Android源码分析之Activity启动与View绘制流程(二)

续前节,接下来我们分析ViewRootImpl#performTraversals方法,代码如下:

/frameworks/base/core/java/android/view/ViewRootImpl.java

private void performTraversals() {
    ...
    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
    // Ask host how big it wants to be
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
    if (measureAgain) {
        ...
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    }           
    ...
        performLayout(lp, mWidth, mHeight);
    ...
        performDraw();
    ...
}

这个函数做了很多的事情,但最重要的是调用MeasuerLayoutDraw三个过程。这里首先看到有获取子View的MeasureSpec的方法,代码如下:

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;
}

可以看到,如果是WRAP_CONTENT对应的模式就是AT_MOSTMATCH_PARENT或其他值,也就是具体数值,对应的模式是EXACTLY。这个MeasureSpec想必大家都了解过,它的高两位用来表示模式SpecMode,低30位用来表示大小SpecSizeSpecMode共有以下三种类型:

  • UNSPECIFIED:父容器不作限制,子View想多大就多大,一般用于系统内部。
  • EXACTLY:精确模式,大小为SpecSize,父容器完全决定子View的大小,对应LayoutParams中的match_parent和具体数值。
  • AT_MOST:最大模式,大小不能大于SpecSize,也就是子View的大小有上限,对应于LayoutParams中的warp_content。

接下来我们分析下测量的过程,代码如下:

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

这里Measure交给View来完成,代码如下:

/frameworks/base/core/java/android/view/View.java

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    ...
}

measure主要的工作是回调onMeasure,在自定义View时也经常会重写此方法,代码如下:

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

可以看到,如果不重写此方法,系统会设置一个默认的大小给子View,这里先看下这个默认大小的实现,代码如下:

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;
}

也就是说,无论是EXACTLY还是AT_MOST,都按照测量结果进行设置。继续来看setMeasuredDimension方法,代码如下:

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    ...
    setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

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

    mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

也就是最终将测量的结果保存在mMeasuredWidthmMeasuredHeight变量中。ViewGroup的测量流程和此一致,只是其在onMeasure时需要测量子View,我们结合DecorView来分析,它继承自FrameLayout,以下是它的onMeasure方法实现:

/frameworks/base/core/java/com/android/internal/policy/DecorView.java

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
    final boolean isPortrait =
            getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;

    final int widthMode = getMode(widthMeasureSpec);
    final int heightMode = getMode(heightMeasureSpec);

    boolean fixedWidth = false;
    mApplyFloatingHorizontalInsets = false;
    if (widthMode == AT_MOST) {
        final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
        if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
            final int w;
            if (tvw.type == TypedValue.TYPE_DIMENSION) {
                w = (int) tvw.getDimension(metrics);
            } else if (tvw.type == TypedValue.TYPE_FRACTION) {
                w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
            } else {
                w = 0;
            }
            if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed width: " + w);
            final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
            if (w > 0) {
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        Math.min(w, widthSize), EXACTLY);
                fixedWidth = true;
            } else {
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        widthSize - mFloatingInsets.left - mFloatingInsets.right,
                        AT_MOST);
                mApplyFloatingHorizontalInsets = true;
            }
        }
    }

    mApplyFloatingVerticalInsets = false;
    if (heightMode == AT_MOST) {
        final TypedValue tvh = isPortrait ? mWindow.mFixedHeightMajor
                : mWindow.mFixedHeightMinor;
        if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
            final int h;
            if (tvh.type == TypedValue.TYPE_DIMENSION) {
                h = (int) tvh.getDimension(metrics);
            } else if (tvh.type == TypedValue.TYPE_FRACTION) {
                h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels);
            } else {
                h = 0;
            }
            if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed height: " + h);
            final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
            if (h > 0) {
                heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        Math.min(h, heightSize), EXACTLY);
            } else if ((mWindow.getAttributes().flags & FLAG_LAYOUT_IN_SCREEN) == 0) {
                heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        heightSize - mFloatingInsets.top - mFloatingInsets.bottom, AT_MOST);
                mApplyFloatingVerticalInsets = true;
            }
        }
    }

