So far, layout has been a linear process that handles open tags and close tags independently. But web pages are trees, and look like them: borders and backgrounds visually nest inside one another. To support that, this chapter switches to tree-based layout, where the tree of elements is transformed into a tree of layout objects for the visual elements of the page. In the process, we’ll make our web pages more colorful with backgrounds.

The layout tree

Right now, our browser lays out an element’s open and close tags separately. Both tags modify global state, like the cursor_x and cursor_y variables, but they aren’t otherwise connected, and information about the element as a whole, like its width and height, is never computed. That makes it pretty hard to draw a background color behind text. So web browsers structure layout differently.

In a browser, layout is about producing a layout tree, whose nodes are layout objects, each associated with an HTML element,Elements like <script> don’t generate layout objects, and some elements generate multiple (<li> elements have a layout object for the bullet point!), but mostly it’s one layout object each. and each with a size and a position. The browser walks the HTML tree to produce the layout tree, then computes the size and position for each layout object, and finally draws each layout object to the screen.

Let’s start by looking how the existing Layout class is used:

class Browser:
    def load(self, url):
        # ...
        self.display_list = Layout(self.nodes).display_list
        #...

Here, a Layout object is created briefly and then thrown away. Let’s instead make it the beginning of our layout tree by storing it in a Browser field:

class Browser:
    def load(self, url):
        # ...
        self.document = Layout(self.nodes)
        self.document.layout()
        self.display_list = self.document.display_list
        #...

Note that I’ve renamed the Layout constructor to a layout method, so that constructing a layout object and actually laying it out can be different steps. The constructor now just stores the node it was passed:

class Layout:
    def __init__(self, node):
        self.node = node

So far, we still don’t have a tree—we just have a single Layout object. To make it into a tree, we’ll need add child and parent pointers. I’m also going to add a pointer to the previous sibling, because that’ll be useful for computing sizes and positions later:

class Layout:
    def __init__(self, node, parent, previous):
        self.node = node
        self.parent = parent
        self.previous = previous
        self.children = []

That said, requiring a parent and previous element now makes it tricky to construct a Layout object in Browser, since the root of the layout tree obviously can’t have a parent. To rectify that, let me add a second kind of layout object to serve as the root of the layout tree.I don’t want to just pass None for the parent, because the root layout object also computes its size and position differently, as we’ll see later this chapter. I think of that root as the document itself, so let’s call it DocumentLayout:

class DocumentLayout:
    def __init__(self, node):
        self.node = node
        self.parent = None
        self.children = []

    def layout(self):
        child = Layout(self.node, self, None)
        self.children.append(child)
        child.layout()
        self.display_list = child.display_list

Note an interesting thing about this new layout method: its role is to create the child layout objects, and then recursively call its children’s layout methods. This is a common pattern for constructing trees, and we’ll be seeing it a lot throughout this book.

Now when we construct a DocumentLayout object inside load, we’ll be building a tree! A very short tree, more of a stump for now, but it’s something!

By the way, since we now have DocumentLayout, let’s rename Layout so it’s less ambiguous. I like BlockLayout as a name, because we ultimately want Layout to represent a block of text, like a paragraph or a heading:

class BlockLayout:
    # ...

Make sure to rename the Layout constructor call in DocumentLayout as well. Test your browser and make sure that after all of these refactors, everything still works.

Block layout

So far, we’ve focused on text layout—and text is laid out horizontally in lines.In European languages, at least! But web pages are really constructed out of larger blocks, like headings, paragraphs, and menus, that are stacked vertically one after another. We need to add support for this kind of layout to our browser, and the way we’re going to do that involves expanding on the layout tree we’ve already built.

The core idea is that we’ll have a whole tree of BlockLayout objects (with a DocumentLayout at the root). Some will represent leaf blocks that contain text, and they’ll lay out their contents the way we’ve already implemented. But there will also be new, intermediate BlockLayouts with BlockLayout children, and they will stack their children vertically.

