ERB Tags

We're getting the page title from a URL parameter and 0:00 we're using it to load in page content from a text file. 0:02 Now we just need to convert our response to actual HTML and 0:06 we can do that using ERB. 0:09 Let's talk about the embedding part of embedded Ruby. 0:12 You store some static text, usually HTML code, in a file. 0:16 Then using specially ERB tags you embed Ruby code into the static 0:20 text that displays changing dynamic data. 0:24 Each time the template is rendered each piece of Ruby code is evaluated to 0:27 get a result. 0:31 Those results are inserted into the template in place of the ERB tags, 0:32 giving you text that changes each time the template is rendered. 0:36 Let's set up a temporary playground where we can try out ERB text. 0:40 I'm going to create a new get route with a path of /test. 0:44 It's important to place this before the get title route in 0:51 your code to avoid a conflict. 0:54 We'll talk about the reason for that later. 0:55 Within the route block, we'll call the ERB method and 0:58 have it render a template called test. 1:02 Then we'll create a new file in the views directory called test.erb. 1:05 The next time we visit the /test path, it will render the contents of this file. 1:13 There are two kinds of ERB, tags, regular embedding tags and output embedding tags. 1:18 Regular embedding tags start with <% and 1:24 end with %>, they contain some Ruby code in between those symbols. 1:28 The code gets evaluated but the results aren't placed directly into the output. 1:32 So let's say I had a regular tag like this, grade = 54. 1:37 When the tag gets evaluated the grade variable will be set but 1:42 nothing will appear in the output. 1:45 Regular embedding tags are most frequently used for two things, loops and 1:48 including or excluding parts of the template based on the condition. 1:51 If you include a conditional like an IF expression in a regular embedding tag, 1:55 any text inside the conditional will only be included in the output if 1:59 the condition is true. 2:03 So let's say that I had a statement up here, 2:05 a regular ERB tag setting the grade variable to 97. 2:09 And then I'll put insert another ERB tagged with a conditional in it. 2:13 if grade, if the grade variable, if the value in it is greater than 60, 2:18 I'll put separate ERB tag down here with the end keyword to end the if statement. 2:23 Now anything contained inside that conditional is only going to be printed 2:32 if the conditional is true. 2:35 So let's put a paragraph tag in here saying you passed. 2:37 If grade were lower than sixty, you passed would not be included in the output. 2:42 But since it's set to ninety seven, it will be. 2:47 Let's save this. 2:49 And now let's restart our server. 2:51 And try visiting the /test path. 2:57 It loads the test.arb tag, 3:02 evaluates the Ruby code setting the grade variable to 97. 3:04 Then it checks the conditional testing whether the grade variable is greater 3:08 than 60. 3:12 And it is, so it prints the paragraph tag, you passed. 3:13 If we change the variable's value to say 54, 3:17 save that, and reload the page, the condition 3:21 will no longer be true and the you passed HTML will be excluded from the output. 3:27 By the way you may have noticed that I didn't restart the app after making 3:33 changes to the ERB template. 3:36 That's because, 3:38 unlike the main Ruby code, ERB templates are reloaded each time they're rendered. 3:39 So if you save changes to the ERB template, 3:44 they'll show up as soon as you refresh your browser. 3:46 The other common use of regular embedding tags is in loops. 3:49 If you place a loop in a regular embedding tag, 3:52 any text inside the loop lobby will be output repeatedly. 3:54 So let's say that I wanted to output three paragraphs containing the word fish. 3:58 We'll create a regular embedding tag up here and 4:03 we'll loop three times by calling the times method on the integer object three. 4:06 Times method takes a block which loops that number of times. 4:12 And since it's a do end block we need the end keyword down here in a separate tag. 4:18 Now any text we put inside the loop here will be repeated that number of times. 4:23 So let's put an HTML paragraph tag, and we'll just put the word fish inside it. 4:27 Save this and reload our page. 4:33 And you see it loops three times, 4:36 each time outputting an HTML paragraph with the word fish. 4:38 In addition to regular embedding tags, ERB also has output 4:42 embedding tags which begin with <%= and end with %>. 4:47 The equal sign at the start is the only difference in how you type output tags 4:53 versus regular tags. 4:56 Like regular embedding tags the Ruby code inside an output tag gets evaluated when 4:59 the template is rendered. 5:03 But unlike regular tags, the return value of the Ruby code gets included in 5:04 the template in place of the tag. 5:08 So if we have an output embedding tag with the math operation 2 + 2, 5:11 when the template is rendered the code will be evaluated and the result, four, 5:14 will be included in the output. 5:19 Similarly, we can place a call to the time.now method in an output tag and 5:22 we'll get the current time in the output. 5:26 And because the values are updated every time we render the template, 5:30 we can refresh the page and see a new result every time. 5:33 We can mix regular embedding tags and output embedding tags too. 5:37 For example, we can nest an output embedding tag inside an each block and 5:41 the output tag will be evaluated repeatedly. 5:45 So let's say we wanted to print an HTML paragraph element for 5:48 each item in an array. 5:52 We can set up the array within a regular embedding tag. 5:54 And the each method, if you recall, loops through each element of the array, 6:05 passing it to the argument here in the block. 6:09 Again this is a multi-line block starting with do, so 6:13 we're going to need the end keyword in a separate tag down here. 6:15 And any text here within the loop will get repeated. 6:20 So we're going to output an HTML paragraph tag and we're going to embed within it 6:23 the output number, the current value of the number variable. 6:28 Let's save this and reload our page. 6:35 And we get the output 1 fish, 5 fish, 25 fish. 6:38 It loops through each value and our array, passes it into the number argument to 6:41 the block, and then within the loop body this output embedding tag gets evaluated. 6:47 The current value of number is the result, and that gets embedded into the output. 6:53 So that's how to embed Ruby code into an HTML template using ERB. 6:58 We're done trying out ERB code for now, so 7:02 let's go into the views directory and delete the test.erb file. 7:04 We'll also want to go into the main app code and delete the get /text route 7:10