ViewModel & Caching

A lot of Presto is built around the concept of a ViewModel. In Presto a ViewModel is a class that defines the list of fields & some metadata for some piece of data (eg. a record from a single database table, some data joined from multiple data sources, a form filled out by a user etc).

It's not specific to a backend implementation rather it's concerned with describing what the data is so that a UI can be generated from it & transformations can be done on the data (eg. from user input to specific format or vice versa).

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Fields

A field is defined using the Field class:

new Field({ name: 'email', label: 'Email' });

This tells us the field name is email and when a label is needed (eg. on a form field or when displaying the value) Email address should be used. The first advantage of this is simply the DRY principle - you can render the label from the field instead of hardcoding it throughout your app.

The second advantage is that it provides the basis for automatically generating UI for managing instances of the ViewModel For example using the details provided on a Field a form for entering those details can be rendered.

More specific field classes provide more details about how that field should be handled.

See the ViewModel documentation for the list of available fields or the Field documentation for how to extend and implement your own.

new IntegerField({ name: 'age', label: 'Age' });

The UI code now knows the field rendered should be treated as a numeric entry. More details can be specified:

new BooleanField({
    name: 'optInCommunications',
    label: 'Received Updates',
    helpText: 'Receive periodic updates from us',
    defaultValue: true,
});

UI components now know the default value when creating a record should be true and there's some additional text that should be rendered around the field.

ViewModel Factory

Fields are grouped together on class called a ViewModel that is created with the viewModelFactory function:

const userFields = {
    name: new CharField(),
    emailAddress: new EmailField(),
};
const User = viewModelFactory(userFields);

We can omit the name and label options for the fields when defining them with a viewModelFactory. The name must always match the object key it's defined against so can always be inferred. Label is inferred from the name - emailAddress becomes Email Address.

viewModelFactory just returns a class so it can be extended and have extra properties or methods attached:

class User extends viewModelFactory(userFields) {
    static label = 'User';
    static labelPlural = 'Users';
}

The static properties label and labelPlural are useful for referring to the name of a single or many instances of a ViewModel.

In some cases it may be desirable to have a base class that contains some field definitions and extend that base class with some new fields. This is possible using augment:

class StaffUser extends User.augment({ isSuperUser: new BooleanField() }) {
    static label = 'Staff User';
    static labelPlural = 'Staff';
}

StaffUser will inherited all the same fields as User and add a new field called isSuperUser.

We can create a record from a ViewModel class by instantiating it with it's data:

const staffUser = new StaffUser({
    id: 1,
    name: 'Bob',
    email: 'bob@example.com',
    isSuperUser: true,
});

The id field hasn't been mentioned thus far but must be present on all records to uniquely identify it. You can specify your own primary key field (or fields for compound keys) but by default a field called id is created. See viewModelFactory documentation for details on customising the primary key name.

You can always access the primary key for a record using the _key property regardless of what the underlying field(s) are.

staffUser._key === 1;
// true

Sometimes you only have some of the fields on the record. Presto supports the concept of a partial record which is simply a record with a subset of the fields filled in. To create a partial record simply instantiate the record with only the fields you have available. This concept becomes important when we start to deal with caching.

NOTE

A partial record is not the same as a record with null values. A partial record has no value for some fields whereas a regular record may just have some values set to null.

Fields can be accessed via the fields property or getField property. getField supports traversing related ViewModel fields which will be discussed later.

Related ViewModel's

One ViewModel often refers to another (eg. a foreign key from one table to another in a database). To model this use RelatedViewModelField.

class PhoneNumber extends viewModelFactory({
    phoneNumber: new CharField(),
    userId: new IntegerField(),
    user: new RelatedViewModelField({ to: User, sourceFieldName: 'userId' }),
}) {}

There's two fields required: the field that stores the actual ID of the related record and a field to store a link to actual related record itself.

The to option can either be a ViewModel class directly, a function that returns a ViewModel class or a function that returns a Promise that resolves to a ViewModel class. The second form is useful when the class hasn't been defined yet and the Promise form can be used to dynamically import code. See RelatedViewModelField for how that works.

As mentioned above getField can be used to traverse related ViewModel fields. This is done using array notation where each entry in the array is a field name (all entries apart from the last must be a RelatedViewModelField).

PhoneNumber.getField(['user', 'email']);

This will return the email field from the User ViewModel.

Circular references are also supported using the function form for to:

class User extends viewModelFactory({
    name: new CharField(),
    emailAddress: new EmailField(),
    defaultPhoneNumberId: new IntegerField(),
    defaultPhoneNumber: new RelatedViewModelField({
        to: () => PhoneNumber,
        sourceFieldName: 'defaultPhoneNumberId',
    }),
}) {}
class PhoneNumber extends viewModelFactory({
    phoneNumber: new CharField(),
    userId: new IntegerField(),
    user: new RelatedViewModelField({ to: User, sourceFieldName: 'userId' }),
}) {}

Although somewhat useless in this case you can retrieve fields across the relations:

User.getField(['defaultPhoneNumber', 'user', 'defaultPhoneNumber', 'phoneNumber']);

Relations become really useful when used in conjunction with caching.

