One of the features of the method based on the definition of Beckoff site is that:
All data of a method are temporary and are only valid while the method
is executed (stack variables). This means that TwinCAT re-initializes
all variables and function blocks, which you have declared in a
method, with each call of the method.
Is there any way to use a method in the plc loop as warm start!
it means that we use the method without re_initializing and method declare variations run just once at the first time we call it and the rest of the time that is called the variables retain their own values?
Yes, this is possible through VAR_INST or VAR_STAT.
Just declare your variables as VAR_INST/VAR_STAT instead, then they will retain their values between the calls.
VAR_INST means it will be unique for every instantiation of the function block of where the method resides, while VAR_STAT will act as a static/global (all instances will point to the same memory location).
https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_plc_intro/2528798091.html&id=
https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_plc_intro/2528787339.html&id=
Related
It seems like there are two styles for writing NEAR smart contracts in assembly script
Bag of functions like Meme Museum
Singleton style like Lottery.
I was wondering under what circumstance one style is recommended over the other.
When should you use one over the other? What are the advantages/disadvantages of each style?
The big differences is in initialization of the contract.
Bag of Functions (BoF)
For the first style, usually a persistent collection is declared at the top level. All top level code in each file is placed into a function and one start function calls each. The Wasm binary format allows to specify the start function so that anytime the binary is instantiated (e.i. loaded by a runtime) that function is called before any exported functions can be called.
For persistent collections this means allocating the objects, but no data is required from storage, until something is read, including the length or size of the collection.
Singleton
A top level instance of the contract is declared. Then storage is checked for the "STATE" key, which contains the state of the instance. If it is present storage is read and the instance is deserialized from storage. Then each "method" of the contract is an exported function that uses the global instance, e.g. instance.method(...), passing the arguments to the method. If the method is decorated with #mutateState then the instance is written back to storage after the method call.
Which To Use
Singleton's provide a nice interface for the contract and is generally easier to understand. However, since you are reading from storage at every method call, it can be more expensive.
The other advantage of the BoF is it's easier to extend as you can export more functions from a dependency.
However, it's also possible to use a combination of both as well.
So really it's whatever makes sense to you.
Sandy Metz says (POODR book, page 26):
Because it is possible to wrap every instance variable in a method and to therefore treat any variable as if it's just another object, the distinction between data and a regular object begins to disappear.
I am not sure if I understand what she is explaining. When we define the accessors, we are wrapping the instance variables (data) on a method but methods are not objects. So what does she mean when she says that we can treat variables as if they're just another object?
The primary difference between data and objects is behaviour. Objects can modify their internal state without changing their interfaces, while data are static structures.
When we wrap data access within a method, we get the same benefits of an object - the interface remains static to consumers even if the underlying data structure needs to change.
I'm not sure what's expected for me to leave here, but basically, I've passed an object of type AwesomeMenu into an ActionControl object's (subclass of NSOBject) initializer class so that the ActionControl object has a reference to the AwesomeMenu. However, in one of the ActionControl functions, there is a call like
[self.menu updateButton];
Where self.menu is the AwesomeMenu and updateButton is a function within AwesomeMenu. For some reason, XCode never enters updateButton. I've tried setting up breakpoints inside updateButton. They don't ever trip. I tried stepping INTO updateButton (it just shows me the parameters and then it skips past the line without taking me into the function), etc. I don't get any errors either. My chosen path through the program takes me over that function call multiple times but it never actually calls.
What's happening?
I did not assign self.menu prior to calling one of its functions; self.menu's value was nil. I just had to switch my initialization statements around to get it to work.
I am using FxCop to look for improvements on our application. One of the rules we are often breaking is "Mark members as static" which is marked as a performance rule.
Certainly we have a lot of class methods that do not action on any of the class members, which could be marked as static, but is there really a performance gain to this?
My understanding is that static will be intantiated once at execution time. If the method is never invoked that it would have been a waste. If the method is invoked multiple times than there might be a small benefit.
