如何優化你的佈局層級結構之RelativeLayout和LinearLayout及FrameLayout效能分析
工作一段時間後，經常會被領導說，你這個進入速度太慢了，競品的進入速度很快，你搞下優化吧？每當這時，你會怎麼辦？功能實現都有啊，進入時要載入那麼多view，這也沒辦法啊，等等。
先看一些現象吧：用Android studio，新建一個Activity自動生成的佈局檔案都是RelativeLayout，或許你會認為這是IDE的預設設定問題，其實不然，這是由 android-sdk\tools\templates\activities\EmptyActivity\root\res\layout\activity_simple.xml.ftl 這個檔案事先就定好了的，也就是說這是Google的選擇，而非IDE的選擇。那SDK為什麼會預設給開發者新建一個預設的RelativeLayout佈局呢？當然是因為RelativeLayout的效能更優，效能至上嘛。但是我們再看看預設新建的這個RelativeLayout的父容器，也就是當前視窗的頂級View——DecorView，它卻是個垂直方向的LinearLayout，上面是標題欄，下面是內容欄。那麼問題來了，Google為什麼給開發者預設新建了個RelativeLayout，而自己卻偷偷用了個LinearLayout，到底誰的效能更高，開發者該怎麼選擇呢？
View的一些基本工作原理
先通過幾個問題，簡單的瞭解寫android中View的工作原理吧。
View是什麼？
簡單來說，View是Android系統在螢幕上的視覺呈現，也就是說你在手機螢幕上看到的東西都是View。
View是怎麼繪製出來的？
View的繪製流程是從ViewRoot的performTraversals（）方法開始，依次經過measure（），layout（）和draw（）三個過程才最終將一個View繪製出來。
View是怎麼呈現在介面上的？
Android中的檢視都是通過Window來呈現的，不管Activity、Dialog還是Toast它們都有一個Window，然後通過WindowManager來管理View。Window和頂級View——DecorView的通訊是依賴ViewRoot完成的。
View和ViewGroup什麼區別？
不管簡單的Button和TextView還是複雜的RelativeLayout和ListView，他們的共同基類都是View。所以說，View是一種介面層控制元件的抽象，他代表了一個控制元件。那ViewGroup是什麼東西，它可以被翻譯成控制元件組，即一組View。ViewGroup也是繼承View，這就意味著View本身可以是單個控制元件，也可以是多個控制元件組成的控制元件組。根據這個理論，Button顯然是個View，而RelativeLayout不但是一個View還可以是一個ViewGroup，而ViewGroup內部是可以有子View的，這個子View同樣也可能是ViewGroup，以此類推。
RelativeLayout和LinearLayout效能PK
基於以上原理和大背景，我們要探討的效能問題，說的簡單明瞭一點就是：當RelativeLayout和LinearLayout分別作為ViewGroup，表達相同佈局時繪製在螢幕上時誰更快一點。上面已經簡單說了View的繪製，從ViewRoot的performTraversals（）方法開始依次呼叫perfromMeasure、performLayout和performDraw這三個方法。這三個方法分別完成頂級View的measure、layout和draw三大流程，其中perfromMeasure會呼叫measure，measure又會呼叫onMeasure，在onMeasure方法中則會對所有子元素進行measure，這個時候measure流程就從父容器傳遞到子元素中了，這樣就完成了一次measure過程，接著子元素會重複父容器的measure，如此反覆就完成了整個View樹的遍歷。同理，performLayout和performDraw也分別完成perfromMeasure類似的流程。通過這三大流程，分別遍歷整棵View樹，就實現了Measure，Layout，Draw這一過程，View就繪製出來了。那麼我們就分別來追蹤下RelativeLayout和LinearLayout這三大流程的執行耗時。
如下圖，我們分別用兩用種方式簡單的實現佈局測試下
LinearLayout
Measure：0.762ms
Layout：0.167ms
draw：7.665ms
RelativeLayout
Measure：2.180ms
Layout：0.156ms
draw：7.694ms
從這個資料來看無論使用RelativeLayout還是LinearLayout，layout和draw的過程兩者相差無幾，考慮到誤差的問題，幾乎可以認為兩者不分伯仲，關鍵是Measure的過程RelativeLayout卻比LinearLayout慢了一大截。
Measure都幹什麼了
RelativeLayout的onMeasure()方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mDirtyHierarchy) {
mDirtyHierarchy = false;
sortChildren();
}
int myWidth = -1;
int myHeight = -1;
int width = 0;
int height = 0;
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
// Record our dimensions if they are known;
if (widthMode != MeasureSpec.UNSPECIFIED) {
myWidth = widthSize;
}
if (heightMode != MeasureSpec.UNSPECIFIED) {
myHeight = heightSize;
}
if (widthMode == MeasureSpec.EXACTLY) {
width = myWidth;
}
if (heightMode == MeasureSpec.EXACTLY) {
height = myHeight;
}
View ignore = null;
int gravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
final boolean horizontalGravity = gravity != Gravity.START && gravity != 0;
gravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final boolean verticalGravity = gravity != Gravity.TOP && gravity != 0;
int left = Integer.MAX_VALUE;
int top = Integer.MAX_VALUE;
int right = Integer.MIN_VALUE;
int bottom = Integer.MIN_VALUE;
boolean offsetHorizontalAxis = false;
boolean offsetVerticalAxis = false;
if ((horizontalGravity || verticalGravity) && mIgnoreGravity != View.NO_ID) {
ignore = findViewById(mIgnoreGravity);
}
final boolean isWrapContentWidth = widthMode != MeasureSpec.EXACTLY;
final boolean isWrapContentHeight = heightMode != MeasureSpec.EXACTLY;
// We need to know our size for doing the correct computation of children positioning in RTL
// mode but there is no practical way to get it instead of running the code below.