    getOutsets(mOutsets);
    if (mOutsets.top > 0 || mOutsets.bottom > 0) {
        int mode = MeasureSpec.getMode(heightMeasureSpec);
        if (mode != MeasureSpec.UNSPECIFIED) {
            int height = MeasureSpec.getSize(heightMeasureSpec);
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(
                    height + mOutsets.top + mOutsets.bottom, mode);
        }
    }
    if (mOutsets.left > 0 || mOutsets.right > 0) {
        int mode = MeasureSpec.getMode(widthMeasureSpec);
        if (mode != MeasureSpec.UNSPECIFIED) {
            int width = MeasureSpec.getSize(widthMeasureSpec);
            widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                    width + mOutsets.left + mOutsets.right, mode);
        }
    }

    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int width = getMeasuredWidth();
    boolean measure = false;

    widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);

    if (!fixedWidth && widthMode == AT_MOST) {
        final TypedValue tv = isPortrait ? mWindow.mMinWidthMinor : mWindow.mMinWidthMajor;
        if (tv.type != TypedValue.TYPE_NULL) {
            final int min;
            if (tv.type == TypedValue.TYPE_DIMENSION) {
                min = (int)tv.getDimension(metrics);
            } else if (tv.type == TypedValue.TYPE_FRACTION) {
                min = (int)tv.getFraction(mAvailableWidth, mAvailableWidth);
            } else {
                min = 0;
            }
            if (DEBUG_MEASURE) Log.d(mLogTag, "Adjust for min width: " + min + ", value::"
                    + tv.coerceToString() + ", mAvailableWidth=" + mAvailableWidth);

            if (width < min) {
                widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
                measure = true;
            }
        }
    }

    // TODO: Support height?

    if (measure) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

可以看到,主要是对AT_MOST模式下的宽高进行了修正,然后调用父类的方法,我们的关注点在后者,下面是FrameLayout的相关实现:

/frameworks/base/core/java/android/widget/FrameLayout.java

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();

    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            // 对child进行测量
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            // 一些值的修正
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            childState = combineMeasuredStates(childState, child.getMeasuredState());
            if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }

    // Account for padding too
    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

    // Check against our minimum height and width
    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

    // Check against our foreground's minimum height and width
    final Drawable drawable = getForeground();
    if (drawable != null) {
        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
    }

    // 设置测量结果
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));

    count = mMatchParentChildren.size();
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            final View child = mMatchParentChildren.get(i);
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

            final int childWidthMeasureSpec;
            // 根据不同的LP,给child设置不同的测量宽度
            if (lp.width == LayoutParams.MATCH_PARENT) {
                // 使用剩余的全部空间
                final int width = Math.max(0, getMeasuredWidth()
                        - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                        - lp.leftMargin - lp.rightMargin);
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width, MeasureSpec.EXACTLY);
            } else {
                // 重新指定
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }

            final int childHeightMeasureSpec;
            if (lp.height == LayoutParams.MATCH_PARENT) {
                final int height = Math.max(0, getMeasuredHeight()
                        - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                        - lp.topMargin - lp.bottomMargin);
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height, MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }
            // 调用child的measure方法
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

这里首先测量child,根据此来设置ViewGroup的宽高,然后再设定所有的设置属性为MATCH_PARENT的child的宽高,这是因为ViewGroup后来处理了padding与Foreground的值,导致可用空间和测量结果不一致了。我们先看它是如何测量child的,代码如下:

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

    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);
}

可以看到,这里也调用了getChildMeasureSpec方法,和后边处理MATCH_PARENT模式的child唯一不同之处在于padding值的大小,也就是将Foreground的padding值加了进来。现在我们看下这个getChildMeasureSpec的实现:

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 == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size. So be it.
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } 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 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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == 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);
}

这里根据ViewGroup的SpecMode不同,给child设置了不同的模式和大小,以保证child能正确完成测量的过程。

接下来我们分析Layout的流程,代码如下:

