通過源碼分析View的測量

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來設置，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中獲取的測量模式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設置了背景，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有設置背景，那麼size就是android：minWidth和背景最小寬度這兩者的最大值，若沒有設置背景，那麼size的值就是android：minWidth的值。