To create these intermediate BlockLayout children, we can use a loop like this:

class BlockLayout:
    def layout_intermediate(self):
        previous = None
        for child in self.node.children:
            next = BlockLayout(child, self, previous)
            self.children.append(next)
            previous = next

I’ve called this method layout_intermediate, but only so you can add it to the code right away and then compare it with the existing layout method.

This code is tricky, so read it carefully. It involves two trees: the HTML tree, which node and child point to; and the layout tree, which self, previous, and next point to. The two trees have similar structure, so it’s easy to get confused. But remember that this code constructs the layout tree from the HTML tree, so it reads from node.children (in the HTML tree) and writes to self.children (in the layout tree).

So we have two ways to lay out an element: either calling recurse and flush, or this layout_intermediate function. To determine which one a layout object should to use, we’ll need to know what kind of content its HTML node contains: text and text-related tags like <b>, or blocks like <p> and <h1>. That function looks something like this:

def layout_mode(node):
    if isinstance(node, Text):
        return "inline"
    elif node.children:
        if any([isinstance(child, Element) and \
                child.tag in BLOCK_ELEMENTS
                for child in node.children]):
            return "block"
        else:
            return "inline"
    else:
        return "block"

Here the list of BLOCK_ELEMENTS is basically what you expect, a list of all the tags that describe parts of a page instead of formatting:Taken from the HTML living standard.

BLOCK_ELEMENTS = [
    "html", "body", "article", "section", "nav", "aside",
    "h1", "h2", "h3", "h4", "h5", "h6", "hgroup", "header",
    "footer", "address", "p", "hr", "pre", "blockquote",
    "ol", "ul", "menu", "li", "dl", "dt", "dd", "figure",
    "figcaption", "main", "div", "table", "form", "fieldset",
    "legend", "details", "summary"
]

Our layout_mode function has to handle one tricky case, where a node contains both block children like a <p> element but also text children like a text node or a <b> element. I’ve chosen to use block mode in this case, but it’s probably best to think of this as a kind of error on the part of the web developer. And just like with implicit tags in Chapter 4, we use a repair mechanism to make sense of the situation.In real browsers, that repair mechanism is called “anonymous block boxes” and is more complex than what’s described here.

So now BlockLayout can determine what kind of layout to do based on the layout_mode of its HTML node:

class BlockLayout:
    def layout(self):
        mode = layout_mode(self.node)
        if mode == "block":
            previous = None
            for child in self.node.children:
                # ...
        else:
            # ...
            self.recurse(self.node)
            self.flush()

Finally, since BlockLayouts can now have children, the layout method needs to recursively call layout so those children can construct their children, and so on recursively:

class BlockLayout:
    def layout(self):
        # ...
        for child in self.children:
            child.layout()

We also need to gather their display_list fields into a single array:

class BlockLayout:
    def layout(self):
        # ...
        for child in self.children:
            self.display_list.extend(child.display_list)

Our browser is now constructing a whole tree of BlockLayout objects; in fact, if you add a print_tree call to Browser’s load method, you’ll see that large web pages like this chapter produce large and complex layout trees!

Oh, you might also notice that the text on these web pages is now totally unreadable, because it’s all overlapping at the top of the page. Let’s fix that next.

Size and position

In the previous chapter, the Layout object was responsible for the whole web page, so it just laid out its content starting at the top of the page. Now that we have multiple BlockLayout objects each containing a different paragraph of text, we’re going to need to do things a little differently, computing a size and position for each layout object independently.

Let’s start with cursor_x and cursor_y. Instead of having them denote absolute positions on the page, let’s make them relative to the BlockLayout itself; they now need to start from 0 instead of HSTEP and VSTEP, both in layout and flush:

class BlockLayout:
    def layout(self):
        else:
            self.cursor_x = 0
            self.cursor_y = 0

    def flush(self):
        # ...
        self.cursor_x = 0
        # ...

Since these fields are now relative, we’ll need to add the block’s x and y position in flush when computing the display list:

class BlockLayout:
    def flush(self):
        # ...
        for rel_x, word, font in self.line:
            x = self.x + rel_x
            y = self.y + baseline - font.metrics("ascent")
            self.display_list.append((x, y, word, font))
        # ...

