Android View的测量、布局、绘制过程详解（下）

View的measure()方法是final的，因此我们无法在子类中去重写这个方法，在该方法内部会调用onMeasure()方法。所以我们自定义view时，要重写也是重写onMeasure方法。同理，layout、draw也是final的，我们根据需要重写onLayout和onDraw方法即可。

1、measure过程

measure过程要分成两种情况来看：

• 1、如果只是一个view，那么直接调用其measure方法即可完成测量；
• 2、如果是一个ViewGroup，除了完成自己的测量外，还会遍历调用所有子view的measure方法；

首先看第一种情况，也就是View的情况。

1.1、 View的Measure过程

View的measure()方法是final的，我们无法在子类中去重写这个方法，在该方法内部会调用onMeasure()方法，如果自定义时需要，可以重写此方法， 源码如下所示：

// (frameworks\base\core\java\android\view\View.java)

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

上述代码很简洁，就是给view设置宽高，都是通过getDefaultSize方法设置的：

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 = specSize;
        break;
    }
    return result;
}

逻辑很简单，如果是UNSPECIFIED，直接返回系统给的建议值size， 如果是AT_MOST或EXACTLY，就返回在MeasureSpec中的实际测量值，而实际测量值的来源有三种情况：

（1）如果View设置了固定宽高，返回的就是设置的大小；

（2）如果设置了MATCH_PARENT，View的测量模式会有两种情况，不过不管是哪一种，返回最大值的都是父容器剩余大小；

（3）如果是自定义view，设置了WRAP_CONTENT，需要自己在onMeasure中重写代码，计算此种情况下应该是什么大小，否则WRAP_CONTENT的效果可能会失效；

第一二点都好理解，第三点通常发生在自定义view，因为从上述代码可见，如果没自己进行计算，则最后效果MATCH_PARENT是一致的，都是得到父容器剩余大小。常见的解决方式就是在onMeasure里针对WRAP_CONTENT属性直接通过setMeasuredDimension()方法为其指定一个默认的宽高。

1.2 ViewGroup的Measure过程

ViewGroup类是没有onMeasure()方法的，因为不同的ViewGroup子类布局都不一样，所以这个方法是交给子类去实现的。

ViewGroup中定义了一个measureChildren()方法，其中通过for循环来遍历测量子视图的大小，然后就可以确定自己的大小：

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);
        }
    }
}

遍历过程中去调用measureChild， 去调用每个子view的measure方法：

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);
}

2、号外：何时去获取View的宽高是最合适的？

通常来说，在measure后，view的大小就测量出来了，就可获取view的宽高了，但是measure是三大流程中最复杂的一个，有的情况下可能需要多次measure后才能最终确定宽高，所以，一种比较好的方式就是在onLayout中去获取View的最终测量宽高。

此外，如果我们不是在view内部的onLayout中去获取宽高，而是在Activity中想去获取某个View的宽高，应该怎么做呢？ 因为view的绘制流程和Activity的生命周期无关，所以不能保证在哪个Activity的生命周期时，View绘制完毕，此时我们又以下四种方法来解决：

• 1、Activity/View的 onWindowFocusChanged方法： 回调这个方法时，就表示View已经初始化完毕，但onWindowFocusChanged可能会被回调多次，所以我们要做好相应的逻辑判断；
• 2、view.post(Runnable): 通过view的post方法将一个Runnable加入到消息队列，然后Looper调用Runnable时，view也就完成初始化了；
• 3、通过ViewTreeObserver监听View的变化：当View树的状态或可见性改变，会回调给它的onGloableLayout方法，但也会回调多次，需要处理好多次的逻辑；
• 4、调用view.measure：手动调用view的measure方法，此种方法比较复杂，这里不进行扩展；

3、layout过程

layout的作用就是ViewGroup用来确定子view的位置，当ViewGroup的位置确定后，它会在onLayout中遍历所有子view并调用其layout方法，然后layout方法又会调用子view的onLayout方法，layout方法确定view自己的位置，onLayout方法确定所有子view的位置, 以下是ViewGroup的layout：

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;
    }
}

ViewGroup的layout什么都没做，直接调用了view的layout：

public void layout(int l, int t, int r, int b) {
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }

    int oldL = mLeft;
    int oldT = mTop;
    int oldB = mBottom;
    int oldR = mRight;

    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);//注释1

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        onLayout(changed, l, t, r, b);//注释2

        if (shouldDrawRoundScrollbar()) {
            if(mRoundScrollbarRenderer == null) {
                mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
            }
        } else {
            mRoundScrollbarRenderer = null;
        }

        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            ArrayList<OnLayoutChangeListener> listenersCopy =
                    (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
            int numListeners = listenersCopy.size();
            for (int i = 0; i < numListeners; ++i) {
                listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
            }
        }
    }

    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;

    if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
        mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
        notifyEnterOrExitForAutoFillIfNeeded(true);
    }
}

上述代码中：

• 注释1： 通过setFrame，完成对View四个顶点的初始化，也就确定了view相对父view的位置，并且会判断视图的大小是否发生过变化，避免未发生变化时进行重绘操作；
• 注释2：调用view的onLayout方法，完成真正的layout，onLayout在View和ViewGroup中均是抽象方法，需要子view实现；

4、draw过程

measure和layout的过程都结束后，就进入到对View进行绘制的draw过程了。

首先会创建出一个Canvas对象，然后调用View的draw()方法来执行具体的绘制工作，大致可以分为四步：

• 1、绘制背景background.draw(canvas);
• 2、绘制自己 onDraw；
• 3、绘制childview（dispatchDraw）；
• 4、绘制装饰（onDrawScrollBars）；