With variables there are obvious implications as to whether or not they are marked static, and it is critical to the operation of your application how they are defined. For methods though I don't believe there is any functional affect on whether or not they are marked static if they do not reference any instance variables or methods.
Am I missing the point here? Is the standard to mark all of these methods as static?
Performance becomes better because static method doesn't have hidden "this" pointer.
Every instance (non-static) method has hidden "this" pointer, which is passed to the method to access instance members. If no non-static members are used, "this" pointer remains unused. Passing additional parameter on the stack or in CPU register takes a time which can be saved by declaring such method as static.
"My understanding is that static will be instantiated once at execution time."
Both static and non-static methods exist only once in the program code. Only non-staic data members are duplicated, when there are different class instances. Non-static class method works with specific instance using class reference (hidden parameter). Code itself is never duplicated.
As you said, when a method is marked as static, it is instantiated once, the first time it is used. Whenever subsequent calls are made, the runtime environment doesn't need to validate it, because it is guaranteed to exist. Here is the Microsoft doc for it: http://msdn.microsoft.com/en-us/library/ms245046%28v=vs.80%29.aspx
Here's some more guidance: Method can be made static, but should it?
I've written a very simple GUI in MATLAB that will convert temperatures. It is meant to serve as a tutorial for a class of students. A strange thing has happened though. As with any MVC design pattern, there is a model object, a view object and a controller function. In order to set the output field of the GUI (the converted temperature), you can run this line in the controller function itself:
set(views.outputTextField,'string',num2str(round(model.outTemp)));
where views.outputTextField is a GUI text field to display the converted temperature and model.outTemp is the converted temperature. Pretty straightforward. The views object has references to all the GUI uicontrols and this updates the field with the newly converted temperature in the model object.
However, I would rather have view functions in the view object, so I attempted to create a line like this:
views.updateOutputField = #()set(views.outputTextField,'string',...
num2str(round(model.outTemp)));
Same line as before, just that now it is an anonymous function in the view object. This way I could call the function from the controllers as simply views.updateOutputField(); and keep the view logic out of the controller logic. But this method won't work! (It will work with the get() function.)
Instead I have to do the following:
views.updateOutputField = #updateOutputField
function updateOutputField()
set(views.outputTextField,'string',num2str(round(model.outTemp)));
end
By separating out the function (redundantly) instead of just using an anonymous function, it works again. What!? This makes no sense to me. The view and model objects are global and the anonymous function works with get(). Does anyone have a clue what's going on here?
Both approaches are not equivalent. Values in the body of anonymous function (aka lambda) are being frozen, see example below:
>> ii = 2;
>> val = #() ii+2;
>> val()
ans =
4
>> ii=5;
>> val()
ans =
4
You can do following to make it work:
views.updateOutputField = #(outTemp) ...
If you want to know how MATLAB captures the workspace context, use function FUNCTIONS on anonymous function.
Your example is a little bit more complicated because your view and model exist in the nested workspace but the essence is the same.
As side note: kudos for teaching also an important design pattern (MVC) in Matlab class!
Mikhail has the right answer. I'll elaborate a bit...
From the MATLAB documentation for anonymous functions:
Anonymous functions commonly include
two types of variables:
Variables specified in the argument
list. These often vary with each
function call.
Variables specified in the body of the
expression. MATLAB captures these
variables and holds them constant
throughout the lifetime of the
function handle.
When you make a call to SET inside your anonymous function, you access fields of your two structure variables views and model. These values are held fixed at what they were when the anonymous function was created. That doesn't matter for the graphics handles stored in views, since these never change (unless you are deleting and recreating graphics objects). This is why calling GET in your anonymous function works fine, since it only uses the unchanged graphics handles in views. However, the values in model change, so you would want to pass them in to the anonymous function as an argument. For example:
views.updateOutputField = #(model) set(views.outputTextField,'String',...
num2str(round(model.outTemp)));
When you instead create your updateOutputField function, you are creating a nested function. Nested functions have access to the variables that exist in the outer function within which they are nested, which is why you don't have to pass views and model as arguments. When these variables change in the outer function, that change is visible in the nested function as well (unlike anonymous functions).