Android 自定义View

Android 自定义View

一 简单的自定义View

Android View 的绘制流程主要有 onMeasure, onLayout, onDraw. UI 内容的绘制主要在onDraw 中完成。

1.1 首先定义View类MyView

MyView 继承View, 实现两个构造方法。MyView(Context context, @Nullable AttributeSet attrs)是在XML中定义时解析时使用的构造方法。XML中的属性被解析为AttributeSet类。

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

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

MyView 有View的基本属性，所以在layout 中可以配置View的基本属性：

<com.dawang.androidexample.view.MyView
    android:layout_height="match_parent"
    android:layout_width="match_parent"/>

1.2 重写onDraw 方法

在这里重写onDraw 方法，使用Canvas 类完成图形的绘制.

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if(mDyDraw) {
            drawDynamicCircle(canvas);
        } else {
            drawText(canvas);
            drawLine(canvas);
            drawRoundRect(canvas);
            drawCircleStroke(canvas);
            drawCircleFill(canvas);
            drawPath(canvas);
            drawOval(canvas);
            drawArc(canvas);
        }
    }

在onDraw 方法中，绘制了各种图形，主要的内容涉及 Canvas 类和Paint 类。方法如下

void drawText(Canvas canvas){
        mPaint.setTextSize(30);
        mPaint.setShadowLayer(10, 55, 22, Color.GREEN);
        canvas.drawText("View Hello World", 0, 30, mPaint);
    }

    void drawLine(Canvas canvas){
        mPaint.setColor(0xffffff00);
        mPaint.setStrokeWidth(10);
        canvas.drawLine(0, 60, 200, 60, mPaint);
    }

    void drawRoundRect(Canvas canvas){
        mPaint.setARGB(255, 255, 0, 255);
        Rect rect = new Rect(10, 60, 100, 100);
        mPaint.setShadowLayer(30, 5, 2, Color.DKGRAY);
        canvas.drawRoundRect(new RectF(rect), 10, 10, mPaint);
    }

    void drawCircleStroke(Canvas canvas){
        mPaint.setAntiAlias(true);
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(5);
        mPaint.setShadowLayer(5, 15, 15, Color.LTGRAY);

//        canvas.drawRGB(255, 255,255);
        canvas.drawCircle(150, 200, 100, mPaint);
    }

    void drawCircleFill(Canvas canvas){
        mPaint.setAntiAlias(true);
        mPaint.setColor(Color.RED);
        mPaint.setStyle(Paint.Style.FILL);
        mPaint.setStrokeWidth(5);

        canvas.drawCircle(380, 200, 100, mPaint);
    }

    void drawPath(Canvas canvas){
        mPaint.reset();
        mPaint.setColor(0xff00ff00);
        mPaint.setStrokeWidth(10);
        mPaint.setStyle(Paint.Style.STROKE);

        Path path = new Path();
        path.moveTo(0, 500);
        path.lineTo(450, 200);
        path.quadTo(0, 500, 40, 570);

        canvas.drawPath(path, mPaint);
    }

    void  drawOval(Canvas canvas){
        Rect rect = new Rect(100, 500, 400, 600);

        canvas.drawRect(rect, mPaint);
        mPaint.setColor(Color.BLUE);
        canvas.drawOval(new RectF(rect), mPaint);

    }
    void drawArc(Canvas canvas){
//        Rect rect = new Rect(400, 500, 600, 600);
        Rect rect = new Rect(100, 500, 400, 600);
        mPaint.setColor(Color.RED);

        canvas.drawArc(new RectF(rect), 0, 90, false, mPaint);
    }

    void drawDynamicCircle(Canvas canvas){
        mPaint.setAntiAlias(true);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeWidth(10);
        int width = getWidth();
        int height = getHeight();
        int min = width < height ? width:height;
        Rect rect = new Rect(width/2 - min/2, (height-min)/2, width/2+min/2, (height+min)/2);

        if(mDyCircle % 4 == 0) {
            mPaint.setColor(Color.RED);
            canvas.drawArc(new RectF(rect), 0, 90, false, mPaint);
        } else if(mDyCircle % 4 == 1) {

            mPaint.setColor(Color.BLUE);
            canvas.drawArc(new RectF(rect), 90, 90, false, mPaint);
        } else if(mDyCircle % 4 == 2) {
            mPaint.setColor(Color.DKGRAY);
            canvas.drawArc(new RectF(rect), 180, 90, false, mPaint);
        } else if(mDyCircle % 4 == 3) {
            mPaint.setColor(Color.YELLOW);
            canvas.drawArc(new RectF(rect), 270, 90, false, mPaint);
        }

        mDyCircle ++;
        postInvalidateDelayed(1000);
    }

1.3 自定义属性

View 可以通过xml直接设置属性，MyView 实现一个属性dy_draw，表示是动态画一个圆还是静态画一些图形。

res/valus/attrs.xml 中定义属性：

format 表示类型：有integer boolean fraction refence color等类型。

<resources>
    <declare-styleable name="MyView" >
        <attr name="dy_draw" format="boolean" />
    declare-styleable>
resources>

