The previous section explained the thread scheduling, and gave two practical examples, including a login example, I wonder if you have thought about such a problem, if a new user must register first, and then automatic login after the registration is successful.
Obviously, this is a nested network request, first need to request registration, after successful registration callback to request login interface.
Of course we can take it for granted:
(Actually can write such code is also very good, at least not written together…)
Such code works, but is not elegant, and this section will allow us to solve this problem in a more elegant way.
To the chase
Let’s start with the simplest transformation operator, Map
Map
Map is one of the simplest transformation operators in RxJava. Its function is to apply a function to each event sent upstream, making each event change according to the specified function. As shown in the event graph:
The map function converts a circular event to a rectangular event, resulting in a rectangular event received downstream. To represent this example in code:
Upstream we send numbers, downstream we receive strings, and in the middle we convert to the map operator. The result is:
With Map, you can convert events sent from upstream to any type, either an Object or a collection. Wouldn’t you like to try such a powerful operator?
Next, let’s look at another well-known operator, flatMap.
FlatMap
FlatMap is a very powerful operator, first illustrated with a rather esoteric concept:
FlatMap transforms an upstream Observables that send events into multiple Observables that send events, and then merges their emitted events into a single Observable.
This sentence is more difficult to understand, let’s first easy to understand the picture to explain in detail, first of all, to look at a picture of the whole:
Let’s look at upstream. Upstream sends three events, one, two, three, and notice the colors.
The middle flatMap converts circular events into a new upstream Observable that sends rectangular and triangular events.
Still can’t understand? Don’t worry, let’s look at the breakdown:
This is very easy to understand!!
For each event sent upstream, flatMap creates a new water pipe, then sends the new event after the transformation, and the downstream receives the data sent by these new water pipes. It is important to note here that flatMap does not guarantee the order of events, as seen in the figure, that event 1 precedes event 2. If you want to ensure order, use concatMap.
Having said that, let’s look at how code is written in practice:
As shown in the code, we convert each event sent upstream into a new pipe that sends three String events in the flatMap. To see that the flatMap result is unordered, we add a delay of 10 milliseconds. Let’s see the result:
And the results do confirm what we said before.
ConcatMap is exactly the same as flatMap, except that its results are sent in exactly the same order as those sent upstream.
Just change the previous flatMap to concatMap and leave the rest unchanged. The result is as follows:
As you can see, the result is still in order.
RxJava has a large number of built-in operators. Map and flatMap are just a primer for other operators. As long as you follow the ideas of this article to understand, and then carefully read the documentation, There should be no problem, if you need to list the operators that need to be explained, I can explain it according to your needs.
practice
By learning the FlatMap operator, we can answer the question posed at the beginning of this article.
How to elegantly resolve nested requests is to use flatMap transformations.
Let’s review the request interface from the previous section:
As you can see, both login and registration return an upstream Observable, and the function of our flatMap operator is to convert one Observable to another, so the result is obvious:
This example also shows how easy it is to switch threads.
That’s it for this tutorial. In the next section we will learn about Flowable and understand the concept of Backpressure. Stay tuned.
