I need to compare the old object with an updated object with or without loopback hooks or maybe mixins irrespective to the calling API.
==> context.currentInstance is available only via prototype.updateAttributes() which is a little different from others(save/create etc) as
when a 'before save' hook triggered by prototype.updateAttributes(), ctx.data will be populated with the data to be changed as you’d expect for an update operation. This is old data ready to be changed and I can compare it with updated data context.data or context.instance. But I am not getting currentInstance in all cases...
I am using "before save" hook of loopback.
Let's say oldObject = data before update.
and updatedObject = data after the update.
Based on who is calling the hook function I am receiving
either
ctx.currentInstance(This is oldObject mean data without changes) + ctx.data(this is updatedObject)
or
ctx.instance (This is new data)
case-1
If the object is new than no comparison can be done.
if(ctx.instance){//true}
case-2
If the object is updated then we can compare data and find the changes that which property is changed.
if(ctx.currentInstance && ctx.data){//true}
Now my query is related to case-2.
If I want to find the diff between oldObject and updatedObject then I can use lodash or I can use deep-diff (deep-diff is a javascript/node.js module providing utility functions for determining the structural differences between objects and includes some utilities for applying differences across objects.) But This solution fails when any hook say before save hook triggered by anything other than prototype.updateAttributes()
how to get affected attributes/properties of the persisted model? The condition is that the entire object is received in ctx.currentInstance and ctx.data.
Model.observe("before save", async function (ctx) {
ctx.hookState.olddata = ctx.data; // or ctx.currentInstance or anyhting that gives the old object (just before updation)
});
I can pass ctx.hookState between hooks. So setting the value in any loopback hook will work. Condition is that I am receiving the whole object and not just the updated part. Any loopback hook triggered by any call say save/create/update/delete etc.
In many cases, LoopBack does not have the old object data. If you want to access that data, you have to fetch it from the database in your "before save" hook.
A mock-up to illustrate my point:
Model.observe("before save", async function (ctx) {
if (ctx.currentInstance) {
ctx.hookState.oldData = [ctx.currentInstance];
} else if (ctx.isNewInstance) {
// new instance being created, no old data
ctx.hookState.oldData = [];
} else if (ctx.where) {
// updating multiple records via `updateAll`
ctx.hookState.oldData = await ctx.Model.find(ctx.where);
} else {
ctx.hookState.oldData = [await ctx.Model.findById(ctx.instance.id)];
}
});
Please note that "before save" hook is fired by operations updating multiple instances too (e.g. updateAll), you need to handle that case too.
In my Electron project, I'm trying to make a module singleton by setting it as a global. Since I use jquery in this module, I import it in the renderer process and then send it to the main via ipc and set it as global there. Here is related part of my code:
main.js:
ipcMain.on( "setGlobal", ( event, global_var) => {
global[global_var[0]] = global_var[1];
console.log(global_var);
event.returnValue = 1;
} );
renderer.js:
const favourites = require("./components/favourites");
console.log(favourites);
ipcRenderer.sendSync("setGlobal", ["favourites", favourites]);
console.log(remote.getGlobal("favourites"));
The outputs of console.logs in the renderer process are in the image below:
And the output of the main process is:
[ 'favourites', { favourites: [] } ]
As you see, the object (module) I sent from the ipcRenderer is changed in the ipcMain, it lost its add and init functions. Do you have any idea what is the reason of this behavior and how to fix it?
PS: To be sure, I tested it with a simple objects that contains functions instead of require("favourites"). They are also behaving in the same way. I did a workaround by using only the entities as global and passing them to all of the functions as arguments. However, it's not a good way in sense of code readability.
You cannot use IPC like that. As noted in docs (eg. sendSync)
Send a message to the main process synchronously via channel, you can also send arbitrary arguments. Arguments will be serialized in JSON internally and hence no functions or prototype chain will be included.
Your functions are simply not making it to main process.
For making a module singleton you should just use the singleton pattern in your module, and use require in main process and remote.require in renderer. Since require using cache (at least by default), the same module should be returned. (more or less. Further reading can be found here)
For example if you export a class:
let _instance = null
class MyClass {
constructor() {
if (_instance === null) _instance = this
return _instance
}
...
