1. What is a callback function?
A callback function is essentially a function (put aside the dispute between functions and methods, just take these two things seriously). It consists of three parts: "declaration", "implementation" and "call".
In the above example, I can see that the declaration and call of the function amount (actually Block) is in Class A, while the implementation part is in Class B. In other words, Class B implements the amount function, but does not have permission to call it, and it is eventually triggered by Class A. We call this mechanism "callback". It means "Although the implementation of a function is written in Class B, the real call still needs to be completed by Class A." Normal function "function declaration and implementation are completed in one class."
In plain language, I understand the concept of "callback": "Although the implementation part of the function is not in my hometown (Class A), the final call is still completed by my hometown." Such a function is called a callback function. "If your hometown calls you, it will callback because you originally belong to your hometown.
Understand the concept of "callback function" using "Infernal Affairs":
* Police Force (Category):
A policeman Zhang San (declares a function), and trained and trained (implemented the function).
A policeman, Chen Rengui (declaration function), was recruited, but he was not trained, but was sent to the triad. But when there are tasks, the police will call Chen Rengui (callback function).
Independent Commission Against Corruption (Class): Use Zhang San from the Police Department (ordinary call).
Triads (category): Cultivate and train Chen Rengui (implement functions).
The second question: Under what circumstances should the callback function be used?
Suppose there are two categories A and B.
(1) Class A has many forms, and the callback function should be implemented in Class B. If Class A is a network request open source class ASIHttpRequest, it may succeed in the request or fail in the request. At this time, Class B needs to deal with different treatments based on the above two situations.
(2) When the form of Class A is determined by Class B, the callback function should be implemented in Class B. For example, the UITableView class will provide many callback functions (iOS professional term is called "delegate" method)
(3) When Class A needs to pass data to Class B, callback functions can be implemented in Class B (Class A is generally a time-consuming operation class for the data layer). As shown in the example of paying wages. In actual programming, one advantage of this mechanism is that it can improve the user's operating experience. For example, if a user jumps from page X to page Y and needs to request data from the network, and it is time-consuming, what should we do? There are three solutions: the first is to display a rotation indicator on page X, and when data sent back from the network is received, the Y page is displayed. The second type is to use a callback function. The user jumps directly from the X page to the Y page. The Y page needs to go to the data layer for execution. When the data is received, it will be displayed on the Y page. The third type is to enable multi-threading in the Y page. Let a child thread go to the background to fetch data. Overall, the second type is more simple and easy to understand, and the code is compact.
The third question: What are the benefits of using callback functions?
(1) The implementer can perform different processing and operations according to the various forms of the callback party. (ASIHttpRequest)
(2) The implementer can customize the different forms of the callback party according to his or her needs. (UITableView)
(3) Time-consuming operations can be hidden in the callback party, and does not affect the display of other information of the implementer.
(4) Make the logic of the code more concentrated and easier to read.
What is a callback function? ——The function called by the class that declares the function is called a callback function. A normal function can make any class call.
Who is the subject of "callback"? ——The class that declares the "callback function".
Block, delegation, notification, and callback functions. Although their names are different, their principles are the same. They are all concrete implementations of the idea of "callback mechanism"!
When iOS 10, NSTimer added a new API with block:
Apple's official documentation says that passing this timer itself as a parameter to the block to avoid circular references:
/// - parameter: block The execution body of the timer; the timer itself is passed as the parameter to this block when executed to aid in avoiding cyclical references
With this API, you no longer need to manually log out the timer. Combined with weakSelf, you can easily handle circular references, such as:
__weak typeof(self) weakSelf = self;
= [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
__strong typeof(self) strongSelf = weakSelf;
[strongSelf printNum];
}];
Before this API appeared, the reference relationship between self and timer was: self->timer->self
The current reference relationship is: self->timer->weakSelf
However, only systems in iOS 10 and beyond can use this API, and we generally adapt to iOS 8, so it is necessary to expand.
How to expand?
To be simple, write a category, copy Apple's API directly (you save time thinking about API design 😎), and then add the prefix:
+ (NSTimer *)cq_scheduledTimerWithTimeInterval:(NSTimeInterval)interval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block {
return [self scheduledTimerWithTimeInterval:interval target:self selector:@selector(cq_callBlock:) userInfo:[block copy] repeats:repeats];
}
+ (void)cq_callBlock:(NSTimer *)timer {
void (^block)(NSTimer *timer) = ;
!block ?: block(timer);
}
Didn't you pass the timer as a parameter to the block? Then I will do the same.
Then you can use it like using the system API:
__weak typeof(self) weakSelf = self;
= [NSTimer cq_scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer *timer) {
__strong typeof(self) strongSelf = weakSelf;
[strongSelf printNum];
}];
Finally, provide a specific demo used by this timer:/CaiWanFeng/CQCountDownButton
The above is all the content of this article. I hope it will be helpful to everyone's study and I hope everyone will support me more.