I know that a block is a reusable chunk of executable code in Objective-C. Is there a reason I shouldn't put that same chunk of code in a function and just called the function when I need that code to run?
It depends on what you're trying to accomplish. One of the cool things about blocks is that they capture local scope. You can achieve the same end result with a function, but you end up having to do something like pass around a context object full of relevant values. With a block, you can do this:
int num1 = 42;
void (^myBlock)(void) = ^{
NSLog(#"num1 is %d", num1);
};
num1 = 0; // Changed after block is created
// Sometime later, in a different scope
myBlock(); // num1 is 42
So simply by using the variable num1, its value at the time myBlock was defined is captured.
From Apple's documentation:
Blocks are a useful alternative to traditional callback functions for
two main reasons:
They allow you to write code at the point of invocation that is
executed later in the context of the method implementation. Blocks are
thus often parameters of framework methods.
They allow access to local variables. Rather than using callbacks
requiring a data structure that embodies all the contextual
information you need to perform an operation, you simply access local
variables directly.
As Brad Larson comments in response to this answer:
Blocks will let you define actions that take place in response to an
event, but rather than have you write a separate method or function,
they allow you to write the handling code right where you set up the
listener for that event. This can save a mess of code and make your
application much more organized.
A good example of which i can give you is of alert view, it will be good if i decided at time of creation of alert view what will happen when i dismiss that instead i write the delegate method and wait for that to call. So it will be much easier to understand and implement and also it provides fast processing.
Related
I think the correct description for what I'm trying to do is be able to pass an expression or function/handler into another handler as a parameter/argument. Some code to be evaluated inside the receiving handler. Similar to Javascript callbacks, I think.
For example, something like this:
on waitFor(theConditionExpression)
timeout_start(5) -- start a 5 second timer
repeat until (theConditionExpression or timeout_isExpired())
delay 0.1
end repeat
return theConditionExpression
end waitFor
theConditionExpression should be some expression or function that evaluates to a boolean result.
not really relevant to the question, but just FYI, timeout_start(…) and timeout_isExpired() are two simple handlers I've written that do exactly what they say. (…start() doesn't return anything, …isExpired() returns a boolean).
Of course, typically if I pass in some boolean expression, it will evaluate that expression once, at the time I pass it in. But I want it to evaluate it every time it's referenced in the code inside the handler.
Some languages (not sure about AS) have some kind of eval() function that you can pass it some code as a string and it will execute that string as code. Theoretically that could solve this, but: (a) I don't know if AS has anything like that, but even if it does, (b) it's not desired for various reasons (performance, injection risks, etc.)
So I'm thinking something more like eg. JavaScript's ability to pass in a function (named or anonymous) as function parameter/argument that can be re-evaluated every iteration in a loop, etc. (eg. like the compareFn argument in JS's Array.sort(compareFn)).
Can AS do anything like this, and if so how?
Thanks!
I'm going to suggest (pro forma) that an AppleScript application with an on idle handler is generally a better solution for wait conditions than a repeat/delay loop. It's more efficient for the system, and doesn't freeze up the script. But that would involve reconceptualizing your script, and I'm not certain it would work in this case, given the way you formed the problem.
There's an old but good site called AppleScript Power Handlers that shows a bunch of nifty-neato tricks for sophisticated use of AppleScript handlers: passing handlers as values or parameters; creating Script Objects within handlers; making closures and constructors. I'm pretty sure the answer to your request is in there. aLikely you'll want to set up a bunch of handlers that serve as condition expressions, then pass them as parameters to the evaluating handler. Or maybe you'll want to set up a script object containing the condition handlers and call it as needed?
At any rate, see what you can do with it, and ask more specific questions if you run into problems.
First, I have had a look at this excellent article already.
I have a MATLAB script, called sdp. I have another MATLAB script called track. I run track after sdp, as track uses some of the outputs from sdp. To run track I need to call a function called action many many times. I have action defined as a function in a separate MATLAB file. Each call of this action has some inputs, say x1,x2,x3, but x2,x3are just "data" which will never change. They were the same in sdp, same in track, and will remain the same in action. Here, x2,x3 are huge matrices. And there are many of them (think like x2,x3,...x10)
The lame way is to define x2,x3 as global in sdp and then in track, so I can call action with only x1. But this slows down my performance incredibly. How can I call action again and again with only x1 such that it remembers what x2,x3 are? Each call is very fast, and if I do this inline for example, it is super fast.
Perhaps I can use some persistent variables. But I don't understand exactly if they are applicable to my example. I don't know how to use them exactly either.
