I do not have any specific id available in the response JSON body (I can not change the body). That is why I can not use
RKEntityMapping *mapping = [RKEntityMapping mappingForEntityForName:....
mapping.identificationAttributes = #[#"specificId"];
Is it possible to configure the mapping in such a way that there is no new NSManagedObject created but always the previous one is updated, if such object exists?
I would like to fetch data for ui update from a single response object (of specific class). Yes I can delete the previous instance of the response before the new one is received but the approach required in this question is cleaner and I do not need to keep the reference/id to the response entity.
I am reading the documentation for RKManagedObjectRequestOperation but it is not clear whether this approach is supported by Restkit.
Thank you for comment.
I have made a hack that is not acceptable but it works: I am using a special attribute in each special "singleton" NSManagedObject's subclass: e.g. unique which I use for the identification on the class level:
In RKManagedObjectMappingOperationDataSource there is the condition modified to allow passing entities with the special unique attribute:
// If we have found the entity identification attributes, try to find an existing instance to update
if ([entityIdentifierAttributes count] || [self.managedObjectCache isUniqueEntityClass:entity])...
In RKFetchRequestManagedObjectCache and in RKInMemoryManagedObjectCache there is the new method defined:
- (BOOL) isUniqueEntityClass:(NSEntityDescription*)entity {
__block BOOL isUniqueEntityClass = NO;
[[[entity attributesByName] allKeys] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
isUniqueEntityClass = [obj isEqualToString:#"unique"];
if(isUniqueEntityClass) {
*stop = YES;
return;
}
}];
return isUniqueEntityClass;
}
In the method
- (NSSet *)managedObjectsWithEntity:(NSEntityDescription *)entity
attributeValues:(NSDictionary *)attributeValues
inManagedObjectContext:(NSManagedObjectContext *)managedObjectContext...
isUniqueEntityClass decides if we should use predicate based on attributeValues to fetch the entity or directly fetch the entity without the predicate.
Related
I have some code that I am using to convert arbitrary QObject subclasses to JSON. I able to convert them if they are pointers to a subclass, but am curious whether it is possible to convert instances (provided the subclass implements a copy constructor). Is there some crazy way to use something like templates or the type information provided by QMetaType to copy an instance of a QObject subclass without knowing what it is? The ToJson code is in a class that has no knowledge of the subclass.
I think it might be possible with QMetaType::create or something similar but I haven't been able to figure out how to actually copy the properties of the subclass instance.
Here's my code for converting:
QJsonValue ToJson(QVariant value){
switch(value.type()){
case QVariant::Int:
case QVariant::Double:
return value.toDouble();
////Other cases, etc...
case QVariant::UserType:
QObject* obj_ptr = qvariant_cast<QObject*>(value);
if(obj_ptr) // value was originally a pointer to a QObject, works correctly
return ToJson(obj_ptr);
else { // value was orginally an instance of a QObject subclass
std::string t = value.typeName(); //returns "MyQObject"
int id = QMetaType::type(t.c_str()); //returns the id of the derived class
void* v = QMetaType::create(id, &value); //passing &value does nothing
obj_ptr = static_cast<QObject*>(v);
return ToJson(obj_ptr); //works, but resulting fields are all default
}
}
}
QJsonObject ToJson(QObject* o){
QJsonObject obj;
auto mo = o->metaObject();
for (int i = mo->propertyOffset(); i < mo->propertyCount(); ++i){
QVariant value = o->property(mo->property(i).name());
obj[mo->property(i).name()] = ToJson(value);
}
return obj;
}
Sample code use case:
qRegisterMetaType<MyQObject>();
MyQObject obj;
obj.db = 11.1;
QVariant test1 = QVariant::fromValue(obj);
QVariant test2 = QVariant::fromValue(&obj);
QJsonValue v1 = ToJson(test1); // default constructed values
QJsonValue v2 = ToJson(test2); // db = 11.1
Sample QObject subclass:
class MyQObject : public QObject {
Q_OBJECT
Q_PROPERTY(double DB MEMBER db)
Q_PROPERTY(int I MEMBER i)
public:
MyQObject();
MyQObject(const MyQObject& other) : QObject() {
i = other.i;
db = other.db;
}
int i = 50;
double db = 1.5;
};
Q_DECLARE_METATYPE(MyQObject)
Is there any way to handle the case illustrated by test1 above?
Long-story-short: nope. There is no way to store QObjects by value in containers or QVariant.
Qt forbids the copy of QObjects and all inheriting classes. The mandatory the Q_OBJECT macro will disable any copy constructor also in newly defined classes.
