Android 绘制流程

绘制流程的起点

从 ViewRootImpl 的performTraversals()方法开始：

private void performTraversals() {
    ...
    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
    ...
    //执行测量流程
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
    //执行布局流程
    performLayout(lp, desiredWindowWidth, desiredWindowHeight);
    ...
    //执行绘制流程
    performDraw();
}

理解 MeasureSpec

MeasureSpec 源码

MeasureSpec 是 View 类的一个静态内部类，用来说明应该如何测量这个 View。

MeasureSpec 表示的是一个32位的整形值，它的高2位表示测量模式 mode，低30位表示某种测量模式下的规格大小 size。

public static class MeasureSpec {
    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK = 0X3 << MODE_SHIFT;

    // 不指定测量模式, 父视图没有限制子视图的大小，子视图可以是想要
    // 的任何尺寸，通常用于系统内部，应用开发中很少用到。
    public static final int UNSPECIFIED = 0 << MODE_SHIFT;

    // 精确测量模式，视图宽高指定为match_parent或具体数值时生效，
    // 表示父视图已经决定了子视图的精确大小，这种模式下View的测量
    // 值就是SpecSize的值。
    public static final int EXACTLY = 1 << MODE_SHIFT;

    // 最大值测量模式，当视图的宽高指定为wrap_content时生效，此时
    // 子视图的尺寸可以是不超过父视图允许的最大尺寸的任何尺寸。
    public static final int AT_MOST = 2 << MODE_SHIFT;

    // 根据指定的大小和模式创建一个MeasureSpec
    public static int makeMeasureSpec(int size, int mode) {
        if (sUseBrokenMakeMeasureSpec) {
            return size + mode;
        } else {
            return (size & ~MODE_MASK) | (mode & MODE_MASK);
        }
    }

    // 微调某个MeasureSpec的大小
    static int adjust(int measureSpec, int delta) {
        final int mode = getMode(measureSpec);
        if (mode == UNSPECIFIED) {
            // No need to adjust size for UNSPECIFIED mode.
            return make MeasureSpec(0, UNSPECIFIED);
        }
        int size = getSize(measureSpec) + delta;
        if (size < 0) {
            size = 0;
        }
        return makeMeasureSpec(size, mode);
    }
}

DecorView 的 MeasureSpec 创建

对于 DecorView 而言，它的 MeasureSpec 由窗口尺寸和其自身的 LayoutParams 共同决定；

//desiredWindowWidth和desiredWindowHeight是屏幕的尺寸
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

private static int getRootMeaureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {
        case ViewGroup.LayoutParams.MATRCH_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;
}

普通 View 的 MeasureSpec 创建

对于普通的 View，它的 MeasureSpec 由父视图的 MeasureSpec 和其自身的 LayoutParams 共同决定。

// ViewGroup.java
protected void measureChildWithMargins(View child,
    int parentWidthMeasureSpec, int widthUsed,
    int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

    // 子元素的MeasureSpec的创建与父容器的MeasureSpec和子元素本身
    // 的LayoutParams有关，此外还和View的margin及padding有关
    final int childWidthMeasureSpec = getChildMeasureSpec(
    parentWidthMeasureSpec,
    mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed, 
    lp.width);

    final int childHeightMeasureSpec = getChildMeasureSpec(
    parentHeightMeasureSpec,
    mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed, 
    lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

下面源码对应的计算规则：

parent	child	specMode
MeasureSpec.EXACTLY	match_parent	MeasureSpec.EXACTLY
MeasureSpec.EXACTLY	wrap_content	MeasureSpec.AT_MOST
MeasureSpec.AT_MOST	match_parent	MeasureSpec.AT_MOST
MeasureSpec.AT_MOST	wrap_content	MeasureSpec.AT_MOST

// ViewGroup.java
public static int getChildMeasureSpec(int spec, int padding, int childDimesion) {
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);

