Auto merge of #7648 - mrobinson:print-tree, r=glennw

Improve printing of DisplayLists

Use box tree characters to make DisplayLists easier to scan when
printing them out.

This is what the output looked like before:
```
#### start printing display list.
"####" Stacking context at Rect(800px×608px at (0px,0px)) with overflow Rect(800px×608px at (0px,0px)):
#### SolidColor(0,0,0,0). Rect(800px×608px at (0px,0px))
#### SolidColor(0,0,0,0). Rect(784px×0px at (8px,0px))
#### SolidColor(0,0.5019608,0,1). Rect(100px×100px at (0px,0px))


#### Child layers list length: 1
"########" Stacking context at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
######## SolidColor(1,0,0,1). Rect(100px×100px at (0px,0px))


######## Child layers list length: 1
"############" Stacking context at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
```

This is what it looks like after this patch:
```
┌ DisplayList
│  ├─ Layered StackingContext at Rect(800px×608px at (0px,0px)) with overflow Rect(800px×608px at (0px,0px)):
│  │  ├─ Block Backgrounds and Borders
│  │  │  ├─ SolidColor rgba(0, 0, 0, 0) @ Rect(800px×608px at (0px,0px)) (7f926f46f1f0)
│  │  │  └─ SolidColor rgba(0, 0, 0, 0) @ Rect(784px×0px at (8px,0px)) (7f926f46f2e0)
│  │  ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│  │  │  ├─ Backgrounds and Borders
│  │  │  │  └─ SolidColor rgba(1, 0, 0, 1) @ Rect(100px×100px at (0px,0px)) (7f926f46f310)
│  │  │  ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│  │  ├─ Layered StackingContext at Rect(100px×100px at (0px,0px)) with overflow Rect(100px×100px at (0px,0px)):
│  │  │  ├─ Backgrounds and Borders
│  │  │  │  └─ SolidColor rgba(0, 0.5019608, 0, 1) @ Rect(100px×100px at (0px,0px)) (7f926f46f3a0)
```

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7648)
<!-- Reviewable:end -->

components/gfx/display_list/mod.rs

@@ -47,6 +47,7 @@ use util::geometry::{self, Au, MAX_RECT, ZERO_RECT};
 use util::linked_list::prepend_from;
 use util::mem::HeapSizeOf;
 use util::opts;
+use util::print_tree::PrintTree;
 use util::range::Range;
 
 // It seems cleaner to have layout code not mention Azure directly, so let's just reexport this for
@@ -59,8 +60,6 @@ pub mod optimizer;
 /// items that involve a blur. This ensures that the display item boundaries include all the ink.
 pub static BLUR_INFLATION_FACTOR: i32 = 3;
 
-const MIN_INDENTATION_LENGTH: usize = 4;
-
 /// An opaque handle to a node. The only safe operation that can be performed on this node is to
 /// compare it to another opaque handle or to another node.
 ///
@@ -201,62 +200,44 @@ impl DisplayList {
         result
     }
 
-    // Print the display list. Only makes sense to call it after performing reflow.
-    pub fn print_items(&self, indentation: String) {
-        // Closures are so nice!
-        let doit = |items: &Vec<DisplayItem>| {
-            for item in items {
-                match *item {
-                    DisplayItem::SolidColorClass(ref solid_color) => {
-                        println!("{} SolidColor({},{},{},{}). {:?}",
-                                 indentation,
-                                 solid_color.color.r,
-                                 solid_color.color.g,
-                                 solid_color.color.b,
-                                 solid_color.color.a,
-                                 solid_color.base.bounds)
-                    }
-                    DisplayItem::TextClass(ref text) => {
-                        println!("{} Text. {:?}", indentation, text.base.bounds)
-                    }
-                    DisplayItem::ImageClass(ref image) => {
-                        println!("{} Image. {:?}", indentation, image.base.bounds)
-                    }
-                    DisplayItem::BorderClass(ref border) => {
-                        println!("{} Border. {:?}", indentation, border.base.bounds)
-                    }
-                    DisplayItem::GradientClass(ref gradient) => {
-                        println!("{} Gradient. {:?}", indentation, gradient.base.bounds)
-                    }
-                    DisplayItem::LineClass(ref line) => {
-                        println!("{} Line. {:?}", indentation, line.base.bounds)
-                    }
-                    DisplayItem::BoxShadowClass(ref box_shadow) => {
-                        println!("{} Box_shadow. {:?}", indentation, box_shadow.base.bounds)
-                    }
-                }
-            }
-            println!("\n");
-        };
+    pub fn print(&self, title: String) {
+        let mut print_tree = PrintTree::new(title);
+        self.print_with_tree(&mut print_tree);
+    }
 