The copy constructor that you are defining in the MyObject class is missing the base class constructor call. If QObject had a copy constructor it would be something like this:
MyQObject(const MyQObject& other) :
QObject(other) // this will NEVER compile
{
i = other.i;
db = other.db;
}
Probably, the compiler is giving you a warning, but allows you to have such a constructor, even if it will result in undefined behavior or slicing an instance of MyObject every time it is passed by value.
Furthermore, the Qt docs states the following:
The values stored in the various containers can be of any assignable
data type. To qualify, a type must provide a default constructor, a
copy constructor, and an assignment operator. This covers most data
types you are likely to want to store in a container, including basic
types such as int and double, pointer types, and Qt data types such as
QString, QDate, and QTime, but it doesn't cover QObject or any QObject
subclass (QWidget, QDialog, QTimer, etc.).
So you can't store QObject and derived classes inside a Qt container unless you store them as pointers, as copy of QObjects is disabled by design.
Furthermore, if you want to exploit polymorphic behavior you must use pointers, even if there is no explicit need to cast to derived classes in your code, as far as I can see. If you really need to resort to casting in some place, you could consider making your ToJson a template function.
There is a solution, but use caution as it is only reasonable/applicable in the following scenario:
Classes in question are primarily data storage classes
The classes in question would be entirely copy-able if they didn't inherit from QObject
Most importantly, the ONLY reason you have the class inherit from QObject is so that it can have meta properties.
If your code uses the class as a QObject for any reason other than to get meta information, you are almost certainly using it incorrectly if you are trying to store it by value (as explained by G. Giordano in their answer).
Misuse considerations aside, in order to JSON-ify a QVariant that stores a QObject subclass by value, you can use the QMetaType::create method and pass it the user type id and yourQVariant.constData().
Example:
MyQObject obj;
obj.db = 11.1;
QVariant value = QVariant::fromValue(obj);
std::string t = value.typeName();
int id = QMetaType::type(t.c_str());
void* v = QMetaType::create(id, value.constData());
obj_ptr = static_cast<QObject*>(v);
QJsonValue json = ToJson(obj_ptr); //json contains db = 11.1
There is already some questions about map and weak maps, like this: What's the difference between ES6 Map and WeakMap? but I would like to ask in which situation should I favor the use of these data structures? Or what should I take in consideration when I favor one over the others?
Examples of the data structures from:https://github.com/lukehoban/es6features
// Sets
var s = new Set();
s.add("hello").add("goodbye").add("hello");
s.size === 2;
s.has("hello") === true;
// Maps
var m = new Map();
m.set("hello", 42);
m.set(s, 34);
m.get(s) == 34;
// Weak Maps
var wm = new WeakMap();
wm.set(s, { extra: 42 });
wm.size === undefined
// Weak Sets
var ws = new WeakSet();
ws.add({ data: 42 });
// Because the added object has no other references, it will not be held in the set
Bonus. Which of the above data structures will produce the same/similar result of doing: let hash = object.create(null); hash[index] = something;
This is covered in §23.3 of the specification:
If an object that is being used as the key of a WeakMap key/value pair is only reachable by following a chain of references that start within that WeakMap, then that key/value pair is inaccessible and is automatically removed from the WeakMap.
So the entries in a weak map, if their keys aren't referenced by anything else, will be reclaimed by garbage collection at some point.
In contrast, a Map holds a strong reference to its keys, preventing them from being garbage-collected if the map is the only thing referencing them.
MDN puts it like this:
The key in a WeakMap is held weakly. What this means is that, if there are no other strong references to the key, then the entire entry will be removed from the WeakMap by the garbage collector.
And WeakSet does the same.
...in which situation should I favor the use of this data structures?
Any situation where you don't want the fact you have a map/set using a key to prevent that key from being garbage-collected. Here are some examples:
Having instance-specific information which is truly private to the instance, which looks like this: (Note: This example is from 2015, well before private fields were an option. Here in 2021, I'd use private fields for this.)
let Thing = (() => {
var privateData = new WeakMap();
class Thing {
constructor() {
privateData[this] = {
foo: "some value"
};
}
doSomething() {
console.log(privateData[this].foo);
}
}
return Thing;
})();
There's no way for code outside that scoping function to access the data in privateData. That data is keyed by the instance itself. You wouldn't do that without a WeakMap because it would be a memory leak, your Thing instances would never be cleaned up. But WeakMap only holds weak references, and so if your code using a Thing instance is done with it and releases its reference to the instance, the WeakMap doesn't prevent the instance from being garbage-collected; instead, the entry keyed by the instance is removed from the map.