// So, instead of running the code twice, we just set the width to a "default display width"
// before the computation and then, as a last pass, we will update their real position with
// an offset equals to "DEFAULT_WIDTH - width".
final int layoutDirection = getLayoutDirection();
if (isLayoutRtl() && myWidth == -1) {
myWidth = DEFAULT_WIDTH;
}
View[] views = mSortedHorizontalChildren;
int count = views.length;
for (int i = 0; i < count; i++) {
View child = views[i];
if (child.getVisibility() != GONE) {
LayoutParams params = (LayoutParams) child.getLayoutParams();
int[] rules = params.getRules(layoutDirection);
applyHorizontalSizeRules(params, myWidth, rules);
measureChildHorizontal(child, params, myWidth, myHeight);
if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
offsetHorizontalAxis = true;
}
}
}
views = mSortedVerticalChildren;
count = views.length;
final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE) {
final LayoutParams params = (LayoutParams) child.getLayoutParams();
applyVerticalSizeRules(params, myHeight, child.getBaseline());
measureChild(child, params, myWidth, myHeight);
if (positionChildVertical(child, params, myHeight, isWrapContentHeight)) {
offsetVerticalAxis = true;
}
if (isWrapContentWidth) {
if (isLayoutRtl()) {
if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
width = Math.max(width, myWidth - params.mLeft);
} else {
width = Math.max(width, myWidth - params.mLeft - params.leftMargin);
}
} else {
if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
width = Math.max(width, params.mRight);
} else {
width = Math.max(width, params.mRight + params.rightMargin);
}
}
}
if (isWrapContentHeight) {
if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
height = Math.max(height, params.mBottom);
} else {
height = Math.max(height, params.mBottom + params.bottomMargin);
}
}
if (child != ignore || verticalGravity) {
left = Math.min(left, params.mLeft - params.leftMargin);
top = Math.min(top, params.mTop - params.topMargin);
}
if (child != ignore || horizontalGravity) {
right = Math.max(right, params.mRight + params.rightMargin);
bottom = Math.max(bottom, params.mBottom + params.bottomMargin);
}
}
}
// Use the top-start-most laid out view as the baseline. RTL offsets are
// applied later, so we can use the left-most edge as the starting edge.
View baselineView = null;
LayoutParams baselineParams = null;
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE) {
final LayoutParams childParams = (LayoutParams) child.getLayoutParams();
if (baselineView == null || baselineParams == null
|| compareLayoutPosition(childParams, baselineParams) < 0) {
baselineView = child;
baselineParams = childParams;
}
}
}
mBaselineView = baselineView;
if (isWrapContentWidth) {
// Width already has left padding in it since it was calculated by looking at
// the right of each child view
width += mPaddingRight;
if (mLayoutParams != null && mLayoutParams.width >= 0) {
width = Math.max(width, mLayoutParams.width);
}
width = Math.max(width, getSuggestedMinimumWidth());
width = resolveSize(width, widthMeasureSpec);
if (offsetHorizontalAxis) {
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE) {
final LayoutParams params = (LayoutParams) child.getLayoutParams();
final int[] rules = params.getRules(layoutDirection);
if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_HORIZONTAL] != 0) {
centerHorizontal(child, params, width);
} else if (rules[ALIGN_PARENT_RIGHT] != 0) {
final int childWidth = child.getMeasuredWidth();
params.mLeft = width - mPaddingRight - childWidth;
params.mRight = params.mLeft + childWidth;
}
}
}
}
}
if (isWrapContentHeight) {
// Height already has top padding in it since it was calculated by looking at
// the bottom of each child view
height += mPaddingBottom;
if (mLayoutParams != null && mLayoutParams.height >= 0) {
height = Math.max(height, mLayoutParams.height);
}
height = Math.max(height, getSuggestedMinimumHeight());
height = resolveSize(height, heightMeasureSpec);
if (offsetVerticalAxis) {
