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Data flow being at the center of almost all things Angular, change detection is something worth knowing about, as it will help you trace bugs much more easily and give you an opportunity to further optimize your apps when working with a complex data set.

In this article, you will learn how Angular detects changes in its data structures and how you can make them immutable to make the most out of Angular’s change detection strategies.

When you change any of your models, Angular detects the changes and immediately updates the views. This is change detection in Angular. The purpose of this mechanism is to make sure the underlying views are always in sync with their corresponding models. This core feature of Angular is what makes the framework tick and is partly the reason why Angular is a neat choice for developing modern web apps.

A model in Angular can change as a result of any of the following scenarios:

All Angular apps are made up of a hierarchical tree of components. At runtime, Angular creates a separate change detector class for every component in the tree, which then eventually forms a hierarchy of change detectors similar to the hierarchy tree of components.

Whenever change detection is triggered, Angular walks down this tree of change detectors to determine if any of them have reported changes.

The change detection cycle is always performed once for every detected change and starts from the root change detector and goes all the way down in a sequential fashion. This sequential design choice is nice because it updates the model in a predictable way since we know component data can only come from its parent.

The change detectors provide a way to keep track of the component’s previous and current states as well as its structure in order to report changes to Angular.

If Angular gets the report from a change detector, it instructs the corresponding component to re-render and update the DOM accordingly.

In order to understand what a change detection strategy is and why it works, we must first understand the difference between value types and reference types in JavaScript. If you are already familiar with how this works, you can skip this section.

To get started, let’s review value types and reference types and their classifications.

Value Types

For simplicity, one can imagine that these types simply store their value on the stack memory (which is technically not true but it’s sufficient for this article). See the stack memory and its values in the image below for example.

Reference Types

These types are a bit more complicated as they store a reference on the stack memory, which points to their actual value on the heap memory. You can see how stack memory and heap memory work together in the example image below. We see the stack memory references the actual values of the reference type in the heap memory.

The important distinction to make between value types and reference types is that, in order to read the value of the value type, we just have to query the stack memory, but in order to read the value of a reference type, we need to first query the stack memory to get the reference and then secondly use that reference to query the heap memory to locate the value of the reference type.

As we stated earlier, Angular monitors changes on the model in order to make sure it catches all of the changes. It will check for any differences between the previous state and current state of the overall application model.

The question that Angular asks in the default change detection strategy is: Has any value in the model changed? But for a reference type, we can implement strategies so that we can ask a better question. This is where OnPush change detection strategy comes in.

The main idea behind the OnPush strategy manifests from the realization that if we treat reference types as immutable objects, we can detect if a value has changed much faster. When a reference type is immutable, this means every time it is updated, the reference on the stack memory will have to change. Now we can simply check: Has the reference (in the stack) of the reference type changed? If yes, only then check all the values (on the heap). Refer back to the previous stack heap diagrams if this is confusing.

The OnPush strategy basically asks two questions instead of one. Has the reference of the reference type changed? If yes, then have the values in heap memory changed?

For example, assume we have an immutable array with 30 elements and we want to know if there are any changes. We know that, in order for there to be any updates to the immutable array, the reference (on the stack) of it would have to have changed. This means we can initially check to see if the reference to the array is any different, which would potentially save us from doing 30 more checks (in the heap) to determine which element is different. This is called the OnPush strategy.

So, you might ask, what does it mean to treat reference types as immutable? It means we never set the property of a reference type, but instead reassign the value all together. See below:

Treating objects as mutable:

Treating objects as immutable:

Note that, in the examples above, we are “treating” reference types as immutable by convention, so in the end we are still working with mutable objects, but just “pretending” they are immutable.

So how do you implement OnPush strategy for a component? All you need to do is add the changeDetection parameter in their @Component annotation.

It is a good idea to enforce immutability if one decides to use the OnPush strategy on an Angular component. That is where Immutable.js comes in.

Immutable.js is a library created by Facebook for immutability in JavaScript. They have many immutable data structures, like List, Map, and Stack. For the purposes of this article, List and Map will be illustrated. For more reference, please check out the official documentation here.

In order to add Immutable.js to your projects, please make sure to go into your terminal and run:

Also make sure to import the data structures you are using from Immutable.js in the component where you are using it.

This is how an Immutable.js Map can be used:

And, an Array can be used:

There are a couple main arguable drawbacks of using Immutable.js.

Men have been giving gifts to women since the first Neanderthal offered his crush a polished wolf femur. In the millennia that have followed, homo sapiens invented the wheel, the laptop, the foam fan finger, and put a man on the moon. But men have not yet developed a system for buying the women in their lives meaningful gifts for special occasions. As a public service, I offer this gifting system for homo sapiens and homo inermis.