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    mLayoutRequested = false;
    mScrollMayChange = true;
    mInLayout = true;

    final View host = mView;
    ...
    try {
        // 先调用mView的layout方法
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

        mInLayout = false;
        int numViewsRequestingLayout = mLayoutRequesters.size();
        if (numViewsRequestingLayout > 0) {
            // requestLayout() was called during layout.
            // If no layout-request flags are set on the requesting views, there is no problem.
            // If some requests are still pending, then we need to clear those flags and do
            // a full request/measure/layout pass to handle this situation.
            // 获取到需要进行layout的View的个数
            ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
                    false);
            if (validLayoutRequesters != null) {
                // Set this flag to indicate that any further requests are happening during
                // the second pass, which may result in posting those requests to the next
                // frame instead
                mHandlingLayoutInLayoutRequest = true;

                // Process fresh layout requests, then measure and layout
                int numValidRequests = validLayoutRequesters.size();
                for (int i = 0; i < numValidRequests; ++i) {
                    final View view = validLayoutRequesters.get(i);
                    Log.w("View", "requestLayout() improperly called by " + view +
                            " during layout: running second layout pass");
                    // 调用它们的requestLayout方法,
                    view.requestLayout();
                }
                // 再次进行测量
                measureHierarchy(host, lp, mView.getContext().getResources(),
                        desiredWindowWidth, desiredWindowHeight);
                mInLayout = true;
                // 重新layout
                host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

                mHandlingLayoutInLayoutRequest = false;

                // Check the valid requests again, this time without checking/clearing the
                // layout flags, since requests happening during the second pass get noop'd
                validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
                if (validLayoutRequesters != null) {
                    final ArrayList<View> finalRequesters = validLayoutRequesters;
                    // Post second-pass requests to the next frame
                    // 再次检查是否仍有需要layout的View,如果有,就到下一帧再继续
                    getRunQueue().post(new Runnable() {
                        @Override
                        public void run() {
                            int numValidRequests = finalRequesters.size();
                            for (int i = 0; i < numValidRequests; ++i) {
                                final View view = finalRequesters.get(i);
                                Log.w("View", "requestLayout() improperly called by " + view +
                                        " during second layout pass: posting in next frame");
                                view.requestLayout();
                            }
                        }
                    });
                }
            }

        }
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
    mInLayout = false;
}

以上过程是先让DecorView进行layout,然后找到所有需要进行layout的View并调用它们的requestLayout方法,然后对整个View Tree重新测量,再次进行第二次layout过程。

DecorView并没有重写layout方法,它继承自FrameLayout,该方法的实现在ViewGroup中,代码如下:

/frameworks/base/core/java/android/view/ViewGroup.java

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;
    }
}

可以看到,这里最终还是调用了View的方法,所以layout的过程在View和ViewGroup间也是没有差别的,View里的相关实现如下:

/frameworks/base/core/java/android/view/View.java

public void layout(int l, int t, int r, int b) {
    if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
        // 需要重新测量
        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);
    // 需要进行layout
    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        onLayout(changed, l, t, r, b);
        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            // 回调onLayoutChange
            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;
}

可以看到,真正的layout工作是在onLayout中完成的,而这个方法在View中是空实现,在ViewGroup中是抽象的,这也是自定义ViewGroup时必须重写此方法的原因。我们再来看在DecorView中的实现:

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    super.onLayout(changed, left, top, right, bottom);
    getOutsets(mOutsets);
    if (mOutsets.left > 0) {
        offsetLeftAndRight(-mOutsets.left);
    }
    if (mOutsets.top > 0) {
        offsetTopAndBottom(-mOutsets.top);
    }
    if (mApplyFloatingVerticalInsets) {
        offsetTopAndBottom(mFloatingInsets.top);
    }
    if (mApplyFloatingHorizontalInsets) {
        offsetLeftAndRight(mFloatingInsets.left);
    }

    // If the application changed its SystemUI metrics, we might also have to adapt
    // our shadow elevation.
    updateElevation();
    mAllowUpdateElevation = true;

    if (changed && mResizeMode == RESIZE_MODE_DOCKED_DIVIDER) {
        getViewRootImpl().requestInvalidateRootRenderNode();
    }
}

