1.为什么要自定义View？

参考链接之超精辟的自定义view 绘制

• UI的奇葩设计。
• 多界面的组件复用。

2.知识点

2.1 MeasureSpec

MeasureSpec是一个32为的整数值，前两位表示测量的模式Spec
Mode，后30位表示该模式下的规格大小SpecSize。

MeasureSpec核心代码如下：

  public static class MeasureSpec {
        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

        /** @hide */
        @IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
        @Retention(RetentionPolicy.SOURCE)
        public @interface MeasureSpecMode {}

        /**
         * Measure specification mode: The parent has not imposed any constraint
         * on the child. It can be whatever size it wants.
         */
        public static final int UNSPECIFIED = 0 << MODE_SHIFT;

        /**
         * Measure specification mode: The parent has determined an exact size
         * for the child. The child is going to be given those bounds regardless
         * of how big it wants to be.
         */
        public static final int EXACTLY     = 1 << MODE_SHIFT;

        /**
         * Measure specification mode: The child can be as large as it wants up
         * to the specified size.
         */
        public static final int AT_MOST     = 2 << MODE_SHIFT;

        /**
         * Creates a measure specification based on the supplied size and mode.
         *
         * The mode must always be one of the following:
         * <ul>
         *  <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
         *  <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
         *  <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
         * </ul>
         *
         * <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
         * implementation was such that the order of arguments did not matter
         * and overflow in either value could impact the resulting MeasureSpec.
         * {@link android.widget.RelativeLayout} was affected by this bug.
         * Apps targeting API levels greater than 17 will get the fixed, more strict
         * behavior.</p>
         *
         * @param size the size of the measure specification
         * @param mode the mode of the measure specification
         * @return the measure specification based on size and mode
         */
        public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
            if (sUseBrokenMakeMeasureSpec) {
                return size + mode;
            } else {
                return (size & ~MODE_MASK) | (mode & MODE_MASK);
            }
        }

        /**
         * Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
         * will automatically get a size of 0. Older apps expect this.
         *
         * @hide internal use only for compatibility with system widgets and older apps
         */
        public static int makeSafeMeasureSpec(int size, int mode) {
            if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
                return 0;
            }
            return makeMeasureSpec(size, mode);
        }

        /**
         * Extracts the mode from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the mode from
         * @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
         *         {@link android.view.View.MeasureSpec#AT_MOST} or
         *         {@link android.view.View.MeasureSpec#EXACTLY}
         */
        @MeasureSpecMode
        public static int getMode(int measureSpec) {
            //noinspection ResourceType
            return (measureSpec & MODE_MASK);
        }

        /**
         * Extracts the size from the supplied measure specification.
         *
         * @param measureSpec the measure specification to extract the size from
         * @return the size in pixels defined in the supplied measure specification
         */
        public static int getSize(int measureSpec) {
            return (measureSpec & ~MODE_MASK);
        }

        static int adjust(int measureSpec, int delta) {
            final int mode = getMode(measureSpec);
            int size = getSize(measureSpec);
            if (mode == UNSPECIFIED) {
                // No need to adjust size for UNSPECIFIED mode.
                return makeMeasureSpec(size, UNSPECIFIED);
            }
            size += delta;
            if (size < 0) {
                Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
                        ") spec: " + toString(measureSpec) + " delta: " + delta);
                size = 0;
            }
            return makeMeasureSpec(size, mode);
        }

        /**
         * Returns a String representation of the specified measure
         * specification.
         *
         * @param measureSpec the measure specification to convert to a String
         * @return a String with the following format: "MeasureSpec: MODE SIZE"
         */
        public static String toString(int measureSpec) {
            int mode = getMode(measureSpec);
            int size = getSize(measureSpec);

            StringBuilder sb = new StringBuilder("MeasureSpec: ");

            if (mode == UNSPECIFIED)
                sb.append("UNSPECIFIED ");
            else if (mode == EXACTLY)
                sb.append("EXACTLY ");
            else if (mode == AT_MOST)
                sb.append("AT_MOST ");
            else
                sb.append(mode).append(" ");

            sb.append(size);
            return sb.toString();
        }
    }

• 主要方法

方法	用法
makeMeasureSpec	重置MeasureSpec约束
getMode	获取MeasureSpec的模式specMode
getSize	获取Measure的模式specSize

• 三种模式

方法（音标）	用法
UNSPECTIFIED（ʌn'spɛsɪfaɪd）	未指明尺寸
EXACTLY（ɪɡ'zæktli）	精确的尺寸
AT_MOST	父视图允许的最大尺寸

