ViewGroup.LayoutParams 布局参数
View.MeasureSpec
View 和 ViewGroup 中几个重要方法分析
LayoutParams 布局参数
ViewGroup.LayoutParams 类
public static class LayoutParams {
/**
* Special value for the height or width requested by a View.
* FILL_PARENT means that the view wants to be as big as its parent,
* minus the parent's padding, if any. This value is deprecated
* starting in API Level 8 and replaced by {@link #MATCH_PARENT}.
*/
@SuppressWarnings({"UnusedDeclaration"})
@Deprecated
public static final int FILL_PARENT = -1;
/**
* Special value for the height or width requested by a View.
* MATCH_PARENT means that the view wants to be as big as its parent,
* minus the parent's padding, if any. Introduced in API Level 8.
*/
public static final int MATCH_PARENT = -1;
/**
* Special value for the height or width requested by a View.
* WRAP_CONTENT means that the view wants to be just large enough to fit
* its own internal content, taking its own padding into account.
*/
public static final int WRAP_CONTENT = -2;
/**
* Information about how wide the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT
* in API Level 8) or WRAP_CONTENT, or an exact size.
*/
@ViewDebug.ExportedProperty(category = "layout", mapping = {
@ViewDebug.IntToString(from = MATCH_PARENT, to = "MATCH_PARENT"),
@ViewDebug.IntToString(from = WRAP_CONTENT, to = "WRAP_CONTENT")
})
public int width;
/**
* Information about how tall the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT
* in API Level 8) or WRAP_CONTENT, or an exact size.
*/
@ViewDebug.ExportedProperty(category = "layout", mapping = {
@ViewDebug.IntToString(from = MATCH_PARENT, to = "MATCH_PARENT"),
@ViewDebug.IntToString(from = WRAP_CONTENT, to = "WRAP_CONTENT")
})
public int height;
}
LayoutParams 用于保存 View 的布局属性
ViewGroup 中定义了 ViewGroup.LayoutParams 作为布局参数的基类，另外在 ViewGroup 中，也定义了 MarginLayoutParams 类，它继承自 ViewGroup.LayoutParams，众多 ViewGroup 的子类中如：
LinearLayout.LayoutParams，FrameLayout.LayoutParams，RelativeLayout.LayoutParams 都是继承自 ViewGroup.MarginLayoutParams。
在 ViewGroup.LayoutParams 中定义了 width 和 height 属性，对应布局中的 layout_width 和 layout_height 属性
在 ViewGroup.MarginLayoutParams 中定义 margin 属性，对应布局中的 layout_margin 和
layout_marginLeft，layout_marginRight，layout_marginTop，layout_marginRight。
以 LayoutParams.width 为例，它可能是三种情况 LayoutParams.MATCH_PARENT，LayoutParams.WRAP_CONTENT 或者 一个确切的数值。
MeasureSpec
View # MeasureSpec 类
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/** @hide */
@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
@Retention(RetentionPolicy.SOURCE)
public @interface MeasureSpecMode {}
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* Extracts the mode from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the mode from
* @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
* {@link android.view.View.MeasureSpec#AT_MOST} or
* {@link android.view.View.MeasureSpec#EXACTLY}
*/
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
/**
* Extracts the size from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the size from
* @return the size in pixels defined in the supplied measure specification
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
LayoutParams 和 MeasureSpec 的关系
LayoutParams 是 View 自身要求的大小，而 MeasureSpec 是由父布局传递给子View的，用以根据父布局自身宽高、padding等，限制子View的宽高。
widthMeasureSpec 是父布局强加给子View在横向空间上的宽度要求。
heightMeasureSpec 是父布局强加给子View在纵向空间上的高度要求。
那么父布局是如何给子View计算得出 widthMeasureSpec 和 heightMeasureSpec 的呢？让我们通过综合对比 FrameLayout 和 LinearLayout 两个 ViewGroup 的子类，
来彻底弄明白 widthMeasureSpec 和 heightMeasureSpec 的产生原理。
几个重要方法
measure 方法
View # measure(int widthMeasureSpec, int heightMeasureSpec) final 方法，内部调用 onMeasure() 方法
View # onMeasure(int widthMeasureSpec, int heightMeasureSpec) 直接调用 setMeasuredDimension() 方法，用于设置 View 测量出的宽高
ViewGroup 中未重写 onMeasure() 方法，由具体的 ViewGroup 子类实现。
layout 方法
View # layout(int l, int t, int r, int b) 内部调用 onLayout() 方法
View # onLayout(boolean changed, int left, int top, int right, int bottom) 空方法体
ViewGroup # onLayout(boolean changed, int l, int t, int r, int b) 抽象方法，由具体 ViewGroup 的子类实现。
由上面 measure 和 layout 中涉及到的方法及实现方式可以得出，要实现一个 ViewGroup 的子类，必须要重写 onMeasure() 和 onLayout() 方法。
在 ViewGroup # measureChildWithMargins() 方法中，可以看出 childWidthMeasureSpec 和 childHeightMeasureSpec 与父容器的 padding 以及自身 LayoutParams 中存放的 margin 有关。
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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);
}
ViewGroup # getChildMeasureSpec(int spec, int padding, int childDimension) 方法
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
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);
}
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