Have a look at object oriented programming in Matlab. Make an action object where you assign the member variables x2 ... to the results from sdp. You can then call a method of action with only x1. Think of the object as a function with state, where the state information in your case are the constant results of sdp.
Another way to do this would be to use a functional approach where you pass action to track as a function handle, where it can operate on the variables of track.
Passing large matrices in MATLAB is efficient. Semantically it uses call-by-value, but it's implemented as call-by-reference until modified. Wrap all the unchanging parameters in a struct of parameters and pass it around.
params.x2 = 1;
params.x3 = [17 39];
params.minimum_velocity = 19;
action('advance', params);
You've already discovered that globals don't perform well. Don't worry about the syntactic sugar of hiding variables somewhere... there are advantages to clearly seeing where the inputs come from, and performance will be good.
This approach also makes it easy to add new data members, or even auxiliary metadata, like a description of the run, the time it was executed, etc. The structs can be combined into arrays to describe multiple runs with different parameters.
By reading some text, especially the iOS document about delegate, all the protocol method are called hook that the custom delegate object need to implement. But some other books, name these hook as callback, what is the difference between them? Are they just different name but the same mechanism? In addition to Obj-C, some other programming languages, such as C, also got the hook, same situation with Obj-C?
The terminology here is a bit fuzzy. In general the two attempt to achieve similar results.
In general, a callback is a function (or delegate) that you register with the API to be called at the appropriate time in the flow of processing (e.g to notify you that the processing is at a certain stage)
A hook traditionally means something a bit more general that serves the purpose of modifying calls to the API (e.g. modify the passed parameters, monitor the called functions). In this meaning it is usually much lower level than what can be achieved by higher-level languages like Java.
In the context of iOS, the word hook means the exact same thing as callback above
Let me chime in with a Javascript answer. In Javascript, callbacks, hooks and events are all used. In this order, they are each higher level concepts than the other.
Unfortunately, they are often used improperly which leads to confusion.
Callbacks
From a control flow perspective, a callback is a function, usually given as an argument, that you execute before returning from your function.
This is usually used in asynchoronous situations when you need to wait for I/O (e.g. HTTP request, a file read, a database query etc.). You don't want to wait with a synchronous while loop, so other functions can be executed in the meantime.
When you get your data, you (permanently) relinquish control and call the callback with the result.
function myFunc(someArg, callback) {
// ...
callback(error, result);
}
Because the callback function may be some code that hasn't been executed yet, and you don't know what's above your function in the call stack, generally instead of throwing errors you pass on the error to the callback as an argument. There are error-first and result-first callback conventions.
Mostly callbacks have been replaced by Promises in the Javascript world and since ES2017+, you can natively use async/await to get rid of callback-rich spaghetti code and make asynchronous control flow look like it was synchronous.
Sometimes, in special cascading control flows you run callbacks in the middle of the function. E.g. in Koa (web server) middleware or Redux middleware you run next() which returns after all the other middlewares in the stack have been run.
Hooks
Hooks are not really a well-defined term, but in Javascript practice, you provide hooks when you want a client (API/library user, child classes etc.) to take optional actions at well-defined points in your control flow.
So a hook may be some function (given as e.g. an argument or a class method) that you call at a certain point e.g. during a database update:
data = beforeUpdate(data);
// ...update
afterUpdate(result);
Usually the point is that:
Hooks can be optional
Hooks usually are waited for i.e. they are there to modify some data
There is at most one function called per hook (contrary to events)
React makes use of hooks in its Hooks API, and they - quoting their definition - "are functions that let you “hook into” React state and lifecycle features", i.e. they let you change React state and also run custom functions each time when certain parts of the state change.
Events
In Javascript, events are emitted at certain points in time, and clients can subscribe to them. The functions that are called when an event happens are called listeners - or for added confusion, callbacks. I prefer to shun the term "callback" for this, and use the term "listener" instead.
This is also a generic OOP pattern.
In front-end there's a DOM interface for events, in node.js you have the EventEmitter interface. A sophisticated asynchronous version is implemented in ReactiveX.
Properties of events:
There may be multiple listeners/callbacks subscribed (to be executed) for the same event.
They usually don't receive a callback, only some event information and are run synchronously
Generally, and unlike hooks, they are not for modifying data inside the event emitter's control flow. The emitter doesn't care 'if there is anybody listening'. It just calls the listeners with the event data and then continues right away.
Examples: events happen when a data stream starts or ends, a user clicks on a button or modifies an input field.
