android : framelayout 研究

最新推荐文章于 2023-06-30 14:12:34 发布

转载最新推荐文章于 2023-06-30 14:12:34 发布 · 592 阅读

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本文详细解析FrameLayout布局机制，包括其布局参数FrameLayout.LayoutParams的控制方式，以及内部实现的onMeasure与onLayout方法原理。揭示FrameLayout如何计算子View的大小与位置，以及在配置gravity与margin时的相互作用。

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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。

原文：http://www.cnblogs.com/slider/archive/2011/12/30/2302413.html