Re: Hot Wheels Multi Loop Race Off Not Working
Edgars Rob
This page contains a conceptual overview of how concurrent programming works in Dart. It explains the event-loop, async language features, and isolates from a high-level. For more practical code examples of using concurrency in Dart, read the Asynchrony support page and Isolates page.
All Dart code runs in isolates, starting in the default main isolate, and optionally expanding to whatever subsequent isolates you explicitly create. When you spawn a new isolate, it has its own isolated memory, and its own event loop. The event loop is what makes asynchronous and concurrent programming possible in Dart.
This example event loop is synchronous and runs on a single thread. However, most Dart applications need to do more than one thing at a time. For example, a client application might need to execute an HTTP request, while also listening for a user to tap a button. To handle this, Dart offers many async APIs, like Futures, Streams, and async-await. These APIs are built around this event loop.
When this code reaches the event loop, it immediately calls the first clause, http.get, and returns a Future. It also tells the event loop to hold onto the callback in the then() clause until the HTTP request resolves. When that happens, it should execute that callback, passing the result of the request as an argument.
This section summarizes the different types and syntaxes of asynchronous programming in Dart. If you're already familiar with Future, Stream, and async-await, then you can skip ahead to the isolates section.
The main() function uses the await keyword in front of _readFileAsync() to let other Dart code (such as event handlers) use the CPU while native code (file I/O) executes. Using await also has the effect of converting the Future returned by _readFileAsync() into a String. As a result, the contents variable has the implicit type String.
As the following figure shows, the Dart code pauses while readAsString() executes non-Dart code, in either the Dart runtime or the operating system. Once readAsString() returns a value, Dart code execution resumes.
Dart supports concurrency via isolates, in addition to asynchronous APIs. Most modern devices have multi-core CPUs. To take advantage of multiple cores, developers sometimes use shared-memory threads running concurrently. However, shared-state concurrency is error prone and can lead to complicated code.
Instead of threads, all Dart code runs inside isolates. Using isolates, your Dart code can perform multiple independent tasks at once. Isolates are like threads or processes, but each isolate has its own memory and a single thread running an event loop.
Each isolate has its own global fields, ensuring that none of the state in an isolate is accessible from any other isolate. Isolates can only communicate to each other via message passing. No shared state between isolates means concurrency complexities like mutexes or locks and data races won't occur in Dart. That said, isolates don't prevent race conditions all together. For more information on this concurrency model, read about the Actor model.
Using isolates, your Dart code can perform multiple independent tasks at once, using additional processor cores if they're available. Isolates are like threads or processes, but each isolate has its own memory and a single thread running an event loop.
Even single-isolate programs can execute smoothly. Before continuing to the next line of code, these apps use async-await to wait for asynchronous operations to complete. A well-behaved app starts quickly, getting to the event loop as soon as possible. The app then responds to each queued event promptly, using asynchronous operations as necessary.
In a client app, the main isolate's event queue might contain repaint requests and notifications of tap and other UI events. For example, the following figure shows a repaint event, followed by a tap event, followed by two repaint events. The event loop takes events from the queue in first in, first out order.
Event handling happens on the main isolate after main() exits. In the following figure, after main() exits, the main isolate handles the first repaint event. After that, the main isolate handles the tap event, followed by a repaint event.
If a synchronous operation takes too much processing time, the app can become unresponsive. In the following figure, the tap-handling code takes too long, so subsequent events are handled too late. The app might appear to freeze, and any animation it performs might be jerky.
A worker isolate can perform I/O (reading and writing files, for example), set timers, and more. It has its own memory and doesn't share any state with the main isolate. The worker isolate can block without affecting other isolates.
When an isolate calls Isolate.spawn(), the two isolates have the same executable code and are in the same isolate group. Isolate groups enable performance optimizations such as sharing code; a new isolate immediately runs the code owned by the isolate group. Also, Isolate.exit() works only when the isolates are in the same isolate group.
