Android源碼分析之Activity啟動(dòng)與View繪制流程(二)

續(xù)前節(jié),接下來(lái)我們分析ViewRootImpl#performTraversals方法,代碼如下:

/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();
    ...
}

這個(gè)函數(shù)做了很多的事情,但最重要的是調(diào)用MeasuerLayoutDraw三個(gè)過(guò)程。這里首先看到有獲取子View的MeasureSpec的方法,代碼如下:

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對(duì)應(yīng)的模式就是AT_MOSTMATCH_PARENT或其他值,也就是具體數(shù)值,對(duì)應(yīng)的模式是EXACTLY。這個(gè)MeasureSpec想必大家都了解過(guò),它的高兩位用來(lái)表示模式SpecMode,低30位用來(lái)表示大小SpecSizeSpecMode共有以下三種類型:

  • UNSPECIFIED:父容器不作限制,子View想多大就多大,一般用于系統(tǒng)內(nèi)部。
  • EXACTLY:精確模式,大小為SpecSize,父容器完全決定子View的大小,對(duì)應(yīng)LayoutParams中的match_parent和具體數(shù)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,對(duì)應(yīng)于LayoutParams中的warp_content。

接下來(lái)我們分析下測(cè)量的過(guò)程,代碼如下:

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);
    }
}

這里Measure交給View來(lái)完成,代碼如下:

/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主要的工作是回調(diào)onMeasure,在自定義View時(shí)也經(jīng)常會(huì)重寫(xiě)此方法,代碼如下:

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

可以看到,如果不重寫(xiě)此方法,系統(tǒng)會(huì)設(shè)置一個(gè)默認(rèn)的大小給子View,這里先看下這個(gè)默認(rèn)大小的實(shí)現(xiàn),代碼如下:

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;
}

也就是說(shuō),無(wú)論是EXACTLY還是AT_MOST,都按照測(cè)量結(jié)果進(jìn)行設(shè)置。繼續(xù)來(lái)看setMeasuredDimension方法,代碼如下:

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;
}

也就是最終將測(cè)量的結(jié)果保存在mMeasuredWidthmMeasuredHeight變量中。ViewGroup的測(cè)量流程和此一致,只是其在onMeasure時(shí)需要測(cè)量子View,我們結(jié)合DecorView來(lái)分析,它繼承自FrameLayout,以下是它的onMeasure方法實(shí)現(xiàn):

/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,主要是對(duì)AT_MOST模式下的寬高進(jìn)行了修正,然后調(diào)用父類的方法,我們的關(guān)注點(diǎn)在后者,下面是FrameLayout的相關(guān)實(shí)現(xiàn):

/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) {
            // 對(duì)child進(jìn)行測(cè)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());
    }

    // 設(shè)置測(cè)量結(jié)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;
            // 根據(jù)不同的LP,給child設(shè)置不同的測(cè)量寬度
            if (lp.width == LayoutParams.MATCH_PARENT) {
                // 使用剩余的全部空間
                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);
            }
            // 調(diào)用child的measure方法
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

這里首先測(cè)量child,根據(jù)此來(lái)設(shè)置ViewGroup的寬高,然后再設(shè)定所有的設(shè)置屬性為MATCH_PARENT的child的寬高,這是因?yàn)閂iewGroup后來(lái)處理了padding與Foreground的值,導(dǎo)致可用空間和測(cè)量結(jié)果不一致了。我們先看它是如何測(cè)量child的,代碼如下:

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,這里也調(diào)用了getChildMeasureSpec方法,和后邊處理MATCH_PARENT模式的child唯一不同之處在于padding值的大小,也就是將Foreground的padding值加了進(jìn)來(lái)。現(xiàn)在我們看下這個(gè)getChildMeasureSpec的實(shí)現(xiàn):

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);
}

這里根據(jù)ViewGroup的SpecMode不同,給child設(shè)置了不同的模式和大小,以保證child能正確完成測(cè)量的過(guò)程。

接下來(lái)我們分析Layout的流程,代碼如下:

