这是英文文档的第二部分（1）：DRAWEE GUIDE

由于第二部分内容多一些，所以分为2个文章发。方便大家查看。

Using Drawees in XML

Drawees have very extensive customization facilities. The best way to customize your Drawee is to do so in the XML.

Here is an example that sets nearly all possible options:

<com.facebook.drawee.view.SimpleDraweeView    android:id="@+id/my_image_view"    android:layout_width="20dp"    android:layout_height="20dp"    fresco:fadeDuration="300"    fresco:actualImageScaleType="focusCrop"    fresco:placeholderImage="@color/wait_color"    fresco:placeholderImageScaleType="fitCenter"    fresco:failureImage="@drawable/error"    fresco:failureImageScaleType="centerInside"    fresco:retryImage="@drawable/retrying"    fresco:retryImageScaleType="centerCrop"    fresco:progressBarImage="@drawable/progress_bar"    fresco:progressBarImageScaleType="centerInside"    fresco:progressBarAutoRotateInterval="1000"    fresco:backgroundImage="@color/blue"    fresco:overlayImage="@drawable/watermark"    fresco:pressedStateOverlayImage="@color/red"    fresco:roundAsCircle="false"    fresco:roundedCornerRadius="1dp"    fresco:roundTopLeft="true"    fresco:roundTopRight="false"    fresco:roundBottomLeft="false"    fresco:roundBottomRight="true"    fresco:roundWithOverlayColor="@color/corner_color"    fresco:roundingBorderWidth="2dp"    fresco:roundingBorderColor="@color/border_color"  />

Height and width mandatory

Youmustdeclare bothandroid:layout_widthandandroid:layout_height. Without both of these two, the view will not be able to lay the image out correctly.

wrap_content

Drawees do not support thewrap_contentvalue for thelayout_widthandlayout_heightattributes.

The reason for this is that the content's size changes. The size of your downloaded image can be different from your placeholder - and the failure and retry images, if any, can be still different.

Use ofwrap_contentwould force Android to do another layout pass when your image comes in - and for the layout to change before users' eyes, creating a jarring effect.

Fixing the aspect ratio

This is the one time you should usewrap_content.

You can force a DraweeView to be laid out in a particular aspect ratio. If you want a width:height ratio of 4:3, for instance, do this:

<com.facebook.drawee.view.SimpleDraweeView    android:id="@+id/my_image_view"    android:layout_width="20dp"    android:layout_height="wrap_content"    <!-- other attributes -->

Then specify your aspect ratio in Java:

mSimpleDraweeView.setAspectRatio(1.33f);

Using Drawees in Code

Change the image

The easy to way is to call

mSimpleDraweeView.setImageURI(uri);

For more advanced requirements, use acontroller builder.

Customizing the hierarchy

For most apps, specify the parameters of their hierarchy inXMLprovides all the customization they need. In some cases, though, you may need to go further.

We create an instance of the builder and then set it to the view:

List<Drawable> backgroundsList;List<Drawable> overlaysList;GenericDraweeHierarchyBuilder builder =    new GenericDraweeHierarchyBuilder(getResources());GenericDraweeHierarchy hierarchy = builder    .setFadeDuration(300)    .setPlaceholderImage(new MyCustomDrawable())    .setBackgrounds(backgroundList)    .setOverlays(overlaysList)    .build();mSimpleDraweeView.setHierarchy(hierarchy);

DonotcallsetHierarchymore than once on the same view, even if the view is recycled. The hierarchy is expensive to create and is intended to be used more than once. UsesetControllerorsetImageURIto change the image shown in it.

Modifying the hierarchy in-place

Some attributes of the hierarchy can be changed while the application is running.

You would first need to get it from the View:

GenericDraweeHierarchy hierarchy = mSimpleDraweeView.getHierarchy();

Change the placeholder

Then you could modify the placeholder, either with a resource id:

hierarchy.setPlaceholderImage(R.drawable.placeholderId);

or a full-fledgedDrawable:

Drawable drawable; // create your drawablehierarchy.setPlaceholderImage(drawable);

Change the image display

You can change thescale type:

hierarchy.setActualImageScaleType(ScalingUtils.ScaleType.CENTER_INSIDE);

If you have chosen scale typefocusCrop,you'll need to set a focus point:

hierarchy.setActualImageFocusPoint(point);

You can add a color filter to the image:

ColorFilter filter;// create your filterhierarchy.setActualImageColorFilter(filter);

Rounding

All of therounding related params, except the rounding method, can be modified. You get aRoundingParamsobject from the hierarchy, modify it, and set it back again:

RoundingParams roundingParams = hierarchy.getRoundingParams();roundingParams.setCornersRadius(10);hierarchy.setRoundingParams(roundingParams);

Drawee Image Branches

Definitions
Actual
Placeholder
Failure
Retry
Progress Bar
Backgrounds
Overlays
Pressed State Overlay

Definitions

This page outlines the different image branches that can be displayed in a Drawee, and how they are set.

