android ImageView setImageDrawable 和 setImageResource 的区别

/**
     * Sets a drawable as the content of this ImageView.
     *
     * <p class="note">This does Bitmap reading and decoding on the UI
     * thread, which can cause a latency hiccup.  If that‘s a concern,
     * consider using {@link #setImageDrawable(android.graphics.drawable.Drawable)} or
     * {@link #setImageBitmap(android.graphics.Bitmap)} and
     * {@link android.graphics.BitmapFactory} instead.</p>
     *
     * @param resId the resource identifier of the drawable
     *
     * @attr ref android.R.styleable#ImageView_src
     */
    @android.view.RemotableViewMethod
    public void setImageResource(int resId) {
        if (mUri != null || mResource != resId) {
            final int oldWidth = mDrawableWidth;
            final int oldHeight = mDrawableHeight;
            updateDrawable(null);
            mResource = resId;
            mUri = null;
            resolveUri();
            if (oldWidth != mDrawableWidth || oldHeight != mDrawableHeight) {
                requestLayout();
            }
            invalidate();
        }
    }

 /**
     * Sets a drawable as the content of this ImageView.
     * 
     * @param drawable The drawable to set
     */
    public void setImageDrawable(Drawable drawable) {
        if (mDrawable != drawable) {
            mResource = 0;
            mUri = null;
            final int oldWidth = mDrawableWidth;
            final int oldHeight = mDrawableHeight;
            updateDrawable(drawable);
            if (oldWidth != mDrawableWidth || oldHeight != mDrawableHeight) {
                requestLayout();
            }
            invalidate();
        }
    }

/**
     * Sets a Bitmap as the content of this ImageView.
     * 
     * @param bm The bitmap to set
     */
    @android.view.RemotableViewMethod
    public void setImageBitmap(Bitmap bm) {
        // if this is used frequently, may handle bitmaps explicitly
        // to reduce the intermediate drawable object
        setImageDrawable(new BitmapDrawable(mContext.getResources(), bm));
    }