如何在 FloatingActionButton 上显示图标？

我在 FloatingActionButton 上显示图标时遇到问题。图标失去透明度。我有compileSdkVersion 28。

依赖项是：

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    //noinspection GradleCompatible
    implementation 'com.android.support:appcompat-v7:28.0.0-alpha1'
    //noinspection GradleDynamicVersion
    implementation 'com.android.support:recyclerview-v7:28.0.0-rc02'
    implementation 'com.android.support:design:28.+'
    implementation 'com.android.support:cardview-v7:28.0.0-rc01'
    implementation 'com.android.support.constraint:constraint-layout:1.1.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso- 
    core:3.0.2'
    implementation 'com.squareup.picasso:picasso:2.71828'

}

布局是：

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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"
android:background="@color/colorDivider"
tools:context=".activities.MainActivity">

<android.support.v7.widget.RecyclerView
    android:id="@+id/recyclerViewId"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:scrollbars="vertical" />

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fabAddCity"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"
    android:layout_margin="16dp"
    android:clickable="true"
    android:src="@mipmap/ic_plus"
    app:backgroundTint="@color/colorPrimaryDark"
    app:borderWidth="0dp"
    android:focusable="true"/>
</FrameLayout>

它看起来像这样：

最近我使用时可能遇到问题 28.0.0-** 图书馆的

所以我建议你

implementation 'com.android.support:appcompat-v7:27.1.1'
implementation 'com.android.support:design:27.1.1'

和改变

 compileSdkVersion 27
 targetSdkVersion 27

浮动按钮示例

<android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_dialog_email" />

添加维度

dimens.xml
<resources>
    <!-- Default screen margins, per the Android Design guidelines. -->
    <dimen name="activity_horizontal_margin">16dp</dimen>
    <dimen name="activity_vertical_margin">16dp</dimen>
    <dimen name="fab_margin">16dp</dimen>
</resources>

我建议再次查看示例以获得更多许可https://www.androidhive.info/2015/12/android-material-design-floating-action-button/ https://www.androidhive.info/2015/12/android-material-design-floating-action-button/