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Hey buddy this is Caleb with envelopes dot com and and this video we're going to continue talking about

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subjects in ARC swift specifically the behavior subject.

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Now it's different than the published subject which is what we just talked about and to show you the

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differences we're going to go ahead and pull open your ex code playground with our our ex capabilities

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and we're going to go ahead and create a new playground page but we're going to copy these top three

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lines because we're going to need them anyway so go ahead and copy those.

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And right click on publish subject click new playground page and go ahead and name it.

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Behavior subject subject.

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Excellent.

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And select one of the numbered lines press command a and delete everything then paste in our imports

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as well as the playground page needs indefinite execution.

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Now to make sure that it's running properly you might need to build it and clean it.

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And let's just make sure that we have x.

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We don't yet.

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So it looks like it's still having some trouble here.

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So what I'm going to do is I'm going to close out of this and open it again just like so and we'll see

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if that will let us use our ax.

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Kate still saying no such modules so I'm going to build it and see if that fixes our problem and makes

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our error go away.

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All right.

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So we have now successfully built.

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Let's see if when we run the playground if that error goes away.

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OK looks like it may have let's double check by try and create an observable of type observable.

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There we go.

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OK so now we have access.

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So just if you have that same problem you can go ahead and close out of your project and reopen it and

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then build it again.

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And that should solve your problem.

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So like I said we're going to be talking about behavior subjects.

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OK.

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So let's create one var behavior.

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Subject equals behavior subject.

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Now something you need to know is that a behavior subject needs an initial value.

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But before we do that we're going to define behaviors subject now a behavior subject is going to basically

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give the subscriber The most recent element and everything that's emitted after that.

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So unlike the published subject which only gives you which only gives you elements that are added after

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the subscription a behavior subject is going to give you the most recent value as well as everything

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that follows.

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So I do see that I spelled this wrong so change that behavior subject and in order to create one of

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these you need to go ahead and put some parentheses and we're going to pass it a value.

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Now this can be a value of anything but we're just going to make it of type string.

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And I'm going to call this value a.

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OK.

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So we now have a behavior subject with an initial value of a string called value.

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Now what we can do is we can create a subscription.

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Now what we can do is we can create a subscription for our behavior subject and then we can print out

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our first value just like we have been.

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So go ahead and type let subscription.

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I'll just call this subscription one.

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And that's going to be equal to behaviour's subject and we need to subscribe to that so that we can

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actually use the values that are passed into our observable stream.

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So go ahead and type subscribe and we're going to use on next steps with a lower case.

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So on next and give it some curly brackets and inside the closure we're going to print out subscription

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one and we're going to print out the dollar sign zero value which is our temporary variable as we're

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cycling through all of the values.

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As you can see value is called When we use this temporary variable and look at that it prints out subscription

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1 value.

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Pretty cool.

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Now if this were a published subject we would not have access to this because a published subject does

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not require an initial value and it only pays attention to values that are given after the behavior

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or subject has been subscribed.

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So let's go ahead let's add a new value here.

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We're going to say behavior subject dot on next.

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And as you can see we can pass in a new element of type String.

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Let's call this value B and let's see what happens.

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Subscription one now prints out value A and value B.

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Because as I just said it'll give us subscriber of the most recent element as well as all elements that

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follow.

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Once you subscribe.

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So let's add another one and see what happens.

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I guess you probably have a prediction of what will happen if I go ahead and give a value see let's

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see what happens where you write value c prints out as the third value for the subscription.

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So now let's create a new subscription called subscription 2.

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And let's see how this changes how our subscriptions are going to print out.

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So go ahead and type let subscription to equalize behavior subject to subscribe and we're going to go

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ahead and call on next.

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And in the closure we're going to print subscription to let's do a number two and we're going to print

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out our dollar sign zero value which is going to take anything which is going to take the values from

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our on next calls when we pass in value.

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Now you might be wondering well what is the deal here why when it prints out are we only getting one

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value for subscription.

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Two but three values for subscription one.

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Now remember a behavior subject takes the most recent value and anything that comes after the subscription.

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So in this case the first subscription takes the initial value of A as the most recent value.

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Then we add on to new values using on next.

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And that's why those show up.

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But the most recent value for subscription 2 is value c and that's why we get value c first.

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Now if I were to print out another one here behavior's subject on next and I were to pass in a string

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like so and say both subscriptions get this value.

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Watch what happens.

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Subscription 1 and subscription to both print out that string.

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Now of course I spelled this wrong subscriptions.

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There we go.

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But as you can tell a behavior subject gets the initial value or I should say the most recent value

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and then anything after the subscription.

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It can also access.

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So that is what differentiates a behavior subject from a published subject as you can see there a little

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bit different and there are different use cases for each one where one might make more sense than another.

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So that is one thing that makes behavior subjects unique is that they take an initial value and they

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also pay attention to the most recent value and then focus on anything else that comes after the subscription.

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So this is behavior subjects we're going to move on next to variables which is a type of observer.

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So I'll see in the next video.