在项目中会用到颜色渐变，我们通过XML实现

创建xml文件

在drawable文件夹下创建shape资源：
shape_gradient.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:angle="90"
        android:endColor="@color/colorAccent"
        android:startColor="@color/colorPrimary" />
</shape>

注：

[shape] 根标签，声明一个shape 
[gradient] 声明该shape的属性-渐变色,除此外还有其他属性如corners、stroke、size等等 
[android:angle]渐变色的角度，举例来说，0代表从上至下颜色渐变；45代表从左至右颜色渐变；90代表从下至上颜色渐变… 
[android:startColor&android:endColor] 很好理解，渐变开始的颜色和渐变结束时的颜色（从什么颜色变到什么颜色）

自定义View

MyView.java

public class MyView extends View {

    public MyView(Context context) {
        super(context);
    }

    public MyView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }


    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //获取View的宽高
        int width = getWidth();
        int height = getHeight();

        int colorStart = getResources().getColor(R.color.red);
        int color1 = Color.GRAY;
        int colorEnd = getResources().getColor(R.color.star_yellow);

        Paint paint = new Paint();
        LinearGradient backGradient = new LinearGradient(0, 0, 0, height, new int[]{colorStart, color1, colorEnd}, null, Shader.TileMode.CLAMP);
        paint.setShader(backGradient);
        canvas.drawRect(0, 0, width, height, paint);
    }
}

使用在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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.yunlin.xihai.user.gradient.view.MyView
        android:layout_width="match_parent"
        android:layout_height="200dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>