代码很长，但逻辑很清楚：

public void draw(Canvas canvas) {
    final int privateFlags = mPrivateFlags;
    final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
            (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
    mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

    /*
     * Draw traversal performs several drawing steps which must be executed
     * in the appropriate order:
     *
     *      1. Draw the background
     *      2. If necessary, save the canvas' layers to prepare for fading
     *      3. Draw view's content
     *      4. Draw children
     *      5. If necessary, draw the fading edges and restore layers
     *      6. Draw decorations (scrollbars for instance)
     */

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

    if (!dirtyOpaque) {
        drawBackground(canvas);
    }

    // skip step 2 & 5 if possible (common case)
    final int viewFlags = mViewFlags;
    boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
    boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
    if (!verticalEdges && !horizontalEdges) {
        // Step 3, draw the content
        if (!dirtyOpaque) onDraw(canvas);

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

        drawAutofilledHighlight(canvas);

        // Overlay is part of the content and draws beneath Foreground
        if (mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay.getOverlayView().dispatchDraw(canvas);
        }

        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);

        // Step 7, draw the default focus highlight
        drawDefaultFocusHighlight(canvas);

        if (debugDraw()) {
            debugDrawFocus(canvas);
        }

        // we're done...
        return;
    }

    /*
     * Here we do the full fledged routine...
     * (this is an uncommon case where speed matters less,
     * this is why we repeat some of the tests that have been
     * done above)
     */

    boolean drawTop = false;
    boolean drawBottom = false;
    boolean drawLeft = false;
    boolean drawRight = false;

    float topFadeStrength = 0.0f;
    float bottomFadeStrength = 0.0f;
    float leftFadeStrength = 0.0f;
    float rightFadeStrength = 0.0f;

    // Step 2, save the canvas' layers
    int paddingLeft = mPaddingLeft;

    final boolean offsetRequired = isPaddingOffsetRequired();
    if (offsetRequired) {
        paddingLeft += getLeftPaddingOffset();
    }

    int left = mScrollX + paddingLeft;
    int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
    int top = mScrollY + getFadeTop(offsetRequired);
    int bottom = top + getFadeHeight(offsetRequired);

    if (offsetRequired) {
        right += getRightPaddingOffset();
        bottom += getBottomPaddingOffset();
    }

    final ScrollabilityCache scrollabilityCache = mScrollCache;
    final float fadeHeight = scrollabilityCache.fadingEdgeLength;
    int length = (int) fadeHeight;

    // clip the fade length if top and bottom fades overlap
    // overlapping fades produce odd-looking artifacts
    if (verticalEdges && (top + length > bottom - length)) {
        length = (bottom - top) / 2;
    }

    // also clip horizontal fades if necessary
    if (horizontalEdges && (left + length > right - length)) {
        length = (right - left) / 2;
    }

    if (verticalEdges) {
        topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
        drawTop = topFadeStrength * fadeHeight > 1.0f;
        bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
        drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
    }

    if (horizontalEdges) {
        leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
        drawLeft = leftFadeStrength * fadeHeight > 1.0f;
        rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
        drawRight = rightFadeStrength * fadeHeight > 1.0f;
    }

    saveCount = canvas.getSaveCount();

    int solidColor = getSolidColor();
    if (solidColor == 0) {
        final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;

        if (drawTop) {
            canvas.saveLayer(left, top, right, top + length, null, flags);
        }

        if (drawBottom) {
            canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
        }

        if (drawLeft) {
            canvas.saveLayer(left, top, left + length, bottom, null, flags);
        }

        if (drawRight) {
            canvas.saveLayer(right - length, top, right, bottom, null, flags);
        }
    } else {
        scrollabilityCache.setFadeColor(solidColor);
    }

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

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

    // Step 5, draw the fade effect and restore layers
    final Paint p = scrollabilityCache.paint;
    final Matrix matrix = scrollabilityCache.matrix;
    final Shader fade = scrollabilityCache.shader;

    if (drawTop) {
        matrix.setScale(1, fadeHeight * topFadeStrength);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, right, top + length, p);
    }

    if (drawBottom) {
        matrix.setScale(1, fadeHeight * bottomFadeStrength);
        matrix.postRotate(180);
        matrix.postTranslate(left, bottom);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, bottom - length, right, bottom, p);
    }

    if (drawLeft) {
        matrix.setScale(1, fadeHeight * leftFadeStrength);
        matrix.postRotate(-90);
        matrix.postTranslate(left, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(left, top, left + length, bottom, p);
    }

    if (drawRight) {
        matrix.setScale(1, fadeHeight * rightFadeStrength);
        matrix.postRotate(90);
        matrix.postTranslate(right, top);
        fade.setLocalMatrix(matrix);
        p.setShader(fade);
        canvas.drawRect(right - length, top, right, bottom, p);
    }

    canvas.restoreToCount(saveCount);

    drawAutofilledHighlight(canvas);

    // Overlay is part of the content and draws beneath Foreground
    if (mOverlay != null && !mOverlay.isEmpty()) {
        mOverlay.getOverlayView().dispatchDraw(canvas);
    }

    // Step 6, draw decorations (foreground, scrollbars)
    onDrawForeground(canvas);

    if (debugDraw()) {
        debugDrawFocus(canvas);
    }
}

View的绘制流程就讲完了，如果您觉得还可以，麻烦点个赞吧~~