Android View的绘制流程

一、measure过程

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ViewGroup measure过程

View的measure()方法是final的，无法被子类重写。
View的onMeasure()方法中传进来的参数的逻辑：父容器会根据自己的和子View的LayoutParams共同确定要给子view的measure()方法中参入什么参数

child \ parent	EXACTLY	AT_MOST
MATCH_PARENT	EXACTLY	AT_MOST
WRAP_CONTENT	AT_MOST	AT_MOST
dp/px	EXACTLY	EXACTLY

ViewGroup会遍历自己的子View，根据上表中的逻辑完成自身和子view的测量，这个共同确定子view的MeasureSpec逻辑主要是getChildMeasureSpec()方法：

    protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        for (int i = 0; i < mChildrenCount; ++i) {
            measureChild(children[i], widthMeasureSpec, heightMeasureSpec);
        }
    }

    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);
    }

    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
            // Parent has imposed an exact size on us
            case MeasureSpec.EXACTLY:
                if (childDimension >= 0) {
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == ViewGroup.LayoutParams.MATCH_PARENT) {
                    // Child wants to be our size. So be it.
                    resultSize = size;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == ViewGroup.LayoutParams.WRAP_CONTENT) {
                    // Child wants to determine its own size. It can't be
                    // bigger than us.
                    resultSize = size;
                    resultMode = MeasureSpec.AT_MOST;
                }
                break;

            // Parent has imposed a maximum size on us
            case MeasureSpec.AT_MOST:
                if (childDimension >= 0) {
                    // Child wants a specific size... so be it
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == ViewGroup.LayoutParams.MATCH_PARENT) {
                    // Child wants to be our size, but our size is not fixed.
                    // Constrain child to not be bigger than us.
                    resultSize = size;
                    resultMode = MeasureSpec.AT_MOST;
                } else if (childDimension == ViewGroup.LayoutParams.WRAP_CONTENT) {
                    // Child wants to determine its own size. It can't be
                    // bigger than us.
                    resultSize = size;
                    resultMode = MeasureSpec.AT_MOST;
                }
                break;

            // Parent asked to see how big we want to be
            case MeasureSpec.UNSPECIFIED:
                if (childDimension >= 0) {
                    // Child wants a specific size... let him have it
                    resultSize = childDimension;
                    resultMode = MeasureSpec.EXACTLY;
                } else if (childDimension == ViewGroup.LayoutParams.MATCH_PARENT) {
                    // Child wants to be our size... find out how big it should
                    // be
                    resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                    resultMode = MeasureSpec.UNSPECIFIED;
                } else if (childDimension == ViewGroup.LayoutParams.WRAP_CONTENT) {
                    // Child wants to determine its own size.... find out how
                    // big it should be
                    resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                    resultMode = MeasureSpec.UNSPECIFIED;
                }
                break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

View measure过程

   
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

View默认的的测量逻辑：

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

    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
            case MeasureSpec.UNSPECIFIED:
                result = size;
                break;
            case MeasureSpec.AT_MOST:
            case MeasureSpec.EXACTLY:
                result = specSize;
                break;
        }
        return result;
    }

    protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }

    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);
    }

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

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

二、Layout过程

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ViewGroup.java

    @Override
    public final void layout(int l, int t, int r, int b) {
        if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
            if (mTransition != null) {
                mTransition.layoutChange(this);
            }
            super.layout(l, t, r, b);
        } else {
            // record the fact that we noop'd it; request layout when transition finishes
            mLayoutCalledWhileSuppressed = true;
        }
    }

它也是final的方法，基本就是直接调了View的layout，当发现左右上下坐标发生了变化时，就会调用onLayout方法。
View.java

    public void layout(int l, int t, int r, int b) {

        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            // 调用了onLayout()
            onLayout(changed, l, t, r, b);

        }

        // ...
}

setFrame()方法主要是设置了view的左上右下几个坐标值

    protected boolean setFrame(int left, int top, int right, int bottom) {

        if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
            changed = true;

            mLeft = left;
            mTop = top;
            mRight = right;
            mBottom = bottom;

            if (sizeChanged) {
                sizeChange(newWidth, newHeight, oldWidth, oldHeight);
            }

        }
        return changed;
    }

而sizeChange()最终调用了onSizeChanged()方法。

我们自定义ViewGroup的时候，需要负责在onLayout中，对子view进行layout。

三、draw过程

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measure过程确定了view的宽高，layout过程确定了view的位置，那么draw就可以开始绘制了。

draw的过程很简单

View.java

    public void draw(Canvas canvas) {

        //...

        // Step 1, draw the background, if needed
        int saveCount;

        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

        // Step 4, draw the children
        dispatchDraw(canvas);

        //...

}

ViewGroup.java

    @Override
    protected void dispatchDraw(Canvas canvas) {

        for (int i = 0; i < childrenCount; i++) {
                    more |= drawChild(canvas, transientChild, drawingTime);

                   //...
        }

    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        return child.draw(canvas, this, drawingTime);
    }