iOS --- 理解Runtime机制及其使用场景

Runtime是iOS中比较难以理解, 但又非常强大的技术.

The Objective-C language defers as many decisions as it can from compile time and link time to runtime. Whenever possible, it does things dynamically. This means that the language requires not just a compiler, but also a runtime system to execute the compiled code. The runtime system acts as a kind of operating system for the Objective-C language; it’s what makes the language work.

所谓运行时, 就是尽可能地把决定从编译器推迟到运行期, 就是尽可能地做到动态. 只是在运行的时候才会去确定对象的类型和方法的. 因此利用Runtime机制可以在程序运行时动态地修改类和对象中的所有属性和方法.
Objective-C中调用对象的方法时, 会向该对象发送一条消息, runtime根据该消息做出反应.
Runtime是一套比较底层的纯C语言的API, Objective-C是运行在Runtime上的, 因此在Runtime中动态添加和实现一些非常强大的功能也就不足为奇了.
在Objective-C代码中使用Runtime, 需要引入

#import <objc/runtime.h>

实例变量

OC中类和对象的实例变量, 实际上是一个指向objc_ivar结构体的指针Ivar.

/// An opaque type that represents an instance variable.
typedef struct objc_ivar *Ivar;

通过runtime方法class_copyIvarList可获取一个指向类和对象的所有实例变量的Ivar指针.

u_int count = 0;
Ivar *ivars = class_copyIvarList([UIView class], &count);
for (int i = 0; i < count; i++) {
    const char *ivarName = ivar_getName(ivars[i]);
    NSString *str = [NSString stringWithCString:ivarName encoding:NSUTF8StringEncoding];
    NSLog(@"ivarName : %@", str);
}

获取到实例变量的Ivar指针后, 即可通过ivar_getName方法获取该实例变量的名称, 由C语言的字符串表示.
部分打印结果如下:

ivarName : _constraintsExceptingSubviewAutoresizingConstraints
ivarName : _cachedTraitCollection
ivarName : _layer
ivarName : _layerRetained
ivarName : _gestureInfo
ivarName : _gestureRecognizers
ivarName : _subviewCache
ivarName : _templateLayoutView
ivarName : _charge
ivarName : _tag
ivarName : _viewDelegate
ivarName : _backgroundColorSystemColorName
ivarName : _countOfMotionEffectsInSubtree
ivarName : _countOfTraitChangeRespondersInDirectSubtree
ivarName : _cachedScreenScale
ivarName : _viewFlags
ivarName : _retainCount

可以看到, 其中包含了UIView中的很多我们经常使用的实例变量.
对于属性, 如layer, tag等, 会自动生成 _ 前缀的成员变量

属性

OC中类和对象的属性, 是一个指向objc_property结构体的指针objc_property_t.

/// An opaque type that represents an Objective-C declared property.
typedef struct objc_property *objc_property_t;

通过runtime方法class_copyPropertyList可获取一个指向类和对象的所有属性的objc_property_t指针.

u_int count = 0;
objc_property_t *properties = class_copyPropertyList([UIView class], &count);
for (int i = 0; i < count; i++) {
    const char *propertyName = property_getName(properties[i]);
    NSString *str = [NSString stringWithCString:propertyName encoding:NSUTF8StringEncoding];
    NSLog(@"propertyName : %@", str);
}
free(properties);

获取到属性的objc_property_t指针后, 即可通过property_getName方法获取该属性的名称, 由C语言的字符串表示.
部分打印结果如下:

propertyName : hash
propertyName : superclass
propertyName : description
propertyName : debugDescription
propertyName : hash
propertyName : superclass
propertyName : description
propertyName : debugDescription
propertyName : userInteractionEnabled
propertyName : tag
propertyName : layer
propertyName : focused
propertyName : hash
propertyName : superclass
propertyName : description
propertyName : center
propertyName : bounds
propertyName : transform

方法

OC中类和对象的方法, 是一个指向objc_method结构体的指针Method.