Except for the actual image, all of them can be set by an XML attribute. The value in XML must be either an Android drawable or color resource.

They can also be set by a method in theGenericDraweeHierarchyBuilderclass, ifsetting programmatically. In code, the value can either be from resources or be a custom subclass ofDrawable.

Some of the items can even be changed on the fly after the hierarchy has been built. These have a method in theGenericDraweeHierarchyclass.

Several of the drawables can bescaled.

Actual

Theactualimage is the target; everything else is either an alternative or a decoration. This is specified using a URI, which can point to an image over the Internet, a local file, a resource, or a content provider.

This is a property of the controller, not the hierarchy. It therefore is not set by any of the methods used by the other Drawee components.

Instead, use thesetImageURImethod orset a controllerprogrammatically.

In addition to the scale type, the hierarchy exposes other methods only for the actual image. These are:

the focus point (used for thefocusCropscale type only)
a color filter

Default scale type:centerCrop

Placeholder

Theplaceholderis shown in the Drawee when it first appears on screen. After you have calledsetControllerorsetImageURIto load an image, the placeholder continues to be shown until the image has loaded.

In the case of a progressive JPEG, the placeholder only stays until your image has reached the quality threshold, whether the default, or one set by your app.

XML attribute:placeholderImage
Hierarchy builder method:setPlaceholderImage
Hierarchy method:setPlaceholderImage
Default value: a transparentColorDrawable
Default scale type:centerInside

Failure

Thefailureimage appears if there is an error loading your image. The most common cause of this is an invalid URI, or lack of connection to the network.

XML attribute:failureImage
Hierarchy builder method:setFailureImage
Default value: The placeholder image
Default scale type:centerInside

Retry

Theretryimage appears instead of the failure image if you have set your controller to enable the tap-to-retry feature.

You mustbuild your own Controllerto do this. Then add the following line

.setTapToRetryEnabled(true)

The image pipeline will then attempt to retry an image if the user taps on it. Up to four attempts are allowed before the failure image is shown instead.

XML attribute:retryImage
Hierarchy builder method:setRetryImage
Default value: The placeholder image
Default scale type:centerInside

Progress Bar

If specified, theprogress barimage is shown as an overlay over the Drawee until the final image is set.

Currently the progress bar remains the same throughout the image load; actually changing in response to progress is not yet supported.

XML attribute:progressBarImage
Hierarchy builder method:setProgressBarImage
Default value: None
Default scale type:centerInside

Backgrounds

Backgrounddrawables are drawn first, "under" the rest of the hierarchy.

Only one can be specified in XML, but in code more than one can be set. In that case, the first one in the list is drawn first, at the bottom.

Background images don't support scale-types and are scaled to the Drawee size.

XML attribute:backgroundImage
Hierarchy builder method:setBackground,setBackgrounds
Default value: None
Default scale type: N/A

Overlays

Overlaydrawables are drawn last, "over" the rest of the hierarchy.

Only one can be specified in XML, but in code more than one can be set. In that case, the first one in the list is drawn first, at the bottom.

Overlay images don't support scale-types and are scaled to the Drawee size.

XML attribute:overlayImage
Hierarchy builder method:setOverlay,setOverlays
Default value: None
Default scale type: N/A

Pressed State Overlay

Thepressed state overlayis a special overlay shown only when the user presses the screen area of the Drawee. For example, if the Drawee is showing a button, this overlay could have the button change color when pressed.

The pressed state overlay doesn't support scale-types.

XML attribute:pressedStateOverlayImage
Hierarchy builder method:setPressedStateOverlay
Default value: None
Default scale type: N/A

Scaling

You can specify a different scale type for each of thedifferent drawablesin your Drawee. The

Available scale types

Scale Type	Explanation
center	Center the image in the view, but perform no scaling.
centerCrop	Scales the image so that both dimensions will be greater than or equal to the corresponding dimension of the parent. One of width or height will fit exactly. The image is centered within parent's bounds.
focusCrop	Same as centerCrop, but based around a caller-specified focus point instead of the center.
centerInside	Downscales the image so that it fits entirely inside the parent. Unlike`fitCenter,`no upscaling will be performed. Aspect ratio is preserved. The image is centered within parent's bounds.
fitCenter	Scales the image so that it fits entirely inside the parent. One of width or height will fit exactly. Aspect ratio is preserved. The image is centered within the parent's bounds.
fitStart	Scales the image so that it fits entirely inside the parent. One of width or height will fit exactly. Aspect ratio is preserved. The image is aligned to the top-left corner of the parent.
fitEnd	Scales the image so that it fits entirely inside the parent. One of width or height will fit exactly. Aspect ratio is preserved. The image is aligned to the bottom-right corner of the parent.
fitXY	Scales width and height independently, so that the image matches the parent exactly. Aspect ratio is not preserved.
none	Used for Android's tile mode.