    // padding是指父容器中已占用的空间大小，因此子元素可用的
    // 大小为父容器的尺寸减去padding
    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;

    switch (sepcMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimesion == 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 == 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 == 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 == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size....
                // find out how big it should be
                resultSize = 0;
                resultMode == MeasureSpec.UNSPECIFIED;
            }
            break;
        }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

measure 流程

从 ViewRootImpl 开始：

// ViewRootImpl.java
private void perormMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    ...
    // 具体的测量操作分发给ViewGroup
    mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
}

遍历测量 ViewGroup 中所有的 child view：

// ViewGroup.java
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];
        // 当View的可见性处于GONE状态时，不对其进行测量
        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}

测量单个 child：

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

    // 根据父容器的MeasureSpec和子View的LayoutParams等信息计算
    // 子View的MeasureSpec
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height);
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

在 measure()中，具体测量行为交由onMeasure()来定义：

// View.java
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...
    // ViewGroup没有定义测量的具体过程，因为ViewGroup是一个
    // 抽象类，其测量过程的onMeasure方法需要各个子类去实现
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    ...
}

ViewGroup 没有重写onMeasure()方法，View 的 onMeasure()：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // setMeasureDimension方法用于设置View的测量宽高
    // getSuggestedMinimumWidth() 的作用是：
    // 如果View没有设置背景，那么返回android:minWidth这个属性所指定的值，这个值可以为0；
    // 如果View设置了背景，则返回android:minWidth和背景的最小宽度这两者中的最大值。
    setMeasureDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), 
    getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

View 的默认测量行为是使用getDefaultSize()来测量宽高，从这里可以看出，View 在默认情况下，wrap_content 和 match_parent 的行为是一样的：

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 = sepcSize;
            break;
    }
    return result;
}

layout 流程

开始布局：

// ViewRootImpl.java
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) {
    ...
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
    ...
}

具休 layout 行为，交给 onLayout()：

// View.java
public void layout(int l, int t, int r, int b) {
    ...
    // 通过setFrame方法来设定View的四个顶点的位置，即View在父容器中的位置
    boolean changed = isLayoutModeOptical(mParent) ? 
    set OpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    ...
    onLayout(changed, l, t, r, b);
    ...
}

// View.java
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {

}

ViewGroup 重写了onLayout()方法，并将它改为抽象方法，子类必须实现此方法：

// ViewGroup.java
@Override
protected abstract void onLayout(boolean changed,
        int l, int t, int r, int b);

draw 流程

开始绘制：

// ViewRootImpl.java
private void performDraw() {
    ...
    draw(fullRefrawNeeded);
    ...
}

// ViewRootImpl.java
private void draw(boolean fullRedrawNeeded) {
    ...
    if (!drawSoftware(surface, mAttachInfo, xOffest, yOffset, 
    scalingRequired, dirty)) {
        return;
    }
    ...
}

// ViewRootImpl.java
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, 
    int xoff, int yoff, boolean scallingRequired, Rect dirty) {
    ...
    mView.draw(canvas);
    ...
}

具体绘制行为，为6个步骤，其中onDraw()用于绘制内容：

// View.java
public void draw(Canvas canvas) {
    ...
    // 步骤一：绘制View的背景
    drawBackground(canvas);

    ...
    // 步骤二：如果需要的话，保持canvas的图层，为fading做准备
    saveCount = canvas.getSaveCount();
    ...
    canvas.saveLayer(left, top, right, top + length, null, flags);

    ...
    // 步骤三：绘制View的内容
    onDraw(canvas);

    ...
    // 步骤四：绘制View的子View
    dispatchDraw(canvas);

    ...
    // 步骤五：如果需要的话，绘制View的fading边缘并恢复图层
    canvas.drawRect(left, top, right, top + length, p);
    ...
    canvas.restoreToCount(saveCount);

    ...
    // 步骤六：绘制View的装饰(例如滚动条等等)
    onDrawForeground(canvas)
}

参考：Android View的绘制流程