I was reading Linq to Sql. Here I created a dbml file where I found that an autogenerated property has been created
[Column(Storage="_RecursionLevel", DbType="Int")]
public System.Nullable<int> RecursionLevel
{
get
{
return this._RecursionLevel;
}
set
{
if ((this._RecursionLevel != value))
{
this._RecursionLevel = value;
}
}
}
Here why if ((this._RecursionLevel != value)) line is written. What is the purpose. Why not directly assigned the value. What benefit they got
The garbage collector keeps track of memory regions that are written to since the last GC. If a region is not written to, the garbage collector can skip a lot of checks.
A simple assignment, marks the memory as dirty, even if the memory written contains the same value.
Check this article. Read from "Making Generations Work with Write Barriers" and "Too many object writes".
It's basically checking to see whether the value you are attempting to assign to the property is different from what it currently is. It will then only make the assignment if the value is different. Presumably this is an optimisation that saves unnecessary assignments when they are the same and stop any "OnChange" events firing for no good reason.
Usually with linq to sql classes (or at least when I used 'em) there's also a propertynotification event fired. Sooo I'm assuming the line firing the event is missing.
Because if the value wasn't changed, no need to change it and thus no need to fire that event.
Related
I have a situation where I need to perform dependent asynchronous operations. For example, check the database for data, if there is data, perform a database write (insert/update), if not continue without doing anything. I have written myself a promise based database API using promise-as3. Any database operation returns a promise that is resolved with the data of a read query, or with the Result object(s) of a write query. I do the following to nest promises and create one point of resolution or rejection for the entire 'initialize' operation.
public function initializeTable():Promise
{
var dfd:Deferred = new Deferred();
select("SELECT * FROM table").then(tableDefaults).then(resolveDeferred(dfd)).otherwise(errorHandler(dfd));
return dfd.promise;
}
public function tableDefaults(data:Array):Promise
{
if(!data || !data.length)
{
//defaultParams is an Object of table default fields/values.
return insert("table", defaultParams);
} else
{
var resolved:Deferred = new Deferred();
resolved.resolve(null);
return resolved.promise;
}
}
public function resolveDeferred(deferred:Deferred):Function
{
return function resolver(value:*=null):void
{
deferred.resolve(value);
}
}
public function rejectDeferred(deferred:Deferred):Function
{
return function rejector(reason:*=null):void
{
deferred.reject(reason);
}
}
My main questions:
Are there any performance issues that will arise from this? Memory leaks etc.? I've read that function variables perform poorly, but I don't see another way to nest operations so logically.
Would it be better to have say a global resolved instance that is created and resolved only once, but returned whenever we need an 'empty' promise?
EDIT:
I'm removing question 3 (Is there a better way to do this??), as it seems to be leading to opinions on the nature of promises in asynchronous programming. I meant better in the scope of promises, not asynchronicity in general. Assume you have to use this promise based API for the sake of the question.
I usually don't write those kind of opinion based answers, but here it's pretty important. Promises in AS3 = THE ROOTS OF ALL EVIL :) And I'll explain you why..
First, as BotMaster said - it's weakly typed. What this means is that you don't use AS3 properly. And the only reason this is possible is because of backwards compatibility. The true here is, that Adobe have spent thousands of times so that they can turn AS3 into strongly type OOP language. Don't stray away from that.
The second point is that Promises, at first place, are created so that poor developers can actually start doing some job in JavaScript. This is not a new design pattern or something. Actually, it has no real benefits if you know how to structure your code properly. The thing that Promises help the most, is avoiding the so called Wall of Hell. But there are other ways to fix this in a natural manner (the very very basic thing is not to write functions within functions, but on the same level, and simply check the passed result).