2.2 Measure

Measure操作用来计算View的实际大小，用于确定当前View或ViewGroup的实际宽高。
常用方法如下：

• onMeasure的源码

 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

由onMeasure的源码可以看出，该方法的目的，就是为了确定view的具体宽高；

• setMeasuredDimension的源码

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }

用于设置View的宽和高，在每一次view尺寸运算完成使用，可以理解为，onMeasure中最后一个必调的方法；

• setMeasuredDimensionRaw的源码

private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

设置每一行的宽高，很少使用，略；

• getMeasuredWidth的源码

public final int getMeasuredWidth() {
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }

获取view的宽度；

• getLayoutParams()的源码

 @ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
    public ViewGroup.LayoutParams getLayoutParams() {
        return mLayoutParams;
    }

获取View的Layoutparams(备注：layout_开头的所有参数都放在了里面)；

• measureChildren的源码（ViewGroup）

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

遍历所有的子View，测量全部View的宽高。一般与getMeasuredWidth()或getMeasuredHeight()配合使用；

• measureChild的源码（ViewGroup）

protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

用于测量具体子view的宽和高；一般与getMeasuredWidth()或getMeasuredHeight()配合使用；

• getChildAt(i)（ViewGroup）
  获取子View

• getChildCount()（ViewGroup）
  获取子View的数量

• 备注：layout_width 三种设置值的方式，以及对应的模式流程

赋值方式	模式流程
wrap_content	AT_MOST
match_parent	AT_MOST --> EXACTLY
100dp	EXACTLY

2.3 Layout（ViewGroup）

Layout的过程是用于确定View在父布局中的位置。由父布局获取参数，然后将参数传递给子View的layout方法中，将view放在具体的位置；
在onLayout中没有太复杂的逻辑需要处理，相应的参数都可以在onMeasure中获得。建议使用LayoutParams进行传值，若子view的值固定，可以使用makeMeasureSpec进行重置约束，通过setMeasuredDimension或onMeasure进行设置子view的约束。

实例代码如下

@Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        for (int i = 0; i < getChildCount(); i++) {
            View child = getChildAt(i);
            child.layout(l, t, r, b);//这里不能直接使用onLayout的参数，具体的参数值可以通过，全局变量或LayoutParams进行传递；
        }
    }

2.4 Draw（多用于View）

onDraw操作用于绘制view的具体界面，可以绘制常见的图形、文字等，通过对view的操作，也可以实现具体的动画，如MD动画的实现。
同时，由于ViewGroup大多为容器，用户承载view，很少会使用onDraw，当然，也可以使用Draw绘制背景等。

核心知识点

• Paint 画笔
  Paint方法用于在Canvas上绘制内容，可以设置Paint的宽度、颜色、笔触、以及对图片进行滤镜处理等。
  详细的Paint效果，请查看Android 画笔Paint

常用的Paint代码：

 private void initPaint() {
        mPaint = new Paint();
        mPaint.setStyle(Paint.Style.STROKE);//设置画笔样式
        mPaint.setStrokeWidth(10);// 设置画笔宽度
        mPaint.setColor(ContextCompat.getColor(getContext(), R.color.colorAccent));//设置画笔颜色
        mPaint.setAntiAlias(true);//设置抗锯齿
        mPaint.setTextSize(60);//设置文字尺寸
    }

设置

笔触类型	描述
FILL_AND_STROKE	填充内部且描边
STROKE	描边
FILL	填充内部

• Canvas 画布

  绘制文字

        /**
         * @param text  文本
         * @param x     水平方向起点
         * @param y     竖直方向的文字底部
         * @param paint 画笔
         */
        canvas.drawText("画圆：", 50, 1000, mPaint);

绘制矩形

       /**
         * RectF : 屏幕左上角是坐标原点，4个参数：left，top，right，bottom
         * left：屏幕左边到矩形左边的距离
         * top：屏幕顶部到矩形顶边的距离
         * right：屏幕左边到矩形右边的距离
         * bottom：屏幕顶边到矩形底边的距离
         *
         * @desc 绘制一个宽50的矩形
         */
        canvas.drawRect(new RectF(50,50,100,100),mPaint);

** 将整个画布绘制成画笔的颜色**

       /**
         * 将整个画布绘制成画笔的颜色
         */
        canvas.drawPaint(mPaint);

** 绘制一条水平直线**

      /**
         * @param startX X轴的起点
         * @param startY Y轴的起点
         * @param stopX X轴的终点
         * @param stopY Y轴的终点
         * @param paint
         */
        canvas.drawLine(10,10,100,10,mPaint);
   

** 绘制一条水平直线**

      /**
         * @param startX X轴的起点
         * @param startY Y轴的起点
         * @param stopX X轴的终点
         * @param stopY Y轴的终点
         * @param paint
         */
        canvas.drawLine(10,10,100,10,mPaint);
   

