How to implement asynchronous programming patterns in JavaScript

Methods to implement asynchronous programming mode in JavaScript: 1. Callback function, which is the most basic method of asynchronous programming; 2. Event listening; 3. Publishing or subscribing; 4. Promises object.

The operating environment of this tutorial: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer.

Methods to implement asynchronous programming mode in JavaScript:

1. Callback function

This is the most basic of asynchronous programming method.

Suppose there are two functions f1 and f2, and the latter waits for the execution result of the former.

The code is as follows:

f1();
f2();

If f1 is a time-consuming task, you can consider rewriting f1 and writing f2 as the callback function of f1.

The code is as follows:

　　function f1(callback){
　　　　setTimeout(function () {
　　　　　　// f1的任务代码
　　　　　　callback();
　　　　}, 1000);
　　}

The execution code becomes the following:

The code is as follows:

　　f1(f2);

In this way, we change the synchronization operation into It has become an asynchronous operation. F1 will not block the running of the program. It is equivalent to executing the main logic of the program first and postponing the execution of time-consuming operations.

The advantage of the callback function is that it is simple, easy to understand and deploy. The disadvantage is that it is not conducive to reading and maintaining the code. The various parts are highly coupled (Coupling), the process will be very confusing, and each task can only be specified A callback function.

2. Event monitoring

Another way of thinking is to use the event-driven model. The execution of a task does not depend on the order of the code, but on whether an event occurs.

Let’s take f1 and f2 as an example. First, bind an event to f1 (jQuery is used here).

The code is as follows:

　　f1.on('done', f2);

The above line of code means that when the done event occurs in f1, f2 will be executed. Then, rewrite f1:

The code is as follows:

　　function f1(){
　　　　setTimeout(function () {
　　　　　　// f1的任务代码
　　　　　　f1.trigger('done');
　　　　}, 1000);
　　}

f1.trigger('done') means that after the execution is completed, the done event will be triggered immediately, thus starting to execute f2.

The advantage of this method is that it is relatively easy to understand, can bind multiple events, each event can specify multiple callback functions, and can be "decoupled" (Decoupling), which is conducive to modularization. The disadvantage is that the entire program has to become event-driven, and the running process will become very unclear.

3. Publish/Subscribe

The "event" in the previous section can be understood as a "signal".

We assume that there is a "signal center". When a task is completed, it "publish" a signal to the signal center. Other tasks can "subscribe" to the signal center. So you know when you can start executing. This is called the "publish-subscribe pattern" (publish-subscribe pattern), also known as the "observer pattern" (observer pattern).

There are many implementations of this pattern. The one below is Ben Alman’s Tiny Pub/Sub, which is a plug-in for jQuery.

First, f2 subscribes to the "done" signal from "Signal Center" jQuery.

The code is as follows:

　　jQuery.subscribe("done", f2);

Then, f1 is rewritten as follows:

The code is as follows:

　　function f1(){
　　　　setTimeout(function () {
　　　　　　// f1的任务代码
　　　　　　jQuery.publish("done");
　　　　}, 1000);
　　}

jQuery.publish("done") means , after the execution of f1 is completed, the "done" signal is released to the "signal center" jQuery, thereby triggering the execution of f2.

In addition, after f2 completes execution, you can also unsubscribe.

The code is as follows:

　　jQuery.unsubscribe("done", f2);

The nature of this method is similar to "event listening", but it is obviously better than the latter. Because we can monitor the operation of the program by looking at the "Message Center" to see how many signals exist and how many subscribers each signal has.

4. Promises object

The Promises object is a specification proposed by the CommonJS working group to provide a unified interface for asynchronous programming.

Simply put, the idea is that each asynchronous task returns a Promise object, which has a then method that allows a callback function to be specified. For example, the callback function f2 of f1 can be written as:

The code is as follows:

　　f1().then(f2);

f1 needs to be rewritten as follows (the jQuery implementation is used here):

The code is as follows :

　　function f1(){
　　　　var dfd = $.Deferred();
　　　　setTimeout(function () {
　　　　　　// f1的任务代码
　　　　　　dfd.resolve();
　　　　}, 500);
　　　　return dfd.promise;
　　}

The advantage of writing this way is that the callback function becomes a chain writing method, the program flow can be seen clearly, and there is a complete set of supporting methods that can realize many powerful functions.

For example, specify multiple callback functions:

The code is as follows:

　　f1().then(f2).then(f3);

Another example, specify the callback function when an error occurs:

The code is as follows:

　　f1().then(f2).fail(f3);

Moreover, it has an advantage that the previous three methods do not have: if a task has been completed and a callback function is added, the callback function will be executed immediately. So you don't have to worry about missing an event or signal. The disadvantage of this method is that it is relatively difficult to write and understand.

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