The most important here is the nature of Promises. Very few people know what they actually do behind the scenes. Because of the nature of JavaScript (and ECMA script at all), there is no real way to tell if a function completed properly or not. If you return false / null / undefined - they are all regular return values. The only way they could actually say "this operation failed" is by throwing an error. So every promisified method, can potentially throw an error. And each error must be handled, or otherwise your code can stop working properly. What this means, is that every single action inside Promise is within try-catch block! Every time you do absolutely basic stuff, you wrap it in try-catch. Even this block of yours:
else
{
var resolved:Deferred = new Deferred();
resolved.resolve(null);
return resolved.promise;
}
In a "regular" way, you would simply use else { return null }. But now, you create tons of objects, resolvers, rejectors, and finally - you try-catch this block.
I cannot stress more on this, but I think you are getting the point. Try-catch is extremely slow! I understand that this is not a big problem in such a simple case like the one I just mentioned, but imagine you are doing it more and on more heavy methods. You are just doing extremely slow operations, for what? Because you can write lame code and just enjoy it..
The last thing to say - there are plenty of ways to use asynchronous operations and make them work one after another. Just by googling as3 function queue I found a few. Not to say that the event-based system is so flexible, and there are even alternatives to it (using callbacks). You've got it all in your hands, and you turn to something that is created because lacking proper ways to do it otherwise.
So my sincere advise as a person worked with Flash for a decade, doing casino games in big teams, would be - don't ever try using promises in AS3. Good luck!
var dfd:Deferred = new Deferred();
select("SELECT * FROM table").then(tableDefaults).then(resolveDeferred(dfd)).otherwise(errorHandler(dfd));
return dfd.promise;
This is the The Forgotten Promise antipattern. It can instead be written as:
return select("SELECT * FROM table").then(tableDefaults);
This removes the need for the resolveDeferred and rejectDeferred functions.
var resolved:Deferred = new Deferred();
resolved.resolve(null);
return resolved.promise;
I would either extract this to another function, or use Promise.when(null). A global instance wouldn't work because it would mean than the result handlers from one call can be called for a different one.
From the performance point of view, which function definition is better/faster? Making an object and adding functions to that, or making functions one by one?
var myCollection:Object = {
first: function(variable:int):void {
},
second: function(variable:int):void {
}
}
myCollection.first(1);
or
private function first(variable:int):void {
}
private function second(variable:int):void {
}
first(1);
The latter. The performance hit will be negligible, except on a massive scale, but the second one will be slightly faster.
Basically it boils down to scope. To get a function from an object, you have to find the memory reference to the object within the scope of the class and then find the memory reference to the function within the scope of the object. With the second, you just have to find the Function object (all functions are objects) memory reference within the scope of the class.
The second method cuts out the middle man, essentially, in identifying the correct function. Now, each one will be found in less than a millisecond. As far as you are concerned, it is instant. But if you are doing this 100k times in a row? Yeah, you might see a bit of a performance boost by declaring within the class.
As an additional note, you are also adding another object to memory with the first one. Add enough of these (again, needs to be on a massive scale), and you will see a slowdown just from the superfluous objects stored in memory
You should also look at usability and readability, though. Declaring in an object means that the functions are not available as soon as the class is instantiated so you have to be careful you don't call the function before the object is instantiated. Additionally, you would lose code hinting and it is not the common way to write your code (meaning another dev, or even yourself a year from now, would have to figure out how it is working without any help from a hinter or from the standards they have already learned before they could do any modifications)
I'm learning flex/flash and I'm lost on this one. I have used the "data" in a bunch of views and it works fine. For some reason it isn't working here.
I set a field to a string here:
function LoginLoaded (e:Event):void {
trace(e.target.data);
var ServerReturn:String;
ServerReturn = new String(e.target.data);
data.UserCommonReturnData = ServerReturn;
navigator.pushView(CommonPlaces, data);
}
and here in the common places view I try to load it back:
var CommonPlacesData:String = new String();
var CurrentSelect:String = new String();
CommonPlacesData = new String(data.UserCommonReturnData);
This gives the error "Access of undefined property data" I don't get it because calling on something like data.PickUpTime (also a string) works fine in other views.