绘制圆弧或扇形

        /**
           * oval：圆弧所在的RectF对象。
           * startAngle：圆弧的起始角度。
           * sweepAngle：圆弧的结束角度。
           * useCenter：是否显示半径连线，true表示显示圆弧与圆心的半径连线，false          表示不显示。
           * paint：绘制时所使用的画笔。
           */
       //绘制扇形
        RectF oval1 = new RectF(50, 50, 300, 300);
        canvas.drawArc(oval1,90,90,true,mPaint);
        //绘制圆弧
        mPaint.setStyle(Paint.Style.STROKE);
        RectF oval2 = new RectF(150, 150, 600, 600);
        canvas.drawArc(oval2,90,90,false,mPaint);
       

绘制圆角矩形

/**
         * RectF : 屏幕左上角是坐标原点，4个参数：left，top，right，bottom
         * left：屏幕左边到矩形左边的距离
         * top：屏幕顶部到矩形顶边的距离
         * right：屏幕左边到矩形右边的距离
         * bottom：屏幕顶边到矩形底边的距离
         * @param rx 圆角X轴半径
         * @param ry 圆角Y轴半径
         * @desc 绘制一个宽50的矩形
         */
        canvas.drawRoundRect(new RectF(50,50,100,100),mPaint);

绘制图片

Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);
/**
 * @param bitmap The bitmap to be drawn
 * @param left   左上角x坐标
 * @param top    左上角y坐标
 * @param paint  画笔
 */
canvas.drawBitmap(bitmap, 360, 1300, mPaint);

绘制点

/**
 * x,y坐标
 */
//画一个点
canvas.drawPoint(500, 1200, p);
//画多个点
canvas.drawPoints(new float[]{600, 1200, 650, 1250, 700, 1200}, mPaint);

绘制三角形

Path path = new Path();
path.moveTo(500, 750);// 此点为多边形的起点
path.lineTo(400, 850);
path.lineTo(600, 850);
path.close(); // 使这些点构成封闭的多边形
canvas.drawPath(path, mPaint);

绘制贝塞尔曲线

Path path2 = new Path();
path2.moveTo(500, 1050);//设置Path的起点
//设置贝塞尔曲线的控制点坐标和终点坐标
path2.quadTo(600, 950, 700, 1050);
path2.quadTo(800, 1150, 900, 1050); 
canvas.drawPath(path2, mPaint);

备注：画线的时候，一定要将画笔设置为Paint.Style.STROKE

2.5 自定义xml中的属性

1. 创建attrs.xml文件

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="FlowLayout">
        <attr name="space" format="dimension"></attr>
    </declare-styleable>
</resources>

declare-styleable 设置自定义view的名称，attr中设置view中需要使用的具体属性，以及相应属性的具体格式；

所有属性的格式如下：

格式	描述	使用
reference	资源ID	@drawable/图片ID
color	颜色值	#ffffff
boolean	布尔值	false or true
dimension	尺寸dp值	100dp or 100dip
float	浮点值	1.0
integer	整型值	100
string	字符串	"str"
fraction	百分数	100%
enum	枚举类型	使用enum标签设置
flag	位或运算	类事枚举，使用flag标签设置

2. 在View中引用
获取自定义属性值

  void init(Context context, AttributeSet attrs) {
        TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.FlowLayout);
        space = ta.getDimensionPixelSize(R.styleable.FlowLayout_space, 0);
        ta.recycle();
    }

init方法应该放在以下的构造方法中。（备注：默认都会调用以下构造方法，无论是否进行findViewById）

public FlowLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
        Log.e(TAG, "FlowLayout(Context context, AttributeSet attrs)");
        init(context, attrs);
        Log.e(TAG,space+"");
    }

3. 在布局文件中使用

<?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="study.zxh.com.viewdemo.MainActivity">

    <study.zxh.com.viewdemo.FlowLayout
        android:id="@+id/fl"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:space="10dp" />

</android.support.constraint.ConstraintLayout>

在跟标签引入

xmlns:app="http://schemas.android.com/apk/res-auto"

在自定义的布局中使用

app:space="10dp"

2.6 LayoutParam

• 自定义LayoutParams

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams;
    }

    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }

    @Override
    protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
        return new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    private static class LayoutParams extends ViewGroup.LayoutParams {
        private int left;
        private int top;
        private int right;
        private int bottom;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

        public LayoutParams(int width, int height) {
            super(width, height);
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }

        public LayoutParams(LayoutParams source) {
            super(source);
            this.left = source.left;
            this.top = source.top;
            this.right = source.right;
            this.bottom = source.bottom;
        }
    }

