I don't understand the following code snippet in the container/heap package.
type Interface interface {
sort.Interface //Is this line a method?
Push(x interface{})
Pop() interface{}
}
This is a type declaration.
The heap.Interface interface embeds the sort.Interface interface.
You can see it as a kind of inheritance/specialization : it means that the structs implementing the heap.Interface interface are defined as the ones that implement the sort.Interface methods and the Push and Pop methods.
Interface embeding is described in Effective Go : http://golang.org/doc/effective_go.html#embedding
Related
Thank you all so much! I just started in Kotlin which probably should be called the K language (like C and F), and have found so many solutions here on this site...it's awesome!
I have an independent class file called AppTime.kt and it's declared in the AndroidManifest.xml file:
<application
android:name=".AppTime"
class AppTime : Application() {
fun burntToast(sMsg: String) {
Toast.makeText(this.applicationContext, "!", Toast.LENGTH_LONG).show()
}
}
It doesn't run when called anywhere from a Fragment class:
class FirstFragment : Fragment() {...
AppTime().burntToast()
I've tried every approach using parameters for the Toast following makeText(...
and then to call it from a Fragment with or without context or string parameters.
Is it the type of class I have?
Functions defined inside a class can only be called on an instance of that class, as you already found.
But you cannot simply instantiate an arbitrary Application and expect it to work. Android does a lot of behind-the-scenes setup of framework classes before they are usable. Any Application or Activity that you instantiate yourself is useless. You have to use the instances that are provided to you through the lifecycle of the Activities that get launched in your application.
If you want to call this function from your Fragment, you will have to get an instance of your application, which you can get from its associated Activity. Since the Activity class doesn't know about your specific subclass of Application, you must also cast the application to your specific subclass to be able to call its unique functions. You can get the Activity by using requireActivity().
(requireActivity().application as AppTime).burntToast()
Java allows the following code to compile. Excuse the naming. I would have chosen better names if I knew why one would write this code, but that is the whole reason for my question. Note that IFace3 extends IFace1 both directly and indirectly (through IFace2). Is there a practical use for this capability?
public interface IFace1 {
public void meth1();
}
interface IFace2 extends IFace1 {
public void meth2();
}
interface IFace3 extends IFace1, IFace2 {
public void meth3();
}
The situation where multiple inheritance causes problems is when a member is defined different ways on different inheritance paths. At least in the days before Java allowed default interface implementations in the middle of a hierarchy, it wasn't possible for part of an interface to be defined in conflicting ways on different inheritance paths, since the actual implementation of every interface member would need to be contained in the class that implements it. Even if an interface includes Interface1.Method1 both directly and by extension through Interface2, the same method in an implementing class will be used to satisfy the members of both interfaces.
This involves autofac and c#. I have an interface derived from a parent interface:
public interface IJ4JLogger<out TCalling>
{
}
public interface IJ4JSmsLogger<out TCalling> : IJ4JLogger<TCalling>
{
}
Certain classes depend on being supplied an instance of the parent interface during construction:
public FileHistoryConfiguration( IJ4JLogger<FileHistoryConfiguration> histLogger, IJ4JLogger<FileHistoryService> svcLogger )
{
}
But if I register the type like this with autofac:
builder.RegisterGeneric( typeof(J4JSmsLogger<>) )
.As(typeof(IJ4JSmsLogger<>))
.SingleInstance();
where J4JSmsLogger<> is a class implementing IJ4JSmsLogger<>, then this call fails with an error that it can't find anything registered to provide an IJ4JLogger<> interface:
_fhConfig = _svcProvider.GetRequiredService<IFileHistoryConfiguration>();
I can work around the problem by changing the As<> clause in the registration of J4JSmsLogger<> to treat it as a IJ4JLogger<> instance, and then cast the result of resolving that interface to IJ4JSmsLogger<> whenever I need the extra capabilities of the child interface.
But I don't understand why I have to do that. Is there an additional step I need to take during registration of the types with autofac so that objects implementing the child interface will satisfy a need for the parent interface?
Cleaner Workaround
Reading more about autofac I learned something new: you can define as many As<>() clauses (including AsSelf()) as you want. So changing my autofac configuration to:
builder.RegisterGeneric( typeof(J4JSmsLogger<>) )
.As(typeof(IJ4JSmsLogger<>))
.As(typeof(IJ4JLogger<>))
.SingleInstance();
provides a cleaner solution than constantly casting resolved instances.
I'm not going to submit it as an answer, though, because I am curious why autofac doesn't do this kind of downcasting automatically, and whether any other DI frameworks do.
Autofac won't cast to base types for you like that. It generally assumes wiring is exact. You could run into some real problems if it didn't, like if someone has a constructor like...
public class BadTimes
{
public BadTimes(object input) { }
}
Which object does it put in there? Everything casts down to object.