主要工作还是交给了父类来处理,以下是FrameLayout的相关实现:

protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}

void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
    final int count = getChildCount();

    final int parentLeft = getPaddingLeftWithForeground();
    final int parentRight = right - left - getPaddingRightWithForeground();

    final int parentTop = getPaddingTopWithForeground();
    final int parentBottom = bottom - top - getPaddingBottomWithForeground();

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            final int width = child.getMeasuredWidth();
            final int height = child.getMeasuredHeight();

            int childLeft;
            int childTop;

            int gravity = lp.gravity;
            if (gravity == -1) {
                gravity = DEFAULT_CHILD_GRAVITY;
            }

            final int layoutDirection = getLayoutDirection();
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
            final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;

            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity.CENTER_HORIZONTAL:
                    childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
                    lp.leftMargin - lp.rightMargin;
                    break;
                case Gravity.RIGHT:
                    if (!forceLeftGravity) {
                        childLeft = parentRight - width - lp.rightMargin;
                        break;
                    }
                case Gravity.LEFT:
                default:
                    childLeft = parentLeft + lp.leftMargin;
            }

            switch (verticalGravity) {
                case Gravity.TOP:
                    childTop = parentTop + lp.topMargin;
                    break;
                case Gravity.CENTER_VERTICAL:
                    childTop = parentTop + (parentBottom - parentTop - height) / 2 +
                    lp.topMargin - lp.bottomMargin;
                    break;
                case Gravity.BOTTOM:
                    childTop = parentBottom - height - lp.bottomMargin;
                    break;
                default:
                    childTop = parentTop + lp.topMargin;
            }

            child.layout(childLeft, childTop, childLeft + width, childTop + height);
        }
    }
}

现在我们就明白了,FrameLayout就是把子View根据其设置的Gravity放置在相应的位置,根据先后顺序进行叠加。

Layout完毕后,就到了最后一个流程:Draw。我们从入口函数看起,代码如下:

private void performDraw() {
    ...
    try {
        draw(fullRedrawNeeded);
    } finally {
        mIsDrawing = false;
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    ...

    if (mReportNextDraw) {
        ...
        try {
            mWindowSession.finishDrawing(mWindow);
        } catch (RemoteException e) {
        }
    }
}

这里调用了draw函数,代码如下:

private void draw(boolean fullRedrawNeeded) {
    ...

    if (!sFirstDrawComplete) {
        synchronized (sFirstDrawHandlers) {
            sFirstDrawComplete = true;
            final int count = sFirstDrawHandlers.size();
            for (int i = 0; i< count; i++) {
                mHandler.post(sFirstDrawHandlers.get(i));
            }
        }
    }

    // 滑动到指定区域
    scrollToRectOrFocus(null, false);

    // 分发OnScrollChanged事件
    if (mAttachInfo.mViewScrollChanged) {
        mAttachInfo.mViewScrollChanged = false;
        mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
    }

    ...

    // RootView滑动回调
    if (mCurScrollY != curScrollY) {
        mCurScrollY = curScrollY;
        fullRedrawNeeded = true;
        if (mView instanceof RootViewSurfaceTaker) {
            ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
        }
    }

    ...

    // 获取需要绘制的区域
    final Rect dirty = mDirty;
    ...

    ...

    // 分发onDraw
    mAttachInfo.mTreeObserver.dispatchOnDraw();

    ...

    if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
        if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
            // 硬件加速
        } else {
            ...
            if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
                return;
            }
        }
    }
    ...
}

这里主要处理了一些回调事件,以及是否设置了硬件加速,我们先不考虑硬件加速的部分,接下来看绘制的过程,代码如下:

private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
        boolean scalingRequired, Rect dirty) {

    // Draw with software renderer.
    final Canvas canvas;
    try {
        ...

        canvas = mSurface.lockCanvas(dirty);

        ...