Similarly, to wrap lines, we can’t compare cursor_x to WIDTH, because cursor_x is a relative measure while WIDTH is an absolute measure; instead, we’ll wrap lines when cursor_x reaches the block’s width:

class BlockLayout:
    def text(self, node):
        for word in node.text.split():
            # ...
            if self.cursor_x + w > self.width:
                # ...
            # ...

So now that leaves us with the problem of computing these x, y, and width fields. Let’s recall that BlockLayouts represent blocks of text like paragraphs or headings, and are stacked vertically one atop another. That means each one starts at its parent’s left edge:

class BlockLayout:
    def layout(self):
        self.x = self.parent.x
        # ...

Its vertical position depends on the position and height of their previous sibling. If there is no previous sibling, they start at the parent’s top edge:

class BlockLayout:
    def layout(self):
        if self.previous:
            self.y = self.previous.y + self.previous.height
        else:
            self.y = self.parent.y
        # ...

Note that in each of these cases, to compute one block’s x and y, the x and y of its parent block must already have been computed. That means these computations have to go before the recursive layout call, so those children can compute their x and y based on this block’s x and y. Similarly, since the y position of a block depends on its previous sibling’s y position, the recursive layout calls have to start at the first sibling and iterate through the list forward—which is how we’ve already done it, but which will be an important constraint in later chapters.

Now we’ll need compute widths and heights. Width is easy: blocks are as wide as their parents:In the next chapter, we’ll add support for author-defined styles, which in real browsers modify these layout rules by setting custom widths or changing how x and y position are computed.

class BlockLayout:
    def layout(self):
        self.width = self.parent.width
        # ...

Height, meanwhile, is a little tricky. A BlockLayout that contains other blocks should be tall enough to contain all of its children, so its height should be the sum of its children’s heights:

class BlockLayout:
    def layout(self):
        # ...
        if mode == "block":
            self.height = sum([
                child.height for child in self.children])

However, a BlockLayout that contains text doesn’t have children; instead, it needs to be tall enough to contain all its text, which we can conveniently read off of cursor_y:Since the height is just equal to cursor_y, why not rename cursor_y to height instead? You could, it would work fine, but I would rather not. As you can see from, say, the y computation, the height field is a public field, read by other layout objects to compute their positions. As such I’d rather make sure it always has the right value, whereas cursor_y changes as we lay out a paragraph of text and therefore sometimes has the “wrong” value. Keeping these two fields separate avoids a whole class of nasty bugs where the height field is read “too soon” and therefore gets the wrong value.

class BlockLayout:
    def layout(self):
        # ...
        else:
            self.height = self.cursor_y

Let’s think again about dependencies. Height has the opposite dependencies compared to x, y, and width: the height of a block depends on its children’s heights. While x, y, and width must be computed before the recursive call, height has to be computed after, at the very end of layout.

Finally, even DocumentLayout needs some layout code, though since the document always starts in the same place it’s pretty simple:

class DocumentLayout:
    def layout(self):
        # ...
        self.width = WIDTH - 2*HSTEP
        self.x = HSTEP
        self.y = VSTEP
        child.layout()
        self.height = child.height + 2*VSTEP

Note that there’s some padding around the contents—HSTEP on the left and right, and VSTEP above and below. That’s so the text won’t run into the very edge of the window and get cut off.

For all three types of layout object, the order of the steps in the layout method should be the same:

• When layout is called, it first computes the width, x, and y fields, reading from the parent and previous layout objects.
• Next, it creates a child layout object for each child element.
• Then, the child layout nodes are recursively laid out by calling their layout methods.
• Finally, layout computes the height field, reading from the child layout objects.

You can see these steps in action in this widget:

This kind of dependency reasoning is crucial to layout and more broadly to any kind of computation on trees. If you get the order of operations wrong, some layout object will try to read a value that hasn’t been computed yet, and the browser will have a bug. We’ll come back to this issue of dependencies later, where it will become even more important.

