android : framelayout 研究

　　感覺FrameLayout很神秘，是因為用到它的地方少，一直覺得它雞肋，原來是自己無知。最近需要實現一些layer的效果，就用到了它。它的用法很簡單，這裡就不多說了，這裡就說說它的原理吧。

　　FrameLayout類裡面沒有什麼東西，主要說的還是它的自身的布局參數FrameLayout.LayoutParams，布局參數類繼承MarginLayoutParams。看名詞就知道，就是控制view的外邊距的，FrameLayout.LayoutParams本身自己定義的參數只有一個gravity。

　　好了，既然有了布局參數，那就會有空間的一些屬性吧。首先FrameLayout也是一個View，所以他必然有Padding(view的內邊距)相關屬性。這裡我們重點關注它的onMeasure與onLayout：

　　onMeasure意思就是計算出該ViewGroup的布局大小及孩子View的布局大小。

onMeasure

 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);
            }
        }
    }

　　簡單的說父容器的width就是所有孩子View的width取最大值加View外邊距和它的內邊距：child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin+getPaddingLeftWithForeground() + getPaddingRightWithForeground();如果ViewGroup有圖片，還有跟圖片比較取較大值,容器的height也是同樣的道理。FrameLayout孩子View的布局大小跟普通View大小確定是一樣的。

　　

onLayout

 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);
            }
        }
    }

　　從代碼中可以看出，孩子View左上方座標的X值就是ViewGroup的左內邊距加上view的外邊距，Y值同理。

　　哎，怎麼感覺自己寫著寫著，還把問題說複雜了點，感覺還是看代碼能明白。

　　還有，我在2.2版本的代碼上，gravity與margin屬性必須同時設定，margin才能發揮作用，如果只有margin屬性而沒有設定gravity,就沒有效果。如果有誰知道，歡迎指教哈。

　　在上一篇中說了下android裡drag and drop 一個View，當時那個imageView放在一個LinearLayout裡面，事實上放在FrameLayout同樣適用，因為他們的布局參數都繼承MarginLayoutParams類，所以通過margin控制他們的布局位置同樣在FrameLayout使用。事實上GridLayout,LinearLayout,RelativeLayout,FrameLayout的布局參數都是繼承MarginLayoutParams的。