These are mostly the same as those supported by the AndroidImageViewclass.

The one unsupported type ismatrix.In its place, Fresco offersfocusCrop,which will usually work better.

How to set

Actual, placeholder, retry, and failure images can all beset in XML, using attributes likefresco:actualImageScaleType. You can also set themin codeusing theGenericDraweeHierarchyBuilderclass.

Even after your hierarchy is built, the actual image scale type can be modified on the fly usingGenericDraweeHierarchy.

However, donotuse theandroid:scaleTypeattribute, nor the.setScaleTypemethod. These have no effect on Drawees.

focusCrop

Android, and Fresco, offer acenterCropscale type, which will fill the entire viewing area while preserving the aspect ratio of the image, cropping as necessary.

This is very useful, but the trouble is the cropping doesn't always happen where you need it. If, for instance, you want to crop to someone's face in the bottom right corner of the image,centerCropwill do the wrong thing.

By specifying a focus point, you can say which part of the image should be centered in the view. If you specify the focus point to be at the top of the image, such as (0.5f, 0f), we guarantee that, no matter what, this point will be visible and centered in the view as much as possible.

Focus points are specified in a relative coordinate system. That is, (0f, 0f) is the top-left corner, and (1f, 1f) is the bottom-right corner. Relative coordinates allow focus points to be scale-invariant, which is highly useful.

A focus point of (0.5f, 0.5f) is equivalent to a scale type ofcenterCrop.

To use focus points, you must first set the right scale type in your XML:

  fresco:actualImageScaleType="focusCrop"

In your Java code, you must programmatically set the correct focus point for your image:

PointF focusPoint;// your app populates the focus pointmSimpleDraweeView    .getHierarchy()    .setActualImageFocusPoint(focusPoint);

none#

If you are using Drawables that make use of Android's tile mode, you need to use thenonescale type for this to work correctly.

Rounded Corners and Circles

Not every image is a rectangle. Apps frequently need images that appear with softer, rounded corners, or as circles. Drawee supports a variety of scenarios, all without the memory overhead of copying bitmaps.

What

Images can be rounded in two shapes:

As a circle - setroundAsCircleto true.
As a rectangle, but with rounded corners. SetroundedCornerRadiusto some value.

Rectangles support having each of the four corners have a different radius, but this must be specified in Java code rather than XML.

How

Images can be rounded with two different methods:

BITMAP_ONLY- Uses a shader to draw the bitmap with rounded corners. This is the default rounding method. This works only on the actual image and theplaceholder. Other branches, like failure and retry images, are not rounded. Furthermore, this rounding method doesn't support animations.
OVERLAY_COLOR- Draws rounded corners by overlaying a solid color, specified by the caller. The Drawee's background should be static and of the same solid color. UseroundWithOverlayColorin XML, orsetOverlayColorin code, for this effect.

In XML

TheSimpleDraweeViewclass will forward several attributes over toRoundingParams:

<com.facebook.drawee.view.SimpleDraweeView   ...   fresco:roundedCornerRadius="5dp"   fresco:roundBottomLeft="false"   fresco:roundBottomRight="false"   fresco:roundWithOverlayColor="@color/blue"   fresco:roundingBorderWidth="1dp"   fresco:roundingBorderColor="@color/red"

In code#

Whenconstructing a hierarchy, you can pass an instance ofRoundingParamsto yourGenericDraweeHierarchyBuilder:

RoundingParams roundingParams = RoundingParams.fromCornersRadius(7f);roundingParams.setOverlayColor(R.color.green);// alternatively use fromCornersRadii or asCirclegenericDraweeHierarchyBuilder    .setRoundingParams(roundingParams);

You can also change most of the rounding parameters on the fly:

RoundingParams roundingParams =     mSimpleDraweeView.getHierarchy().getRoundingParams();roundingParams.setBorder(R.color.red, 1.0);roundingParams.setRoundAsCircle(true);mSimpleDraweeView.getHierarchy().setRoundingParams(roundingParams);

The one exception to this is that theRoundingMethodcannot be changed when changing dynamically. Attempting to do so will throw anIllegalStateException.

Android(安卓)Fresco图片处理库用法API英文原文文档2-1（Facebook开源Android图片库）