这是一个很有趣的现象 大家稍作测试就会发现
你复制代码1可以正常使用,代码2就会发现layout_marginHorizontal无效了
· 代码1
android:id="@+id/view_phone"
android:layout_width="0dp"
android:layout_height="52dp"
android:layout_marginHorizontal="45dp"
android:background="@color/red"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.3" />
· 代码2
先做一个简单的知识补充,对于我们平常设置的 marginStart,marginEnd的时候如果左右的间距一样那么可以通过设置layout_marginHorizontal即可以实现避免我们重复写两个代码
至于是为什么会发现这个问题呢,是今天同事问我为什么设置layout_marginHorizontal无效了 给我贴一段代码,基本就是代码2,我也一脸懵逼,看了半天也没发现,最后是同事自己发现多加了android:layout_marginStart
导致的
接下来分析一下为什么会这样,我自测了一下发现marginLeft,marginRihgt不会导致无效,只有marginStart,marginEnd会导致
那么我们带着问题来找答案
首先我们知道layout_marginHorizontal是ViewGroup的属性,所以就全局搜索这个字段
可以发现是在如下代码进行设置的
public MarginLayoutParams(Context c, AttributeSet attrs) {
super();
TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_MarginLayout);
setBaseAttributes(a,
R.styleable.ViewGroup_MarginLayout_layout_width,
R.styleable.ViewGroup_MarginLayout_layout_height);
int margin = a.getDimensionPixelSize(
com.android.internal.R.styleable.ViewGroup_MarginLayout_layout_margin, -1);
if (margin >= 0) {
leftMargin = margin;
topMargin = margin;
rightMargin= margin;
bottomMargin = margin;
} else {
int horizontalMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginHorizontal, -1);
int verticalMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginVertical, -1);
if (horizontalMargin >= 0) { //这里左右margin
leftMargin = horizontalMargin;
rightMargin = horizontalMargin;
} else {
leftMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginLeft,
UNDEFINED_MARGIN);
if (leftMargin == UNDEFINED_MARGIN) {
mMarginFlags |= LEFT_MARGIN_UNDEFINED_MASK;
leftMargin = DEFAULT_MARGIN_RESOLVED;
}
rightMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginRight,
UNDEFINED_MARGIN);
if (rightMargin == UNDEFINED_MARGIN) {
mMarginFlags |= RIGHT_MARGIN_UNDEFINED_MASK;
rightMargin = DEFAULT_MARGIN_RESOLVED;
}
}
//这里是获取start,endMargin
startMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginStart,
DEFAULT_MARGIN_RELATIVE);
endMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginEnd,
DEFAULT_MARGIN_RELATIVE);
if (verticalMargin >= 0) {
topMargin = verticalMargin;
bottomMargin = verticalMargin;
} else {
topMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginTop,
DEFAULT_MARGIN_RESOLVED);
bottomMargin = a.getDimensionPixelSize(
R.styleable.ViewGroup_MarginLayout_layout_marginBottom,
DEFAULT_MARGIN_RESOLVED);
}
//注意这里,后续要用到,这里可以看下 isMarginRelative()方法
if (isMarginRelative()) {
mMarginFlags |= NEED_RESOLUTION_MASK;
}
}
final boolean hasRtlSupport = c.getApplicationInfo().hasRtlSupport();
final int targetSdkVersion = c.getApplicationInfo().targetSdkVersion;
if (targetSdkVersion < JELLY_BEAN_MR1 || !hasRtlSupport) {
mMarginFlags |= RTL_COMPATIBILITY_MODE_MASK;
}
// Layout direction is LTR by default
mMarginFlags |= LAYOUT_DIRECTION_LTR;
a.recycle();
}
isMarginRelative方法如下
public boolean isMarginRelative() {
return (startMargin != DEFAULT_MARGIN_RELATIVE || endMargin != DEFAULT_MARGIN_RELATIVE);
}
这里我们知道因为设置了startMargin,肯定是返回的true
接着往下分析,既然是layout_marginHorizontal无效 通过上诉代码我们知道 Viewgroup是将layout_marginHorizontal设置给Leftmarign,rightMargin ,但是实际情况是设置endMargin后layout_marginHorizontal无效,同时endMargin生效,那么我们直接搜索endMargin看他是在哪里使用的就行了
最后通过搜索可以定位到如下代码
private void doResolveMargins() {
if ((mMarginFlags & RTL_COMPATIBILITY_MODE_MASK) == RTL_COMPATIBILITY_MODE_MASK) {
// if left or right margins are not defined and if we have some start or end margin
// defined then use those start and end margins.
if ((mMarginFlags & LEFT_MARGIN_UNDEFINED_MASK) == LEFT_MARGIN_UNDEFINED_MASK
&& startMargin > DEFAULT_MARGIN_RELATIVE) {
leftMargin = startMargin;
}
if ((mMarginFlags & RIGHT_MARGIN_UNDEFINED_MASK) == RIGHT_MARGIN_UNDEFINED_MASK
&& endMargin > DEFAULT_MARGIN_RELATIVE) {
rightMargin = endMargin;
}
} else {
// We have some relative margins (either the start one or the end one or both). So use
// them and override what has been defined for left and right margins. If either start
// or end margin is not defined, just set it to default "0".
switch(mMarginFlags & LAYOUT_DIRECTION_MASK) {
case View.LAYOUT_DIRECTION_RTL:
leftMargin = (endMargin > DEFAULT_MARGIN_RELATIVE) ?
endMargin : DEFAULT_MARGIN_RESOLVED;
rightMargin = (startMargin > DEFAULT_MARGIN_RELATIVE) ?
startMargin : DEFAULT_MARGIN_RESOLVED;
break;
case View.LAYOUT_DIRECTION_LTR:
default:
leftMargin = (startMargin > DEFAULT_MARGIN_RELATIVE) ?
startMargin : DEFAULT_MARGIN_RESOLVED;
rightMargin = (endMargin > DEFAULT_MARGIN_RELATIVE) ?
endMargin : DEFAULT_MARGIN_RESOLVED;
break;
}
}
mMarginFlags &= ~NEED_RESOLUTION_MASK;
}
这里我们回到MarginLayoutParams的构造方法这里看如下代码
if (targetSdkVersion < JELLY_BEAN_MR1 || !hasRtlSupport) {
mMarginFlags |= RTL_COMPATIBILITY_MODE_MASK;
}
// Layout direction is LTR by default
mMarginFlags |= LAYOUT_DIRECTION_LTR
由此可以定位到这个if...else...方法体是走else里的内容,然后通过mMarginFlags |= LAYOUT_DIRECTION_LTR
这一段可以确定是case到了
case View.LAYOUT_DIRECTION_LTR:
default:
leftMargin = (startMargin > DEFAULT_MARGIN_RELATIVE) ?
startMargin : DEFAULT_MARGIN_RESOLVED;
rightMargin = (endMargin > DEFAULT_MARGIN_RELATIVE) ?
endMargin : DEFAULT_MARGIN_RESOLVED;
break;
这里
那么就明白了 left,right被重置到了start,end的大小。
以上是知道了为什么被重置了,接下来看下是通过代码引用看下是在哪里被重置。
首先看doResolveMargins()
方法的引用如下,依次查看被引用的方法图1,图2,图3,图4
.
·
看过之后我们知道图2,图3都是被调用后才会调用,只有图4最可能,继续追查最后在View的measure内找到调用代码
measure->resolveRtlPropertiesIfNeeded->resolveLayoutParams