}
module.exports = MyClass
After #pergy's answer, I decided to drop IPC and use only the globals. So, here is the workaround I find:
main process:
global.provider = {};
renderer process:
const favourites = require("./components/favourites");
remote.getGlobal("provider").favourites = favourites;
other modules:
const favourites = remote.getGlobal("provider").favourites;
Note: My question is about the way of including/passing the dispatcher instance around, not about how the pattern is useful.
I am studying the Flux Architecture and I cannot get my head around the concept of the dispatcher (instance) potentially being included everywhere...
What if I want to trigger an Action from my Model Layer? It feels weird to me to include an instance of an object in my Model files... I feel like this is missing some injection pattern...
I have the impression that the exact PHP equivalent is something (that feels) horrible similar to:
<?php
$dispatcher = require '../dispatcher_instance.php';
class MyModel {
...
public function someMethod() {
...
$dispatcher->...
}
}
I think my question is not exactly only related to the Flux Architecture but more to the NodeJS "way of doing things"/practices in general.
TLDR:
No, it is not bad practice to pass around the instance of the dispatcher in your stores
All data stores should have a reference to the dispatcher
The invoking/consuming code (in React, this is usually the view) should only have references to the action-creators, not the dispatcher
Your code doesn't quite align with React because you are creating a public mutable function on your data store.
The ONLY way to communicate with a store in Flux is via message passing which always flows through the dispatcher.
For example:
var Dispatcher = require('MyAppDispatcher');
var ExampleActions = require('ExampleActions');
var _data = 10;
var ExampleStore = assign({}, EventEmitter.prototype, {
getData() {
return _data;
},
emitChange() {
this.emit('change');
},
dispatcherKey: Dispatcher.register(payload => {
var {action} = payload;
switch (action.type) {
case ACTIONS.ADD_1:
_data += 1;
ExampleStore.emitChange();
ExampleActions.doThatOtherThing();
break;
}
})
});
module.exports = ExampleStore;
By closing over _data instead of having a data property directly on the store, you can enforce the message passing rule. It's a private member.
Also important to note, although you can call Dispatcher.emit() directly, it's not a good idea.
There are two main reasons to go through the action-creators:
Consistency - This is how your views and other consuming code interacts with the stores
Easier Refactoring - If you ever remove the ADD_1 action from your app, this code will throw an exception rather than silently failing by sending a message that doesn't match any of the switch statements in any of the stores
Main Advantages to this Approach
Loose coupling - Adding and removing features is a breeze. Stores can respond to any event in the system with by adding one line of code.
Less complexity - One way data flow makes wrapping head around data flow a lot easier. Less interdependencies.
Easier debugging - You can debug every change in your system with a few lines of code.
debugging example:
var MyAppDispatcher = require('MyAppDispatcher');
MyAppDispatcher.register(payload => {
console.debug(payload);
});
I need one simple thing:
var Base = function(module){
this.outsideMethod = function(arg1)
{
// run method in new context - sandbox
return vm.runInNewContext(module.insideMethod, arg1);
}
}
is something like this possible in nodejs? thx very much
If the insideMethod function does not call or use functions/vlues from outside the context it shall run in, yes.
You can convert any function in Javascript to a string.
Doing vm.runInNewContext('('+module.insideMethod+')('+JSON.stringify(arg1)+"); could be what you want.
