlingTODO，Aspects會(huì)自動(dòng)調(diào)用super方法，并且使用起來(lái)更加簡(jiǎn)單、方便。
Aspects利用OC的消息轉(zhuǎn)發(fā)機(jī)制，hook消息。這樣會(huì)有一些性能開銷，因此不要把Aspects加到經(jīng)常被使用的方法里面。Aspects是用來(lái)設(shè)計(jì)給View/Controller代碼使用的，而不是用來(lái)hook每秒調(diào)用1000次的方法的。因此Aspects不應(yīng)該被用在for循環(huán)這些方法里面，會(huì)造成很大的性能損耗。
Aspects是不支持hook 靜態(tài)static方法的。

這里還有一些簡(jiǎn)介需要翻譯，后續(xù)補(bǔ)上。TODO

使用方法

Aspects是NSObject的一個(gè)extension，只要是NSObject，都可以使用這兩個(gè)方法。一個(gè)是用來(lái)hook類方法的，一個(gè)是hook實(shí)例方法的。

/// Adds a block of code before/instead/after the current `selector` for a specific class.
///
/// @param block Aspects replicates the type signature of the method being hooked.
/// The first parameter will be `id<AspectInfo>`, followed by all parameters of the method.
/// These parameters are optional and will be filled to match the block signature.
/// You can even use an empty block, or one that simple gets `id<AspectInfo>`.
///
/// @note Hooking static methods is not supported.
/// @return A token which allows to later deregister the aspect.
+ (id<AspectToken>)aspect_hookSelector:(SEL)selector
                           withOptions:(AspectOptions)options
                            usingBlock:(id)block
                                 error:(NSError **)error;

/// Adds a block of code before/instead/after the current `selector` for a specific instance.
- (id<AspectToken>)aspect_hookSelector:(SEL)selector
                           withOptions:(AspectOptions)options
                            usingBlock:(id)block
                                 error:(NSError **)error;

添加Aspects后，會(huì)返回一個(gè)AspectToken，用來(lái)注銷hook方法。所有的調(diào)用都是線程安全的。

/// Deregister an aspect.
/// @return YES if deregistration is successful, otherwise NO.
id<AspectToken> aspect = ...;
[aspect remove];

這里的AspectToken是隱式的，允許我們調(diào)用remove去撤銷一個(gè)hook。remove方法返回YES代表撤銷成功，返回NO就撤銷失敗。

Hook方法的參數(shù)

block第一個(gè)參數(shù)遵循AspectInfo協(xié)議。我們甚至可以使用一個(gè)空的block。AspectInfo協(xié)議里面的參數(shù)是可選的，主要是用來(lái)匹配block簽名的。

AspectOptions類型：

typedef NS_OPTIONS(NSUInteger, AspectOptions) {
    AspectPositionAfter   = 0,            /// Called after the original implementation (default)；默認(rèn)值，在原方法執(zhí)行完之后調(diào)用block
    AspectPositionInstead = 1,            /// Will replace the original implementation.替換原方法
    AspectPositionBefore  = 2,            /// Called before the original implementation.在原方法之前調(diào)用block
    
    AspectOptionAutomaticRemoval = 1 << 3 /// Will remove the hook after the first execution.在hook執(zhí)行完后自動(dòng)移除。
};

AspectInfo對(duì)象：

/// The AspectInfo protocol is the first parameter of our block syntax.
@protocol AspectInfo <NSObject>

/// The instance that is currently hooked. 
/// 返回當(dāng)前被hook的實(shí)例。
- (id)instance;

/// The original invocation of the hooked method.
/// 返回被hooked方法的原始的Invocation。
- (NSInvocation *)originalInvocation;

/// All method arguments, boxed. This is lazily evaluated.
/// 返回selector方法的所有參數(shù)。是懶加載實(shí)現(xiàn)的。
- (NSArray *)arguments;

@end

錯(cuò)誤碼類型：

typedef NS_ENUM(NSUInteger, AspectErrorCode) {
    AspectErrorSelectorBlacklisted,                   /// Selectors like release, retain, autorelease are blacklisted.
    AspectErrorDoesNotRespondToSelector,              /// Selector could not be found.
    AspectErrorSelectorDeallocPosition,               /// When hooking dealloc, only AspectPositionBefore is allowed.
    AspectErrorSelectorAlreadyHookedInClassHierarchy, /// Statically hooking the same method in subclasses is not allowed.
    AspectErrorFailedToAllocateClassPair,             /// The runtime failed creating a class pair.
    AspectErrorMissingBlockSignature,                 /// The block misses compile time signature info and can't be called.
    AspectErrorIncompatibleBlockSignature,            /// The block signature does not match the method or is too large.

    AspectErrorRemoveObjectAlreadyDeallocated = 100   /// (for removing) The object hooked is already deallocated.
};

extern NSString *const AspectErrorDomain;

.m分析TODO