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE) {
final LayoutParams params = (LayoutParams) child.getLayoutParams();
final int[] rules = params.getRules(layoutDirection);
if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_VERTICAL] != 0) {
centerVertical(child, params, height);
} else if (rules[ALIGN_PARENT_BOTTOM] != 0) {
final int childHeight = child.getMeasuredHeight();
params.mTop = height - mPaddingBottom - childHeight;
params.mBottom = params.mTop + childHeight;
}
}
}
}
}
if (horizontalGravity || verticalGravity) {
final Rect selfBounds = mSelfBounds;
selfBounds.set(mPaddingLeft, mPaddingTop, width - mPaddingRight,
height - mPaddingBottom);
final Rect contentBounds = mContentBounds;
Gravity.apply(mGravity, right - left, bottom - top, selfBounds, contentBounds,
layoutDirection);
final int horizontalOffset = contentBounds.left - left;
final int verticalOffset = contentBounds.top - top;
if (horizontalOffset != 0 || verticalOffset != 0) {
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE && child != ignore) {
final LayoutParams params = (LayoutParams) child.getLayoutParams();
if (horizontalGravity) {
params.mLeft += horizontalOffset;
params.mRight += horizontalOffset;
}
if (verticalGravity) {
params.mTop += verticalOffset;
params.mBottom += verticalOffset;
}
}
}
}
}
if (isLayoutRtl()) {
final int offsetWidth = myWidth - width;
for (int i = 0; i < count; i++) {
final View child = views[i];
if (child.getVisibility() != GONE) {
final LayoutParams params = (LayoutParams) child.getLayoutParams();
params.mLeft -= offsetWidth;
params.mRight -= offsetWidth;
}
}
}
setMeasuredDimension(width, height);
}根據原始碼我們發現RelativeLayout會對子View做兩次measure。這是為什麼呢？首先RelativeLayout中子View的排列方式是基於彼此的依賴關係，而這個依賴關係可能和佈局中View的順序並不相同，在確定每個子View的位置的時候，就需要先給所有的子View排序一下。又因為RelativeLayout允許A，B 2個子View，橫向上B依賴A，縱向上A依賴B。所以需要橫向縱向分別進行一次排序測量。
LinearLayout的onMeasure()方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}與RelativeLayout相比LinearLayout的measure就簡單明瞭的多了，先判斷線性規則，然後執行對應方向上的測量。隨便看一個吧。
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
totalWeight += lp.weight;
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
// Optimization: don't bother measuring children who are going to use
// leftover space. These views will get measured again down below if
// there is any leftover space.
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
} else {
int oldHeight = Integer.MIN_VALUE;
if (lp.height == 0 && lp.weight > 0) {
// heightMode is either UNSPECIFIED or AT_MOST, and this
// child wanted to stretch to fill available space.
// Translate that to WRAP_CONTENT so that it does not end up
// with a height of 0
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
if (useLargestChild) {
largestChildHeight = Math.max(childHeight, largestChildHeight);
}
}父檢視在對子檢視進行measure操作的過程中，使用變數mTotalLength儲存已經measure過的child所佔用的高度，該變數剛開始時是0。在for迴圈中呼叫measureChildBeforeLayout（）對每一個child進行測量，該函式實際上僅僅是呼叫了measureChildWithMargins(),在呼叫該方法時，使用了兩個引數。其中一個是heightMeasureSpec，該引數為LinearLayout本身的measureSpec；另一個引數就是mTotalLength，代表該LinearLayout已經被其子檢視所佔用的高度。 每次for迴圈對child測量完畢後，呼叫child.getMeasuredHeight()獲取該子檢視最終的高度，並將這個高度新增到mTotalLength中。在本步驟中，暫時避開了lp.weight>0的子檢視，即暫時先不測量這些子檢視，因為後面將把父檢視剩餘的高度按照weight值的大小平均分配給相應的子檢視。原始碼中使用了一個區域性變數totalWeight累計所有子檢視的weight值。處理lp.weight>0的情況需要注意，如果變數heightMode是EXACTLY，那麼，當其他子檢視佔滿父檢視的高度後，weight>0的子檢視可能分配不到佈局空間，從而不被顯示，只有當heightMode是AT_MOST或者UNSPECIFIED時，weight>0的檢視才能優先獲得佈局高度。最後我們的結論是：如果不使用weight屬性，LinearLayout會在當前方向上進行一次measure的過程，如果使用weight屬性，LinearLayout會避開設定過weight屬性的view做第一次measure，完了再對設定過weight屬性的view做第二次measure。由此可見，weight屬性對效能是有影響的，而且本身有大坑，請注意避讓。
本文出自逆流的魚：http://blog.csdn.net/hejjunlin/article/details/51159419
小結
從原始碼中我們似乎能看出，我們先前的測試結果中RelativeLayout不如LinearLayout快的根本原因是RelativeLayout需要對其子View進行兩次measure過程。而LinearLayout則只需一次measure過程，所以顯然會快於RelativeLayout，但是如果LinearLayout中有weight屬性，則也需要進行兩次measure，但即便如此，應該仍然會比RelativeLayout的情況好一點。
RelativeLayout另一個性能問題
對比到這裡就結束了嘛？顯然沒有！我們再看看View的Measure（）方法都幹了些什麼？
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
......