-        doit(&(self.all_display_items()));
-        if !self.children.is_empty() {
-            println!("{} Children stacking contexts list length: {}",
-                     indentation,
-                     self.children.len());
-            for stacking_context in &self.children {
-                stacking_context.print(indentation.clone() +
-                                       &indentation[0..MIN_INDENTATION_LENGTH]);
+    pub fn print_with_tree(&self, print_tree: &mut PrintTree) {
+        fn print_display_list_section(print_tree: &mut PrintTree,
+                                      items: &LinkedList<DisplayItem>,
+                                      title: &str) {
+            if items.is_empty() {
+                return;
             }
+
+            print_tree.new_level(title.to_owned());
+            for item in items {
+                print_tree.add_item(format!("{:?}", item));
+            }
+            print_tree.end_level();
         }
-        if !self.layered_children.is_empty() {
-            println!("{} Child layers list length: {}",
-                     indentation,
-                     self.layered_children.len());
-            for paint_layer in &self.layered_children {
-                paint_layer.stacking_context.print(indentation.clone() +
-                                                   &indentation[0..MIN_INDENTATION_LENGTH]);
-            }
+
+        print_display_list_section(print_tree,
+                                   &self.background_and_borders,
+                                   "Backgrounds and Borders");
+        print_display_list_section(print_tree,
+                                   &self.block_backgrounds_and_borders,
+                                   "Block Backgrounds and Borders");
+        print_display_list_section(print_tree, &self.floats, "Floats");
+        print_display_list_section(print_tree, &self.content, "Content");
+        print_display_list_section(print_tree, &self.positioned_content, "Positioned Content");
+        print_display_list_section(print_tree, &self.outlines, "Outlines");
+
+
+        for stacking_context in &self.children {
+            stacking_context.print_with_tree(print_tree);
+        }
+
+        for paint_layer in &self.layered_children {
+            paint_layer.stacking_context.print_with_tree(print_tree);
         }
     }
 }
@@ -375,8 +356,8 @@ impl StackingContext {
             };
 
             if opts::get().dump_display_list_optimized {
-                println!("**** optimized display list. Tile bounds: {:?}", paint_context.page_rect);
-                display_list.print_items("####".to_owned());
+                display_list.print(format!("Optimized display list. Tile bounds: {:?}",
+                                           paint_context.page_rect));
             }
 
             // Set up our clip rect and transform.
@@ -645,25 +626,23 @@ impl StackingContext {
                          self.display_list.background_and_borders.iter().rev())
     }
 
-    pub fn print(&self, mut indentation: String) {
-        // We cover the case of an empty string.
-        if indentation.is_empty() {
-            indentation = "####".to_owned();
+    pub fn print(&self, title: String) {
+        let mut print_tree = PrintTree::new(title);
+        self.print_with_tree(&mut print_tree);
+    }
+
+    fn print_with_tree(&self, print_tree: &mut PrintTree) {
+        if self.layer_id.is_some() {
+            print_tree.new_level(format!("Layered StackingContext at {:?} with overflow {:?}:",
+                                             self.bounds,
+                                             self.overflow));
+        } else {
+            print_tree.new_level(format!("StackingContext at {:?} with overflow {:?}:",
+                                             self.bounds,
+                                             self.overflow));
         }
-
-        // We grow the indentation by 4 characters if needed.
-        // I wish to push it all as a slice, but it won't work if the string is a single char.
-        while indentation.len() < MIN_INDENTATION_LENGTH {
-            let c = indentation.char_at(0);
-            indentation.push(c);
-        }
-
-        println!("{:?} Stacking context at {:?} with overflow {:?}:",
-                 indentation,
-                 self.bounds,
-                 self.overflow);
-
-        self.display_list.print_items(indentation);
+        self.display_list.print_with_tree(print_tree);
+        print_tree.end_level();
     }
 }
 
@@ -1301,13 +1280,18 @@ impl fmt::Debug for DisplayItem {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{} @ {:?} ({:x})",
             match *self {
-                DisplayItem::SolidColorClass(_) => "SolidColor",
-                DisplayItem::TextClass(_) => "Text",
-                DisplayItem::ImageClass(_) => "Image",
-                DisplayItem::BorderClass(_) => "Border",
-                DisplayItem::GradientClass(_) => "Gradient",
-                DisplayItem::LineClass(_) => "Line",
-                DisplayItem::BoxShadowClass(_) => "BoxShadow",
+                DisplayItem::SolidColorClass(ref solid_color) =>
+                    format!("SolidColor rgba({}, {}, {}, {})",
+                            solid_color.color.r,
+                            solid_color.color.g,
+                            solid_color.color.b,
+                            solid_color.color.a),
+                DisplayItem::TextClass(_) => "Text".to_owned(),
+                DisplayItem::ImageClass(_) => "Image".to_owned(),
+                DisplayItem::BorderClass(_) => "Border".to_owned(),
+                DisplayItem::GradientClass(_) => "Gradient".to_owned(),
+                DisplayItem::LineClass(_) => "Line".to_owned(),
+                DisplayItem::BoxShadowClass(_) => "BoxShadow".to_owned(),
             },
             self.base().bounds,
             self.base().metadata.node.id()

components/gfx/paint_task.rs

@@ -429,7 +429,6 @@ impl<C> PaintTask<C> where C: PaintListener + Send + 'static {
                                        &perspective,
                                        parent_id)
             }
-
         }
     }
 }

components/layout/layout_task.rs

@@ -1060,8 +1060,7 @@ impl LayoutTask {
                                                   stacking_context.clone());
 
                 if opts::get().dump_display_list {
-                    println!("#### start printing display list.");
-                    stacking_context.print("#".to_owned());
+                    stacking_context.print("DisplayList".to_owned());
                 }
                 if opts::get().dump_display_list_json {
                     println!("{}", serde_json::to_string_pretty(&stacking_context).unwrap());

components/util/lib.rs

@@ -61,6 +61,7 @@ pub mod mem;
 pub mod opts;
 pub mod persistent_list;
 pub mod prefs;
+pub mod print_tree;
 pub mod range;
 pub mod resource_files;
 pub mod str;

components/util/print_tree.rs

@@ -0,0 +1,59 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/// A struct that makes it easier to print out a pretty tree of data, which
+/// can be visually scanned more easily.
+pub struct PrintTree {
+    /// The current level of recursion.
+    level: u32,
+
+    /// An item which is queued up, so that we can determine if we need
+    /// a mid-tree prefix or a branch ending prefix.
+    queued_item: Option<String>,
+}
+
+impl PrintTree {
+    pub fn new(title: String) -> PrintTree {
+        println!("┌ {}", title);
+        PrintTree {
+            level: 1,
+            queued_item: None,
+        }
+    }
+
+    /// Descend one level in the tree with the given title.
+    pub fn new_level(&mut self, title: String) {
+        self.flush_queued_item("├─");
+
+        self.print_level_prefix();
+        println!("├─ {}", title);
+
+        self.level = self.level + 1;
+    }
+
+    /// Ascend one level in the tree.
+    pub fn end_level(&mut self) {
+        self.flush_queued_item("└─");
+        self.level = self.level - 1;
+    }
+
+    /// Add an item to the current level in the tree.
+    pub fn add_item(&mut self, text: String) {
+        self.flush_queued_item("├─");
+        self.queued_item = Some(text);
+    }
+
+    fn print_level_prefix(&self) {
+        for _ in 0..self.level {
+            print!("│  ");
+        }
+    }
+
+    fn flush_queued_item(&mut self, prefix: &str) {
+        if let Some(queued_item) = self.queued_item.take() {
+            self.print_level_prefix();
+            println!("{} {}", prefix, queued_item);
+        }
+    }
+}