The data begins in the first view like this:
[Bindable] public var User:ObjectProxy = new ObjectProxy();
User.ID = "2314084";
navigator.pushView(TaxiNowOrLaterView, User);
then in later views I call on it like this: (works fine)
var PickUpString:String = new String(data.ID);
Any help would be great!! Thanks!!!
There are few things about your code. First, you really should make it a habit to name the identifiers as appropriate to the language. Identifiers that start with the capital letter and subsequently use minuscule letters for the rest of the logical part of the word (system alternatively known as PascalCase) is employed to only name classes. The rest of identifiers should use camelCase (similar to PascalCase, but the first letter isn't capitalized)**. This greatly reduces the effort at understanding your code. A seasoned AS3 programmer would interpret your code as follows:
// Static constant (!) ID of the class User is assigned (?) a value of "2314084"
User.ID = "2314084";
// invoke a method pushView of a local variable navigator with arguments
// of which first is the class TaxiNowOrLater, the second is the class User
navigator.pushView(TaxiNowOrLaterView, User);
while, perhaps, you didn't mean it.
new String();
In the context of AS3 makes no sense at all. Strings are never references, are immutable and have literal syntax the majority of programmers agreed upon. The code above is identical to "". In the similar way, new String(anotherString) has exact same effect as anotherString.
Your question: event.target may be many different things, some of them may have property called "data" and some may not. The general approach to this problem is that you need to cast the value of event.currentTarget or event.target to the type you expect to dispatch the event. Suppose you are expecting an event from an instance of a Button class, then:
private function clickHandler(event:MouseEvent):void {
if (Button(event.currentTarget).enabled) // do things
}
This will not protect you from an error, if the same event was dispatched by an object, which is not a button, but will make the error reporting more conscious, because it would tell you what class was it trying to cast to what other class, when it failed.
If your program logic requires that the handler be aware of events it shouldn't handle (why?) you could then write it like this:
private function clickHandler(event:MouseEvent):void {
var button:Button = event.currentTarget as Button;
if (button && button.enabled) // do things
}
event.target vs event.currentTarget - very-very rarely you would need the event.target, most of the time you need the currentTarget. I'm not saying it is wrong, but, it looks like it might be the problem. target is the object that was the first cause of the event. Events may bubble, which means they may travel the display list hierarchy up and down, first from the parent to child, then in reverse direction. In the example below, suppose there was a label on the button, which was clicked once the event was generated - in such case, even if you added a listener to the button, event.target would be the button label, not the button. currentTarget is, on the contrary, the immediate object which dispatched the event into the handler.
Few more things: ObjectProxy is an idiotic class, you probably shouldn't use at any event. It serves no purpose and, perhaps, buggy, but very few cared to discover its bugs so far. In a nutshell, what it does is as follows: it creates an object, which "watches" dynamic creation, assignment and removal of its properties and dispatches events when these events happen. This behavior is prone to a lot of errors and implicit bugs. For example, is foo.bar = "baz"; foo.bar = "baz"; a reassignment of the same property or not? Is foo.bar.baz = "fizzbuzz"; a modification of foo.bar? What if property name is not a sting? And so on.
Why you shouldn't use it: there is always a better way to treat your data. A more transparent, easier to debug, more efficient. This class is a prototype, which had never really worked. Besides having the behavior described above, it is huge in terms of lines of code that were used to write it. Debugging the errors that happen inside of it requires a lot of time and patience, which are certainly not worth it.
If you need an object to represent a user, define a class, with properties you expect the user to have and just use that class - this will make debugging and understanding your code much easier.