The two term are very similar and are sometimes used interchangably. A hook is an option in a library were the user code can link a function to change the behavior of the library. The library function need not run concurrent with the user code; as in a destructor.
A callback is a specific type of hook where the user code is going to initiate the library call, usually an I/O call or GUI call, which gives contol over to the kernel or GUI subsystem. The controlling process then 'calls back' the user code on an interupt or signal so the user code can supply the handler.
Historically, I've seen hook used for interupt handlers and callback used for GUI event handlers. I also see hook used when the routine is to be static linked and callback used in dynamic code.
Two great answers already, but I wanted to throw in one more piece of evidence the terms "hook" and "callback" are the same, and can be used interchangeably: FreeRTOS favors the term "hook" but recognizes "callback" as an equivalent term, when they say:
The idle task can optionally call an application defined hook (or callback) function - the idle hook.
The tick interrupt can optionally call an application defined hook (or callback) function - the tick hook.
The memory allocation schemes implemented by heap_1.c, heap_2.c, heap_3.c, heap_4.c and heap_5.c can optionally include a malloc() failure hook (or callback) function that can be configured to get called if pvPortMalloc() ever returns NULL.
Source: https://www.freertos.org/a00016.html
I am pretty new to iPhone development. i need some help on how to synchronize a callback method and a for loop.
For example:
I have a for loop say 1 to 3.
Within this loop, first i send message to a receiver. The result from the receiver is obtained in a callback function. With this result i need to perform some parsing. Now how can i continue with the loop??
BR,
Suppi
Edited with Code:
-(void)requestData{
for (int i=1; i<3; i++) {
completeMessage = [self generateMessage:message];
[self sendMessageToReceiver:completeMessage];
//now it goes to the callback function to read message from receiver. How do i return to this point?? to continue the loop.
[self dosomething:result];
}
}
I don't know much about iPhone development but based on my asynchronous function calling experience you might have to reconsider your approach - assuming this is an asynchronous function call.
When you go through the loop the first time, your code is going to call all the asynchronous functions and move on. It is not going to wait. If you want it to wait for each function call then you either shouldn't use asynchronous functions or use a thread.wait or thread.sleep function in the loop. You could also use some kind of thread synchronization and signalling in the loop. For example, you could make the asynchronous call and then your thread waits until it gets a signal from your callback to continue.
You may want to take your custom end processing out of the loop and do it after all your callbacks are done. You could put state in a common location for each of your callbacks and use it after the callbacks are done.
Of course, you would need to wait until all the callbacks are done before you can continue.
Hope this helps.
Launch the message in a separate thread:
[receiver performSelectorInBackground:#selector(doSomething)];
use performSelectorInBackground:withObject: if you wish to pass a parameter.
Convert your "for" loop into the equivalent goto statements. Then break the goto basic blocks into methods and method calls without goto's. Then break the method containing the wait into 2 methods and use an asynchronous call and callback in between them. You may have to save some of the local and for loop's implicit state in instance variables.
Goto's are not always bad. They are just implicit in more readable structured and/or OOP messaging constructs. Sometimes the compiler can't do the conversion for you, so you need to know enough about raw program control sequencing to do it yourself.
Is it a function?
Is it a function being called from the source?
Or, is it a function being returned from the destination?
Or, is it just executing a function at the destination?
Or, is it a value returned from a function passed to the destination?
A callback is the building block of asynchronous processing.
Think of it this way: when you call someone and they don't answer, you leave a message and your phone number. Later on, the person calls you back based on the phone number you left.
A callback works in a similar manner.
You ask an API for a long running operation and you provide a method from within your code to be called with the result of the operation. The API does its work and when the result is ready, it calls your callback method.
From the great Wikipedia:
In computer programming, a callback is
executable code that is passed as an
argument to other code. It allows a
lower-level software layer to call a
subroutine (or function) defined in a
higher-level layer.
Said another way, when you pass a callback to your method, it's as if you are providing additional instructions (e.g., what you should do next). An attempt at making a simple human example follows:
Paint this wall this shade of green (where "paint" is analagous to the method called, while "wall" and "green" are similar to arguments).
When you have finished painting, call me at this number to let me know that you're done and I'll tell you what to do next.
In terms of practical applications, one place where you will sometimes see callbacks is in situations with asynchronous message passing. You might want to register a particular message as an item of interest for class B.
However, without something like a callback, there's no obvious way for class A to know that class B has received the message. With a callback, you can tell class B, here's the message that I want you to listen for and this is the method in class A that I want you to call when you receive it.
Here is a Java example of a callback from a related question.