Using simple injector with the command pattern described here and the query pattern described here. For one of the commands, I have 2 handler implementations. The first is a "normal" implementation that executes synchronously:
public class SendEmailMessageHandler
: IHandleCommands<SendEmailMessageCommand>
{
public SendEmailMessageHandler(IProcessQueries queryProcessor
, ISendMail mailSender
, ICommandEntities entities
, IUnitOfWork unitOfWork
, ILogExceptions exceptionLogger)
{
// save constructor args to private readonly fields
}
public void Handle(SendEmailMessageCommand command)
{
var emailMessageEntity = GetThisFromQueryProcessor(command);
var mailMessage = ConvertEntityToMailMessage(emailMessageEntity);
_mailSender.Send(mailMessage);
emailMessageEntity.SentOnUtc = DateTime.UtcNow;
_entities.Update(emailMessageEntity);
_unitOfWork.SaveChanges();
}
}
The other is like a command decorator, but explicitly wraps the previous class to execute the command in a separate thread:
public class SendAsyncEmailMessageHandler
: IHandleCommands<SendEmailMessageCommand>
{
public SendAsyncEmailMessageHandler(ISendMail mailSender,
ILogExceptions exceptionLogger)
{
// save constructor args to private readonly fields
}
public void Handle(SendEmailMessageCommand command)
{
var program = new SendAsyncEmailMessageProgram
(command, _mailSender, _exceptionLogger);
var thread = new Thread(program.Launch);
thread.Start();
}
private class SendAsyncEmailMessageProgram
{
internal SendAsyncEmailMessageProgram(
SendEmailMessageCommand command
, ISendMail mailSender
, ILogExceptions exceptionLogger)
{
// save constructor args to private readonly fields
}
internal void Launch()
{
// get new instances of DbContext and query processor
var uow = MyServiceLocator.Current.GetService<IUnitOfWork>();
var qp = MyServiceLocator.Current.GetService<IProcessQueries>();
var handler = new SendEmailMessageHandler(qp, _mailSender,
uow as ICommandEntities, uow, _exceptionLogger);
handler.Handle(_command);
}
}
}
For a while simpleinjector was yelling at me, telling me that it found 2 implementations of IHandleCommands<SendEmailMessageCommand>. I found that the following works, but not sure whether it is the best / optimal way. I want to explicitly register this one interface to use the Async implementation:
container.RegisterManyForOpenGeneric(typeof(IHandleCommands<>),
(type, implementations) =>
{
// register the async email handler
if (type == typeof(IHandleCommands<SendEmailMessageCommand>))
container.Register(type, implementations
.Single(i => i == typeof(SendAsyncEmailMessageHandler)));
else if (implementations.Length < 1)
throw new InvalidOperationException(string.Format(
"No implementations were found for type '{0}'.",
type.Name));
else if (implementations.Length > 1)
throw new InvalidOperationException(string.Format(
"{1} implementations were found for type '{0}'.",
type.Name, implementations.Length));
// register a single implementation (default behavior)
else
container.Register(type, implementations.Single());
}, assemblies);
My question: is this the right way, or is there something better? For example, I'd like to reuse the existing exceptions thrown by Simpleinjector for all other implementations instead of having to throw them explicitly in the callback.
Update reply to Steven's answer
I have updated my question to be more explicit. The reason I have implemented it this way is because as part of the operation, the command updates a System.Nullable<DateTime> property called SentOnUtc on a db entity after the MailMessage is successfully sent.
The ICommandEntities and IUnitOfWork are both implemented by an entity framework DbContext class.The DbContext is registered per http context, using the method described here:
container.RegisterPerWebRequest<MyDbContext>();
container.Register<IUnitOfWork>(container.GetInstance<MyDbContext>);
container.Register<IQueryEntities>(container.GetInstance<MyDbContext>);
container.Register<ICommandEntities>(container.GetInstance<MyDbContext>);
The default behavior of the RegisterPerWebRequest extension method in the simpleinjector wiki is to register a transient instance when the HttpContext is null (which it will be in the newly launched thread).
var context = HttpContext.Current;
if (context == null)
{
// No HttpContext: Let's create a transient object.
return _instanceCreator();
...
This is why the Launch method uses the service locator pattern to get a single instance of DbContext, then passes it directly to the synchronous command handler constructor. In order for the _entities.Update(emailMessageEntity) and _unitOfWork.SaveChanges() lines to work, both must be using the same DbContext instance.
NOTE: Ideally, sending the email should be handled by a separate polling worker. This command is basically a queue clearing house. The EmailMessage entities in the db already have all of the information needed to send the email. This command just grabs an unsent one from the database, sends it, then records the DateTime of the action. Such a command could be executed by polling from a different process / app, but I will not accept such an answer for this question. For now, we need to kick off this command when some kind of http request event triggers it.