[Bindable] metadata is evil. I can't tell you you should avoid it, because it is used too often, but, I will. You should avoid it as much as possible. I didn't yet encounter an instance of when the use of this meta would be justified. Much in the similar spirit as ObjectProxy it is a prototype, something designed for lazy programmers w/o much consideration for either performance or corner cases. This is, again, a source of a lot of implicit bugs, typically difficult to spot due to the code generated around this meta "swallowing" the errors. The alternative to this meta is plain addEventListener(...) code with custom events.
Unfortunately, a lot of tutorial will use this kind of code to have you quickly started with the language + framework...
** there are some exceptions to this rule: constants are all upper-case and namespace names use underscores to separate the logical part of the word, but never use uppercase letters.
I have a class that requires some of its methods to be called in a specific order. If these methods are called out of order then the object will stop working correctly. There are a few asserts in the methods to ensure that the object is in a valid state. What naming conventions could I use to communicate to the next person to read the code that these methods need to be called in a specific order?
It would be possible to turn this into one huge method, but huge methods are a great way to create problems. (There are a 2 methods than can trigger this sequence so 1 huge method would also result in duplication.)
It would be possible to write comments that explain that the methods need to be called in order but comments are less useful then clearly named methods.
Any suggestions?
Is it possible to refactor so (at least some of) the state from the first function is passed as a paramter to the second function, then it's impossible to avoid?
Otherwise, if you have comments and asserts, you're doing quite well.
However, "It would be possible to turn this into one huge method" makes it sound like the outside code doesn't need to access the intermediate state in any way. If so, why not just make one public method, which calls several private methods successively? Something like:
FroblicateWeazel() {
// Need to be in this order:
FroblicateWeazel_Init();
FroblicateWeazel_PerformCals();
FroblicateWeazel_OutputCalcs();
FroblicateWeazel_Cleanup();
}
That's not perfect, but if the order is centralised to that one function, it's fairly easy to see what order they should come in.
Message digest and encryption/decryption routines often have an _init() method to set things up, an _update() to add new data, and a _final() to return final results and tear things back down again.
I have a class which is called a number of times. When the application goes to the next stage these all have to be unloaded. Because of that, I created an unload() method in the class.
The problem is that I can't seem to set my uint variable "charId" to null in order to "unset" it. The "delete" command is not possible either as that is only applicable for dynamic variables or something in that kind of way.
Now I wonder, how am I supposed to unset this variable, so it's memory will be re-allocated later on?
The class's unload method:
public function unload():void
{
trace("Unloading character with charname '" + charName + "'.");
enterButton.removeEventListener(MouseEvent.CLICK, enterClicked);
removeChild(enterButton);
enterButton = null;
charName = null;
charId = null; //this is possible but not recommended - what's a better way?
lobbyInterface = null;
}
So yeah, it's practically possible as it changes the variable type - however it's not recommended and raising a warning. So, what's a better way to do it?
Note that this object is also unloaded in it's parent. Does that also free all these variables from memory?
uint, int, Number and Boolean are not nullable in AS3. Number can be NaN, but that is really the best you can get. int and uint are always just 32 bit, so you can't stuff a null-reference in there.
The type of cleanup you are trying to do cannot be accomplished since AS3 has the concept of sealed classes. A sealed class has a fixed size in memory. When it comes to instance variables, think of it as a C struct, you can only dump all of it, or nothing. You can do anything in C of course, it's a fixed block in memory, an entity of one reference per variable.
What you want to do is only work with dynamic variables, which are maintained differently.
You don't need to do this sort of cleanup since Flash has garbage collection like most runtimes nowadays. It also deals with nested and circular references, the only thing you have to be sure about is, that you delete any "outer" references to that class. Things that are generally not collected are objects on the display list, running timers and intervals, and I/O related stuff. As soon as you have a reference chain from there to your object, it will not be collected.
Let us say you have an object A with an event handler for a mouse movement on an object on some list, referencing an object B. B will not be collected, but as soon as there is no chain leading to an object, it will be collected (sooner or later, the GC is quite lazy. But the more memory you use, the more it does its work).