Anyway, with all of the sizes and positions now computed correctly, you should see the browser now correctly display all of the text on the page.

Recursive painting

Our layout method is now doing quite a bit of work: computing sizes and positions; creating child layout objects; recursively laying out those child layout objects; and aggregating the display lists so the text can be drawn to the screen. This is a bit messy, so let’s take a moment to extract just one part of this, the display list part. Along the way, we can stop copying the display list contents over and over again as we go up the layout tree.

I think it’s most convenient to do that by adding a paint function to each layout object, which appends any of its own layout objects to the display list and then recursively paints the child layouts. A neat trick here is to pass the list itself as an argument, and have the recursive function append to that list. For DocumentLayout, which only has one child, the recursion looks like this:

class DocumentLayout:
    def paint(self, display_list):
        self.children[0].paint(display_list)

You can now delete the line that computes a DocumentLayout’s display_list field.

For a BlockLayout with multiple children, paint is called on each child:

class BlockLayout:
    def paint(self, display_list):
        for child in self.children:
            child.paint(display_list)

Again, delete the line that computes a BlockLayout’s display_list field by copying from child layout objects.

Finally for a BlockLayout object with text inside, we need to copy over the display_list field that it computes during recurse and flush:

class BlockLayout:
    def paint(self, display_list):
        display_list.extend(self.display_list)

Now the browser can use paint to collect its own display_list variable:

class Browser:
    def load(self, url):
        # ...
        self.display_list = []
        self.document.paint(self.display_list)
        self.draw()

Check it out: your browser is now using fancy tree-based layout! I recommend pausing to test and debug. Tree-based layout is powerful but complex, and we’re about to add more features. Stable foundations make for comfortable houses.

Backgrounds

Browsers use the layout tree a lot,For example, in Chapter 7, we’ll use the size and position of each link to figure out which one the user clicked on. and one simple and visually compelling use case is drawing backgrounds.

Backgrounds are rectangles, so our first task is putting rectangles in the display list. Conceptually, the display list contains commands, and we want two types of commands:

class DrawText:
    def __init__(self, x1, y1, text, font):
        self.top = y1
        self.left = x1
        self.text = text
        self.font = font
    
class DrawRect:
    def __init__(self, x1, y1, x2, y2, color):
        self.top = y1
        self.left = x1
        self.bottom = y2
        self.right = x2
        self.color = color

Now BlockLayout must add DrawText objects to the display list:Why not change the display_list field inside an BlockLayout to contain DrawText commands directly? I suppose you could, but I think it’s cleaner this way, with all of the draw commands created in one place.

class BlockLayout:
    def paint(self, display_list):
        for x, y, word, font in self.display_list:
            display_list.append(DrawText(x, y, word, font))
        # ...

Note that we must add the block’s x and y, since the positions in the display list are relative to the block’s position.

But it can also add DrawRect commands for backgrounds. Let’s add a gray background to pre tags (which are used for code examples):

class BlockLayout:
    def paint(self, display_list):
        if isinstance(self.node, Element) and self.node.tag == "pre":
            x2, y2 = self.x + self.width, self.y + self.height
            rect = DrawRect(self.x, self.y, x2, y2, "gray")
            display_list.append(rect)
        # ...

Make sure this code comes before the loop that adds DrawText objects and before the recursion into child layout objects: the background has to be drawn below and therefore before any contents. This is again a kind of dependency reasoning with tree traversals!

