先看下图

一.全局替换方式

1.新建assets引入资源

?2.? 自定义application，将第三方的字体，替换当前系统默认字体

class App : Application() {
    override fun onCreate() {
        super.onCreate()
        initTypeface()
    }

    private fun initTypeface() {
        val typefaceBysong = Typeface.createFromAsset(assets, "fonts/bysong.ttf")
        val monospace = Typeface::class.java.getDeclaredField("MONOSPACE")
        monospace.isAccessible = true
        monospace.set(null, typefaceBysong)
    }
}

?3.定义app的style（主要是<item name="android:typeface">monospace</item>）

<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.Study" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_500</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_700</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>

    <style name="AppTheme.NoActionBar" parent="Theme.Study">
        <item name="windowActionBar">false</item>
        <item name="windowNoTitle">true</item>
        <item name="android:typeface">monospace</item>
    </style>
</resources>

4.设置在manifests文件中设置application，替换theme="@style/AppTheme.NoActionBar"

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ming.widgets">

    <application
        android:name=".App"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme.NoActionBar">
      
        <activity
            android:name=".FontActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

?这样全部都替换成第三方字体，对于字体因为太大而导致的问题，可以将字体的扩展名改成zip或者jpg就会解决。

二.单个TextView替换

1.对每个TextView调用 setTypeface(@Nullable Typeface tf)方法

class FontActivity : AppCompatActivity() {
    private lateinit var mBinding: ActivityFontBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mBinding = ActivityFontBinding.inflate(LayoutInflater.from(this))
        setContentView(mBinding.root)


        //得到AssetManager
        val typefaceBysong = Typeface.createFromAsset(assets, "fonts/bysong.ttf")
        //设置字体
        mBinding.textViewThird.typeface = typefaceBysong
    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FontActivity">

    <TextView
        android:id="@+id/textViewThird"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        android:text="三方字体文字"/>
    <TextView
        android:id="@+id/textViewNormal"
        android:layout_marginTop="100dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        android:text="普通文字"/>
</androidx.constraintlayout.widget.ConstraintLayout>

? 但是这样未免有些麻烦，需要对每个TextView调用 setTypeface(@Nullable Typeface tf)方法，于是还可以这样

2在res资源文件夹下新建font文件夹，将三方字体资源文件导入

?对TextView添加android:fontFamily="@font/fzkatong"标签即可

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FontActivity">

          
    <TextView
        android:id="@+id/textViewResFont"
        android:layout_marginTop="200dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        android:fontFamily="@font/fzkatong"
        android:text="引用Res/Font下资源的文字"/>
</androidx.constraintlayout.widget.ConstraintLayout>