如何在Android中创建动画渐变背景。

在进入示例之前，我们应该了解什么是渐变色。根据维基百科，在计算机图形学中，颜色渐变（有时称为颜色渐变或颜色渐变）指定了一系列依赖于位置的颜色，通常用于填充区域。例如，许多窗口管理器允许将屏幕背景指定为渐变。

此示例演示了如何在Android中创建动画渐变背景。

步骤 1 - 在Android Studio中创建一个新项目，转到文件 ⇒ 新建项目，并填写所有必要的信息以创建一个新项目。

步骤 2 - 将以下代码添加到res/layout/activity_main.xml中。

<?xml version = "1.0" encoding = "utf-8"?>
<android.support.constraint.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:id = "@+id/constraintLayout"
   android:layout_width = "match_parent"
   android:layout_height = "match_parent"
   android:background = "@drawable/gradient_animation "
   tools:context = ".MainActivity">
   <!-- Your layout here -->
   <TextView
      android:layout_width = "368dp"
      android:layout_height = "520dp"
      android:layout_marginBottom = "8dp"
      android:layout_marginLeft = "8dp"
      android:layout_marginRight = "8dp"
      android:layout_marginTop = "8dp"
      android:gravity = "center"
      android:text = "@string/app_name"
      android:textAlignment = "center"
      android:textColor = "@android:color/background_light"
      android:textSize = "30sp"
      android:textStyle = "bold"
      app:layout_constraintBottom_toBottomOf = "parent"
      app:layout_constraintLeft_toLeftOf = "parent"
      app:layout_constraintRight_toRightOf = "parent"
      app:layout_constraintTop_toTopOf = "parent"
      tools:text = "@string/app_name"/>
</android.support.constraint.ConstraintLayout>

在上面的代码中，我们将背景设置为drawable中的gradient_animation。现在在drawable文件夹中创建gradient_animation.xml文件并添加以下代码 -

<?xml version = "1.0" encoding = "utf-8"?>
<animation-list xmlns:android = "http://schemas.android.com/apk/res/android">
   <item
      android:drawable = "@drawable/drawable_purple_gradient"
      android:duration = "3000" />
   <item
      android:drawable = "@drawable/drawable_amber_gradient"
      android:duration = "3000" />
   <item
      android:drawable = "@drawable/drawable_green_gradient"
      android:duration = "3000" />
   <item
      android:drawable = "@drawable/drawable_red_gradient"
      android:duration = "3000" />
</animation-list>

在上面的animation-list中，我们添加了4个子drawable并为每个动画视图添加了持续时间，超时后，它将更改背景。在drawable_purple_gradient中，包含紫色背景，因此在drawable文件夹中创建一个名为drawable_purple_gradient.xml的文件并添加以下代码 -

<?xml version = "1.0" encoding = "utf-8"?>
<shape xmlns:android = "http://schemas.android.com/apk/res/android">
   <gradient
      android:angle = "90"
      android:endColor = "#D500F9"
      android:startColor = "#4A148C" />
</shape>

按照上述步骤，在Drawable文件夹中创建drawable_amber_gradient.xml、drawable_green_gradient.xml和drawable_red_gradient.xml，并添加以下代码，如下所示 -

drawable_amber_gradient.xml-

<?xml version = "1.0" encoding = "utf-8"?>
<shape xmlns:android = "http://schemas.android.com/apk/res/android">
   <gradient
      android:angle = "135"
      android:endColor = "#FFC400"
      android:startColor = "#FF6F00" />
</shape>

drawable_green_gradient.xml -

<?xml version = "1.0" encoding = "utf-8"?>
<shape xmlns:android = "http://schemas.android.com/apk/res/android">
   <gradient
      android:angle = "0"
      android:endColor = "#00E676"
      android:startColor = "#1B5E20"/>
</shape>

drawable_red_gradient.xml -

<?xml version = "1.0" encoding = "utf-8"?>
<shape xmlns:android = "http://schemas.android.com/apk/res/android">
   <gradient
      android:angle = "45"
      android:endColor = "#FF1744"
      android:startColor = "#B71C1C" />
</shape>

步骤 3 - 将以下代码添加到src/MainActivity.java中 -

package com.example.andy.myapplication;

import android.graphics.drawable.AnimationDrawable;
import android.support.constraint.ConstraintLayout;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {
   private ConstraintLayout constraintLayout;
   private AnimationDrawable animationDrawable;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      constraintLayout = (ConstraintLayout) findViewById(R.id.constraintLayout);
      animationDrawable = (AnimationDrawable) constraintLayout.getBackground();
      animationDrawable.setEnterFadeDuration(3000);
      animationDrawable.setExitFadeDuration(2000);
   }
   @Override
   protected void onResume() {
      super.onResume();
      if (animationDrawable ! = null && !animationDrawable.isRunning()) {
         animationDrawable.start();
      }
   }
   @Override
   protected void onPause() {
      super.onPause();
      if (animationDrawable ! = null && animationDrawable.isRunning()) {
         animationDrawable.stop();
      }
   }
}

让我们尝试运行您的应用程序。我假设您已将您的实际Android移动设备连接到您的计算机。要从Android Studio运行应用程序，请打开您项目中的一个活动文件，然后单击工具栏中的运行图标。选择您的移动设备作为选项，然后检查您的移动设备，它将显示您的默认屏幕 -