• 引用步骤

1. 实现静态内部类LayoutParams，并继承自ViewGroup.LayoutParams或其子类；
2. 重写checkLayoutParams用于检测其类型。
3. 重写generateLayoutParams实现自定义LayoutParams的构造；
4. 重写generateDefaultLayoutParams()传入默认的LayoutParams，一般传入ViewGroup.LayoutParams。
5. 使用时，直接通过view.getLayoutParams()，获取子view的LayoutParams使用；

3.自定义View之实现MD按钮动画

4.自定义ViewGroup之流式布局

实现的最终效果

实现的最终效果

最终代码

/**
 * Created by zhangxuehui on 2017/6/16.
 * 实现动态添加文字标签
 */
public class FlowLayout extends ViewGroup {
    private static final String TAG = "FlowLayout";
    private int space = 0;//文字间的间距，以及四周的边距；


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

    public FlowLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
        init(context, attrs);
        Log.e(TAG, space + "");
    }

    public FlowLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    //初始化自定义的属性
    void init(Context context, AttributeSet attrs) {
        TypedArray ta = context.obtainStyledAttributes(attrs, R.styleable.FlowLayout);
        space = ta.getDimensionPixelSize(R.styleable.FlowLayout_space, 0);
        ta.recycle();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int curWidth = 0;
        int curHeight = 0;
        int maxWidth = 0;
        int maxHeight = 0;
        int pos = 0;//用于判读最后一行是否有数据
        for (int i = 0; i < this.getChildCount(); i++) {
            View child = this.getChildAt(i);
            //测量child的宽高
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
            LayoutParams lp = (LayoutParams) child.getLayoutParams();
            lp.left = curWidth + space;
            lp.top = maxHeight + space;
            lp.right = lp.left + child.getMeasuredWidth();
            lp.bottom = lp.top + child.getMeasuredHeight();
            curWidth = lp.right;
            curHeight = Math.max(child.getMeasuredHeight(), curHeight);
            if (curWidth > widthSize - space) {//翻页,需要去除右边距，否则，最右侧标签可能会贴紧屏幕；
                //翻页后，初始化相应的参数
                maxWidth = Math.max(maxWidth, curWidth);
                pos = i;
                curHeight = 0;
                curWidth = 0;
                maxHeight = lp.bottom;
                //重新设置超出屏幕的view
                lp.left = curWidth + space;
                lp.top = maxHeight + space;
                lp.right = lp.left + child.getMeasuredWidth();
                lp.bottom = lp.top + child.getMeasuredHeight();
                //取得最新的参数
                curWidth = lp.right;
                curHeight = Math.max(child.getMeasuredHeight(), curHeight);
            }
            Log.e(TAG, lp.toString());
        }
        if (getChildCount() > pos) {
            maxHeight += curHeight + space * 2;
        }
        widthMeasureSpec = MeasureSpec.makeMeasureSpec(maxWidth, MeasureSpec.EXACTLY);//重建约束
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(maxHeight, MeasureSpec.EXACTLY);//重建约束
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, MeasureSpec.EXACTLY), resolveSizeAndState(maxHeight, heightMeasureSpec, MeasureSpec.EXACTLY));
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        for (int i = 0; i < this.getChildCount(); i++) {
            View child = this.getChildAt(i);
            LayoutParams lp = (LayoutParams) child.getLayoutParams();
            child.layout(lp.left, lp.top, lp.right, lp.bottom);
        }
    }

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams;
    }

    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return new LayoutParams(p);
    }

    @Override
    protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
        return new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
    }

    private static class LayoutParams extends ViewGroup.LayoutParams {
        private int left;
        private int top;
        private int right;
        private int bottom;

        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }

        public LayoutParams(int width, int height) {
            super(width, height);
        }

        public LayoutParams(ViewGroup.LayoutParams source) {
            super(source);
        }

        public LayoutParams(LayoutParams source) {
            super(source);
            this.left = source.left;
            this.top = source.top;
            this.right = source.right;
            this.bottom = source.bottom;
        }

        public void clear() {
            this.left = 0;
            this.top = 0;
            this.right = 0;
            this.bottom = 0;
        }

        @Override
        public String toString() {
            return "LayoutParams{" +
                    "left=" + left +
                    ", top=" + top +
                    ", right=" + right +
                    ", bottom=" + bottom +
                    '}';
        }
    }
}

5.组合view之微信聊天界面

最终效果
实现思想
首先整个聊天界面通过系统的ListView实现，通过组合控件实现聊天气泡和输入框，全部代码可以划分为三个组合控件，WeChatMsgList、WeChatMsgInput、WeChatBubble
最终代码

• WeChatMsgList

• WeChatMsgInput

• WeChatBubble

参考地址：
Canvas绘图
Android 画笔Paint
官方api Canvas