Holding information for objects you don't control. Suppose you get an object from some API and you need to remember some additional information about that object. You could add properties to the object itself (if it's not sealed), but adding properties to objets outside of your control is just asking for trouble. Instead, you can use a WeakMap keyed by the object to store your extra information.
One use case for WeakSet is tracking or branding: Suppose that before "using" an object, you need to know whether that object has ever been "used" in the past, but without storing that as a flag on the object (perhaps because if it's a flag on the object, other code can see it [though you could use a private field to prevent that]; or because it's not your object [so private fields wouldn't help]). For instance, this might be some kind of single-use access token. A WeakSet is a simple way to do that without forcing the object to stay in memory.
Which of the above data structures will produce the same/similar result of doing: let hash = Object.create(null); hash[index] = something;
That would be nearest to Map, because the string index (the property name) will be held by a strong reference in the object (it and its associated property will not be reclaimed if nothing else references it).
Basically I have been using a Dictionary object in my program that basically took ints as its keys and stored RTMFP peer IDs in the appropriate locations. Each int was unique and represented one user.
Now I'm needing to expand on this where users are identified by a combination of the int and a Boolean value, kind of like this:
private var m_iUID:int;
private var m_blnIsCurrent:Boolean;
Only the combination between those two really uniquely identifies the user. That being said I was just about to use a new class made out of this for the Dictionary keys; but then it occurred to me that instead of doing it this way, I could just add the peer ID to the class definition and turn the Dictionary object into an ArrayCollection:
private var m_iUID:int;
private var m_blnIsCurrent:Boolean;
public var m_strNearID:String;
So now I'm wondering which is really better in this scenario. And that question has led to a bigger question: where do you really draw the line between these two collection types in general? They're suddenly starting to not seem all that different after all, except where you're trying to avoid messing with class definitions. I guess I'm really asking for advice about both the specific scenario and the general question. Thanks!
ArrayCollection is just a wrapper for an Array, and is only available in Flex.
In AS3 you really have 3 fundamental hash table types: Array, Object, and Dictionary. You choose which one to use based on the type of key you want to use: an integer, a string, or an object reference. Arrays will convert any key to an int, Object will convert any key to a string. Dictionary works like Object for string keys (and will convert primitives to a string) but what it is really good at is using object references as keys.
It you want to use a single int as the unique key, use an array. If you want to use a single string as the unique key, use an object. If you want to use object references as the unique key, use a Dictionary.
In your case you should probably use an Object, and a custom toString() method on your "key" class. This is because you want to use a composite of primitive values (NOT an object reference) as your unique key. There is no way to do this natively, so you'll have to mash the values together as a single string. Objects are the best (fastest) hash table for string keys, so that is the collection you should use.
Example:
class User {
private var m_iUID:int;
private var m_blnIsCurrent:Boolean;
public var m_strNearID:String;
public function User(UID:int, IsCurrent:Boolean) {
m_iUID = UID;
m_blnIsCurrent = IsCurrent;
}
// Custom toString to mash together primitives
public function toString() {
return m_iUID.toString() + "-" + (m_blnIsCurrent ? "1" : "0");
}
}
// Later:
var allUsers:Object = {}
var user1:User = new User(231049, true);
var user2:User = new User(0x2309, false);
// Implicitly calls toString():
allUsers[user1] = "User 1";
allUsers[user2] = "User 2";
// All of the following will successfully retrieve the value for user1 ("User 1"):
// ONLY the first would work if allUsers was a Dictionary
trace(allUsers[user1]);
trace(allUsers[user1.toString()]);
trace(allUsers["231049-1"]);
trace(allUsers[new User(231049, true)]);
Dictionary and ArrayCollection have some important differences:
Dictionary maps objects to other objects, while ArrayCollection is just a list of objects.
ArrayCollection is Flex only, so unusable in a generic AS3 project.
Which one you should use really depends on what you need in your app:
Will you be using the "identity" object (with user id and "is current") somewhere else, without an associated peer id? In that case, make it a separate Identity class or so and use a Dictionary to map Identity instances to peer ids.
Do you need to perform lookups based on identities? In other words, do you need to ask "which peer id is associated with this identity?". If so, go for Dictionary + Identity once more, to avoid looping through a list instead.
I'm sure there are more considerations, but these should get you started.