        // TODO: Do this in native
        canvas.setDensity(mDensity);
    } catch (Surface.OutOfResourcesException e) {
        ...
    }

    try {
        ...
        try {
            canvas.translate(-xoff, -yoff);
            if (mTranslator != null) {
                mTranslator.translateCanvas(canvas);
            }
            canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
            attachInfo.mSetIgnoreDirtyState = false;

            mView.draw(canvas);

            drawAccessibilityFocusedDrawableIfNeeded(canvas);
        } finally {
            if (!attachInfo.mSetIgnoreDirtyState) {
                // Only clear the flag if it was not set during the mView.draw() call
                attachInfo.mIgnoreDirtyState = false;
            }
        }
    } finally {
        try {
            surface.unlockCanvasAndPost(canvas);
        } catch (IllegalArgumentException e) {
            ...
    }
    return true;
}

可以看到,这里是通过Surface来生成画布Canvas,相关操作在Native层进行,我们以后再分析。然后调用View#draw方法在画布上进行绘制,接下来我们看下DecorView的draw方法做了什么,代码如下:

public void draw(Canvas canvas) {
    super.draw(canvas);

    if (mMenuBackground != null) {
        mMenuBackground.draw(canvas);
    }
}

可以看到这里没有做什么特别的事情,主要的工作还是在View中完成的,代码如下:

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);

        // 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);

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

    ...
}

注释里写的很清楚,要依次执行6个步骤,不过步骤2和5是可以跳过的,这里我们主要看步骤3和步骤4。步骤3会调用View本身的onDraw方法,这个方法是空实现,如果自定义View一般需要自行实现。dispatchDraw在View中也是空实现,这个方法主要是针对ViewGroup的,所以我们看下ViewGroup中相应的实现,代码如下:

protected void dispatchDraw(Canvas canvas) {
    ...
    for (int i = 0; i < childrenCount; i++) {
        while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
            final View transientChild = mTransientViews.get(transientIndex);
            if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
                    transientChild.getAnimation() != null) {
                more |= drawChild(canvas, transientChild, drawingTime);
            }
            transientIndex++;
            if (transientIndex >= transientCount) {
                transientIndex = -1;
            }
        }

        final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
        final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
            more |= drawChild(canvas, child, drawingTime);
        }
    }
    ...
}

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

最终依然是由View来处理,代码如下:

boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
    ...

    if (!drawingWithRenderNode) {
        // apply clips directly, since RenderNode won't do it for this draw
        if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
            if (offsetForScroll) {
                canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
            } else {
                if (!scalingRequired || cache == null) {
                    canvas.clipRect(0, 0, getWidth(), getHeight());
                } else {
                    canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
                }
            }
        }

        if (mClipBounds != null) {
            // clip bounds ignore scroll
            canvas.clipRect(mClipBounds);
        }
    }

    if (!drawingWithDrawingCache) {
        if (drawingWithRenderNode) {
            mPrivateFlags &= ~PFLAG_DIRTY_MASK;
            ((DisplayListCanvas) canvas).drawRenderNode(renderNode);
        } else {
            // Fast path for layouts with no backgrounds
            if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
                mPrivateFlags &= ~PFLAG_DIRTY_MASK;
                dispatchDraw(canvas);
            } else {
                draw(canvas);
            }
        }
    } else if (cache != null) {
        mPrivateFlags &= ~PFLAG_DIRTY_MASK;
        if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
            // no layer paint, use temporary paint to draw bitmap
            Paint cachePaint = parent.mCachePaint;
            if (cachePaint == null) {
                cachePaint = new Paint();
                cachePaint.setDither(false);
                parent.mCachePaint = cachePaint;
            }
            cachePaint.setAlpha((int) (alpha * 255));
            canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
        } else {
            // use layer paint to draw the bitmap, merging the two alphas, but also restore
            int layerPaintAlpha = mLayerPaint.getAlpha();
            if (alpha < 1) {
                mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
            }
            canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
            if (alpha < 1) {
                mLayerPaint.setAlpha(layerPaintAlpha);
            }
        }
    }

    ...
}

这里主要是使用了缓存,如果没有缓存,就会调用View#draw方法进行绘制,或者是通知子View进行绘制。

至此,View的绘制流程我们就分析完毕了。

上一篇:Android源码分析之Activity启动与View绘制流程(一)

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