ViewModel Caching

Each ViewModel class has an associated cache that is created automatically. You can provide your own by extending ViewModelCache and assigning it to the static cache property on the ViewModel class.

To cache an entry call the add method with either an instance of the record or the data directly:

// These two statements are equivalent
User.cache.add(new User({ id: 1, name: 'John', email: 'john@example.com' });
User.cache.add({ id: 1, name: 'John', email: 'john@example.com'  });

If you have nested data you can populate multiple caches based on defined RelatedViewModelField fields:

User.cache.add({
    id: 1,
    name: 'John',
    email: 'john@example.com',
    phoneNumber: { id: 5, phoneNumber: '(03) 5550 1234', userId: 1 },
});

This will populate the User cache and PhoneNumber cache.

To get an entry use get or getList methods using the primary key of the record and the list of fields you want:

const john = User.cache.get(1, ['name']);
// User({
//   id: 1,
//   name: 'John',
// })

This will return a partial record that contains only the name field. Accessing any other field will result in a warning telling you the data was not available. Partial records are always kept in sync with the latest data where possible (ie. when new data is cached that is a superset of the fields on the partial record).

You can also retrieve the latest version of a record by passing a record instance:

const latestJohn = User.cache.get(john);

NOTE

The primary key is always returned regardless of whether you request it or not. If you request a RelatedViewModelField then the associated sourceFieldName is also returned regardless of whether you request it.

getList can be used to retrieve multiple records. The options are the same as get except you pass an array instead of a single id or record:

const people = User.cache.getList([1, 2, 3], ['name']);

or with records

User.cache.getList([frodo, samwise]);

Any records missing will be null by default but they can be removed by passing the final argument removeNulls which, when true, will filter out any missing records.

You can also retrieve data across caches by using array notation for fields:

User.cache.get(1, ['name', ['phoneNumber', 'phone']]);
// User({
//   id: 1,
//   name: 'John',
//   phoneNumber: PhoneNumber({
//     id: 5,
//     phoneNumber: '(03) 5550 1234',
//   })
// })

NOTE

Cache get and getList always return instances of a ViewModel even if the data is cached directly. RelatedViewModelField's retrieved are also instances of the relevant ViewModel class.

To get notified when something in the case changes use addListener or addListenerList. To listen to any change just pass a function:

cache.addListener(() => console.log('Change detected!'));

It will be called when anything at all changes in the cache.

To listen to specific record changes pass the ID and field names:

User.cache.addListener(1, ['name'], (prev, current) => {
    console.log('Record changed from', prev, 'to', current);
});

or to multiple values:

User.cache.addListenerList([1, 2, 3, 4], ['name'], (prev, current) => {
    console.log('Records changed from', prev, 'to', current);
});

useViewModelCache

useViewModelCache returns data from the cache and automatically re-renders whenever the underlying data in the cache changes. With this you can write UI that responds immediately to changes to the cached data it renders:

const user = useViewModelCache(User, cache => cache.get(1, ['name', 'email']));

The second argument is just a selector function that gets passed the cache and can return any value. The selector will be called anytime the cache changes and will re-render your component if the value returned from the selector differs from the last time it was called.

The selector can return anything. Here we return user records grouped by a field:

const usersByGroup = cache =>
    cache.getAll(['groupId', 'firstName', 'email']).reduce((acc, record) => {
        acc[record.groupId] = acc[record.firstName] || [];
        acc[record.groupId].push(record);
        return acc;
    }, {});
const groupedUsers = useViewModelCache(User, usersByGroup);

By default useViewModelCache will compare the previous and current value from the selector function using a strict equality check. In the example above usersByGroup returns a new object each time and so will fail the equality check. In critical parts of you application this may have a performance impact so for those times you can provide your own equality check:

import { isDeepEqual } from '@prestojs/util';

const groupedUsers = useViewModelCache(User, usersByGroup, [], isDeepEqual);

Here it uses the isDeepEqual function to do a deep equality to avoid re-rendering when the data is identical.

The third argument is a list of arguments to pass to the selector function. This allows you to write re-usable selectors that can just be passed straight through to useViewModelCache that will only be called whenever any of the arguments change.

// Define a selector that selects a record from a cache
const selectRecord = (cache, id, fieldNames) => cache.get(id, fieldNames);

const fieldNames = ['name', 'id'];
const id = 1;

// selectRecord will be called only if id or fieldNames changes
const record = useViewModelCache(User, selectRecord, [id, fieldNames]);

// Alternatively you can use an arrow function. The difference is that
// selectRecord will be called every time the containing component or
// hook renders.
const record = useViewModelCache(user, cache => selectRecord(cache, id, fieldNames));