How do I check that the model is correct for an object read from the database.
var myModelObject = theDB.myDbContext.myModelObject.Find(1234);
Now I would like to control if it is correct according to the rules / attributy in the model... but how?
It is expected that data read from database are correct according to validation rules because that validation rules also defines the mapping. Inconsistency can lead in some cases to exception in object materialization.
If you want to executed validation based on data annotations manually you can use:
using System.ComponentModel.DataAnnotations;
var results = new List<ValidationResult>();
var context = new ValidationContext(myModelObject, null, null);
bool isValid = Validator.TryValidateObject(myModelObject, context, results, true);
I've been spending some of my spare time working a set of collections for ActionScript 3 but I've hit a pretty serious roadblock thanks for the way ActionScript 3 handles equality checks inside Dictionary Objects.
When you compare a key in a dictionary, ActionScript uses the === operator to perform the comparison, this has a bit of a nasty side effect whereby only references to the same instance will resolve true and not objects of equality. Here's what I mean:
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
const table : Dictionary = new Dictionary();
table[jonny1] = "That's me";
trace(table[jonny1]) // traces: "That's me"
trace(table[jonny2]) // traces: undefined.
The way I am attempting to combat this is to provide an Equalizer interface which looks like this:
public interface Equalizer
{
function equals(object : Object) : Boolean;
}
This allows to to perform an instanceOf-esq. check whenever I need to perform an equality operation inside my collections (falling back on the === operator when the object doesn't implement Equalizer); however, this doesn't get around the fact that my underlying datastructure (the Dictionary Object) has no knowledge of this.
The way I am currently working around the issue is by iterating through all the keys in the dictionary and performing the equality check whenever I perform a containsKey() or get() operation - however, this pretty much defeats the entire point of a hashmap (cheap lookup operations).
If I am unable to continue using a Dictionary instance as the backing for map, how would I go about creating the hashes for unique object instances passed in as keys so I can still maintain equality?
How about you compute a hash code for your objects when you insert them, and then look them up by the hash code in your backing dictionary? The hashcode should compare === just fine. Of course, that would require you to have a Hashable interface for your object types instead of your Equalizer interface, so it isn't much less work than you are already doing, but you do get the cheap lookups.
How about rather doing this:
public interface Hashable {
function hash():String;
}
personally, I ask myself, why you want to do this ... hashing objects to obtain keys makes little sense if they are mutable ...
also, you might consider using a different approach, as for example this factory:
package {
public class Person {
/**
* don't use this!
* #private
*/
public function Person(name:String, age:int) {
if (!instantiationAllowed)
throw new Error("use Person.getPerson instead of constructor");
//...
}
private static var instantiationAllowed:Boolean = false;
private static var map:Object = {};
private static function create(name:String, age:int):Person {
instantiationAllowed = true;
var ret:Person = new Person(name, age);
instantiationAllowed = false;
}
public static function getPerson(name:String, age:int):Person {
var ageMap:Array = map[name];
if (ageMap == null) {
map[name] = ageMap = [];
return ageMap[age] = Person.create(name, age);
}
if (ageMap.hasOwnProperty(age))
return ageMap[age];
return ageMap[age] = Person.create(name, age);
}
}
}
it ensures, there's only one person with a given name and age (if that makes any sense) ...
Old thread I know, but still worth posting.
const jonny1 : Person = new Person("jonny", 26); const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3
The problem with AS3/JavaScript/EcmaScript is not that they create two different, equivalent objects.
The problem is that they cannot equate those two equivalent objects--only identity works, since there is no equals or hashCode methods that can be overriden with class-specific comparison logic.
For Map implementations such as dynamic Object or Dictionary, this means that you have to either use Strings or references as keys: you cannot recover objects from a map using different but equivalent objects.
To work around that problem, people either resort to strict toString implementations (for Object maps) which is undesirable, or to instance control for Dictionaries, as in #back2dos example, which introduces different problems (Also, note that #back2dos solution does not really guarantee unique Person instances since there is a time window during which asynchronous threads will be allowed to instantiate new Persons).
#A.Levy's solution is good except that in general, hashCodes are not strictly required to issue unique values (they are meant to map entries to buckets allowing for fast lookups, wherein fine-grained differentiation is done through equals method).
You need both a hashCode and an equals method, e.g.
public interface IEquable
{
function equals(object : Object) : Boolean;
function hash():String;
}
In any programming language,
const jonny1 : Person = new Person("jonny", 26);
const jonny2 : Person = new Person("jonny", 26);
is creating two completely different objects that will not compare using ==, guess I don't see why it's any more of a road block because of as3