Android （单帧布局）FrameLayout

川长思鸟来 2022-01-14 21:21 356阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

感觉FrameLayout很神秘，是因为用到它的地方少，一直觉得它鸡肋，原来是自己无知。最近需要实现一些layer的效果，就用到了它。它的用法很简单，这里就不多说了，这里就说说它的原理吧。

FrameLayout类里面没有什么东西，主要说的还是它的自身的布局参数FrameLayout.LayoutParams，布局参数类继承MarginLayoutParams。看名词就知道，就是控制view的外边距的，FrameLayout.LayoutParams本身自己定义的参数只有一个gravity。

好了，既然有了布局参数，那就会有空间的一些属性吧。首先FrameLayout也是一个View，所以他必然有Padding(view的内边距)相关属性。这里我们重点关注它的onMeasure与onLayout：

onMeasure意思就是计算出该ViewGroup的布局大小及孩子View的布局大小

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) \{  
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);  
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());  
\}

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();  
int childWidthMeasureSpec;  
int childHeightMeasureSpec;  
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);  
\}

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

简单的说父容器的width就是所有孩子View的width取最大值加View外边距和它的内边距：child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin+getPaddingLeftWithForeground() + getPaddingRightWithForeground();如果ViewGroup有图片，还有跟图片比较取较大值,容器的height也是同样的道理。FrameLayout孩子View的布局大小跟普通View大小确定是一样的。

protected void onLayout(boolean changed, int left, int top, int right, int bottom) \{  
final int count = getChildCount();

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

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

mForegroundBoundsChanged = true;  
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 = getResolvedLayoutDirection();  
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);  
final int verticalGravity = gravity & Gravity.VERTICAL\_GRAVITY\_MASK;

switch (absoluteGravity & Gravity.HORIZONTAL\_GRAVITY\_MASK) \{  
case Gravity.LEFT:  
childLeft = parentLeft + lp.leftMargin;  
break;  
case Gravity.CENTER\_HORIZONTAL:  
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +  
lp.leftMargin - lp.rightMargin;  
break;  
case Gravity.RIGHT:  
childLeft = parentRight - width - lp.rightMargin;  
break;  
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);  
\}  
\}  
\}

从代码中可以看出，孩子View左上角坐标的X值就是ViewGroup的左内边距加上view的外边距，Y值同理。

哎，怎么感觉自己写着写着，还把问题说复杂了点，感觉还是看代码能明白。

还有，我在2.2版本的代码上，gravity与margin属性必须同时设置，margin才能发挥作用，如果只有margin属性而没有设置gravity,就没有效果。如果有谁知道，欢迎指教哈。

在上一篇中说了下android里drag and drop 一个View，当时那个imageView放在一个LinearLayout里面，事实上放在FrameLayout同样适用，因为他们的布局参数都继承MarginLayoutParams类，所以通过margin控制他们的布局位置同样在FrameLayout使用。事实上GridLayout,LinearLayout,RelativeLayout,FrameLayout的布局参数都是继承MarginLayoutParams的。

转载于:https://my.oschina.net/gavinjin/blog/42213

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220114/3224de1c9a9d46f7955c51b63cd74ee4.png