Facebook的Shimmer源码阅读

源码的github地址：

https://github.com/facebook/Shimmer

使用示例：

FBShimmeringView *shimmeringView = [[FBShimmeringView alloc] initWithFrame:self.view.bounds];
[self.view addSubview:shimmeringView];

UILabel *loadingLabel = [[UILabel alloc] initWithFrame:shimmeringView.bounds];
loadingLabel.textAlignment = NSTextAlignmentCenter;
loadingLabel.text = NSLocalizedString(@"Shimmer", nil);
shimmeringView.contentView = loadingLabel;

// Start shimmering.
shimmeringView.shimmering = YES;

contentView为FBShimmeringView的subView，setContentView时，添加contentView，获取contentView的layer，并将其强转为FBShimmeringLayer。

Shimmer的效果是由FBShimmeringLayer的subLayer，FBShimmeringMaskLayer（CAGradientLayer的子类）实现。

View和contentView，以及Layer和contentLayer。

/* An object that will receive the CALayer delegate methods defined

 * below (for those that it implements). The value of this property is

 * not retained. Default value is nil. */

@property(weak) id delegate;

这是一个informal protocol，NSObject的Category，CALayerDelegate。

__bridge

A bridged cast is a C-style cast annotated with one of three keywords:

• (__bridge T) op casts the operand to the destination type T. If T is a retainable object pointer type, then op must have a non-retainable pointer type. If T is a non-retainable pointer type, then op must have a retainable object pointer type. Otherwise the cast is ill-formed. There is no transfer of ownership, and ARC inserts no retain operations.
• (__bridge_retained T) op casts the operand, which must have retainable object pointer type, to the destination type, which must be a non-retainable pointer type. ARC retains the value, subject to the usual optimizations on local values, and the recipient is responsible for balancing that +1.
• (__bridge_transfer T) op casts the operand, which must have non-retainable pointer type, to the destination type, which must be a retainable object pointer type. ARC will release the value at the end of the enclosing full-expression, subject to the usual optimizations on local values.

These casts are required in order to transfer objects in and out of ARC control; see the rationale in the section on conversion of retainable object pointers.

Using a __bridge_retained or __bridge_transfer cast purely to convince ARC to emit an unbalanced retain or release, respectively, is poor form.

官方文档中指出：__bridge兼顾ARC和非ARC两种内存管理系统，两者互不影响，而__bridge_retained和__bridge_transfer会在两种内存管理系统中切换。