Android Path 圆角分析

Android_Drawable

Java层的Path.java 对应C层的Path.cpp,SkPath.cpp
对应的C代码位置分别在：
android / platform / frameworks / base / core / jni / android / graphics / Path.cpp
android / platform / external / skia / src / core / SkPath.cpp

API 18+开始SkPath.cpp的实现有大变动,在API 18之前设置radius>矩形的宽高时不会变成半圆形效果

Q:为什么在xml文件中定义自定义shape时，设置radius属性大于控件本身的大小时就产生了圆角矩形效果
A:xml文件自定义的shape对应到GradientDrawable,设置圆角的具体实现在Path类中。

如果要实现半边圆角矩形效果，则需要矩形的rx/ry分别为矩形的高的一半即可。

基于API 17的源码分析
Path.java

public void addRoundRect(float left, float top, float right, float bottom, float[] radii,
        Direction dir) {
    if (radii.length < 8) {
        throw new ArrayIndexOutOfBoundsException("radii[] needs 8 values");
    }
    isSimplePath = false;
    native_addRoundRect(mNativePath, left, top, right, bottom, radii, dir.nativeInt);
}
// ① 4个rx相同的情况下 -> Path.addRoundRectXY
private static native void native_addRoundRect(long nPath, float left, float top,
                                                   float right, float bottom,
                                                   float rx, float ry, int dir);
// ② 4个rx分别设置的情况 -> Path.addRoundRect8                                                   
private static native void native_addRoundRect(long nPath, float left, float top,
                                                   float right, float bottom,
                                                   float[] radii, int dir);

Path.cpp

// ①
static void addRoundRectXY(JNIEnv* env, jobject clazz, jlong objHandle, jfloat left, jfloat top,
        jfloat right, jfloat bottom, jfloat rx, jfloat ry, jint dirHandle) {
    SkRect rect = SkRect::MakeLTRB(left, top, right, bottom);
    SkPath* obj = reinterpret_cast<SkPath*>(objHandle);
    SkPath::Direction dir = static_cast<SkPath::Direction>(dirHandle);
    obj->addRoundRect(rect, rx, ry, dir); // -> SkPath.addRoundRect ->  SkPaht.add_corner_arc
}
// ②
static void addRoundRect8(JNIEnv* env, jobject, jlong objHandle, jfloat left, jfloat top,
        jfloat right, jfloat bottom, jfloatArray array, jint dirHandle) {
    SkRect rect = SkRect::MakeLTRB(left, top, right, bottom);
    SkPath* obj = reinterpret_cast<SkPath*>(objHandle);
    SkPath::Direction dir = static_cast<SkPath::Direction>(dirHandle);
    AutoJavaFloatArray  afa(env, array, 8);
#ifdef SK_SCALAR_IS_FLOAT
    const float* src = afa.ptr();
#else
    #error Need to convert float array to SkScalar array before calling the following function.
#endif
    obj->addRoundRect(rect, src, dir); // -> SkPath.addRoundRect()
}

SkPath.cpp

// ① 画圆角
static void add_corner_arc(SkPath* path, const SkRect& rect,
                           SkScalar rx, SkScalar ry, int startAngle,
                           SkPath::Direction dir, bool forceMoveTo) {
    // rx取 指定的rx或者矩形宽度一半的较小的一个
    rx = SkMinScalar(SkScalarHalf(rect.width()), rx);
    ry = SkMinScalar(SkScalarHalf(rect.height()), ry);
    SkRect   r;
    r.set(-rx, -ry, rx, ry);
    switch (startAngle) {
        case   0:
            r.offset(rect.fRight - r.fRight, rect.fBottom - r.fBottom);
            break;
        case  90:
            r.offset(rect.fLeft - r.fLeft,   rect.fBottom - r.fBottom);
            break;
        case 180: r.offset(rect.fLeft - r.fLeft,   rect.fTop - r.fTop); break;
        case 270: r.offset(rect.fRight - r.fRight, rect.fTop - r.fTop); break;
        default: SkDEBUGFAIL("unexpected startAngle in add_corner_arc");
    }
    SkScalar start = SkIntToScalar(startAngle);
    SkScalar sweep = SkIntToScalar(90);
    if (SkPath::kCCW_Direction == dir) {
        start += sweep;
        sweep = -sweep;
   }
   path->arcTo(r, start, sweep, forceMoveTo);
}
// ①
void SkPath::addRoundRect(const SkRect& rect, const SkScalar rad[],
                          Direction dir) {
    // abort before we invoke SkAutoPathBoundsUpdate()
    if (rect.isEmpty()) {
        return;
    }
    SkAutoPathBoundsUpdate apbu(this, rect);
    if (kCW_Direction == dir) {
        add_corner_arc(this, rect, rad[0], rad[1], 180, dir, true);
        add_corner_arc(this, rect, rad[2], rad[3], 270, dir, false);
        add_corner_arc(this, rect, rad[4], rad[5],   0, dir, false);
        add_corner_arc(this, rect, rad[6], rad[7],  90, dir, false);
    } else {
        add_corner_arc(this, rect, rad[0], rad[1], 180, dir, true);
        add_corner_arc(this, rect, rad[6], rad[7],  90, dir, false);
        add_corner_arc(this, rect, rad[4], rad[5],   0, dir, false);
        add_corner_arc(this, rect, rad[2], rad[3], 270, dir, false);
    }
    this->close();
}

// ②
void SkPath::addRoundRect(const SkRect& rect, SkScalar rx, SkScalar ry, Direction dir) {
    SkScalar    w = rect.width();
    SkScalar    halfW = SkScalarHalf(w); // rect.width * 0.5f
    SkScalar    h = rect.height();
    SkScalar    halfH = SkScalarHalf(h);
    if (halfW <= 0 || halfH <= 0) {
        return;
    }
    // x方向的宽度大于矩形宽度的一半则跳过
    bool skip_hori = rx >= halfW;
    bool skip_vert = ry >= halfH;
    // 变成椭圆了
    if (skip_hori && skip_vert) {
        this->addOval(rect, dir);
        return;
    }
    SkAutoPathBoundsUpdate apbu(this, rect);
    // 这里就是关键了
    // 椭圆宽度==矩形宽度/2
    if (skip_hori) {
        rx = halfW;
    } else if (skip_vert) {
        ry = halfH;
   }

所以在shape中设定的宽高大于控件的宽高时就会变成圆角矩形了。

以下基于API 25的源码分析