In some special cases, you might need to use Isolate.spawnUri(), which sets up the new isolate with a copy of the code that's at the specified URI. However, spawnUri() is much slower than spawn(), and the new isolate isn't in its spawner's isolate group. Another performance consequence is that message passing is slower when isolates are in different groups.
All Dart apps can use async-await, Future, and Stream for non-blocking, interleaved computations. The Dart web platform, however, does not support isolates. Dart web apps can use web workers to run scripts in background threads similar to isolates. Web workers' functionality and capabilities differ somewhat from isolates, though.
For instance, when web workers send data between threads, they copy the data back and forth. Data copying can be very slow, though, especially for large messages. Isolates do the same, but also provide APIs that can more efficiently transfer the memory that holds the message instead.
Creating web workers and isolates also differs. You can only create web workers by declaring a separate program entrypoint and compiling it separately. Starting a web worker is similar to using Isolate.spawnUri to start an isolate. You can also start an isolate with Isolate.spawn, which requires fewer resources because it reuses some of the same code and data as the spawning isolate. Web workers don't have an equivalent API.
Road racing is one of the fundamental competition formats for cyclists. Although riders have access to an ever-growing variety of events, including gravel, mountain bike, and cyclocross, many incorporate road cycling and road racing as a component of their cycling season. Many cyclists get interested in road racing after riding on their own, joining group rides, and participating in mass-start events like charity rides and gran fondos.
Road racing encompasses a range of competitions, including traditional road races, circuit races, criteriums, time trials, hill climbs, and stage races. Click here for specifics on the parameters for these race formats. Almost all road races are mass start events with competitors separated into categories based on gender, skill, and/or age groups. Exceptions to this include individual time trials and hill climbs. These are timed events in which competitors start one-by-one and drafting is not allowed. For road races, circuit races, and criteriums, riders who start together as a peloton are allowed to draft and cooperate with each other throughout the race, and the winner is the first rider to cross the finish line. Stage races and omniums combine multiple races, typically over a period of days. The winner of a stage race is the rider with the lowest cumulative time through all stages. Omniums, on the other hand, are decided by cumulative points earned during individual races.
Unlike gravel races and gran fondos that feature mass starts that include all participants, road race fields are segregated into categories based on gender, skill level, and sometimes age group. Categories for road racing start at Novice (replaces Category 5), progresses to Category 4, and then numerically to Category 1 (elite). Although male and female competitors race in separate categories, race promoters can combine skill categories to encourage competitive fields (e.g. Categories 3 and 4 racing together in a criterium). Similarly, promoters have discretion to separate or combine age groups within skill categories (i.e. Male 40-44 Category 3 and Male 45-49 Category 3).
Athletes are encouraged to upgrade to more advanced skill categories as they gain experience and fitness. Racers earn upgrade points through race results. You can review the road category upgrade policy here. Riders can upgrade from Novice to Category 4 voluntarily at any time and must upgrade to Category 4 upon earning 10 upgrade points in a 12-month period. Upgrading from Category 4 to Category 3 and beyond can be accomplished voluntarily with results that meet the minimum requirements laid out in the upgrade policy linked to above. To encourage fair competitions at all levels, mandatory upgrades are required when cyclists earn sufficient upgrade points within a 12-month period.
Bicycles used for road racing typically feature multiple gears with a freewheel and derailleurs that allow for coasting and shifting into harder and easier gear ratios. The bike must be equipped with brakes for both wheels. Although flat handlebars are technically legal, the vast majority of racers compete on drop-bar bikes. However, no matter whether you have flat bars or drop bars, forward extensions (i.e. aero bars or bar ends) are only allowed in some time trial events.
The short answer is, yes. The differences between bicycles marketed for road, gravel, and cyclocross disciplines are mainly focused on tire clearance and frame geometry. Traditionally, road racing bikes feature tight tolerances for tire clearance and angles suited to high-speed cornering and stability for descents. Cyclocross bikes have increased tire clearance and higher bottom brackets to handle the grass, mud, and barriers that are part of cyclocross racing.
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