Gifts show love and affection and can also signal appreciation. Coincidentally, gifts have also been used throughout history to demonstrate wealth, success, and power. Trust me, when you become a great gift giver, your status and desirability as a romantic partner increase exponentially. The only thing women love more than getting a meaningful gift is showing it off to their envious friends.

Giving your significant other a gift deepens the warmth, trust, affection, and intimacy in your relationship, and even increases her admiration of you. This kindling effect of gift giving shouldn’t be reserved only for major anniversaries and holidays, but used to keep the flame burning during your “regular” life together as well.

Gifts don’t have to cost a fortune (and can even be free, or close to it); most women really do appreciate the gesture more than the price tag. In fact, a gift that costs too much can create more angst than joy, especially if it strains the budget or cuts in to other spending priorities she considers important. It really is the thought that counts, so put some into your choice.

If you are starved for ideas, start a file somewhere with the answers to the following questions. Ask them occasionally and subtly (not all at the same time), and record her answers (including the why) carefully:

Keep this guide handy for frequent reference. With it, you can always find an inexpensive way to show her you pay attention and know what she likes. You’ll also have this list in mind as you go about your business. If you see something that fits her, buy it and stash it for the next gift-giving occasion. Buying at the last minute puts a lot of pressure on you; buying when you happen to find something means you’ll never have to panic.

Some other guidelines:

Buy her things that have a story. Tell her about the artist who made the bracelet or glazed the pottery. Tell her about where you found it and why you thought it would be perfect for her. Find pieces that are unique, one of a kind, or that have a long history, and let her know that.

You can also give gifts that make life easier — as long as they’re not too utilitarian. Giving her a book of smoothie recipes is thoughtful; buying her a blender is not. Giving her a set of file folders with a beautiful pattern for her home office is thoughtful; buying the latest edition of Turbo Tax is not.

Don’t forget that acts of service can make her heart sing. Fix something without being asked. Borrow her car for an errand and bring it back washed and vacuumed with the gas tank filled. Do the dishes, clean the junk drawer, wash and fold a load of laundry, or do another task that she dislikes or that normally wouldn’t be your job. (No extra credit for doing what you should as your fair share.)

Make dinner. Or reservations. Hire a sitter for the night and plan a walk in the park or on the beach. Take on the planning or logistics of something she normally has to handle. Tell her why you’re doing it: “You deserve a night off.” “You’ve been working so hard lately.” “I just wanted to steal a few hours of your time.” It will cost you almost nothing and bring you lots of gratitude in return.

Buy her things whose only quality is that they make life more beautiful. Scented soaps, practical items with beautiful designs (think desk accessories or a bud vase for her office). A set of (only) two wine glasses, coffee mugs, or dessert plates for the two of you to share at special moments. Replace something ugly, old, or utilitarian with something beautiful: baking or kitchen utensils, water bottles or teacups, calendars or planners, even something as small as a bookmark can delight her. Think of something she uses often. Let her know that she deserves beautiful things around her every day.

For more ideas on how to speak her specific gifting language (through acts of service and other methods), read Dr. Gary Chapman’s book The 5 Love Languages.

Timing and presentation matter enormously as to how your gift will be received. Here are some ideas on timing.

Surprise her, but only when she has time to be fully present. Don’t give her a gift:

Here is how to time your gift for optimal joy and appreciation:

If you’re not sure about specific dates, ask — she’ll almost certainly know. (Write them down; once she’s told you, she’ll assume remembering it means as much to you as it does to her.) If you want extra points, celebrate days that only you will know (and, coincidentally, you alone can verify). “Thursday, March 12th was the day I realized I was in love with you.”

Presentation matters. You don’t have to have great wrapping skills to make a gift look special, but you do have to put some thought into your presentation:

Don’t:

Do:

If you don’t have a separate bank account for purchasing gifts, think about saving up and paying cash for your gift. If she has to pay the bill for your present, it definitely kills the buzz (especially if she will think you over or under-paid for it). If finances are an issue, save up your spare change to spend on her (you can even let her know that’s your plan).

What if your best effort falls flat? It’s possible the gift of clothing, jewelry, or other item will not be a great fit for her personal style. If this happens, it’s as difficult for her as it is for you. She wants very much to honor your gift without encouraging more purchases like it. You’ll have to read her body language very closely; you should be able to tell the difference between love and not-so-much-love for your effort.

If you can see it doesn’t work for her, it’s okay to acknowledge it out loud. “I can see you love the idea, but maybe not so much the execution. Should we exchange it and find something that you love? I want you to feel as beautiful wearing it as I think you look.” If she exchanges your gift, pay attention to (or ask) what she likes better about her choice. Make notes, learn, and grow. She will definitely give you credit for trying.

If you need the bumper sticker version of this entire guide, here it is: “Simple. Personal. Meaningful.” It’ll work every time.

__________________________________________

Candace Moody is a writer based in Jacksonville, Florida. She has been married for many years to the love of her life and a great gift giver. Find her at candacemoody.com.

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