关于getView中图片重复加载

在ListView或GridView中，经常会出现getView反复调用的问题。究其原因，是视图进行绘制前多次onMeasure的结果。

要解决这个问题，就要避免父视图反复测量以适应整个布局，尽量避免使用wrap_content属性。

普通的getView反复调用问题并不会有太大的影响，会产生影响的往往是在getView中实行的操作，比如在getView中加载图片之类的。

例如，我要在ScrollView中嵌入一个GridView中，然后在GridView显示图片，为了避免一些奇怪的问题产生（比如滑动冲突或者干脆没法滑动），就要覆写onMeasure方法使其自适应：

public class AlbumGridView extends GridView {

public AlbumGridView(Context context) {
super(context);
}

public AlbumGridView(Context context, AttributeSet attrs) {
super(context, attrs);
}

public AlbumGridView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int height =MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, height);
}
}

我将AlbumGridView放入ScrollView中，然后用AlbumGridView加载图片，得到以下Log：

05-31 08:21:56.729 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.729 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.730 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.730 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (3).jpg
05-31 08:21:56.731 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (2).jpg
05-31 08:21:56.734 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (1).jpg
05-31 08:21:56.800 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.800 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.817 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.817 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.850 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.851 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.878 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:21:56.879 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg

可以看到，getView在重复调用，且第一张图片img (4).jpg更是反复调用了11次。总共4张图片，调用了14次，试验多次，第一张图片总是会反复调用多次——单纯调用getView影响并不大，但因为是图片加载，要在getView中根据uri加载图片，这样问题就大了。

于是改写AlbumGridView并在getView中判断是否在测量中，如果在测量中，就先不加载图片，否则就开始加载图片，这样一来会节省很多资源：

public class AlbumGridView extends GridView {
public boolean isOnMeasure;

public AlbumGridView(Context context) {
super(context);
}

public AlbumGridView(Context context, AttributeSet attrs) {
super(context, attrs);
}

public AlbumGridView(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
isOnMeasure = true;
int height = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
super.onMeasure(widthMeasureSpec, height);
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
isOnMeasure = false;
super.onLayout(changed, l, t, r, b);
}

public boolean isOnMeasure() {
return isOnMeasure;
}

加一个isOnMeasure标记以判断视图当前状态；

public class PhotosAdapter extends BaseAdapter {

......

@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view;
if (convertView == null) {
view = layoutInflater.inflate(R.layout.item, parent, false);//inflate item
} else {
view = convertView;
}

if (parent instanceof AlbumGridView) {
if (((AlbumGridView) parent).isOnMeasure()) {
return view;
} else {
Log.w("load",""+url);
//加载图片
}
}
return view;
}

}

这样一来，getView其实还是反复在调用，但是图片加载却不会再反复加载了。其Log如下：

05-31 08:40:54.750 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (4).jpg
05-31 08:40:54.751 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (3).jpg
05-31 08:40:54.751 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (2).jpg
05-31 08:40:54.752 1689-1689/com.xter.picbrowser W/load: file:///storage/emulated/0/Download/img/img (1).jpg