Part 3 – Expression Syntax 1/2

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In part 3 of the Data Binding tutorial, we will start learning the syntax of the expressions we can use in our data binding layout files. Data binding expressions have their very own syntax with similarities to both Java and Kotlin, but also some differences and caveats that we have to keep in mind.

String resource placeholder syntax explained:
https://stackoverflow.com/questions/3656371/is-it-possible-dynamically-to-add-string-to-string-xml-in-android/40715374#40715374

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fragment_example.xmlstrings.xmlExampleFragment.ktactivity_main.xmlMainActivity.ktUser.kt

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools">

    <data>

        <import type="android.view.View" />

        <import type="com.codinginflow.databindingexample.User" />

        <variable
            name="user"
            type="User" />
    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:orientation="vertical"
        tools:context=".ExampleFragment">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{@string/hello(user.firstName, user.lastName)}"
            android:textAppearance="@style/TextAppearance.AppCompat.Large" />

        <TextView
            android:id="@+id/text_view_first_name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{user.firstName, default = `First Name`}"
            android:textAppearance="@style/TextAppearance.AppCompat.Large" />

        <TextView
            android:id="@+id/text_view_last_name"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text='@{user.lastName, default = "Last Name"}'
            android:textAppearance="@style/TextAppearance.AppCompat.Large" />

        <TextView
            android:id="@+id/text_view_age"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(user.age) + ` years old`}"
            android:textAppearance="@style/TextAppearance.AppCompat.Large"
            android:textColor="@{user.age &lt; 18 ? @android:color/holo_red_light : @android:color/black}" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="You are not old enough to use this app"
            android:visibility="@{user.age > 17 ? View.GONE : View.VISIBLE}" />

    </LinearLayout>
</layout>

<resources>
    <string name="app_name">Data Binding Example</string>
    <string name="hello">Hello, %1$s %2$s</string>
</resources>

package com.codinginflow.databindingexample

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import com.codinginflow.databindingexample.databinding.FragmentExampleBinding

class ExampleFragment : Fragment() {

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val binding = FragmentExampleBinding.inflate(inflater,
            container, false)

        binding.user = User("Florian",  "Walther", 17)

        return binding.root
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <FrameLayout
        android:id="@+id/fragment_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

package com.codinginflow.databindingexample

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        supportFragmentManager.beginTransaction().replace(R.id.fragment_container,
            ExampleFragment()).commit()
    }
}

package com.codinginflow.databindingexample

data class User(val firstName: String, val lastName: String, val age: Int)