UI绘制流程(measure，layout，draw)

ViewRootImpl--

一、Measure （performMeasure）

1.measureSpace

ViewRootImpl--getRootMeasureSpec()，返回了一个int类型的measureSpace（以下简称ms），ms是个32位的int数值，他的前2位表示mode，后30位表示尺寸。

例：
    00      000000000000000000000000000000
specMode ↑  specSize

SpecMode分为3种EXACTLY、AT_MOST、UNSPECIFIED

        /**
         * Measure specification mode: The parent has not imposed any constraint
         * on the child. It can be whatever size it wants.
         */
        00000000000000000000000000000000
        1.父容器不对view做任何限制，系统内部使用
        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.
         */
        01000000000000000000000000000000
        2.父容器检测出view的大小，view的大小就是SpecSize 
        子控件的LayoutParams的match_parent 以及 固定大小
        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.
         */
        10000000000000000000000000000000
        3.父容器指定一个指定大小，view大小不能超过这个值， LayoutParams wrap_content
        public static final int AT_MOST     = 2 << MODE_SHIFT;

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

通过View--makeMeasureSpec()我们可以知道，
mode + size = MeasureSpec

        private static final int MODE_SHIFT = 30;
        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;
/**
         * Creates a measure specification based on the supplied size and mode.
         *
         * The mode must always be one of the following:
         * 

         *  
{@link android.view.View.MeasureSpec#UNSPECIFIED}
         *  
{@link android.view.View.MeasureSpec#EXACTLY}
         *  
{@link android.view.View.MeasureSpec#AT_MOST}
         * 
         *
         * 
Note: 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.
         *
         * @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);
            }
        }

通过对MODE_MASK和~MODE_MASK 取 & 操作，取得mode 和 size

 MODE_MASK = 11000000000000000000000000000000
~MODE_MASK = 00111111111111111111111111111111

View的测量：

一、确定DecorViewde的MesasureSpec

DecorView的MesaureSpec有窗口大小和自身LayoutParams决定，遵循如下规则：
1.LayoutParams.MATCH_PARENT：精确模式，窗口大小
2.LayoutParams.WRAP_CONTENT：最大模式，最大为窗口大小
3.固定模式：精确模式，大小为LayoutParams大小

二、确定View的MeasureSpec

image.png

measure--onMeasure--setMeasureDimension--setMeasureDimensionRaw

ViewGroup的测量：

measure--onMeasure（测量子控件宽高）--setMeasureDimension--setMeasureDimensionRaw(保存自己的宽高)

二、Layout （performLayout）

1.View类型:
调用viwe.layout确定自身位置，即确定mLeft, mTop, mRight, mBottom
2.ViewGroup类型：
layout（来确定自己位置，4个点的位置）--onLauout（进行子view的布局）

三、Draw ( performDraw )

1.绘制背景 drawBackground(canvas)
2.绘制自己 onDraw(canvas)
3.绘制子View dispatchDraw(canvas) （view不会有）
4.绘制前景，如滚动条等装饰 onDrawForefround(canvas)

总结

onMeasure---onLayout---onDraw
当我们自定义的是容器的时候，需要去实现onLayout方法，自定义view的时候，可以不用重写它。
onDraw方法，可选，如果onDraw中放的都是系统控件，可以不用重写。