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轻量级低风险 iOS 热更新方案

我们都知道苹果对 Hotfix 抓得比较严，强大好用的 JSPatch 也成为了过去式。但即使测试地再细致，也难保线上 App 不出问题，小问题还能忍忍，大问题就得重新走发布流程，然后等待审核通过，等待用户升级，周期长且麻烦。如果有一种方式相对比较安全，不需要 JSPatch 那么完善，但也足够应付一般场景，使用起来还比较轻量就好了，这也是本文要探讨的主题。

要达到这个目的，Native 层只要透出两种能力就基本可以了：

在任意方法前后注入代码的能力，可能的话最好还能替换掉。
调用任意类/实例方法的能力。

第 2 点不难，只要把 [NSObject performSelector:...] 那一套通过 JSContext 暴露出来即可。难的是第 1 点。其实细想一下，这不就是 AOP 么，而 iOS 有一个很方便的 AOP Library: Aspects，只要把它的几个方法通过 JSContext 暴露给 JS 不就可以了么？

选择 Aspects 的原因是它已经经过了验证，不光是功能上的，更重要的是可以通过 AppStore 的审核。

This is stable and used in hundreds of apps since it’s part of PSPDFKit, an iOS PDF framework that ships with apps like Dropbox or Evernote.

Aspects 使用姿势：

[UIViewController aspect_hookSelector:@selector(viewWillAppear:) withOptions:AspectPositionAfter usingBlock:^(id aspectInfo, BOOL animated) {
    NSLog(@"View Controller %@ will appear animated: %tu", aspectInfo.instance, animated);
} error:NULL];

前插、后插、替换某个方法都可以。使用类的方式很简单，NSClassFromString 即可，Selector 也一样 NSSelectorFromString，这样就能通过外部传入 String，内部动态构造 Class 和 Selector 来达到 Fix 的效果了。

这种方式的安全性在于：

不需要中间 JS 文件，准备工作全部在 Native 端完成。
没有使用 App Store 不友好的类/方法。

Demo

假设线上运行这这样一个 Class，由于疏忽，没有对参数做检查，导致特定情况下会 Crash。

@interface MightyCrash: NSObject
- (float)divideUsingDenominator:(NSInteger)denominator;
@end

@implementation MightyCrash
// 传一个 0 就 gg 了
- (float)divideUsingDenominator:(NSInteger)denominator
{
    return 1.f/denominator;
}
@end

现在我们要避免 Crash，就可以通过这种方式来修复

[Felix fixIt];

NSString *fixScriptString = @" /
fixInstanceMethodReplace('MightyCrash', 'divideUsingDenominator:', function(instance, originInvocation, originArguments){ /
    if (originArguments[0] == 0) { /
        console.log('zero goes here'); /
    } else { /
        runInvocation(originInvocation); /
    } /
}); /
/
";

[Felix evalString:fixScriptString];

运行一下看看

MightyCrash *mc = [[MightyCrash alloc] init];
float result = [mc divideUsingDenominator:3];
NSLog(@"result: %.3f", result);
result = [mc divideUsingDenominator:0];
NSLog(@"won't crash");

// output
// result: 0.333
// Javascript log: zero goes here
// won't crash

It Works, 是不是有那么点意思了。以下是可以正常运行的代码，仅供参考。

#import #import #import @interface Felix: NSObject
+ (void)fixIt;
+ (void)evalString:(NSString *)javascriptString;
@end


@implementation Felix
+ (Felix *)sharedInstance
{
    static Felix *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });

    return sharedInstance;
}

+ (void)evalString:(NSString *)javascriptString
{
    [[self context] evaluateScript:javascriptString];
}

+ (JSContext *)context
{
    static JSContext *_context;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _context = [[JSContext alloc] init];
        [_context setExceptionHandler:^(JSContext *context, JSValue *value) {
            NSLog(@"Oops: %@", value);
        }];
    });
    return _context;
}

+ (void)_fixWithMethod:(BOOL)isClassMethod aspectionOptions:(AspectOptions)option instanceName:(NSString *)instanceName selectorName:(NSString *)selectorName fixImpl:(JSValue *)fixImpl {
    Class klass = NSClassFromString(instanceName);
    if (isClassMethod) {
        klass = object_getClass(klass);
    }
    SEL sel = NSSelectorFromString(selectorName);
    [klass aspect_hookSelector:sel withOptions:option usingBlock:^(id aspectInfo){
        [fixImpl callWithArguments:@[aspectInfo.instance, aspectInfo.originalInvocation, aspectInfo.arguments]];
    } error:nil];
}