There are indeed easier ways to do this. For instance, instead of registering a BatchRegistrationCallback as you did in your last code snippet, you can make use of the OpenGenericBatchRegistrationExtensions.GetTypesToRegister method. This method is used internally by the RegisterManyForOpenGeneric methods, and allows you to filter the returned types before you send them to an RegisterManyForOpenGeneric overload:
var types = OpenGenericBatchRegistrationExtensions
.GetTypesToRegister(typeof(IHandleCommands<>), assemblies)
.Where(t => !t.Name.StartsWith("SendAsync"));
container.RegisterManyForOpenGeneric(
typeof(IHandleCommands<>),
types);
But I think it would be better to make a few changes to your design. When you change your async command handler to a generic decorator, you completely remove the problem altogether. Such a generic decorator could look like this:
public class SendAsyncCommandHandlerDecorator<TCommand>
: IHandleCommands<TCommand>
{
private IHandleCommands<TCommand> decorated;
public SendAsyncCommandHandlerDecorator(
IHandleCommands<TCommand> decorated)
{
this.decorated = decorated;
}
public void Handle(TCommand command)
{
// WARNING: THIS CODE IS FLAWED!!
Task.Factory.StartNew(
() => this.decorated.Handle(command));
}
}
Note that this decorator is flawed because of reasons I'll explain later, but let's go with this for the sake of education.
Making this type generic, allows you to reuse this type for multiple commands. Because this type is generic, the RegisterManyForOpenGeneric will skip this (since it can't guess the generic type). This allows you to register the decorator as follows:
container.RegisterDecorator(
typeof(IHandleCommands<>),
typeof(SendAsyncCommandHandler<>));
In your case however, you don't want this decorator to be wrapped around all handlers (as the previous registration does). There is an RegisterDecorator overload that takes a predicate, that allows you to specify when to apply this decorator:
container.RegisterDecorator(
typeof(IHandleCommands<>),
typeof(SendAsyncCommandHandlerDecorator<>),
c => c.ServiceType == typeof(IHandleCommands<SendEmailMessageCommand>));
With this predicate applied, the SendAsyncCommandHandlerDecorator<T> will only be applied to the IHandleCommands<SendEmailMessageCommand> handler.
Another option (which I prefer) is to register a closed generic version of the SendAsyncCommandHandlerDecorator<T> version. This saves you from having to specify the predicate:
container.RegisterDecorator(
typeof(IHandleCommands<>),
typeof(SendAsyncCommandHandler<SendEmailMessageCommand>));
As I noted however, the code for the given decorator is flawed, because you should always build a new dependency graph on a new thread, and never pass on dependencies from thread to thread (which the original decorator does). More information about this in this article: How to work with dependency injection in multi-threaded applications.
So the answer is actually more complex, since this generic decorator should really be a proxy that replaces the original command handler (or possibly even a chain of decorators wrapping a handler). This proxy must be able to build up a new object graph in a new thread. This proxy would look like this:
public class SendAsyncCommandHandlerProxy<TCommand>
: IHandleCommands<TCommand>
{
Func<IHandleCommands<TCommand>> factory;
public SendAsyncCommandHandlerProxy(
Func<IHandleCommands<TCommand>> factory)
{
this.factory = factory;
}
public void Handle(TCommand command)
{
Task.Factory.StartNew(() =>
{
var handler = this.factory();
handler.Handle(command);
});
}
}
Although Simple Injector has no built-in support for resolving Func<T> factory, the RegisterDecorator methods are the exception. The reason for this is that it would be very tedious to register decorators with Func dependencies without framework support. In other words, when registering the SendAsyncCommandHandlerProxy with the RegisterDecorator method, Simple Injector will automatically inject a Func<T> delegate that can create new instances of the decorated type. Since the proxy only refences a (singleton) factory (and is stateless), we can even register it as singleton:
container.RegisterSingleDecorator(
typeof(IHandleCommands<>),
typeof(SendAsyncCommandHandlerProxy<SendEmailMessageCommand>));
Obviously, you can mix this registration with other RegisterDecorator registrations. Example:
container.RegisterManyForOpenGeneric(
typeof(IHandleCommands<>),
typeof(IHandleCommands<>).Assembly);
container.RegisterDecorator(
typeof(IHandleCommands<>),
typeof(TransactionalCommandHandlerDecorator<>));
container.RegisterSingleDecorator(
typeof(IHandleCommands<>),
typeof(SendAsyncCommandHandlerProxy<SendEmailMessageCommand>));
container.RegisterDecorator(
typeof(IHandleCommands<>),
typeof(ValidatableCommandHandlerDecorator<>));
This registration wraps any command handler with a TransactionalCommandHandlerDecorator<T>, optionally decorates it with the async proxy, and always wraps it with a ValidatableCommandHandlerDecorator<T>. This allows you to do the validation synchronously (on the same thread), and when validation succeeds, spin of handling of the command on a new thread, running in a transaction on that thread.