在构造函数中获取属性

    public MyView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        TypedArray array = context.obtainStyledAttributes(attrs, R.styleable.MyView);
        mDyDraw = array.getBoolean(R.styleable.MyView_dy_draw, false);
        array.recycle();
    }

显示效果

二 android.graphics 概念

Canvas 和 Paint 概念涉及Android 图形知识。Android 的二维图形绘制上基于Skia 完成。Skia 是一个基于C++开发的图形库。Android Java 层的图形绘制概念基本上来自于Skia. Canvas Paint Bitmap native等都是对Skia中各个 概念的继承, 了解Skia API 对理解view的绘制有很大帮助。在 android.graphics包中，这个包的注释中：

Provides low level graphics tools such as canvases, color filters, points, and
rectangles that let you handle drawing to the screen directly.

java 代码：frameworks/base/graphics/java/android/graphics/
jni 代码： frameworks/base/core/jni/android/graphics/
frameworks/base/core/jni/android_graphics_*.cpp

以Canvas drawText 方法为例，看下具体的实现：
Canvas#drawText 通过Jni 调用drawTextString，然后通过get_canvas 把canvasHandle 获取到native Canvas, Jni native canvasHandle 对应于java 的mNativeCanvasWrapper

public void drawText(@NonNull String text, float x, float y, @NonNull Paint paint) {
        native_drawText(mNativeCanvasWrapper, text, 0, text.length(), x, y, paint.mBidiFlags,
                paint.getNativeInstance(), paint.mNativeTypeface);
}

native_drawText:
static void drawTextString(JNIEnv* env, jobject, jlong canvasHandle, jstring text,
                           jint start, jint end, jfloat x, jfloat y, jint bidiFlags,
                           jlong paintHandle, jlong typefaceHandle) {
    Paint* paint = reinterpret_cast(paintHandle);
    Typeface* typeface = reinterpret_cast(typefaceHandle);
    const int count = end - start;
    const jchar* jchars = env->GetStringChars(text, NULL);
    get_canvas(canvasHandle)->drawText(jchars + start, 0, count, count, x, y,
                                       bidiFlags, *paint, typeface);
    env->ReleaseStringChars(text, jchars);
}

static Canvas* get_canvas(jlong canvasHandle) {
    return reinterpret_cast(canvasHandle);
}

Java 的 mNativeCanvasWrapper 在Canvas 构造的视同通过initRaster获取，initRaster又是一个Jni调用，最终通过Canvas::create_canvas 得到真正的Canvas, 饶了一圈回到了native。

public Canvas() {
        if (!isHardwareAccelerated()) {
            // 0 means no native bitmap
            mNativeCanvasWrapper = initRaster(null);
            mFinalizer = NoImagePreloadHolder.sRegistry.registerNativeAllocation(
                    this, mNativeCanvasWrapper);
        } else {
            mFinalizer = null;
        }
    }


static jlong initRaster(JNIEnv* env, jobject, jobject jbitmap) {
    SkBitmap bitmap;
    if (jbitmap != NULL) {
        GraphicsJNI::getSkBitmap(env, jbitmap, &bitmap);
    }
    return reinterpret_cast(Canvas::create_canvas(bitmap));
}

Canvas

Android 对绘制要求四个基本的元素，Canvas Bitmap primitive(Rect, Path, text, Bitmap) Paint.
对Canvas的定义, Canvas 拥有draw 方法，可以绘制各种文字，图片 Path等。

/**
 * The Canvas class holds the "draw" calls. To draw something, you need
 * 4 basic components: A Bitmap to hold the pixels, a Canvas to host
 * the draw calls (writing into the bitmap), a drawing primitive (e.g. Rect,
 * Path, text, Bitmap), and a paint (to describe the colors and styles for the
 * drawing).
 *

Bitmap

Bitmap 就是描述绘制内容的的内存，用来保存像素。

Paint 类

Paint 表示绘制用的画笔，比如可以设置绘制文字的大小，颜色等。
自定义控件三部曲之绘图篇(七)——Paint之函数大汇总