Once you learn how to use D3 transitions, you begin thinking about ways to define them in ways that will allow you to reuse the same transition on multiple elements. To demonstrate how to do this, we've got a simple example here with a style called black, which is just a 50x50 pixel gray square. We've got two of those defined here, and we also have an extra class defined on each of these, A and B, so that we can address them individually. We'll switch over here to our app.js file.
Previously we've seen transitions defined on selections, but there's actually a d3.transition method. Here we'll say d3.transition and give it a delay of 1 second, and a duration of 1 second. Then we'll see how we can reuse this transition instance across our different blocks. The first thing that we'll do is we'll select all of our blocks by saying d3.selectAll.block, and then once we have that selection, we can then say transition and actually pass in this transition instance like that.
In this case we want that transition to effect the width of our block, and so we're going to say width is 400 pixels. If we save this and it runs, we'll see that our blocks do grow to 400 pixels in width, so we know that things are working. Now the next thing I want to do is affect just one of these blocks, so we'll say d3.select.A, so that's just going to be our first block there. We'll again pass in the transition instance to the transition method, but this time we're going to make the background color fade to orange.
We'll just copy this block of code, and make this block address the B block, and we'll change this background to blue. Save that. Now we see that those transitions are in fact getting reused. You can see how this is useful because we've defined it one time up here, we've defined the delay and the duration in just one place, and now we're using it in three places. This is a totally valid approach, but there is something to be aware of when you use this approach.
If we wanted this function to run in response to a button click instead of when the page simply loads, let's see what we would run into. So we're just going to create a button here called Go that calls a function named go, and we'll come over here and we'll just take all of these and wrap them inside of our function. Now we have a function named go, and within that function we use all of our transitions, and we're going to call that function when we click this button here.
If we come over here and click our Go button, that didn't' look right. That was a lot faster than one second in duration, and there was no delay at all. If we see that again, come here, we reload the page, and we click Go. It's not respecting that, and what's going on here is that we're seeing a side effect of the fact that when you call d3.transition, it's creating it and starting it. Once this transition ends, so essentially two seconds after this page loads, that transition is destroyed and it no longer exists.
If we reload this page and then quickly click that, you can see that we do get to use our transition that we defined, but if we wait too long, we don't. This can be really unpredictable, and really confusing, and so if you wanted to use this approach, you just have to make sure that you're using it as soon as it's created, you can't create it ahead of time.
Another approach is to use the transition.call method which is similar to selection.call, where you're able to specify a function that should be called and any optional parameters that should be passed to it. In this case we'll create a function called configure, and just like selection.call passes the selection as the first argument, transition.call passes the transition as the first argument.
The first argument will be T, and then we're going to specify a delay and a duration parameter, so that those things can be configured when using this with transition.call. In the body of the function, we'll simply take the transition instance that's passed in and call its delay method and duration method, passing in the arguments provided. Now if we create a new function, and we'll call it goNow, we're going to select all of our blocks, and then we'll call .transition as normal.
We're calling the transition method of the selection, and then we're calling the .call method of the transition and telling it that we want it to call the configure method and pass in the arguments that we have provided here. Then finally, transition.call is going to return that transition so we can again add another property on there, and so we'll tell it to make the height 300 pixels tall. We'll come over here, and we will create a new button that will call our goNow function.
This page has refreshed, we'll give a couple seconds to make sure we're not catching that transition before it dies, and now we'll go ahead and click Go Now, and we in fact get a nice delayed, slow transition that grows our blocks to 300 pixels tall.