Since some of your dependencies are registered Per Web Request, this means that they would get a new (transient) instance an exception is thrown when there is no web request, which is they way this is implemented in the Simple Injector (as is the case when you start a new thread to run the code). As you are implementing multiple interfaces with your EF DbContext, this means Simple Injector will create a new instance for each constructor-injected interface, and as you said, this will be a problem.
You'll need to reconfigure the DbContext, since a pure Per Web Request will not do. There are several solutions, but I think the best is to make an hybrid PerWebRequest/PerLifetimeScope instance. You'll need the Per Lifetime Scope extension package for this. Also note that also is an extension package for Per Web Request, so you don't have to use any custom code. When you've done this, you can define the following registration:
container.RegisterPerWebRequest<DbContext, MyDbContext>();
container.RegisterPerLifetimeScope<IObjectContextAdapter,
MyDbContext>();
// Register as hybrid PerWebRequest / PerLifetimeScope.
container.Register<MyDbContext>(() =>
{
if (HttpContext.Current != null)
return (MyDbContext)container.GetInstance<DbContext>();
else
return (MyDbContext)container
.GetInstance<IObjectContextAdapter>();
});
UPDATE
Simple Injector 2 now has the explicit notion of lifestyles and this makes the previous registration much easier. The following registration is therefore adviced:
var hybrid = Lifestyle.CreateHybrid(
lifestyleSelector: () => HttpContext.Current != null,
trueLifestyle: new WebRequestLifestyle(),
falseLifestyle: new LifetimeScopeLifestyle());
// Register as hybrid PerWebRequest / PerLifetimeScope.
container.Register<MyDbContext, MyDbContext>(hybrid);
Since the Simple Injector only allows registering a type once (it doesn't support keyed registration), it is not possible to register a MyDbContext with both a PerWebRequest lifestyle, AND a PerLifetimeScope lifestyle. So we have to cheat a bit, so we make two registrations (one per lifestyle) and select different service types (DbContext and IObjectContextAdapter). The service type is not really important, except that MyDbContext must implement/inherit from that service type (feel free to implement dummy interfaces on your MyDbContext if this is convenient).
Besides these two registrations, we need a third registration, a mapping, that allows us to get the proper lifestyle back. This is the Register<MyDbContext> which gets the proper instance back based on whether the operation is executed inside a HTTP request or not.
Your AsyncCommandHandlerProxy will have to start a new lifetime scope, which is done as follows:
public class AsyncCommandHandlerProxy<T>
: IHandleCommands<T>
{
private readonly Func<IHandleCommands<T>> factory;
private readonly Container container;
public AsyncCommandHandlerProxy(
Func<IHandleCommands<T>> factory,
Container container)
{
this.factory = factory;
this.container = container;
}
public void Handle(T command)
{
Task.Factory.StartNew(() =>
{
using (this.container.BeginLifetimeScope())
{
var handler = this.factory();
handler.Handle(command);
}
});
}
}
Note that the container is added as dependency of the AsyncCommandHandlerProxy.
Now, any MyDbContext instance that is resolved when HttpContext.Current is null, will get a Per Lifetime Scope instance instead of a new transient instance.