修改Xcode自带代码片段CodeSnippets

Xcode默认只能编辑用户自建的代码片段，而系统自带的代码片段在Xcode中是不支持直接编辑的。但是可以通过以下方法进行编辑修改

1. 先找到文件存放位置，Xcode自带代码片段存放位置：
   Xcode 9 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/SystemCodeSnippets.codesnippets
   Xcode 10 /Applications/Xcode.app/Contents/PlugIns/IDESourceEditor.framework/Versions/A/Resources/SystemCodeSnippets.codesnippets
2. 将该文件复制出来将后缀改为plist，打开编辑该文件即可

例：由于加@符Xcode不会有代码提示，可以将@protocol、@interface、@implementation中的@符去掉，这样Xcode自带的代码片段就可以起作用，不用自己写了。
附：(一些常用的Xcode自带OC代码片段)

initialize

    + (void)initialize
    {
        if (self == [<#ClassName#> class]) {
            <#statements#>
        }
    }

initWithFrame

    - (instancetype)initWithFrame:(CGRect)frame
    {
        self = [super initWithFrame:frame];
        if (self) {
            <#statements#>
        }
        return self;
    }

init
    - (instancetype)init
    {
        self = [super init];
        if (self) {
            <#statements#>
        }
        return self;
    }

forin

    for (<#type *object#> in <#collection#>) {
        <#statements#>
    }

delloc

    - (void)dealloc
    {
        <#statements#>
    }

description

    - (NSString *)description
    {
        return [NSString stringWithFormat:@"<#format string#>", <#arguments#>];
    }

debugDescription

    - (NSString *)debugDescription
    {
        return [NSString stringWithFormat:@"<%@: %p> <#additional format string#>", [self class], self, <#additional arguments#>];
    }

encodeWithCoder

    - (void)encodeWithCoder:(NSCoder *)coder
    {
        [super encodeWithCoder:coder];
        <#statements#>
    }

initWithCoder

    - (instancetype)initWithCoder:(NSCoder *)coder
    {
        self = [super initWithCoder:coder];
        if (self) {
            <#statements#>
        }
        return self;
    }

isEqual

    - (BOOL)isEqual:(id)other
    {
        if (other == self) {
            return YES;
        } else if (![super isEqual:other]) {
            return NO;
        } else {
            return <#comparison expression#>;
        }
    }

    - (NSUInteger)hash
    {
        return <#hash expression#>;
    }

typedefBlock

    typedef <#returnType#>(^<#name#>)(<#arguments#>);

typedef

    typedef <#existing#> <#new#>;

dispatch_once snippet

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        <#code to be executed once#>
    });

dispatch_source timer

    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, <#dispatchQueue#>);
    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, <#intervalInSeconds#> * NSEC_PER_SEC, <#leewayInSeconds#> * NSEC_PER_SEC);
    dispatch_source_set_event_handler(timer, ^{
        <#code to be executed when timer fires#>
    });
    dispatch_resume(timer);

dispatch_after snippet

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(<#delayInSeconds#> * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        <#code to be executed after a specified delay#>
    });

inlineBlock

    <#returnType#>(^<#blockName#>)(<#parameterTypes#>) = ^(<#parameters#>) {
        <#statements#>
    };

enumdef

    typedef enum : NSUInteger {
        <#MyEnumValueA#>,
        <#MyEnumValueB#>,
        <#MyEnumValueC#>,
    } <#MyEnum#>;

nsenum

    typedef NS_ENUM(NSUInteger, <#MyEnum#>) {
        <#MyEnumValueA#>,
        <#MyEnumValueB#>,
        <#MyEnumValueC#>,
    };

nsoptions

    typedef NS_OPTIONS(NSUInteger, <#MyEnum#>) {
        <#MyEnumValueA#> = 1 << 0,
        <#MyEnumValueB#> = 1 << 1,
        <#MyEnumValueC#> = 1 << 2,
    };

structdef

    struct <#struct name#> {
        <#struct fields#>
    };

if

    if (<#condition#>) {
        <#statements#>
    }

ifelse
    
    if (<#condition#>) {
        <#statements#>
    } else {
        <#statements#>
    }

switch 

    switch (<#expression#>) {
        case <#constant#>:
            <#statements#>
            break;
            
        default:
            break;
    }

while

    while (<#condition#>) {
        <#statements#>
    }

dowhile

    do {
        <#statements#>
    } while (<#condition#>);

for

    for (<#initialization#>; <#condition#>; <#increment#>) {
        <#statements#>
    }    

test

    - (void)test<#Name#> {
        <#statements#>
    }

observeValueForKeyPath 

    - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
    {
        if (context == <#context#>) {
            <#code to be executed upon observing keypath#>
        } else {
            [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        }
    }


带@符号的


@protocol

    @protocol <#protocol name#> <NSObject>

    <#methods#>

    @end

@interface

    @interface <#class name#> : <#superclass#>

    @end

@interface-extension

    @interface <#class name#> ()

    @end

@interface-category

    @interface <#class name#> (<#category name#>)

    @end

@implementation

    @implementation <#class#>

    <#methods#>

    @end

@implementation-category

    @implementation <#class#> (<#category name#>)

    <#methods#>

    @end

@try

    @try {
        <#Code that can potentially throw an exception#>
    } @catch (NSException *exception) {
        <#Handle an exception thrown in the @try block#>
    } @finally {
        <#Code that gets executed whether or not an exception is thrown#>
    }

@autoreleasepool

    @autoreleasepool {
        <#statements#>
    }

@synchronized

    @synchronized (<#token#>) {
        <#statements#>
    }