通過源碼分析View的測量，源碼分析View測量

通過源碼分析View的測量，源碼分析View測量

 

要理解View的測量，首先要瞭解MeasureSpec，系統在測量view的寬高時，要先確定MeasureSpec。

MeasureSpec（32為int值）由兩部分組成：

SpecMode（高2位）:測量模式。

SpecSize（低30位）：某種測量模式下的規格大小。

 

SpecMode有3類：

UNSPECIFIED: 父容器不對view做大小限制，一般用於系統內部，表示一種測量狀態。

EXACTLY:精確模式。對應於：LayoutPrams中的match_parent和具體數值。

AT_MOST:最大值模式。對應於LayoutParam中的wrap_content模式。

 

接下來我們看看View的onMeasure方法：

/**     * <p>     * Measure the view and its content to determine the measured width and the     * measured height. This method is invoked by {@link #measure(int, int)} and     * should be overridden by subclasses to provide accurate and efficient     * measurement of their contents.     * </p>     *protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));    }

該方法是由measure來調用的，view寬高的測量值最終是通過setMeasuredDimension來設定，具體實現不去深究。

我們看看getDefaultSize方法：

/**     * Utility to return a default size. Uses the supplied size if the     * MeasureSpec imposed no constraints. Will get larger if allowed     * by the MeasureSpec.     *     * @param size Default size for this view     * @param measureSpec Constraints imposed by the parent     * @return The size this view should be.     */    public static int getDefaultSize(int size, int measureSpec) {        int result = size;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        switch (specMode) {        case MeasureSpec.UNSPECIFIED:            result = size;            break;        case MeasureSpec.AT_MOST:        case MeasureSpec.EXACTLY:            result = specSize;            break;        }        return result;    }

 

先從MeasureSpec中擷取的測量模式specMode和該模式下的大小specSize，然後對測量模式進行判斷，如果是UNSPECIFIED，那麼結果就是參數size，對於這個size是什麼等會討論。如果是AT_MOST和EXACTLY，那麼結果就是view測量後的大小specSize。

那麼這個參數size到底是什麼呢？它是由實參getSuggestedMinimumWidth()和getSuggestedMinimumHeight()傳下來的，我們進去getSuggestedMinimumWidth()看看。

/**     * Returns the suggested minimum width that the view should use. This     * returns the maximum of the view's minimum width)     * and the background's minimum width     *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).     * <p>     * When being used in {@link #onMeasure(int, int)}, the caller should still     * ensure the returned width is within the requirements of the parent.     *     * @return The suggested minimum width of the view.     */    protected int getSuggestedMinimumWidth() {        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());    }

 

從代碼我們可以看出如果該view設定了背景，那麼就返回max(mMinWidth, mBackground.getMinimumWidth())，否則返回mMinWidth。
mMinWidth是什麼呢？它是對應android：minWidth這個屬性的值，若是不指定，那麼預設為0.

max(mMinWidth, mBackground.getMinimumWidth())：返回mMinWidth和mBackground.getMinimumWidth()兩者中的最大值。

mBackground.getMinimumWidth()的代碼如下：

/**     * Returns the minimum width suggested by this Drawable. If a View uses this     * Drawable as a background, it is suggested that the View use at least this     * value for its width. (There will be some scenarios where this will not be     * possible.) This value should INCLUDE any padding.     *     * @return The minimum width suggested by this Drawable. If this Drawable     *         doesn't have a suggested minimum width, 0 is returned.     */    public int getMinimumWidth() {        final int intrinsicWidth = getIntrinsicWidth();        return intrinsicWidth > 0 ? intrinsicWidth : 0;    }

它返回的是Drawable的原始寬度，如果Drawable有原始寬度的話。否則返回0.

 

因此若view有設定背景，那麼size就是android：minWidth和背景最小寬度這兩者的最大值，若沒有設定背景，那麼size的值就是android：minWidth的值。