+ (id)_runClassWithClassName:(NSString *)className selector:(NSString *)selector obj1:(id)obj1 obj2:(id)obj2 {
    Class klass = NSClassFromString(className);
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    return [klass performSelector:NSSelectorFromString(selector) withObject:obj1 withObject:obj2];
#pragma clang diagnostic pop
}

+ (id)_runInstanceWithInstance:(id)instance selector:(NSString *)selector obj1:(id)obj1 obj2:(id)obj2 {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
    return [instance performSelector:NSSelectorFromString(selector) withObject:obj1 withObject:obj2];
#pragma clang diagnostic pop
}

+ (void)fixIt
{
    [self context][@"fixInstanceMethodBefore"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:NO aspectionOptions:AspectPositionBefore instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"fixInstanceMethodReplace"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:NO aspectionOptions:AspectPositionInstead instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"fixInstanceMethodAfter"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:NO aspectionOptions:AspectPositionAfter instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"fixClassMethodBefore"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:YES aspectionOptions:AspectPositionBefore instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"fixClassMethodReplace"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:YES aspectionOptions:AspectPositionInstead instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"fixClassMethodAfter"] = ^(NSString *instanceName, NSString *selectorName, JSValue *fixImpl) {
        [self _fixWithMethod:YES aspectionOptions:AspectPositionAfter instanceName:instanceName selectorName:selectorName fixImpl:fixImpl];
    };

    [self context][@"runClassWithNoParamter"] = ^id(NSString *className, NSString *selectorName) {
        return [self _runClassWithClassName:className selector:selectorName obj1:nil obj2:nil];
    };

    [self context][@"runClassWith1Paramter"] = ^id(NSString *className, NSString *selectorName, id obj1) {
        return [self _runClassWithClassName:className selector:selectorName obj1:obj1 obj2:nil];
    };

    [self context][@"runClassWith2Paramters"] = ^id(NSString *className, NSString *selectorName, id obj1, id obj2) {
        return [self _runClassWithClassName:className selector:selectorName obj1:obj1 obj2:obj2];
    };

    [self context][@"runVoidClassWithNoParamter"] = ^(NSString *className, NSString *selectorName) {
        [self _runClassWithClassName:className selector:selectorName obj1:nil obj2:nil];
    };

    [self context][@"runVoidClassWith1Paramter"] = ^(NSString *className, NSString *selectorName, id obj1) {
        [self _runClassWithClassName:className selector:selectorName obj1:obj1 obj2:nil];
    };

    [self context][@"runVoidClassWith2Paramters"] = ^(NSString *className, NSString *selectorName, id obj1, id obj2) {
        [self _runClassWithClassName:className selector:selectorName obj1:obj1 obj2:obj2];
    };

    [self context][@"runInstanceWithNoParamter"] = ^id(id instance, NSString *selectorName) {
        return [self _runInstanceWithInstance:instance selector:selectorName obj1:nil obj2:nil];
    };

    [self context][@"runInstanceWith1Paramter"] = ^id(id instance, NSString *selectorName, id obj1) {
        return [self _runInstanceWithInstance:instance selector:selectorName obj1:obj1 obj2:nil];
    };

    [self context][@"runInstanceWith2Paramters"] = ^id(id instance, NSString *selectorName, id obj1, id obj2) {
        return [self _runInstanceWithInstance:instance selector:selectorName obj1:obj1 obj2:obj2];
    };

    [self context][@"runVoidInstanceWithNoParamter"] = ^(id instance, NSString *selectorName) {
        [self _runInstanceWithInstance:instance selector:selectorName obj1:nil obj2:nil];
    };

    [self context][@"runVoidInstanceWith1Paramter"] = ^(id instance, NSString *selectorName, id obj1) {
        [self _runInstanceWithInstance:instance selector:selectorName obj1:obj1 obj2:nil];
    };

    [self context][@"runVoidInstanceWith2Paramters"] = ^(id instance, NSString *selectorName, id obj1, id obj2) {
        [self _runInstanceWithInstance:instance selector:selectorName obj1:obj1 obj2:obj2];
    };

    [self context][@"runInvocation"] = ^(NSInvocation *invocation) {
        [invocation invoke];
    };

    // helper
    [[self context] evaluateScript:@"var console = {}"];
    [self context][@"console"][@"log"] = ^(id message) {
        NSLog(@"Javascript log: %@",message);
    };
}
@end

--EOF--

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