However, you could always register it as both types and call it a day:
builder.RegisterGeneric(typeof(J4JSmsLogger<>))
.As(typeof(IJ4JSmsLogger<>))
.As(typeof(IJ4JLogger<>))
.SingleInstance();
Edit: So far it looks like the answer to my question is, "You can't do that in Swift." I currently have a solution whereby the subclass names are listed in an array and I loop around and instantiate them to trigger the process I'm describing below. If this is the best that can be done, I'll switch it to a plist so that least it's externally defined. Another option would be to scan a directory and load all files found, then I would just need to make sure the compiler output for certain classes is put into that directory...
I'm looking for a way to do something that I've done in C++ a few times. Essentially, I want to build a series of concrete classes that implement a particular protocol, and I want to those classes to automatically register themselves such that I can obtain a list of all such classes. It's a classic Prototype pattern (see GoF book) with a twist.
Here's my approach in C++; perhaps you can give me some ideas for how to do this in Swift 4? (This code is grossly simplified, but it should demonstrate the technique.)
class Base {
private:
static set<Base*> allClasses;
Base(Base &); // never defined
protected:
Base() {
allClasses.put(this);
}
public:
static set<Base*> getAllClasses();
virtual Base* clone() = 0;
};
As you can see, every time a subclass is instantiated, a pointer to the object will be added to the static Base::allClasses by the base class constructor.
This means every class inherited from Base can follow a simple pattern and it will be registered in Base::allClasses. My application can then retrieve the list of registered objects and manipulate them as required (clone new ones, call getter/setter methods, etc).
class Derived: public Base {
private:
static Derived global; // force default constructor call
Derived() {
// initialize the properties...
}
Derived(Derived &d) {
// whatever is needed for cloning...
}
public:
virtual Derived* clone() {
return new Derived(this);
}
};
My main application can retrieve the list of objects and use it to create new objects of classes that it knows nothing about. The base class could have a getName() method that the application uses to populate a menu; now the menu automatically updates when new subclasses are created with no code changes anywhere else in the application. This is a very powerful pattern in terms of producing extensible, loosely coupled code...
I want to do something similar in Swift. However, it looks like Swift is similar to Java, in that it has some kind of runtime loader and the subclasses in this scheme (such as Derived) are not loaded because they're never referenced. And if they're not loaded, then the global variable never triggers the constructor call and the object isn't registered with the base class. Breakpoints in the subclass constructor shows that it's not being invoked.
Is there a way to do the above? My goal is to be able to add a new subclass and have the application automatically pick up the fact that the class exists without me having to edit a plist file or doing anything other than writing the code and building the app.
Thanks for reading this far — I'm sure this is a bit of a tricky question to comprehend (I've had difficulty in the past explaining it!).
I'm answering my own question; maybe it'll help someone else.
My goal is to auto initialize subclasses such that they can register with a central authority and allow the application to retrieve a list of all such classes. As I put in my edited question, above, there doesn't appear to be a way to do this in Swift. I have confirmed this now.
I've tried a bunch of different techniques and nothing seems to work. My goal was to be able to add a .swift file with a class in it and rebuild, and have everything automagically know about the new class. I will be doing this a little differently, though.
I now plan to put all subclasses that need to be initialized this way into a particular directory in my application bundle, then my AppDelegate (or similar class) will be responsible for invoking a method that scans the directory using the filenames as the class names, and instantiating each one, thus building the list of "registered" subclasses.
When I have this working, I'll come back and post the code here (or in a GitHub project and link to it).
Same boat. So far the solution I've found is to list classes manually, but not as an array of strings (which is error-prone). An a array of classes such as this does the job:
class AClass {
class var subclasses: [AClass.Type] {
return [BClass.self, CClass.self, DClass.self]
}
}
As a bonus, this approach allows me to handle trees of classes, simply by overriding subclasses in each subclass.
I am using Visual Paradigm for UML to model our class hierarchy. I often have the case where one of our interfaces has a method requires an implementation of another of our interfaces as parameter to a method. Example (C++, interface = abstract class):
class IFoo {
public:
virtual void bla() = 0;
};
class IBar {
public:
virtual void meep(IFoo &) = 0;
};
I have no problem modeling both interfaces, but I am wondering which type of association to use for visually representing the relation of these two interfaces. Currently I am using the Usage-relation provided by Visual Paradigm, but I am not sure if this is indeed intended for this scenario. Is this the correct relation to use? If not, how can I model this relationship?
At least in a class diagram there is no visual representation of what's going on.
The Usage that you suggested would make it some kind of "meta" information, I guess. An alternative would be to specify it in a Usecase diagram. But that'd also be what I'd call "out-of-band" or on the meta-level, as Usecases are usually only used to communicate a warm, fluffy feeling of having documented something...
You can use a dependency arrow pointing from the interface with the dependent operation to the interface used as a parameter in the operation. You can then model the specifics of the dependency by providing the full signature of the dependent operations in your interface model element.
If you wanted to provide even more detail describing the nature of the dependency, you could attach a note to the dependency arrow.
Using your example: