Add a layout debug module. This outputs a trace of the layout process to a JSON

file which can be viewed in an external tool. It provides a timelapse view of
how the flow tree and fragments changed during the layout process, which makes
it easier to debug layout bugs.

1 files changed, 112 insertions, 0 deletions

diff --git a/src/components/layout/layout_debug.rs b/src/components/layout/layout_debug.rs
new file mode 100644
index 00000000000..bf4cbbfca2a
--- /dev/null
+++ b/src/components/layout/layout_debug.rs
@@ -0,0 +1,112 @@
+/* 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/. */
+
+//! Supports writing a trace file created during each layout scope
+//! that can be viewed by an external tool to make layout debugging easier.
+
+use flow_ref::FlowRef;
+use serialize::json;
+use std::cell::RefCell;
+use std::io::File;
+use std::sync::atomics::{AtomicUint, SeqCst, INIT_ATOMIC_UINT};
+
+local_data_key!(state_key: RefCell<State>)
+
+static mut DEBUG_ID_COUNTER: AtomicUint = INIT_ATOMIC_UINT;
+
+pub struct Scope;
+
+#[deriving(Encodable)]
+struct ScopeData {
+    name: String,
+    pre: String,
+    post: String,
+    children: Vec<Box<ScopeData>>,
+}
+
+impl ScopeData {
+    fn new(name: String, pre: String) -> ScopeData {
+        ScopeData {
+            name: name,
+            pre: pre,
+            post: String::new(),
+            children: vec!(),
+        }
+    }
+}
+
+struct State {
+    flow_root: FlowRef,
+    scope_stack: Vec<Box<ScopeData>>,
+}
+
+/// A layout debugging scope. The entire state of the flow tree
+/// will be output at the beginning and end of this scope.
+impl Scope {
+    pub fn new(name: String) -> Scope {
+        let maybe_refcell = state_key.get();
+        match maybe_refcell {
+            Some(refcell) => {
+                let mut state = refcell.borrow_mut();
+                let flow_trace = json::encode(&state.flow_root.get());
+                let data = box ScopeData::new(name, flow_trace);
+                state.scope_stack.push(data);
+            }
+            None => {}
+        }
+        Scope
+    }
+}
+
+impl Drop for Scope {
+    fn drop(&mut self) {
+        let maybe_refcell = state_key.get();
+        match maybe_refcell {
+            Some(refcell) => {
+                let mut state = refcell.borrow_mut();
+                let mut current_scope = state.scope_stack.pop().unwrap();
+                current_scope.post = json::encode(&state.flow_root.get());
+                let previous_scope = state.scope_stack.mut_last().unwrap();
+                previous_scope.children.push(current_scope);
+            }
+            None => {}
+        }
+    }
+}
+
+/// Generate a unique ID. This is used for items such as Fragment
+/// which are often reallocated but represent essentially the
+/// same data.
+pub fn generate_unique_debug_id() -> uint {
+    unsafe { DEBUG_ID_COUNTER.fetch_add(1, SeqCst) }
+}
+
+/// Begin a layout debug trace. If this has not been called,
+/// creating debug scopes has no effect.
+pub fn begin_trace(flow_root: FlowRef) {
+    assert!(state_key.get().is_none());
+
+    let flow_trace = json::encode(&flow_root.get());
+    let state = State {
+        scope_stack: vec![box ScopeData::new("root".to_string(), flow_trace)],
+        flow_root: flow_root,
+    };
+    state_key.replace(Some(RefCell::new(state)));
+}
+
+/// End the debug layout trace. This will write the layout
+/// trace to disk in the current directory. The output
+/// file can then be viewed with an external tool.
+pub fn end_trace() {
+    let task_state_cell = state_key.replace(None).unwrap();
+    let mut task_state = task_state_cell.borrow_mut();
+    assert!(task_state.scope_stack.len() == 1);
+    let mut root_scope = task_state.scope_stack.pop().unwrap();
+    root_scope.post = json::encode(&task_state.flow_root.get());
+
+    let result = json::encode(&root_scope);
+    let path = Path::new("layout_trace.json");
+    let mut file = File::create(&path).unwrap();
+    file.write_str(result.as_slice()).unwrap();
+}