/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method;

通过runtime方法class_copyMethodList可获取一个指向类和对象的所有方法的Method指针.

u_int count = 0;
// 获取所有方法
Method *methods = class_copyMethodList([UIView class], &count);
for (int i = 0; i < count; i++) {
    Method method = methods[i];
    // 方法类型是SEL选择器类型
    SEL methodName = method_getName(method);
    NSString *str = [NSString stringWithCString:sel_getName(methodName) encoding:NSUTF8StringEncoding];

    int arguments = method_getNumberOfArguments(method);
    NSLog(@"methodName : %@, arguments Count: %d", str, arguments);
}
free(methods);

获取到方法的Method指针后, 即可通过method_getName方法获取该方法的名称, 由C语言的字符串表示.
使用method_getNumberOfArguments可获取该方法的参数个数.
部分打印结果如下:

2016-04-12 13:32:19.925 DemoRuntime[2244:54083] methodName : captureImageIBAIncludingSubviews:, arguments Count: 3
2016-04-12 13:32:19.925 DemoRuntime[2244:54083] methodName : captureImageFromIOSurfaceIBA, arguments Count: 2
2016-04-12 13:32:19.925 DemoRuntime[2244:54083] methodName : hideSubviewsIBA, arguments Count: 2
2016-04-12 13:32:19.925 DemoRuntime[2244:54083] methodName : safeCaptureImageIBA, arguments Count: 2
2016-04-12 13:32:19.926 DemoRuntime[2244:54083] methodName : unhideSubviewsIBAWithContext:, arguments Count: 3
2016-04-12 13:32:19.926 DemoRuntime[2244:54083] methodName : snapshotViewHierarchyIBA, arguments Count: 2
2016-04-12 13:32:19.926 DemoRuntime[2244:54083] methodName : iba_viewControllerName, arguments Count: 2
2016-04-12 13:32:19.926 DemoRuntime[2244:54083] methodName : iba_viewControllerMemoryAddress, arguments Count: 2
2016-04-12 13:32:19.926 DemoRuntime[2244:54083] methodName : iba_alignmentRectForCurrentFrame, arguments Count: 2
2016-04-12 13:32:19.927 DemoRuntime[2244:54083] methodName : iba_participatingConstraints, arguments Count: 2
2016-04-12 13:32:19.927 DemoRuntime[2244:54083] methodName : setIba_contentHuggingPriorities:, arguments Count: 3

Method Swizzling

通过修改一个已存在类的方法, 来实现方法替换是比较常用的runtime技巧.
如在UIView的load方法中:

+ (void)load {
     Method origin = class_getInstanceMethod([UIView class], @selector(touchesBegan:withEvent:));
     Method custom = class_getInstanceMethod([UIView class], @selector(custom_touchesBegan:withEvent:));
     method_exchangeImplementations(origin, custom);
}

- (void)custom_touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
     // TODO
}

这样, 想要触发UIView的touchesBegan:withEvent:方法时, 实际调用的却是自定义的custom_touchesBegan:withEvent:方法.
另外, 关于runtime method swizzling的一个使用场景, 请参考博客:
iOS — 使用runtime解决3D Touch导致UIImagePicker崩溃的问题

关联对象

Objective-C中的Category无法向既有的类添加属性, 因此可以使用runtime的关联对象(associated objects)来实现.
如将一个字符串关联到一个数组:

static char overviewKey;
NSArray *array = [[NSArray alloc] initWithObjects:@"1", @"2", @"3", nil];
// 为了演示的目的，使用initWithFormat:来确保字符串可以被销毁
NSString *overview = [[NSString alloc] initWithFormat:@"@", @"first three numbers"];
objc_setAssociatedObject(array, &overviewKey, overview, OBJC_ASSOCIATION_RETAIN);

这样, 当overview被手动释放时, 却不会被销毁. 因为关联策略指明了数组array要保有相关联的对象.
而array也释放时, overview才会被销毁.

设置关联对象, 指定被关联对象array, 关联关键字overviewKey, 关联对象overview即可:

objc_setAssociatedObject(array, &overviewKey, overview, OBJC_ASSOCIATION_RETAIN);

获取关联对象, 需要传递被关联对象array和关联关键字overviewKey:

NSString *associatedObject = (NSString *)objc_getAssociatedObject(array, &overviewKey);

断开关联, 只需要设置关联对象为nil即可, 关联策略就无所谓了.

objc_setAssociatedObject(array, &overviewKey, nil, OBJC_ASSOCIATION_ASSIGN);

使用objc_removeAssociatedObjects可断开所有关联, 把对象恢复至原始状态.

Demo

Demo地址:
DemoRuntime