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

    final View host = mView;
    ...
    try {
        // 先調(diào)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.
            // 獲取到需要進(jìn)行l(wèi)ayout的View的個(gè)數(shù)
            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");
                    // 調(diào)用它們的requestLayout方法,
                    view.requestLayout();
                }
                // 再次進(jìn)行測(cè)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
                    // 再次檢查是否仍有需要layout的View,如果有,就到下一幀再繼續(xù)
                    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;
}

以上過(guò)程是先讓DecorView進(jìn)行l(wèi)ayout,然后找到所有需要進(jìn)行l(wèi)ayout的View并調(diào)用它們的requestLayout方法,然后對(duì)整個(gè)View Tree重新測(cè)量,再次進(jìn)行第二次layout過(guò)程。

DecorView并沒(méi)有重寫(xiě)layout方法,它繼承自FrameLayout,該方法的實(shí)現(xiàn)在ViewGroup中,代碼如下:

/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,這里最終還是調(diào)用了View的方法,所以layout的過(guò)程在View和ViewGroup間也是沒(méi)有差別的,View里的相關(guān)實(shí)現(xiàn)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) {
        // 需要重新測(cè)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);
    // 需要進(jìn)行l(wèi)ayout
    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) {
            // 回調(diào)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,而這個(gè)方法在View中是空實(shí)現(xiàn),在ViewGroup中是抽象的,這也是自定義ViewGroup時(shí)必須重寫(xiě)此方法的原因。我們?cè)賮?lái)看在DecorView中的實(shí)現(xiàn):

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();
    }
}

主要工作還是交給了父類來(lái)處理,以下是FrameLayout的相關(guān)實(shí)現(xiàn):

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);
        }
    }
}

現(xiàn)在我們就明白了,F(xiàn)rameLayout就是把子View根據(jù)其設(shè)置的Gravity放置在相應(yīng)的位置,根據(jù)先后順序進(jìn)行疊加。

Layout完畢后,就到了最后一個(gè)流程:Draw。我們從入口函數(shù)看起,代碼如下:

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

    ...

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

這里調(diào)用了draw函數(shù),代碼如下:

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));
            }
        }
    }

    // 滑動(dòng)到指定區(qū)域
    scrollToRectOrFocus(null, false);

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

    ...

    // RootView滑動(dòng)回調(diào)
    if (mCurScrollY != curScrollY) {
        mCurScrollY = curScrollY;
        fullRedrawNeeded = true;
        if (mView instanceof RootViewSurfaceTaker) {
            ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
        }
    }

    ...

    // 獲取需要繪制的區(qū)域
    final Rect dirty = mDirty;
    ...

    ...

    // 分發(fā)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;
            }
        }
    }
    ...
}

這里主要處理了一些回調(diào)事件,以及是否設(shè)置了硬件加速,我們先不考慮硬件加速的部分,接下來(lái)看繪制的過(guò)程,代碼如下:

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,這里是通過(guò)Surface來(lái)生成畫(huà)布Canvas,相關(guān)操作在Native層進(jìn)行,我們以后再分析。然后調(diào)用View#draw方法在畫(huà)布上進(jìn)行繪制,接下來(lái)我們看下DecorView的draw方法做了什么,代碼如下:

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

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

可以看到這里沒(méi)有做什么特別的事情,主要的工作還是在View中完成的,代碼如下:

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;
    }

    ...
}

注釋里寫(xiě)的很清楚,要依次執(zhí)行6個(gè)步驟,不過(guò)步驟2和5是可以跳過(guò)的,這里我們主要看步驟3和步驟4。步驟3會(huì)調(diào)用View本身的onDraw方法,這個(gè)方法是空實(shí)現(xiàn),如果自定義View一般需要自行實(shí)現(xiàn)。dispatchDraw在View中也是空實(shí)現(xiàn),這個(gè)方法主要是針對(duì)ViewGroup的,所以我們看下ViewGroup中相應(yīng)的實(shí)現(xiàn),代碼如下:

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);
}

最終依然是由View來(lái)處理,代碼如下:

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);
            }
        }
    }

    ...
}

這里主要是使用了緩存,如果沒(méi)有緩存,就會(huì)調(diào)用View#draw方法進(jìn)行繪制,或者是通知子View進(jìn)行繪制。

至此,View的繪制流程我們就分析完畢了。

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