FrameLayout的onMearsure()與onLayout()源碼淺析

前言

之前一直對View繪制過程隻知道個大概,最近看了扔物線的視訊,突然來了興趣,于是找了一個比較簡單的ViewGroup來分析下源碼,鞏固下了解。

onMeasure

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();//子View的數量

        //①判斷是否需要再次測量子View
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear();

        int maxHeight = ;//子View占的最大高度
        int maxWidth = ;//子View占的最大寬度
        int childState = ;

        for (int i = ; i < count; i++) {
            final View child = getChildAt(i);
            //如果子View的Visibility屬性不為GONE,或者mMeasureAllChildren為True則測量子View的寬高
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //②開始測量子View的寬高與MeasureMode
                measureChildWithMargins(child, widthMeasureSpec, , heightMeasureSpec, );
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                //帶上Margin計算出子View占的最大寬度與最大高度
                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());
                //如果測量被測量的子ViewLayoutParams為Match_Parent,添加這些子View到需要重新測量的子View集合
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too FrameLayout最後的寬高還需要加上間距值
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width ,比較目前FrameLayout的寬高是否小于使用者設定的最小值
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        ...

        //③根據最後計算出的最大寬高,設定FrameLayout的寬高
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));


        count = mMatchParentChildren.size();
        if (count > ) {
            //根據LayoutParams的寬高是否為Match_Parent與重新計算過後确定的FrameLayout的寬高,重新計算子View寬高
            for (int i = ; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    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(, 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);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

分析

①判斷是否需要再次測量Match_parent的子View

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

這裡先判斷下FrameLayout本身的MeasureSpec是否都是EXACTLY,精确模式那麼FrameLayout的寬高都是确定了的,子View在測量時是Match_Parent,子View的的MeasureSpec也會變成EXACTLY,這樣就沒有再重新測量子View的必要了。可以看下ViewGroup中的getChildMeasureSpec()方法。

case MeasureSpec.EXACTLY:
            if (childDimension >= ) {
                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;

②開始測量子View的寬高與MeasureMode

measureChildWithMargins(child, widthMeasureSpec, , heightMeasureSpec, );

我們現在具體來看

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);
    }

可以看到這裡就是把Margin和Padding加起來再調用getChildMeasureSpec()方法

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

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

        int resultSize = ;
        int resultMode = ;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= ) {
                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 >= ) {
                // 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 >= ) {
                // 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 ?  : 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 ?  : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

getChildMeasureSpec()這個方法就是根據父View的MesasureSpec來确定FrameLayout期望子View的MearsureSpec,這部分相關很多部落格都說的很好,我就不重複了。

疑惑思考

onMeasure()代碼裡面看到Match_Parent的子View再FrameLayout不是EXACTIVITY的測量模式時計算了兩次,看代碼的時候一直沒有想通,為什麼需要計算兩次呢?

看着代碼想了很久,分别看了FramLayout的幾種情況,最後發現,如果Framelayout的MeasureMode是AT_MOST的話,第一輪測量子View時,全部子View的Size的最大值還不能确定是多少,是以需要等到一輪測量完成後,FrameLayout的寬高确定了,最後才能給LayoutParams為Match_Parent的子View重新算出寬高。

onLayout

FrameLayout中就一個方法layoutChildren()

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

具體來分析下layoutChildren:

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

        //算出各未知的的padding
        final int parentLeft = getPaddingLeftWithForeground();
        final int parentRight = right - left - getPaddingRightWithForeground();

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

    //和onMeasure類似,循環确定各子View的位置并調用子View的Layout方法
        for (int i = ; 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 == -) {
                    gravity = DEFAULT_CHILD_GRAVITY;
                }

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

                //确定子View的Left與Top的坐标
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                //①這裡可能稍微有點抽象需要多想想
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) /  +
                        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) /  +
                        lp.topMargin - lp.bottomMargin;
                        break;
                    case Gravity.BOTTOM:
                        childTop = parentBottom - height - lp.bottomMargin;
                        break;
                    default:
                        childTop = parentTop + lp.topMargin;
                }

                //知道Left和Top加上寬高就能推出四個點的坐标了
                child.layout(childLeft, childTop, childLeft + width, childTop + height);
            }
        }
    }

分析

①這裡可能稍微有點抽象需要多想想,特别是要求子View處于父View中心位置時。

case Gravity.CENTER_HORIZONTAL:
                        childLeft = parentLeft + (parentRight - parentLeft - width) /  +
                        lp.leftMargin - lp.rightMargin;
                        break;

直接看這個可能想象起來可能有點複雜,是以我們要先拆一下

先假定父View的左邊位置parentLeft是0,

(parentRight-parentLeft-width)/2=(parentRight-parentLeft)-width/2

(parentRight-parentLeft):求出ParentView的中點

(parentRight-parentLeft)-width/2:減去子View寬度的一半就是子View左邊的位置

(parentRight-parentLeft)-width/2+lp.leftMargin - lp.rightMargin:

求出子View左邊的位置加上Margin後的位置

parentLeft + (parentRight-parentLeft)-width/2+lp.leftMargin - lp.rightMargin,加上父View的左側的位置就是子View最後的位置

後記

由于個人水準有限,可能某些地方分析的不太好或者有錯誤,特别是有很多地方用到了位操作的地方還是沒看的太明白,希望大家多多了解,給予指正。

參考資料

HenCode

FrameLayout的onMearsure()與onLayout()源碼淺析

前言

onMeasure

分析

①判斷是否需要再次測量Match_parent的子View

②開始測量子View的寬高與MeasureMode

疑惑思考

onLayout

繼續閱讀

專訪鄧凡平：深入了解Android

與專家面對面：Android開發入門問與答

Android MVP架構MVCMVP

mvp初探

Android常用工具類簡介目錄

android常用工具類轉載自：http://www.trinea.cn/android/android-common-utils/Android常用工具類

Android常用工具類ToastUtilsSizeUtilsLogUtilsRetrofitManagerAnimUtil

Android Studio 0.8.0解決 Failure [INSTALL_FAILED_OLDER_SDK] 問題

Android – ListView 中添加按鈕，動态删除添加ItemView的操作

Kotlin的ListView與RecyclerView的簡單應用

關于公司的那些坑：Android Studio篇

android SQLite事務和kotlin協程

無法正常編譯錯誤Compilation failed to complete

關于Gradle配置的小結

AOP程式設計_Android優雅權限架構(1)概念基礎，2021金三銀四前言正文大綱正文

Android APK檔案反編譯Android APK檔案反編譯