Hey there! Let's break down your two gradient drawable questions one by one, with practical examples for both XML and code implementations:

You can create gradient drawables either via XML (the most common approach for static styles) or programmatically in code for dynamic adjustments.

XML Implementation #

Use a <shape> drawable with a <gradient> child tag to define your gradient. Here are examples for the three main gradient types:

Linear Gradient (Left to Right)

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="linear"
        android:startColor="#FF4081"
        android:endColor="#3F51B5"
        android:angle="0"/> <!-- 0 = left to right, 90 = bottom to top -->
</shape>

Radial Gradient (Centered, Fixed Radius)

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="radial"
        android:startColor="#FFEB3B"
        android:endColor="#F44336"
        android:centerX="0.5"
        android:centerY="0.5"
        android:gradientRadius="100dp"/> <!-- Radius of the radial gradient -->
</shape>

Sweep Gradient (Full Circle)

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="sweep"
        android:startColor="#4CAF50"
        android:centerColor="#FF9800"
        android:endColor="#9C27B0"/>
</shape>

Key attributes to tweak for custom styles:

• android:type: Defines gradient type (linear, radial, sweep)
• android:startColor/android:centerColor/android:endColor: Color stops for the gradient
• android:centerX/android:centerY: Position of the gradient center (0-1, default 0.5)
• android:gradientRadius: Required for radial gradients

Programmatic Implementation #

Use the GradientDrawable class to build gradients dynamically:

// Linear Gradient (Top to Bottom)
GradientDrawable linearGradient = new GradientDrawable(
    GradientDrawable.Orientation.TOP_BOTTOM,
    new int[]{Color.parseColor("#FF4081"), Color.parseColor("#3F51B5")}
);

// Radial Gradient
GradientDrawable radialGradient = new GradientDrawable();
radialGradient.setGradientType(GradientDrawable.RADIAL_GRADIENT);
radialGradient.setColors(new int[]{Color.parseColor("#FFEB3B"), Color.parseColor("#F44336")});
radialGradient.setGradientRadius(100f); // Radius in pixels
radialGradient.setGradientCenter(0.5f, 0.5f);

// Sweep Gradient
GradientDrawable sweepGradient = new GradientDrawable();
sweepGradient.setGradientType(GradientDrawable.SWEEP_GRADIENT);
sweepGradient.setColors(new int[]{Color.parseColor("#4CAF50"), Color.parseColor("#FF9800"), Color.parseColor("#9C27B0")});

By default, sweep gradients start at 0 degrees (which maps to the 3 o'clock position) and rotate clockwise. To adjust the starting angle, you can use these methods:

XML Implementation #

Add the android:angle attribute to your <gradient> tag. The angle is measured in degrees, clockwise from the 3 o'clock position:

• 12 o'clock = 270 degrees
• 9 o'clock = 180 degrees
• 6 o'clock = 90 degrees

Example: Start sweep gradient from 12 o'clock (top):

<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
        android:type="sweep"
        android:startColor="#4CAF50"
        android:centerColor="#FF9800"
        android:endColor="#9C27B0"
        android:angle="270"/> <!-- Rotate start point to top -->
</shape>

Programmatic Implementation #

For API 24+

Use the setAngle(float angle) method directly on GradientDrawable:

GradientDrawable sweepGradient = new GradientDrawable();
sweepGradient.setGradientType(GradientDrawable.SWEEP_GRADIENT);
sweepGradient.setColors(new int[]{Color.GREEN, Color.YELLOW, Color.RED});
sweepGradient.setAngle(270f); // Start at 12 o'clock

For API <24

Since setAngle() isn't available before API 24, use SweepGradient shader with a matrix rotation:

// Define center coordinates (e.g., center of your view)
int centerX = view.getWidth() / 2;
int centerY = view.getHeight() / 2;

// Create sweep gradient
SweepGradient sweepShader = new SweepGradient(
    centerX, centerY,
    new int[]{Color.GREEN, Color.YELLOW, Color.RED},
    null // Use default color positions
);

// Rotate the gradient 270 degrees to start at 12 o'clock
Matrix matrix = new Matrix();
matrix.postRotate(270, centerX, centerY);
sweepShader.setLocalMatrix(matrix);

// Apply shader to a view's background or paint
view.getBackground().setShader(sweepShader);

内容的提问来源于stack exchange，提问作者Mohsen Delghandi