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0219-ios-check-type.md

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Explicit returned type from NSDictionary(Store) category

  • Proposal: SDL-0219
  • Author: Misha Vyrko
  • Status: Accepted with Revisions
  • Impacted Platforms: [iOS]

Introduction

This proposal is to specify returned types from NSDictionary (Store). This ensures that the return value is assigned to a variable of the correct type.

Motivation

Method (-(nullable id)sdl_objectForName:(SDLName)name) in NSDictionary (Store) returns id in the future it can be casted to any type. As result wrong casting leads to a crash in runtime. For example:

- (SDLLanguage)languageDesired {
    id object = [parameters sdl_objectForName:SDLNameLanguageDesired];
    return (SDLLanguage)object;
} 

- (void)someFunction:(SDLLanguage)language {
    if ([self.languageDesired isEqualToEnum:language]) { // `-[__NSCFNumber isEqualToEnum:]: unrecognized selector sent to instance`
        
    }
}

Proposed solution

Mark method as private: -(nullable id)sdl_objectForName:(SDLName)name;.

Instead of method above use next methods:

For mandatory values, the following functions try to return object of classType overwise error will be filed. When the object is not of type classType, the error will not be nil - (nullable id)sdl_objectForName:(SDLName)name ofClass:(Class)classType error:(NSError **)error - (nullable NSArray *)sdl_objectsForName:(SDLName)name ofClass:(Class)classType error:(NSError **)error;.

For optional values, can use functions that return object of classType or nil. These functions are syntactic sugar on functions with error where put nil to error parameter. - (nullable id)sdl_objectForName:(SDLName)name ofClass:(Class)classType;. - (nullable NSArray *)sdl_objectsForName:(SDLName)name ofClass:(Class)classType.

For suitable working with SDLEnums create the following methods: - (nullable SDLEnum)sdl_enumForName:(SDLName)name error:(NSError **)error; - (nullable NSArray<SDLEnum> *)sdl_enumsForName:(SDLName)name error:(NSError **)error;

These new methods will assert (which only affects apps in DEBUG mode), log the error using the SDL logging framework, and return nil with an error object containing the faulty data sent by the head unit and an error message.

Examples:

- (NSNumber<SDLInt> *)menuID {
    NSError *error;
    return [parameters sdl_objectForName:SDLNameMenuId ofClass:NSNumber.class error:&error];
}
- (nullable NSString *)fullAppID {
    return [parameters sdl_objectForName:SDLNameFullAppID ofClass:NSString.class];
}
- (nullable NSArray<NSString *> *)vrSynonyms {
    return [parameters sdl_objectsForName:SDLNameVRSynonyms ofClass:NSString.class];
}
- (SDLLanguage)languageDesired {
    return [parameters sdl_enumForName:SDLNameLanguageDesired];
}
- (nullable NSArray<SDLVentilationMode> *)ventilationMode {
    return [store sdl_enumsForName:SDLNameVentilationMode];
}

Proposal implemented in smartdevicelink/sdl_ios#1158

Potential downsides

Store can return wrong type, which would be highlighted by an error. For example:

NSNumber *number = @2;
self.store = @{SDLSomeValue : number};

NSError *error;
NSString *string = [self.store sdl_objectForName:SDLSomeValue ofClass:NSString.class error:&error]; 

//string is NSNumber
//string and error aren't nil

Impact on existing code

No impact

Alternatives considered