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}FrameLayout和LinearLayout效能PK
FrameLayout
LinearLayout
Measure：2.058ms
Layout：0.296ms
draw：3.857ms
FrameLayout
Measure：1.334ms
Layout：0.213ms
draw：3.680ms
從這個資料來使用LinearLayout，僅巢狀一個LinearLayou，在onMeasure就相關2倍時間和FrameLayout相比，layout和draw的過程兩者相差無幾，考慮到誤差的問題，幾乎可以認為兩者不分伯仲
看下FrameLayout的原始碼，做了什麼？
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
//當FrameLayout的寬和高，只有同時設定為match_parent或者指定的size，那麼這個
//measureMatchParentChlidren = false，否則為true。下面會用到這個變數
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0; //寬高的期望型別
for (int i = 0; i < count; i++) { //一次遍歷每一個不為GONE的子view
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//去掉FrameLayout的左右padding，子view的左右margin，這時候，再去
//計運算元view的期望的值
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
/*maxWidth找到子View中最大的寬，高同理，為什麼要找到他，因為在這裡，FrameLayout是wrap
-content.他的寬高肯定受子view的影響*/
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());
/*下面的判斷，只有上面的FragLayout的width和height都設定為match_parent 才不會執行
此處的mMatchParentChlidren的list裡存的是設定為match_parent的子view。
結合上面兩句話的意思，當FrameLayout設定為wrap_content，這時候要把所有寬高設定為
match_parent的子View都記錄下來，記錄下來幹什麼呢？
這時候FrameLayout的寬高同時受子View的影響*/
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());
}
//設定測量過的寬高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();//這個大小就是子view中設定為match_parent的個數
if (count > 1) {
for (int i = 0; i < count; i++) {
//這裡看上去重新計算了一遍
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
int childWidthMeasureSpec;
int childHeightMeasureSpec;
/*如果子view的寬是match_parent，則寬度期望值是總寬度-padding-margin
如果子view的寬是指定的比如100dp，則寬度期望值是padding+margin+width
這個很容易理解,下面的高同理*/
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
//把這部分子view重新計算大小
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}加了一個巢狀，onMeasure時間，多了將近一倍，原因在於：LinearLayout在某一方向onMeasure，發現還存在LinearLayout。將觸發
if (useLargestChild && (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
}結論
1.RelativeLayout會讓子View呼叫2次onMeasure，LinearLayout 在有weight時，也會呼叫子View2次onMeasure
2.RelativeLayout的子View如果高度和RelativeLayout不同，則會引發效率問題，當子View很複雜時，這個問題會更加嚴重。如果可以，儘量使用padding代替margin。
3.在不影響層級深度的情況下,使用LinearLayout和FrameLayout而不是RelativeLayout。
最後再思考一下文章開頭那個矛盾的問題，為什麼Google給開發者預設新建了個RelativeLayout，而自己卻在DecorView中用了個LinearLayout。因為DecorView的層級深度是已知而且固定的，上面一個標題欄，下面一個內容欄。採用RelativeLayout並不會降低層級深度，所以此時在根節點上用LinearLayout是效率最高的。而之所以給開發者預設新建了個RelativeLayout是希望開發者能採用儘量少的View層級來表達佈局以實現效能最優，因為複雜的View巢狀對效能的影響會更大一些。
4.能用兩層LinearLayout，儘量用一個RelativeLayout，在時間上此時RelativeLayout耗時更小。另外LinearLayout慎用layout_weight,也將會增加一倍耗時操作。由於使用LinearLayout的layout_weight,大多數時間是不一樣的，這會降低測量的速度。這只是一個如何合理使用Layout的案例，必要的時候，你要小心考慮是否用layout weight。總之減少層級結構，才是王道，讓onMeasure做延遲載入，用viewStub，include等一些技巧。
第一時間獲得部落格更新提醒，以及更多android乾貨，原始碼分析，歡迎關注我的微信公眾號，掃一掃下方二維碼，即可關注。