With the display list filled out, we need the paint method to run each graphics command. Let’s add an execute method for this. On DrawText it calls create_text:

class DrawText:
    def execute(self, scroll, canvas):
        canvas.create_text(
            self.left, self.top - scroll,
            text=self.text,
            font=self.font,
            anchor='nw',
        )

Note that execute takes the scroll amount as a parameter; this way, each graphics command does the relevant coordinate conversion itself. DrawRect does the same with create_rectangle:

class DrawRect:
    def execute(self, scroll, canvas):
        canvas.create_rectangle(
            self.left, self.top - scroll,
            self.right, self.bottom - scroll,
            width=0,
            fill=self.color,
        )

By default, create_rectangle draws a one-pixel black border, which for backgrounds we don’t want, so make sure to pass width = 0:

We still want to skip offscreen graphics commands, so let’s add a bottom field to DrawText so we know when to skip those:

def __init__(self, x1, y1, text, font):
    # ...
    self.bottom = y1 + font.metrics("linespace")

The browser’s draw method now just uses top and bottom to decide which commands to execute:

def draw(self):
    self.canvas.delete("all")
    for cmd in self.display_list:
        if cmd.top > self.scroll + HEIGHT: continue
        if cmd.bottom < self.scroll: continue
        cmd.execute(self.scroll, self.canvas)

Try your browser on a page—maybe this one—with code snippets on it. You should see each code snippet set off with a gray background.

Here’s one more cute benefit of tree-based layout. Thanks to tree-based layout we now record the height of the whole page. The browser can use that to avoid scrolling past the bottom of the page:

def scrolldown(self, e):
    max_y = self.document.height - HEIGHT
    self.scroll = min(self.scroll + SCROLL_STEP, max_y)
    self.draw()

So those are the basics of tree-based layout! In fact, as we’ll see in the next two chapters, this is just part of the layout tree’s role in the browser. But before we get to that, we need to add some styling capabilities to our browser.

Summary

This chapter was a dramatic rewrite of your browser’s layout engine:

• Layout is now tree-based and produces a layout tree
• Each node in the tree has one of two different layout modes
• Layout computes a size and position for each layout object
• The display list now contains generic commands
• Plus, source code snippets now have backgrounds

Tree-based layout makes it possible to dramatically expand our browser’s styling capabilities. We’ll work on that in the next chapter.

Outline

The complete set of functions, classes, and methods in our browser should look something like this:

Exercises

Links Bar: At the top and bottom of each chapter of this book is a gray bar naming the chapter and offering back and forward links. It is enclosed in a <nav class="links"> tag. Have your browser give this links bar the light gray background a real browser would.

Hidden Head: There’s a good chance your browser is still showing scripts, styles, and page titles at the top of every page you visit. Make it so that the <head> element and its contents are never displayed. Those elements should still be in the HTML tree, but not in the layout tree.

Bullets: Add bullets to list items, which in HTML are <li> tags. You can make them little squares, located to the left of the list item itself. Also indent <li> elements so the text inside the element is to the right of the bullet point.

Scrollbar: At the right edge of the screen, draw a blue, rectangular scrollbar. The ratio of its height to the screen height should be the same as the ratio of the screen height to the document height, and its location should reflect the position of the screen within the document. Hide the scrollbar if the whole document fits onscreen.

Table of Contents: This book has a table of contents at the top of each chapter, enclosed in a <nav id="toc"> tag, which contains a list of links. Add the text “Table of Contents”, with a gray background, above that list. Don’t modify the lexer or parser.

Anonymous block boxes: Sometimes, an element has a mix of text-like and container-like children. For example, in this HTML,

<div><i>Hello, </i><b>world!</b><p>So it began...</p></div>

the <div> element has three children: the <i>, <b>, and <p> elements. The first two are text-like; the last is container-like. This is supposed to look like two paragraphs, one for the <i> and <b> and the second for the <p>. Make your browser do that. Specifically, modify BlockLayout so it can be passed a sequence of sibling nodes, instead of a single node. Then, modify the algorithm that constructs the layout tree so that any sequence of text-like elements gets made into a single BlockLayout.

Run-ins: A “run-in heading” is a heading that is drawn as part of the next paragraph’s text.The exercise names in this section could be considered run-in headings. But since browser support for the display: run-in property is poor, this book actually doesn’t use it; the headings are actually embedded in the next paragraph. Modify your browser to render <h6> elements as run-in headings. You’ll need to implement the previous exercise on anonymous block boxes, and then add a special case for <h6> elements.

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