【第三篇】Volley图片加载之NetworkImageView代码分析

  1 public class NetworkImageView extends ImageView {
  2     /** The URL of the network image to load */
  3     private String mUrl;
  4 
  5     /**
  6      * Resource ID of the image to be used as a placeholder until the network image is loaded.
  7      */
  8     private int mDefaultImageId;
  9 
 10     /**
 11      * Resource ID of the image to be used if the network response fails.
 12      */
 13     private int mErrorImageId;
 14 
 15     /** Local copy of the ImageLoader. */
 16     private ImageLoader mImageLoader;
 17 
 18     /** Current ImageContainer. (either in-flight or finished) */
 19     private ImageContainer mImageContainer;
 20 
 21     public NetworkImageView(Context context) {
 22         this(context, null);
 23     }
 24 
 25     public NetworkImageView(Context context, AttributeSet attrs) {
 26         this(context, attrs, 0);
 27     }
 28 
 29     public NetworkImageView(Context context, AttributeSet attrs, int defStyle) {
 30         super(context, attrs, defStyle);
 31     }
 32 
 33     /**
 34      * Sets URL of the image that should be loaded into this view. Note that calling this will
 35      * immediately either set the cached image (if available) or the default image specified by
 36      * {@link NetworkImageView#setDefaultImageResId(int)} on the view.
 37      *
 38      * NOTE: If applicable, {@link NetworkImageView#setDefaultImageResId(int)} and
 39      * {@link NetworkImageView#setErrorImageResId(int)} should be called prior to calling
 40      * this function.
 41      *
 42      * @param url The URL that should be loaded into this ImageView.
 43      * @param imageLoader ImageLoader that will be used to make the request.
 44      */
 45     public void setImageUrl(String url, ImageLoader imageLoader) {
 46         mUrl = url;
 47         mImageLoader = imageLoader;
 48         // The URL has potentially changed. See if we need to load it.
 49         loadImageIfNecessary(false);
 50     }
 51 
 52     /**
 53      * Sets the default image resource ID to be used for this view until the attempt to load it
 54      * completes.
 55      */
 56     public void setDefaultImageResId(int defaultImage) {
 57         mDefaultImageId = defaultImage;
 58     }
 59 
 60     /**
 61      * Sets the error image resource ID to be used for this view in the event that the image
 62      * requested fails to load.
 63      */
 64     public void setErrorImageResId(int errorImage) {
 65         mErrorImageId = errorImage;
 66     }
 67 
 68     /**
 69      * Loads the image for the view if it isn‘t already loaded.
 70      * @param isInLayoutPass True if this was invoked from a layout pass, false otherwise.
 71      */
 72     void loadImageIfNecessary(final boolean isInLayoutPass) {
 73         int width = getWidth();
 74         int height = getHeight();
 75 
 76         boolean wrapWidth = false, wrapHeight = false;
 77         if (getLayoutParams() != null) {
 78             wrapWidth = getLayoutParams().width == LayoutParams.WRAP_CONTENT;
 79             wrapHeight = getLayoutParams().height == LayoutParams.WRAP_CONTENT;
 80         }
 81 
 82         // if the view‘s bounds aren‘t known yet, and this is not a wrap-content/wrap-content
 83         // view, hold off on loading the image.
 84         boolean isFullyWrapContent = wrapWidth && wrapHeight;
 85         if (width == 0 && height == 0 && !isFullyWrapContent) {
 86             return;
 87         }
 88 
 89         // if the URL to be loaded in this view is empty, cancel any old requests and clear the
 90         // currently loaded image.
 91         if (TextUtils.isEmpty(mUrl)) {
 92             if (mImageContainer != null) {
 93                 mImageContainer.cancelRequest();
 94                 mImageContainer = null;
 95             }
 96             setDefaultImageOrNull();
 97             return;
 98         }
 99 
100         // if there was an old request in this view, check if it needs to be canceled.
101         if (mImageContainer != null && mImageContainer.getRequestUrl() != null) {
102             if (mImageContainer.getRequestUrl().equals(mUrl)) {
103                 // if the request is from the same URL, return.
104                 return;
105             } else {
106                 // if there is a pre-existing request, cancel it if it‘s fetching a different URL.
107                 mImageContainer.cancelRequest();
108                 setDefaultImageOrNull();
109             }
110         }
111 
112         // Calculate the max image width / height to use while ignoring WRAP_CONTENT dimens.
113         int maxWidth = wrapWidth ? 0 : width;
114         int maxHeight = wrapHeight ? 0 : height;
115 
116         // The pre-existing content of this view didn‘t match the current URL. Load the new image
117         // from the network.
118         ImageContainer newContainer = mImageLoader.get(mUrl,
119                 new ImageListener() {
120                     @Override
121                     public void onErrorResponse(VolleyError error) {
122                         if (mErrorImageId != 0) {
123                             setImageResource(mErrorImageId);
124                         }
125                     }
126 
127                     @Override
128                     public void onResponse(final ImageContainer response, boolean isImmediate) {
129                         // If this was an immediate response that was delivered inside of a layout
130                         // pass do not set the image immediately as it will trigger a requestLayout
131                         // inside of a layout. Instead, defer setting the image by posting back to
132                         // the main thread.
133                         if (isImmediate && isInLayoutPass) {
134                             post(new Runnable() {
135                                 @Override
136                                 public void run() {
137                                     onResponse(response, false);
138                                 }
139                             });
140                             return;
141                         }
142 
143                         if (response.getBitmap() != null) {
144                             setImageBitmap(response.getBitmap());
145                         } else if (mDefaultImageId != 0) {
146                             setImageResource(mDefaultImageId);
147                         }
148                     }
149                 }, maxWidth, maxHeight);
150 
151         // update the ImageContainer to be the new bitmap container.
152         mImageContainer = newContainer;
153     }
154 
155     private void setDefaultImageOrNull() {
156         if(mDefaultImageId != 0) {
157             setImageResource(mDefaultImageId);
158         }
159         else {
160             setImageBitmap(null);
161         }
162     }
163 
164     @Override
165     protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
166         super.onLayout(changed, left, top, right, bottom);
167         loadImageIfNecessary(true);
168     }
169 
170     @Override
171     protected void onDetachedFromWindow() {
172         if (mImageContainer != null) {
173             // If the view was bound to an image request, cancel it and clear
174             // out the image from the view.
175             mImageContainer.cancelRequest();
176             setImageBitmap(null);
177             // also clear out the container so we can reload the image if necessary.
178             mImageContainer = null;
179         }
180         super.onDetachedFromWindow();
181     }
182 
183     @Override
184     protected void drawableStateChanged() {
185         super.drawableStateChanged();
186         invalidate();
187     }
188 }