Many ionic applications will definitely benefit from their ability to persist
data across app restocks and app reloads within your application.
So very often when you turn off your mobile device and then turn it on at a later time,
or close your app and then restart your app,
you want any changes that you have made,
or any data that you have input into your application,
to be saved so that it can be retrieved at a future time.
So, this is when the permanent storage that is available on
your mobile devices comes to your aid and we have
seen that both Android and iOS have a way of persisting data within the device.
Although you won't see their support like in
their full fledged file system that you might see on
a standard desktop or a regular computer
but their ability to be able to store and retrieve data is very useful.
Structured data can also be stored using
the database support that is available in mobile devices.
Both Android and iOS make use of
the SQLite database for supporting structured data on your mobile device.
We'll look at SQLite a little bit later.
But ionic itself uses a class called 'the storage' which
provides an all encompassing view of the underlying storage.
So that it separates you from the details of exactly how the storage is done on
a specific mobile device and exports a common API that you can make use of
within your ionic application.
Let's look at the details next.
Ionic storage supports key/value pair storage so
within your application you can store content by addressing them using
a key and then specifying the corresponding value and then either
store this content and then retrieve the content at a later stage by specifying the key.
Now, underneath, the ionic storage uses a variety of storage underneath.
If you are running your ionic application as a native app,
then you can use SQLite underneath.
We will see how we can configure ionic storage to use SQLite in one of the later lessons.
If not, then it results to using the IndexedDB or WebSQL or
Localstorage in that order when you are
using your ionic application as a progressive web application.
Now, if you are going to be using it on
a native device and you want to use
the SQLite storage support that is available within the native device,
be it Android or be it iOS,
then you'll need to install what is called the Cordova SQLite plugin.
We'll look at that in one of the later exercises.
For this moment, we're going to result to using the standard,
the default web ionic wants to store.
So if you don't specify the SQLite, well,
maybe that's also the result of using IndexedDB or WebSQL or local storage in that order,
whatever, ionic can enable to use on a specific device.
In the exercise that follows this lecture,
you will see that our ionic application is using IndexedDB in our browser.
The storage itself is part of
the ionic storage library so you will import the ionic storage module
from the ionic storage library into your app module and then you
configure that within your app module as we will see in the exercise.
And then, wherever you want to make use of storage,
say for example within various components of the various pages in your ionic app,
you will import storage from ionic storage
and then inject the storage service into the constructor of
your component or your page and then it becomes available for you and provides you
with various methods that then you can use to store your content.
Storage itself supports a promise-based methods which we will discuss in the next slide.
The storage itself supports several instance methods.
Here, I'll give you a subset of
these instance methods specifically those that I will make use of in the exercise.
There are a few more than these but these are the key instance methods
that we will make use of with an atypical ionic application.
Now, in your application you may want to first check by calling
the ready method on your storage to ensure that your storage is ready for being accessed.
Now, in your application you can use the get
method which takes the key as a parameter.
The get method will return the value corresponding to the key.
You can return a promise first and then inside the promise you use
the then method of the promise and inside of the then method then the problem resolves,
the data will become available for your application.
So, you will see me using this approach within the exercise.
Similarly, the set method takes the key and the value as the two parameters,
and then stores the data into your underlying storage.
The removed method, when given a key,
will delete the corresponding value that is stored in your ionic storage.
The clear method will clear the entire contents of
your storage so be very careful when you are using the clear method.
You want to be sure that that is exactly what you want to
do when you want to invoke the clear method.
With this quick understanding of ionic storage we will proceed onto
the exercise where we will see the use of
these various methods within our ionic application.
Jogesh K. MuppalaAssociate Professor
